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10 ton asteroid impact	has average interval time 1 year
	is a kind of asteroid impact
100 ton asteroid impact	has average interval time 10 years
	is a kind of asteroid impact
1000 ton asteroid impact	has average interval time 100 years
	is a kind of asteroid impact
16th century scientist	has definition a scientist born between 1500 and 1600 AD
	is a kind of dead person
	is a kind of scientist
17 Leporis	has companion M1 III
	has definition A close binary system (A0 V, M1 III) with a shell-like spectrum indicating that mass transfer may be occurring from the late-type companion onto the A0 primary.
	has primary A0 V
	is a part of Lepus
	is an instance of binary star
	is an instance of shell star
17th century scientist	has definition a scientist born between 1600 and 1700 AD
	is a kind of dead person
	is a kind of scientist
18th century scientist	has definition a scientist born between 1700 and 1800 AD
	is a kind of dead person
	is a kind of scientist
19th century scientist	has definition a scientist born between 1800 and 1900 AD
	is a kind of dead person
	is a kind of scientist
2-meter Telescope	has altitude 331 m
	has aperture 1.34 m
	has comment can also be used in Cassegrain and coude modes
	has creation date 1960
	has focal ratio f/3.00
	has has mirror diameter 2 m
	has latitude 50° 59' N
	has location Tautenberg, Germany
	has longitude 11° 43'E
	has mirror maker Zeiss (Jena)
	has mirror type Schott ZK-7 glass
	has mounting manufacturer Zeiss (Jena)
	has owner Karl Schwarzschild Observatorium
	has synonym Tautenberg Schmidt
	is an instance of Fork equatorial telescope
	is an instance of Schmidt
2.3-meter Telescope	has altitude 725 m
	has aperture 2.33 m
	has creation date 1985
	has focal ratio f/3.25, 13, 43
	has latitude 12° 35' N
	has location Kavalur, Tamil Nadu, India
	has longitude 78° 50' E
	has mirror maker Indian Inst. Astrophys.
	has mirror type Zerodur
	has mounting manufacturer Walchandnagar Industries
	has owner Vainu Bappu Observatory
	has synonym Vainu Bappu 2.3 m
	is an instance of Horseshoe equatorial telescope
	is an instance of optical telescope
	is an instance of reflector
20th century scientist	has definition a scientist born between 1900 and 2000 AD
	is a kind of scientist
26-inch Equatorial	has altitude 92 m
	has aperture 0.66 m
	has creation date 1873
	has focal ratio f/15.0
	has latitude 38° 55' N
	has lens maker Alvan Clark & Sons
	has location Washington, DC, US
	has longitude 77° 04' W
	has mounting manufacturer Warner and Swasey
	has owner US Naval Observatory
	is an instance of German equatorial telescope
	is an instance of refractor
28-inch Visual Refractor	has altitude 47 m
	has aperture 0.711 m
	has comment Dome under repair in 1992
	has creation date 1894
	has focal ratio f/11.9
	has latitude 51° 29' N
	has lens maker Gmbh
	has location Greenwich, London, England
	has longitude 00° 00'
	has mounting manufacturer Ransomes and Sims
	has owner Old Royal Observatory
	has synonym Greenwich refractor
	is an instance of English equatorial telescope
	is an instance of refractor
2U 1543-47	is an instance of transient X-ray source
3 alpha process	has definition A nuclear reaction (3 4He → 12C + γ + 7 MeV) by which helium is transformed into carbon. The process is dominant in red giants. At a temperature of about 2 × 108 K and a density of 105 g cm-3, after core hydrogen is exhausted, three α-particles can fuse to form an excited nucleus of carbon 12, which occasionally decays into a stable carbon 12 nucleus. The overall process can be looked upon as an equilibrium between three helium nuclei and the excited 12C*, with occasional irreversible leakage out of the equilibrium into the ground state of carbon 12. Further capture of α-particles by carbon 12 nuclei produces oxygen 16 and neon 20. (also called the triple-α process)
	is a kind of nucleosynthetic reaction
3-kpc arm	has definition A component of the Sagittarius arm with noncircular gas motions. It is seen in absorption against Sgr A with a velocity of -53 km s-1, implying that at least part of the arm is expanding away from the galactic center. The nearest "edge" is presently at a radius of 4 kpc from the Galactic center.
	is a part of Sagittarius arm
3.5-meter Telescope	has altitude 2168 m
	has aperture 3.50 m
	has creation date 1984
	has focal ratio f/3.5, 3.9, 10, 35
	has latitude 37° 13' N
	has location Calar Alto, Spain
	has longitude 2° 32' W
	has mirror maker Zeiss (Ober.)
	has mirror type Zerodur
	has mounting manufacturer Voith, Heidenheim; Zeiss (Ober.)
	has operator German-Spanish Astronomical Center
	has owner Calar Alto Observatory
	is an instance of Horseshoe equatorial telescope
	is an instance of optical telescope
	is an instance of Ritchey-Chrétien
30 Doradus Nebula	has absolute magnitude Mv = - 19
	has definition A giant H II region, at least 300 pc across - one of the largest known. It is larger and more luminous than any known in the Galaxy. It is the brightest object in the Large Magellanic Cloud at both optical and radio wavelengths, and contains the densest concentration of W-R stars. (The brightest object near the center is a O+ WN star of Mv = - 10.2.) It is characterized by very rapid, disordered, and complex motions.
	has distance 170000 light years
	has image
	has synonym Great Looped Nebula
	has synonym NGC 2070
	has synonym Tarantula Nebula
	is a part of Large Magellanic Cloud
	is an instance of gaseous nebula
36-inch Refractor	has altitude 1290 m
	has aperture 0.895 m
	has comment Front surface of crown element refigured in 1987
	has creation date 1888
	has focal ratio f/19.7
	has latitude 37° 20' N
	has lens maker Alvan Clark & Sons
	has location Mount Hamilton, Calif., US
	has longitude 121° 39' W
	has mounting manufacturer Warner and Swasey
	has owner Lick Observatory
	has synonym 36 inch
	is an instance of German equatorial telescope
	is an instance of refractor
3TA-10 Schmidt Telescope	has altitude 1450 m
	has aperture 1.00 m
	has comment three 1 m diameter objective prisms
	has creation date 1961
	has focal ratio f/2.13
	has latitude 40° 20' N
	has location Mount Aragatz, Armenia
	has longitude 44° 30' E
	has mirror diameter 1.5 m
	has mirror maker LOMO
	has mounting manufacturer LOMO
	has owner Byurakan Astrophysical Observatory
	has synonym Byurakan Schmidt
	is an instance of Schmidt
4-kpc arm	has definition A component of the Sagittarius arm with noncircular gas motions.
	is a part of Sagittarius arm
40 Eridani	has definition A nearby triple system, 5 pc distant. Component A is K0 V; component B is a DA white dwarf; component C is M5e V.
	is a part of Eridanus
	is an instance of triple star
47 Tucanae	has definition A metal-rich globular cluster. It has roughly one-quarter the solar metal abundance. It has a high galactic latitude and low reddening. It is a member of the thick-disk population.
	has distance 15000 light years
	has image
	is a part of halo
	is a part of Tucana
	is an instance of unclassified globular cluster
4N nucleus	has definition Nuclei possessing equal and even numbers of neutrons and protons. 4N nuclei are formed in supernova envelopes at temperatures of at least 2 × 109 K and are very stable.
	has synonym even-even nucleus
	is a kind of Bose-Einstein nucleus
53 Arietis	has definition Runaway star which diverges from a comparatively small area in Orion.
	is a part of Aries
	is an instance of runaway star
61 Cygni	has definition The first star other than the Sun to have its parallax, and hence distance, measured. The star is a double orange dwarf.
	has distance 11.4 light-years
	is a part of Cygnus
	is an instance of high proper motion star
[O III] line	is a kind of forbidden line
[WR] star	has definition WR star that is the central object of a planetary nebula.
	is a kind of Wolf-Rayet
A band	has definition One of about a dozen of the strongest Fraunhofer lines seen in the Solar spectrum, the A band at 7600 angstoms is due to telluric lines of molecular oxygen in the Earth's atmosphere. (originally thought to originate in the Sun by Fraunhofer)
	has species O2
	has wavelength 7600 Å
	is a kind of Fraunhofer line
	is a kind of molecular band
	occurs in Earth's atmosphere
A ring	is a kind of ring
	is a part of Saturn ring system
A star	has absorption line H I
	has color white
	has definition Star with spectral type A in which the spectrum of the Balmer lines of hydrogen attain their greatest strength. Helium lines can no longer be seen. Some metallic lines are present; in late A stars the H and K lines of ionized calcium appear. A0 stars have a color index of zero.
	has surface temperature 10000 K
	is a kind of early star
ablation	has definition Removal of material from a solid by heating, vaporization or collisions
	has example Erosion of a meteorite by the friction generated when it passes through the Earth's atmosphere. The material passes from solid to gas phase.
	has final phase gas
	has initial phase solid
	has inverse process condensation
	is a kind of first order phase transition
absolute magnitude	has definition A measure of the intrinsic brightness of a star or galaxy. Absolute magnitude is defined as the apparent magnitude the star or galaxy would have if it were 32.6 light-years (10 parsecs) from Earth. The lower an object's absolute magnitude, the greater its intrinsic brightness. For example, the Sun has an absolute magnitude of +4.83, while Sirius, whose intrinsic brightness is greater, has an absolute magnitude of +1.43. A star that is one absolute magnitude brighter than another (e.g., +4 versus +5) is 2.5 times intrinsically brighter; a star that is 5 absolute magnitudes brighter is 100 times intrinsically brighter; and a star that is 10 absolute magnitudes brighter is 10000 times intrinsically brighter.
	has definition The absolute magnitude (g) of a solar-system body such as an asteroid is defined as the brightness at zero phase angle when the object is 1 AU from the Sun and 1 AU from the observer.
	is a kind of magnitude
absolute temperature	has definition Temperature measured on the Kelvin scale: 0 Kelvin = -273.15 ° Celsius. Absolute temperature is directly related to (kinetic) energy via the equation E = kBT, where kB is Boltzmann's constant. So, a temperature of 0 K corresponds to zero energy, and room temperature, 300 K = 27 °C, corresponds to an energy of 0.025 eV.
	is a kind of temperature
absolute zero	has definition The lowest possible temperature, attained when a system is at its minimum possible energy. The Kelvin temperature scale sets its zero point at absolute zero (-273.15° on the Celsius scale, and -434.07° on the Fahrenheit scale).
	is an instance of temperature
absorption	has definition Decrease in the intensity of radiation, representing energy converted into excitation or ionization of electrons in the region through which the radiation travels. As contrasted with monochromatic scattering (in which reemission occurs in all directions at the same frequency), the inverse process of emission refers to radiation that is reemitted in general in all directions and at all frequencies.
	has inverse process emission
	is a kind of radiation intensity modification
absorption edge	has definition Sudden rises superposed on the smooth decrease of the curve of the attenuation coefficient, which cause the curve to have a typical sawtooth aspect. They generally occur at the limit of spectral lines.
	is a kind of spectral feature
absorption line	has antonym emission line
	has definition Dark line in a spectrum, produced when light or other electromagnetic radiation coming from a distant source passes through a gas cloud or similar object closer to the observer. Like emission lines, absorption lines betray the chemical composition and velocity of the material that produces them.
	has definition Dark line superposed on a continuous spectrum, caused by the absorption of light passing through a gas of lower temperature than the continuum light source.
	is a kind of spectral line
abstraction	has definition a general concept formed by extracting common features from specific examples
	is a kind of kbTop
acceleration unit	has definition meter per second squared
	has symbol m·s-2
	is a kind of derived SI unit
accretion	has definition A process by which a star accumulates matter as it moves through a dense cloud of interstellar gas; or, more generally, whereby matter surrounding a star flows toward it (as in close binaries).
	has definition Collection of material together, generally to form a single body.
	is a kind of contraction
accumulating oceanic element	has ocean concentration depth indenpendent
	has ocean residence time 105 years or greater
	is a kind of oceanic element
accumulation theory	has definition The theory by which planetesimals are assumed to collide with one another and coalesce, eventually sweeping up enough material to form the planets.
	is a kind of planetary theory
acetaldehyde	has symbol CH3CHO
	is an instance of interstellar molecule
Achernar	has B-V magnitude -0.16
	has declination -57 14 12
	has distance 35 pc
	has right ascension 1 37 42.9
	has spectral type B3Vpe
	has spectral type B5
	has synonym alpha Eridani
	has synonym HR 472
	has V magnitude 0.46
	is a part of Eridanus
	is an instance of B star
	is an instance of naked eye star
	is an instance of subgiant
Achilles	has asteroid number 588
	has definition Trojan asteroid (60°) ahead of Jupiter. It was the first Trojan to be discovered.
	has discovery date 1906
	has eccentricity e = 0.15
	has inclination i = 10°.3
	has orbital period P = 11.98 yr
	has semi-major axis a = 5.2 AU
	is a part of Trojan asteroid
	is an instance of Trojan asteroid
achromatic lens	has definition Lens (or combination of lenses) that brings different wavelenghts within a ray of light to a single focus, thus overcoming chromatic aberration. It was first successfully made by Joseph von Fraunhofer.
	is a kind of lens
achromatic objective	has definition A lens of two or more components with different refraction indices (e.g., crown glass and flint glass), used to correct for chromatic aberration.
	is a kind of achromatic lens
actinide	has atomic number 89 to 103
	has definition Element with atomic number between 89 (Actinium) and 103
	has filling orbital 5f
	is a kind of inner transition metal
	is a kind of inner transition metal
	is a kind of metal
actinium	has abundance minute traces in Earth's crust
	has abundance n.a. in Sun relative to H = 1 × 1012
	has abundance nil in seawater
	has atomic emission line 383.312 nm for Ac II
	has atomic emission line 408.844 nm for Ac II
	has atomic emission line 450.720 nm for Ac II
	has atomic emission line 591.085 nm for Ac II
	has atomic emission line 416.840 nm for Ac II (strong)
	has atomic emission line 438.641 nm for Ac II (strong)
	has atomic number 89
	has atomic radii 188 pm
	has biological role none
	has boiling point 3470 ± 300 K
	has chief source uranium ore contain 0.2 p.p.m.
	has crystal cell dimension a = 531.1 pm
	has crystal type f.c.c.
	has daily dietary intake nil
	has definition soft, silvery-white, radioactive metal which glows in the dark
	has density 10060 kg m-3 for solid at 293 K
	has discoverer Andre Debierne
	has discovery date 1899
	has discovery location Paris, France
	has electron configuration [Rn]6d17s2 in ground state
	has electronegativity 1.1 Pauling
	has hazard never encountered normally, dangerous because it is a powerful source of α-radiation, element found only inside nuclear facilities or research laboratories
	has heat capacity 20.84 J K-1 mol-1 for gas at constant pressure 0.1 MPa at 298.15 K
	has heat capacity 27.2 J K-1 mol-1 for solid at constant pressure 0.1 MPa at 298.15 K
	has heat of fusion 14.2 kJ mol-1
	has heat of sublimation 51.9 ± 0.5 kJ mol-1
	has heat of vaporization 418 kJ mol-1
	has image
	has ionic radii 118 pm for Ac3+
	has isotope mass range 209 to 232
	has level in humans nil
	has linear expansion coefficient 14.9 K-1
	has longest lived isotope actinium 227
	has mass absorption coefficient cm2 g-1 for CuKα X-ray diffraction
	has mass absorption coefficient cm2 g-1 for MoKα X-ray diffraction
	has mass of element in person nil for a 70 kg average person
	has melting point 1320 ± 50 K
	has molar volume 22.6 cm3
	has name origin aktinos from Greek word for 'ray'
	has number of isotopes 26
	has number of protons 89
	has pronunciation ak-tin-iuhm
	has registry number 7440-34-8 for Chemical Abstracts System database
	has relative atomic mass 227.0728 for 227Ac in units of 12C = 12.000
	has space group Fm3m
	has specimen not commerecially available
	has symbol Ac
	has symbol Ac
	has synthesis mechanism bombardment of radium 236 with neutrons
	has synthesis mechanism decay of uranium 235
	has term symbol 2D3/2 in ground state
	has thermal conductivity 12 W m-1 K-1 at 300 K
	has world production probably less than a gram
	is a kind of actinide
	is a kind of radioactive element
	reacts with water to release hydrogen gas
actinium 224	has atomic mass 224.021685
	has decay mode α (6.323 MeV) 9.1%
	has decay mode γ
	has decay mode EC (1.397 MeV) 90.9%
	has half life 2.9 hours
	has nuclear spin I =
	has number of neutrons 135
	has number of nucleons 224
	has symbol 224Ac
	is an instance of actinium
actinium 225	has atomic mass 225.023205
	has decay mode α (5.935 MeV)
	has decay mode γ
	has half life 10.0 days
	has nuclear spin I = 3/2+
	has number of neutrons 136
	has number of nucleons 225
	has symbol 225Ac
	has uses tracer
	is an instance of actinium
actinium 226	has atomic mass 226.026084
	has decay mode α 6 × 10-3%
	has decay mode β- (1.117) 83 %
	has decay mode γ
	has decay mode EC (0.635 MeV) 17%
	has half life 29 hours
	has nuclear spin I = 1-
	has number of neutrons 137
	has number of nucleons 226
	has symbol 226Ac
	is an instance of actinium
actinium 227	has atomic mass 227.027750
	has decay mode α (5.043 Mev) 1.380%
	has decay mode β- (0.0410 Mev) 98.620%
	has decay mode γ
	has decay products radium 223
	has decay products thorium 227
	has half life 21.773 years
	has magnetogyric ratio 3.5 × 107 rad T-1 s-1
	has NMR frequency 13.1 MHz where 1H = 100 MHz; 2.3488 T
	has NMR receptivity where 13C = 1.00
	has nuclear magnetic moment μ = +1.1
	has nuclear quadrupole moment 1.7 × 10-28 m2
	has nuclear spin I = 3/2-
	has number of neutrons 138
	has number of nucleons 227
	has relative NMR sensitivity where 1H = 1.00
	has symbol 227Ac
	has thermal neutron capture cross section 515 barns
	has uses Nuclear Magnetic Resonance
	is an instance of actinium
actinium 228	has atomic mass 228.031015
	has decay mode β- (2.142 MeV)
	has decay mode γ
	has half life 6.15 hours
	has nuclear spin I = 3+
	has number of neutrons 139
	has number of nucleons 228
	has symbol 228Ac
	is an instance of actinium
actinium 229	has atomic mass 229.032980
	has decay mode β- (0.0410 MeV) 99%
	has decay mode γ
	has half life 62.7 minutes
	has nuclear spin I = 3/2+
	has number of neutrons 140
	has number of nucleons 229
	has symbol 229Ac
	is an instance of actinium
active galaxy	has definition Active galactic nuclei are very luminous. Their energy output is in two forms: nonthermal continuum and thermal emission line.
	has definition Any galaxy which is emitting large quantities of non-thermal radiation.
	has luminosity 1043 to 1046 ergs s-1
	is a kind of galaxy
active optics	has definition Controlling the shape of a telescope mirror at a relatively slow rate.
	is a kind of mirror
active satellite	is a kind of artificial satellite
active Sun	has definition The Sun during its 11-year cycle of activity when spots, flares, prominences, and variations in radiofrequency radiation are at a maximum.
	has period 11 years
	is an instance of periodic celestial event
	is an instance of solar event
adaptive optics	has definition Compensating for atmospheric distortions in a wavefront by high-speed changes in the shape of a small, thin mirror.
	is a kind of mirror
advance of the perihelion	has definition The slow rotation of the major axis of a planet's orbit in the same direction as the revolution of the planet itself, due to gravitational interactions with other planets and/or other effects (such as those due to general relativity).
	is a kind of motion
AE Aurigae	has definition Runaway star which diverges from a comparatively small area in Orion.
	has spectral type O9.5 V
	is a part of Auriga
	is an instance of dwarf
	is an instance of O star
	is an instance of runaway star
Ae star	has definition A-type star with emission in one or several Balmer lines.
	has emission line one or several Balmer lines
	is a kind of A star
	is a kind of emission line star
aeon	has definition In astronomical terms, 1000 million years.
	has synonym eon
	is an instance of time unit
aether	has definition In Aristotelian physics, the fifth element, of which the stars and planets are made.
	has definition In Classical physics, an invisible medium that was thought to suffuse all space.
	is a kind of outdated belief
AI Velorum star	has definition A class of dwarf Cepheids. They are all RR Lyrae stars with periods shorter than 0.25 days.
	is a kind of dwarf Cepheid
airborne telescope	is a kind of telescope
airglow	has definition Light in the nighttime sky caused by the collision of atoms and molecules (primarily oxygen, OH, and Ne) in Earth's geocorona with charged particles and X-rays from the Sun or outer space. The airglow varies with time of night, latitude, and season. It is a minimum at zenith and maximum about 10° above the horizon.
	has synonym nightglow
	is a part of geocorona
alchemy	has definition Art of bringing parts of the universe to the perfect state toward which they were thought to aspire - e.g., gold for metals, immortality for human beings.
	is a kind of outdated belief
Aldebaran	has B-V magnitude 1.54
	has declination +16 30 33
	has definition A K5 III subgiant (a foreground star in the Hyades). It has a faint M2 V companion. It is now known to be slowly and irregularly variable.
	has distance 21 parsecs
	has right ascension 4 35 55.2
	has spectral type K5III
	has synonym alpha Tau
	has synonym HR 1457
	has V magnitude 0.85
	is a part of Taurus
	is an instance of giant
	is an instance of K star
	is an instance of naked eye star
Algol	has definition An eclipsing system of at least three components (B8 V, K0, Am). Period of components A and B is about 68.8 hours; period of components A, B, and C is about 1.9 years. Long term observations also indicate a massive, unseen fourth component with a period of about 190 years. Algol is also an erratic radio source of about 0.5 AU diameter.
	has definition The most famous eclipsing binary, Algol was probably the first variable star discovered. It lies in the constellation Perseus and consists of two stars that orbit each other every 2.87 days. When one star passes in front of the other, the light of the system dims.
	has distance 25 pc
	has synonym beta Per
	has synonym Demon star
	is a part of Perseus
	is an instance of beta Persei star
alkali earth metal	has definition Element in the second column of the periodic table (from the left)
alkali Earth metal	has group 2
alkali earth metal	has hardness malleable, extrudable and machinable
alkali Earth metal	has synonym group II element
	is a kind of column grouped element
alkali earth metal	is a kind of metallic element
alkali metal	has appearancee silvery except for francium
	has definition Metal in the first column of the periodic table
	has group 1
	has hardness soft
	has synonym group I element
	is a kind of column grouped element
	is a kind of metallic element
	reacts with water vigorously
Alpha Andromedae	has B-V magnitude -0.11
	has declination +29 05 26
	has right ascension 00 08 23.2
	has spectral type B8IVpMnHg
	has synonym HR 15
	has V magnitude 2.06
	is a part of Andromeda
	is an instance of B star
	is an instance of naked eye star
	is an instance of subgiant
Alpha Antliae	has B-V magnitude 1.45
	has declination -31 04 04
	has right ascension 10 27 09.1
	has spectral type K4III
	has synonym HR 4104
	has V magnitude 4.25
	is a part of Antlia
	is an instance of giant
	is an instance of K star
	is an instance of naked eye star
Alpha Apodis	has B-V magnitude 1.43
	has declination -79 02 41
	has right ascension 14 47 51.6
	has spectral type K2.5III
	has synonym HR 5470
	has V magnitude 3.83
	is a part of Apus
	is an instance of giant
	is an instance of K star
	is an instance of naked eye star
Alpha Arietis	has B-V magnitude 1.15
	has declination +23 27 45
	has right ascension 02 07 10.3
	has spectral type K2IIIabCa-I
	has synonym HR 617
	has V magnitude 2.00
	is a part of Aries
	is an instance of giant
	is an instance of K star
	is an instance of naked eye star
Alpha Caeli	has B-V magnitude 0.34
	has declination -41 51 50
	has right ascension 4 40 33.6
	has spectral type F2V
	has synonym HR 1502
	has V magnitude 4.45
	is a part of Caelum
	is an instance of dwarf
	is an instance of F star
	is an instance of naked eye star
Alpha Cassiopeiae	has B-V magnitude 1.17
	has declination +56 32 15
	has right ascension 0 40 30.4
	has spectral type K0III
	has synonym HR 168
	has V magnitude 2.23
	is a part of Cassiopeia
	is an instance of giant
	is an instance of K star
	is an instance of naked eye star
alpha Centauri	has B-V magnitude 0.71
	has declination -60 50 07
	has definition A binary star whose components have G2 V and K5 V spectral types. The nearest star system to the Sun and the third brightest star in the night sky.
	has distance 4.35 light-years
	has period 80 years
	has proper motion 3'.68 per year
	has right ascension 14 39 36.2
	has spectral type G2V
	has synonym HR 5459
	has synonym Rigil Kent
	has V magnitude -0.01
	is a part of Centaurus
	is an instance of binary star
	is an instance of dwarf
	is an instance of G star
	is an instance of naked eye star
Alpha Cephei	has B-V magnitude 0.22
	has declination +62 35 08
	has right ascension 21 18 34.7
	has spectral type A7V
	has synonym HR 8162
	has V magnitude 2.44
	is a part of Cepheus
	is an instance of A star
	is an instance of dwarf
	is an instance of naked eye star
Alpha Chamaeleontis	has B-V magnitude 0.39
	has declination -76 55 11
	has right ascension 08 18 31.7
	has spectral type F5III
	has synonym HR 3318
	has V magnitude 4.07
	is a part of Chamaeleon
	is an instance of F star
	is an instance of giant
	is an instance of naked eye star
Alpha Circini	has B-V magnitude 0.24
	has declination -64 58 31
	has right ascension 14 42 30.3
	has spectral type ApSrEuCr:
	has synonym HR 5463
	has V magnitude 3.19
	is a part of Circinus
	is an instance of A star
	is an instance of naked eye star
Alpha Columbae	has B-V magnitude -0.12
	has declination -34 04 27
	has right ascension 05 39 38.9
	has spectral type B7IVe
	has synonym HR 1956
	has V magnitude 2.64
	is a part of Columba
	is an instance of B star
	is an instance of naked eye star
	is an instance of subgiant
Alpha Comae Berenices	has B-V magnitude 0.00
	has declination +17 31 46
	has right ascension 13 09 59.2
	has spectral type F5V
	has synonym HR 4969
	has V magnitude 5.22
	is a part of Coma Berenices
	is an instance of dwarf
	is an instance of F star
Alpha Coronae Autralis	has B-V magnitude 0.04
	has declination -37 54 16
	has right ascension 19 09 28.2
	has spectral type A2V
	has synonym HR 7254
	has V magnitude 4.11
	is a part of Corona Australis
	is an instance of A star
	is an instance of dwarf
	is an instance of naked eye star
Alpha Coronae Borealis	has B-V magnitude -0.02
	has declination +26 42 53
	has right ascension 15 34 41.2
	has spectral type A0V
	has synonym HR 5793
	has V magnitude 2.23
	is a part of Corona Borealis
	is an instance of A star
	is an instance of dwarf
	is an instance of naked eye star
alpha decay	has definition Spontaneous emission by a heavier element (such as uranium) of positively charged helium nuclei - alpha particles - comprising 2 protons and 2 neutrons. The result of this radioactive decay is that the original element is very gradually converted into another element, with a decreased atomic number and mass. Alpha particle emission may be simultaneous with beta particle decay.
	has definition The disintegration of an atomic nucleus, in which the final products are an alpha particle and a nucleus with two fewer protons and two fewer neutrons than the original.
	is a kind of nuclear decay
Alpha Doradus	has B-V magnitude -0.10
	has declination -55 2 42
	has right ascension 4 33 59.8
	has spectral type A0IIISi
	has synonym HR 1465
	has V magnitude 3.27
	is a part of Dorado
	is an instance of A star
	is an instance of giant
	is an instance of naked eye star
Alpha Equulei	has B-V magnitude 0.53
	has declination + 5 14 52
	has right ascension 21 15 49.3
	has spectral type G0III+A5V
	has synonym HR 8131
	has V magnitude 3.92
	is a part of Equuleus
	is an instance of G star
	is an instance of giant
	is an instance of naked eye star
Alpha Fornacis	has B-V magnitude 0.52
	has declination -28 59 14
	has right ascension 03 12 04.2
	has spectral type F8V
	has synonym HR 963
	has V magnitude 3.87
	is a part of Fornax
	is an instance of dwarf
	is an instance of F star
	is an instance of naked eye star
Alpha Gruis	has B-V magnitude -0.13
	has declination -46 57 40
	has right ascension 22 8 13.9
	has spectral type B7IV
	has synonym HR 8425
	has V magnitude 1.74
	is a part of Grus
	is an instance of B star
	is an instance of naked eye star
	is an instance of subgiant
Alpha Horologii	has B-V magnitude 1.10
	has declination -42 17 40
	has right ascension 04 14 00.1
	has spectral type K1III
	has synonym HR 1326
	has V magnitude 3.86
	is a part of Horologium
	is an instance of giant
	is an instance of K star
	is an instance of naked eye star
Alpha Hydrae	has B-V magnitude 1.44
	has declination -8 39 31
	has right ascension 9 27 35.2
	has spectral type K3II-III
	has synonym Alphard
	has synonym HR 3748
	has V magnitude 1.98
	is a part of Hydra
	is an instance of bright giant
	is an instance of K star
	is an instance of naked eye star
Alpha Indi	has B-V magnitude 1.00
	has declination -47 17 29
	has right ascension 20 37 34.0
	has spectral type K0IIICNIII-IV
	has synonym HR 7869
	has V magnitude 3.11
	is a part of Indus
	is an instance of giant
	is an instance of K star
	is an instance of naked eye star
Alpha Lacertae	has B-V magnitude 0.01
	has declination +50 16 57
	has right ascension 22 31 17.4
	has spectral type A1V
	has synonym HR 8585
	has V magnitude 3.77
	is a part of Lacerta
	is an instance of A star
	is an instance of dwarf
	is an instance of naked eye star
Alpha Leonis	has B-V magnitude -0.11
	has declination +11 58 2
	has right ascension 10 8 22.3
	has spectral type B7V
	has synonym HR 3982
	has V magnitude 1.35
	is a part of Leo
	is an instance of B star
	is an instance of dwarf
	is an instance of naked eye star
Alpha Leporis	has B-V magnitude 0.21
	has declination -17 49 20
	has right ascension 05 32 43.7
	has spectral type F0Ib
	has synonym HR 1865
	has V magnitude 2.58
	is a part of Lepus
	is an instance of F star
	is an instance of naked eye star
Alpha Lupi	has B-V magnitude -0.20
	has declination -47 23 17
	has right ascension 14 41 55.7
	has spectral type B1.5III
	has synonym HR 5469
	has V magnitude 2.30
	is a part of Lupus
	is an instance of B star
	is an instance of giant
	is an instance of naked eye star
Alpha Lyncis	has B-V magnitude 1.55
	has declination +34 23 33
	has right ascension 9 21 3.2
	has spectral type K7IIIab
	has synonym HR 3705
	has V magnitude 3.13
	is a part of Lynx
	is an instance of bright giant
	is an instance of K star
	is an instance of naked eye star
Alpha Mensae	has B-V magnitude 0.72
	has declination -74 45 11
	has right ascension 06 10 14.6
	has spectral type G6V
	has synonym HR 2261
	has V magnitude 5.09
	is a part of Mensa
	is an instance of dwarf
	is an instance of G star
Alpha Monocerotis	has B-V magnitude 1.02
	has declination -09 33 04
	has right ascension 07 41 14.8
	has spectral type K0III
	has synonym HR 2970
	has V magnitude 3.93
	is a part of Monoceros
	is an instance of giant
	is an instance of K star
	is an instance of naked eye star
Alpha Muscae	has B-V magnitude -0.20
	has declination -69 08 08
	has right ascension 12 37 11.0
	has spectral type B2IV-V
	has synonym HR 4798
	has V magnitude 2.69
	is a part of Musca
	is an instance of B star
	is an instance of naked eye star
	is an instance of subgiant
Alpha Ophiuchi	has B-V magnitude 0.15
	has declination +12 33 36
	has right ascension 17 34 56.0
	has spectral type A5III
	has synonym HR 6556
	has V magnitude 2.08
	is a part of Ophiuchus
	is an instance of A star
	is an instance of giant
	is an instance of naked eye star
alpha particle mass	applies to particle alpha particle
	has symbol mα
	has uncertainty 0.00000052 × 10-27 kg
	has value 6.64465598 × 10-27 kg
	is an instance of particle mass
Alpha Pavonis	has B-V magnitude -0.20
	has declination -56 44 07
	has right ascension 20 25 38.8
	has spectral type B2IV
	has synonym HR 7790
	has V magnitude 1.94
	is a part of Pavo
	is an instance of B star
	is an instance of naked eye star
	is an instance of subgiant
Alpha Persei	has B-V magnitude 0.48
	has declination +49 51 41
	has right ascension 3 24 19.3
	has spectral type F5Ib
	has synonym HR 1017
	has V magnitude 1.79
	is a part of Perseus
	is an instance of F star
	is an instance of naked eye star
	is an instance of supergiant
alpha Persei cluster	has definition A young open cluster with a high mean rotational velocity.
	is a part of Perseus
	is an instance of open cluster
Alpha Phoenicis	has B-V magnitude 1.09
	has declination -42 18 22
	has right ascension 0 26 17.0
	has spectral type K0III
	has synonym HR 99
	has V magnitude 2.39
	is a part of Phoenix
	is an instance of giant
	is an instance of K star
	is an instance of naked eye star
Alpha Pictoris	has B-V magnitude 0.21
	has declination -61 56 29
	has right ascension 06 48 11.4
	has spectral type A7IV
	has synonym HR 2550
	has V magnitude 3.27
	is a part of Pictor
	is an instance of A star
	is an instance of naked eye star
	is an instance of subgiant
Alpha Pyxidis	has B-V magnitude -0.18
	has declination -33 11 11
	has right ascension 08 43 35.5
	has spectral type B1.5III
	has synonym HR 3468
	has V magnitude 3.68
	is a part of Pyxis
	is an instance of B star
	is an instance of giant
	is an instance of naked eye star
Alpha Reticuli	has B-V magnitude 0.91
	has declination -62 28 26
	has right ascension 04 14 25.5
	has spectral type G8II-III
	has synonym HR 1336
	has V magnitude 3.35
	is a part of Reticulum
	is an instance of bright giant
	is an instance of G star
	is an instance of naked eye star
Alpha Sculptoris	has B-V magnitude -0.16
	has declination -29 21 28
	has right ascension 0 58 36.3
	has spectral type B7IIIp
	has synonym HR 280
	has V magnitude 4.31
	is a part of Sculptor
	is an instance of B star
	is an instance of bright giant
	is an instance of naked eye star
Alpha Scuti	has B-V magnitude 1.33
	has declination -08 14 39
	has right ascension 18 35 12.3
	has spectral type K3III-IIIb
	has synonym HR 6973
	has V magnitude 3.85
	is a part of Scutum
	is an instance of giant
	is an instance of K star
	is an instance of naked eye star
Alpha Serpentis	has B-V magnitude 1.17
	has declination +6 25 32
	has right ascension 15 44 16.0
	has spectral type K2IIIbCN1Fe414
	has synonym HR 5854
	has V magnitude 2.65
	is a part of Serpens
	is an instance of giant
	is an instance of K star
	is an instance of naked eye star
Alpha Sextantis	has B-V magnitude -0.04
	has declination -0 22 18
	has right ascension 10 7 56.2
	has spectral type A0III
	has synonym HR 3981
	has V magnitude 4.49
	is a part of Sextans
	is an instance of A star
	is an instance of giant
	is an instance of naked eye star
Alpha Telescopii	has B-V magnitude -0.17
	has declination -45 58 06
	has right ascension 18 26 58.3
	has spectral type B3IV
	has synonym HR 6897
	has V magnitude 3.51
	is a part of Telescopium
	is an instance of B star
	is an instance of naked eye star
	is an instance of subgiant
Alpha Trianguli Australis	has B-V magnitude 1.44
	has declination -69 01 40
	has right ascension 16 48 39.9
	has spectral type K2IIb-IIIa
	has synonym HR 6217
	has V magnitude 1.92
	is a part of Triangulum Australe
	is an instance of bright giant
	is an instance of K star
	is an instance of naked eye star
Alpha Tucanae	has B-V magnitude 1.39
	has declination -60 15 35
	has right ascension 22 18 30.1
	has spectral type K3III
	has synonym HR 8502
	has V magnitude 2.86
	is a part of Tucana
	is an instance of giant
	is an instance of K star
	is an instance of naked eye star
Alpha Vulpeculae	has B-V magnitude 1.50
	has declination +24 39 54
	has right ascension 19 28 42.2
	has spectral type M0III
	has synonym HR 7405
	has V magnitude 4.44
	is a part of Vulpecula
	is an instance of giant
	is an instance of M star
	is an instance of naked eye star
alpha-particle	has definition Nucleus formed by the α-process (q.v.) (see even-even nuclei).
	has definition Particle first discovered in radioactive α decay, and later identified as helium nuclei (two protons and two neutrons bound together).
	has definition The nucleus of a 4He atom, consisting of two protons and two neutrons. Mass of α-particle 4.00260 amu.
	is an instance of 4N nucleus
alpha-process	has definition A hypothetical process of nucleosynthesis, which consisted of redistributing α-particles in the region from neon 20 to iron 56 (and perhaps slightly higher). The α-process has been replaced by explosive and nonexplosive C, O, and Si burning occurring in rapidly evolving or even explosive stages of stellar evolution which at higher temperatures and densities becomes the e-process.
	is a kind of nucleosynthetic reaction
alpha2 CVn	has definition A spectrum variable with spectrum showing strong, profuse lines of rare earths, iron-peak elements, and Si.
	has period 5.469 days
	is a part of Canes Venatici
	is an instance of spectrum variable
Altair	has apparent magnitude mv = 0.78
	has B-V magnitude 0.22
	has declination +08 52 06
	has definition A bright A7 V star.
	has distance 4.8 pc
	has right ascension 19 50 46.9
	has spectral type A7 V
	has synonym alpha Aql
	has synonym HR 7557
	has V magnitude 0.77
	is a part of Aquila
	is an instance of A star
	is an instance of dwarf
	is an instance of naked eye star
altazimuth telescope	has definition A form of mounting similar to that of a radar which allows the telescope tube to be moved horizontally (by rotation in azimuth or compass direction) and vertically (by rotation in altitude or elevation). To follow a star the telescope must be adjusted simultaneously in both axes. (also called alt-az)
	has mounting altazimuth
	is a kind of Earth based telescope
aluminium	has abundance 0.13 × 10-4 p.p.m. in deep Pacific seawater
	has abundance 1.3 × 10-4 p.p.m. in Pacific surface seawater
	has abundance 3.3 × 106 in Sun relative to H = 1 × 1012
	has abundance 5.2 × 10-4 p.p.m. in deep Atlantic seawater
	has abundance 82000 p.p.m. in Earth's crust
	has abundance 9.7 × 10-4 p.p.m. in Atlantic surface seawater
	has atomic emission line 308.215 nm for Al I
	has atomic emission line 309.271 nm for Al I (used in atom absorption spectrometry)
	has atomic emission line 309.281 nm for Al I (used in atom absorption spectrometry)
	has atomic emission line 394.401 nm for Al I
	has atomic emission line 396.152 nm for Al I (strong)
	has atomic number 13
	has atomic radii 143 pm
	has biological role none
	has boiling point 2740 K
	has bulk modulus 75.2 GPa
	has chief source bauxite
	has covalent radii 125 pm
	has crystal cell dimension a = 404.959 pm
	has crystal type f.c.c.
	has daily dietary intake 2.45 mg
	has definition soft and malleable metal
	has density 2390 kg m-3 for liquid at 933.52 K melting point
	has density 2698 kg m-3 for solid at 293 K
	has discoverer Oersted
	has discovery date 1825
	has discovery location Copenhagen, Denmark
	has electrical resistivity 2.6548 × 108 Ω m at 293 K
	has electron affinity 44 kJ mol-1 from Al to Al-
	has electron configuration [Ne]3s23p1 in ground state
	has electronegativity 1.61 Pauling
	has hazard accumulates in the body from daily intake, compounds are used as food additives and in indigestion tablets
	has heat capacity 21.38 J K-1 mol-1 for gas at constant pressure 0.1 MPa at 298.15 K
	has heat capacity 24.35 J K-1 mol-1 for solid at constant pressure 0.1 MPa at 298.15 K
	has heat of fusion 10.67 kJ mol-1
	has heat of vaporization 293.72 kJ mol-1
	has image
	has ionic radii 57 pm for Al3+
	has isotope mass range 22 to 31
	has level in human blood 0.39 mg dm-3
	has level in human bone 4 - 27 p.p.m.
	has level in human liver 3 - 23 p.p.m.
	has level in human muscle 0.7 - 28 p.p.m.
	has linear expansion coefficient 23.03 K-1
	has longest lived isotope aluminium 27 which is stable
	has main mining area Surinam, Jamaica, Ghana, Indonesia, Russia
	has mass absorption coefficient 48.6 cm2 g-1 for CuKα X-ray diffraction
	has mass absorption coefficient 5.16 cm2 g-1 for MoKα X-ray diffraction
	has mass magnetic susceptibility 7.7 × 109 kg-1 m3 for solid
	has mass of element in person 60 mg for a 70 kg average person
	has melting point 933.52 K
	has mineral bauxite, boehmite, diaspore, gibbsite, andalusite, corundum, sillimanite, topaz
	has molar volume 10.00 cm3
	has name origin alumen = alum from latin
	has neutron scattering length 0.3449 × 10-12 cm
	has number of isotopes 11
	has number of protons 13
	has ocean oxidation state III
	has ocean residence time 150 years
	has poisson's ratio 0.345 GPa
	has pronunciation al-oo-min-iuhm
	has registry number 7429-90-5 for Chemical Abstracts System database
	has relative atomic mass 26.981539 in units of 12C = 12.000
	has reserves 6 × 106 tonnes
has rigidity modulus 26.2 GPa
has space group Fm3m
has specimen foil, granules, ingots, pellets, powder, rod, shot or wire. Safe. Aluminum powder can react dangerously with other materials.
has symbol Al
has term symbol 2P1/2 in ground state
has thermal conductivity 237 W m-1 K-1 at 300 K
has toxic intake 5 g
has uses vehicle, aircraft and construction industries
has van der Waals radii 205 pm
has world production 15 × 106 tonnes per year
has young's modulus 70.6 GPa
is a kind of accumulating oceanic element
is a kind of group III element
is a kind of metallic element
reacts with air to produce a thin protective oxide layer
stable isotope aluminium 27
aluminium 26	has atomic mass 25.986892
	has decay mode β+ (4.005 Mev) 82%
	has decay mode γ
	has decay mode EC 18%
	has decay product magnesium 26
	has diagnostic role if decay products found in a solid then solid must have condensed within the first million years after the creation of Al 26
	has half life 7.2 × 105 years
	has nuclear spin I = 5+
	has number of neutrons 13
	has number of nucleons 26
	has symbol 26Al
	has uses research
	is an instance of aluminium
	is an instance of supernova produced radioactive element
aluminium 27	has atomic mass 26.9885386
	has magnetogyric ratio 6.9704 × 107 rad T-1 s-1
	has NMR frequency 26.057 MHz where 1H = 100 MHz; 2.3488 T
	has NMR receptivity 1.17 × 103 where 13C = 1.00
	has nuclear magnetic moment μ = 3.641504
	has nuclear quadrupole moment 0.1403 × 10-28 m2
	has nuclear spin I = 5/2+
	has number of neutrons 14
	has number of nucleons 27
	has relative NMR sensitivity 0.21 where 1H = 1.00
	has symbol 27Al
	has thermal neutron capture cross section 0.231 barns
	has uses Nuclear Magnetic Resonance
	is an instance of aluminium
aluminium 28	has atomic mass 27.981910
	has decay mode β- (4.642 Mev)
	has decay mode γ
	has half life 2.25 minutes
	has nuclear magnetic moment μ = 3.24
	has nuclear spin I = 3+
	has number of neutrons 15
	has number of nucleons 28
	has symbol 28Al
	is an instance of aluminium
aluminium 29	has atomic mass 28.980446
	has decay mode β- (3.68 Mev) %
	has decay mode γ
	has half life 6.5 minutes
	has nuclear spin I = 5/2+
	has number of neutrons 16
	has number of nucleons 29
	has symbol 29Al
	is an instance of aluminium
Am star	has definition A-type or F-type object to which no unique spectral type can be assigned. Usually the classifier provides a classification according to the hydrogen, metallic and calcium lines. Also call metallic-line stars.
	has definition Peculiar star whose metallic lines are as strong as those of the F stars but whose hydrogen lines are so strong as to require that they be classed with the A stars. They are generally short-period (<300d) spectroscopic binaries with high atmospheric turbulence and variable spectra, and are slower rotators than normal A stars.
	is a kind of A star
	is a kind of peculiar star
Amalthea	has definition The innermost satellite of Jupiter.
	has diameter 140 km
	has discoverer Barnard
	has discovery date 1892
	has eccentricity e = 0.0028
	has inclination i = 0°.4
	has orbital period 0.498 days
	has synonym Barnard's satellite
	has synonym Jupiter V
	is a part of Jupiter
	is an instance of natural satellite
americium	has abundance nil in Earth's crust
	has abundance nil in seawater
	has atomic emission line 367.312 nm for Am I
	has atomic emission line 377.750 nm for Am II
	has atomic emission line 392.625 nm for Am II
	has atomic emission line 408.929 nm for Am II
	has atomic emission line 428.926 nm for Am I
	has atomic emission line 450.945 nm for Am II
	has atomic emission line 457.559 nm for Am II
	has atomic emission line 466.279 nm for Am II
	has atomic emission line 605.464 nm for Am I (strong)
	has atomic number 95
	has atomic radii 173 pm
	has biological role none
	has boiling point 2880 K
	has crystal cell dimension a = 346.80 pm for α phase
	has crystal cell dimension a = 489.4 pm for β phase
	has crystal type f.c.c. for β phase
	has crystal type h.c.p. for α phase
	has daily dietary intake nil
	has definition radioactive silvery metal which does not occur naturally
	has density 13670 kg m-3 for solid at 293 K
	has discoverer Glen T. Seaborg, R.A. James, L.O. Morgan, and A. Ghiorso
	has discovery date 1944
	has discovery location Chicago, Illinois, US
	has electrical resistivity 68 × 10-8 Ω m
	has electron configuration [Rn]5f77s2 in ground state
	has electronegativity 1.3 Pauling
	has hazard intense α-radiation, maximum permissible body burden of 241Am is 0.03 μCi - it targets bone. γ radiation is a problem in gram amounts.
	has heat of fusion 14.4 kJ mol-1
	has heat of sublimation 34.2 kJ mol-1
	has heat of vaporization 284 kJ mol-1
	has image
	has ionic radii 107 pm for Am3+
	has ionic radii 80 pm for Am6+
	has ionic radii 86 pm for Am5+
	has ionic radii 92 pm for Am4+
	has isotope mass range 227 to 247
	has level in humans nil
	has longest lived isotope americium 243
	has mass magnetic susceptibility +5 × 10-8 kg-1 m3
	has mass of element in person nil for a 70 kg average person
	has melting point 1445 K
	has mineral none
	has molar volume 17.78 cm3
	has name origin america from English
	has neutron scattering length 0.83 in 10-12 cm units
	has number of isotopes 23 including nuclear isomers
	has number of protons 95
	has phase changed temperature 1347 K at α to β phase crystal transition
	has pronunciation amer-is-iuhm
	has registry number 7440-35-9 for Chemical Abstracts System database
	has relative atomic mass 243.0614 for americium 243 in units of 12C = 12.000
	has space group Fm3m for β phase
	has space group P63/mmc for α phase
	has symbol Am
	has term symbol 8S7/2 in ground state
	has thermal conductivity 10 W m-1 K-1 at 300 K
	has thermal neutron capture cross section 75.3 barns
	has uses source of radiation for γ-radiotherapy
	has world production probably a few kilograms per year
	is a kind of radioactive element
	is a kind of transuranium element
	reacts with air, steam and acids, but not alkalis
americium 237	has atomic mass 237.050050
	has decay mode α (6.20 Mev) 0.02%
	has decay mode γ
	has decay mode EC (1.6 Mev) 99.98%
	has half life 1.22 hours
	has nuclear spin I = 5/2-
	has number of neutrons 142
	has number of nucleons 237
	has symbol 237Am
	is an instance of americium
americium 238	has atomic mass 238.051980
	has decay mode α (6.04 Mev) < 0.1%
	has decay mode γ
	has decay mode EC (2.26 MeV)
	has half life 1.63 hours
	has nuclear spin I = 1+
	has number of neutrons 143
	has number of nucleons 238
	has symbol 238Am
	is an instance of americium
americium 239	has atomic mass 239.053016
	has decay mode α (5.924 Mev) 0.01%
	has decay mode γ
	has decay mode EC (0.800 MeV) 99.99%
	has half life 11.9 hours
	has nuclear spin I = 5/2-
	has number of neutrons 144
	has number of nucleons 239
	has symbol 239Am
	is an instance of americium
americium 240	has atomic mass 240.055278
	has decay mode α (5.592 Mev)
	has decay mode γ
	has decay mode EC (1.38 MeV)
	has half life 2.12 days
	has nuclear spin I = 3-
	has number of neutrons 145
	has number of nucleons 240
	has symbol 240Am
	is an instance of americium
americium 241	has atomic mass 241.056823
	has decay mode α (5.637 Mev) %
	has decay mode γ
	has half life 432.2 years
	has nuclear magnetic moment μ = 1.61
	has nuclear spin I = 5/2-
	has number of neutrons 146
	has number of nucleons 241
	has specimen commercially available, under licence
	has symbol 241Am
	has synthesis mechanism 241Pu subjected to neutron bombardment over a period of years
	has uses research, medical therapy, medical diagnosis
	is an instance of americium
americium 242	has atomic mass 242.056541
	has decay mode β- (0.663 Mev) 83%
	has decay mode γ
	has decay mode EC (0.750 Mev) 17%
	has half life
	has nuclear magnetic moment μ = +0.388
	has nuclear spin I = 1-
	has number of neutrons 147
	has number of nucleons 242
	has symbol 242Am
	is an instance of americium
americium 242m	has decay mode α (5.62 Mev) 0.5%
	has decay mode γ
	has decay mode IT (0.48 Mev) 99.5%
	has half life 141 years
	has nuclear magnetic moment μ = 1.0
	has nuclear spin I = 5-
	has number of neutrons 147
	has number of nucleons 242
	has symbol 242mAm
	is an instance of americium
americium 243	has atomic mass 243.061375
	has decay mode α (5.438 Mev)
	has decay mode γ
	has half life 7370 years
	has magnetogyric ratio 1.54 × 107 rad T-1 s-1
	has NMR frequency 5.76 MHz where 1H = 100 MHz; 2.3488 T
	has nuclear magnetic moment μ = 1.61
	has nuclear quadrupole moment +4.210 × 10-28 m2
	has nuclear spin I = 5/2-
	has number of neutrons 148
	has number of nucleons 243
	has specimen commercially available, under licence
	has symbol 243Am
	has synthesis mechanism 239Pu subjected to neutron bombardment
	has uses research, Nuclear Magnetic Resonance
	is an instance of americium
americium 244	has atomic mass 244.064279
	has decay mode β- (1.427 Mev)
	has decay mode γ
	has half life 10.0 hours
	has number of neutrons 149
	has number of nucleons 244
	has symbol 244Am
	is an instance of americium
americium 245	has atomic mass 245.066444
	has decay mode β- (0.894 Mev)
	has decay mode γ
	has half life 2.05 hours
	has nuclear spin I = 5/2+
	has number of neutrons 150
	has number of nucleons 245
	has symbol 245Am
	is an instance of americium
ammonia	has symbol NH3
	is an instance of interstellar molecule
	is an instance of neutral particle
amount of substance unit	is a kind of unit
	is a unit of amount of substance
amount-of-substance concentration unit	has definition mole per cubic meter
	has symbol mol·m-3
	is a kind of derived SI unit
ampere	has consequence fixes the magnetic constant (permeability of vacuum) at exactly 4 × 10-7 H · m-1
	has definition constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 meter apart in vacuum, would produce between these conductors a force equal to 2 × 10-7 newton per meter of length
	has definition Unit of electric current. "The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 meter apart in vacuum, would produce between these conductors a force equal to 2 × 10-7 newton per meter of length" (CIPM
	has symbol A
	is an instance of base SI unit
	is an instance of current unit
Andromeda	has acronym And
	has boundary
	has genitive Andromedae
	has historical image
	has synonym Chained Lady
	has synonym Princess of Ethiopia
	is a part of celestial sphere
	is an instance of constellation
Andromeda galaxy	has definition A spiral galaxy (kS5 in Morgan's classification) in the Local Group, about 650-700 kpc distant (MV = -21), visible to the naked eye as a fuzzy patch in the constellation of Andromeda. Total mass about 3.1 × 1011 Msun ; i = 77°, heliocentric velocity - 180 km s-1. Its nucleus exhibits noncircular gas motions. It is similar to but slightly larger than our Galaxy.
	has definition Major spiral galaxy, 2.2 million light-years from Earth. Gravitationally bound to the Milky Way with which it shares membership in the Local Group, it is currently approaching us, rather than receding as is the case for most galaxies.
	has definition The largest galaxy in the Local Group. Also known as the Great Spiral and M31. It is about one and a half times the size of our own galaxy, and contains at least 300 globular clusters. Two smaller, elliptical galaxies (M32 and NGC 205) lie close to it.
	has definition The largest member of the local group. It is a giant spiral galaxy that lies 2.4 million light-years away.
	has image
	has NED data
	has synonym M 31
	is a part of Local Group
	is an instance of galaxy containing Cepheids
	is an instance of hypergalaxy
	is an instance of Messier object
	is an instance of naked eye object
	is an instance of Sb spiral
Andromeda I	has definition Dwarf spheroidal galaxy in the Andromeda subgroup of the Local Group. It is among the intrinsically faintest members of the Local Group.
	has discoverer van den Bergh
	has discovery date 1972
	is a part of Local Group
	is an instance of satellite galaxy
	orbits Andromeda galaxy
Andromeda II	has definition Dwarf spheroidal galaxy in the Andromeda subgroup of the Local Group. It is among the intrinsically faintest members of the Local Group.
	has discoverer van den Bergh
	has discovery date 1972
	is a part of Local Group
	is an instance of satellite galaxy
	orbits Andromeda galaxy
Andromeda III	has definition Dwarf spheroidal galaxy in the Andromeda subgroup of the Local Group. It is among the intrinsically faintest members of the Local Group.
	has discoverer van den Bergh
	has discovery date 1972
	is a part of Local Group
	is an instance of satellite galaxy
	orbits Andromeda galaxy
angle	has unit angle unit
	is a kind of quantity
angle unit	is a kind of unit
	is a unit of angle
Anglo-Australian Telescope	has acronym AAT
	has altitude 1149 m
	has aperture 3.893 m
	has creation date 1975
	has focal ratio t/3.3. 8, 15. 36
	has latitude 31° 17' S
	has location Siding Spring Mtn., Australia
	has longitude 149° 04' E
	has mirror maker Grubb-Parsons
	has mirror type Cer-Vit
	has mounting manufacturer Mitsubishi
	has owner Anglo-Australian Observatory
	is an instance of Horseshoe equatorial telescope
	is an instance of optical telescope
	is an instance of reflector
angstrom	has definition A unit of length used when expressing wavelengths.
	has equivalent 10-10 meters
	has symbol Å
	is an instance of length unit
	is an instance of non SI unit
	is named after Anders Jonas Angstrom (1814-1874)
angular momentum	has definition The angular momentum of a system about a specified origin is the sum over all the particles in the system (or an integral over the different elements of the system if it is continuous) of the vector products of the radius vector joining each particle to the origin and the momentum of the particle. For a closed system it is conserved by virtue of the isotropy of space.
	has definition The product of mass and angular velocity for an object in rotation; similar to linear momentum. In quantum mechanics, angular momentum is quantized, i.e., is measured in indivisible units equivalent to Planck's constant divided by 2 pi. This corresponds classically to only certain frequencies of rotation being allowed.
	has symbol L
	is a kind of quantity
angular size	has definition The angle subtended by an object on the sky. For example, the angular size of the moon is 30 arcminutes.
	is a kind of angle
ANIK 1	is an instance of ANIK satellite
ANIK 2	is an instance of ANIK satellite
ANIK 3	is an instance of ANIK satellite
ANIK satellite	is a kind of geosynchronous satellite
anisotropy	breaks isotropy
	has definition The characteristic of being dependent upon direction. (Light coming with equal intensity from all directions is isotropic; a spotlight's beam is anisotropic.) The cosmic background radiation is generally isotropic - i.e., its intensity is the same in all parts of the sky - but small anisotropies have been detected which are thought to reflect the earth's proper motion relative to the framework of the universe as a whole.
	has definition The condition in which the universe appears different in different directions.
	is a kind of symmetry breaking
Annie Jump Cannon	has career Havard College Observatory
	has greatest achievement the Henry Draper Catalog of spectral types
	is an instance of 19th century scientist
	is an instance of astronomer
annular eclipse	has definition A solar eclipse in which the solar disk is never completely covered but is seen as an annulus or ring at maximum eclipse. An annular eclipse occurs when the apparent disk of the Moon is smaller than that of the Sun.
	has definition An eclipse of the Sun in which the Moon is too far from Earth to block out the Sun completely, so that a ring of sunlight appears around the Moon.
	is an instance of solar eclipse
anomalistic month	has definition The interval between two successive perigee passages of the Moon.
	has value 27.555 days
	is an instance of month
anomalistic year	has definition A period of time based on the revolution of the Earth around the Sun, where a year is defined as the mean interval between successive passages of the Earth through perihelion.
	has definition The interval between two successive perihelion passages of Earth.
	has value 365.2596 ephemeris days
	is an instance of year
anomalous Zeeman effect	has cause the influence of magnetic fields on both the orbital angular momentum and the spin angular momentum of electrons in atoms or ions
	has definition Zeeman effet in which spectral lines are split into several components, in contrast to the normal Zeeman effect which results in only two distinct components.
	has effect splitting of spectral lines into more than two components
	is a kind of Zeeman effect
antapex	has definition The direction in the sky away from which the Sun seems to be moving (at a speed of 19.4 km s-1) relative to general field stars in the Galaxy.
	is a part of Columba
	is an instance of equatorial coordinate
	is opposite of solar apex
Antares	has B-V magnitude 1.83
	has declination -26 25 55
	has definition A red M1 Ib supergiant. It has a B3 V companion, which is a radio source.
	has distance 125 parsecs
	has location inner edge of the Orion spiral arm
	has right ascension 16 29 24.4
	has spectral type M1.5Iab-Ib+B4V
	has synonym alpha Sco
	has synonym HR 6134
	has V magnitude 0.96
	is a part of Scorpius
	is an instance of naked eye star
	is an instance of red supergiant
anthropic principle	has definition The doctrine that the value of certain fundamental constants of nature can be explained by demonstrating that, were they otherwise, the universe could not support life and therefore would contain nobody capable of worrying about why they are as they are. Were the strong nuclear force slightly different in strength, for instance, the stars could not shine and life as we know it would be impossible.
	has definition The weak form of the anthropic principle states that life can exist only during a brief period of the history of our universe. The strong form of the principle states that out of all possible values for the fundamental constants of nature and the initial conditions of the universe, only a small fraction could allow life to form at all, at anytime. (See boundary conditions; fundamental constants of nature.)
	is a kind of outdated belief
anthropocentrism	has definition The belief that humans are central to the universe.
	is a kind of outdated belief
anthropomorphism	has definition The projection of human attributes onto nonhuman entities such as animals, the planets, or the universe as a whole.
	is a kind of outdated belief
anti-reflection coating	has definition Also AR coating. A layer of material of lower refractive index of just the right thickness (1/4 wave) is deposited on the optical surface to be coated. More complex coatings are possible which cover a large wavelength range.
	is a kind of coating
anticenter	has definition The direction of the sky opposite to that of the galactic center.
	has definition The point in the galactic plane that lies directly opposite the galactic center. Here we gaze toward the edge of the Galactic disk. The nearest bright star to the anticenter is El Nath, in the constellation Taurus.
	has galactic latitude 0
	has galactic longitude 180 degrees
	is a part of Auriga
	is an instance of galactic coordinate
	is opposite of galactic center
antimony	has abundance 0.2 p.p.m. in Earth's crust
	has abundance 10 in Sun relative to H = 1 × 1012
	has abundance 3 × 10-4 p.p.m. in seawater
	has atomic emission line 206.833 nm for Sb I
	has atomic emission line 217.581 nm for Sb I
	has atomic emission line 252.852 nm for Sb I
	has atomic emission line 259.805 nm for Sb I
	has atomic emission line 231.147 nm for Sb I (strong)
	has atomic number 51
	has atomic radii 182 pm
	has biological role none
	has boiling point 1908 K
	has chief source stibnite, tetrahedrite although mainly a copper ore yields antimony as a by-product
	has covalent radii 141 pm
	has crystal cell dimension a = 298.6 pm for grey cubic form
	has crystal cell dimension a = 336.9 pm for metal form
	has crystal cell dimension a = 430.84 pm for grey rhombohedral form
	has crystal cell dimension c = 1124.7 pm for grey rhombohedral form
	has crystal cell dimension c = 533 pm for metal form
	has crystal type cubic for grey cubic form
	has crystal type h.c.p. for metal form
	has crystal type rhombohedral for grey rhombohedral form
	has daily dietary intake 0.002 to 1.3 mg
	has definition metalloid element with three forms. The metallic form is the more stable and is bright, silvery, hard and brittle
	has density 6483 kg m-3 for liquid at 903.89 K melting point
	has density 6691 kg m-3 for solid at 293 K
	has discovery date 1600 BC probably known to the ancients and certainly to the alchemists
	has electrical resistivity 39.0 × 10-8 Ω m at 273 K
	has electron affinity 101 kJ mol-1 from Sb to Sb-
	has electron configuration [Kr]4d105s25p3 in ground state
	has electronegativity 2.05 Pauling
	has hazard small doses stimulate metabolism, large doses cause liver damage
	has heat capacity 20.79 J K-1 mol-1 for gas at constant pressure 0.1 MPa at 298.15 K
	has heat capacity 25.23 J K-1 mol-1 for solid at constant pressure 0.1 MPa at 298.15 K
	has heat of fusion 20.9 kJ mol-1
	has heat of vaporization 67.91 kJ mol-1
	has image
	has ionic radii 245 pm for Sb2-
	has ionic radii 62 pm for Sb5+
	has ionic radii 89 pm for Sb3+
	has isotope mass range 108 to 136
	has lethal intake 140 mg for antimony potassium tartrate (oral) for LD50
	has level in humans 0.0033 mg dm-3 in blood
	has level in humans 0.01 to 0.6 p.p.m. in bone
	has level in humans 0.011 to 0.42 p.p.m. in liver
	has level in humans 0.042 to 0.191 p.p.m. in muscle
	has linear expansion coefficient 8.5 × 10-6 K-1
	has main mining area China, Italy, Peru, Mexico, Bolivia, France
	has mass absorption coefficient 270 cm2 g-1 for CuKα X-ray diffraction
	has mass absorption coefficient 33.1 cm2 g-1 for MoKα X-ray diffraction
	has mass magnetic susceptibility -1.0 × 10-8 kg-1 m3 for solid
	has mass of element in person 2 mg for a 70 kg average person
	has melting point 903.89 K
	has mineral sibiconite, stibnite, tetrahedrite, ullmannite
	has molar volume 18.20 cm3
	has name origin anti + monos = not alone from greek
	has neutron scattering length 0.557 × 10-12 cm
	has number of isotopes 40
	has number of protons 51
	has ocean oxidation state III
	has ocean residence time 3.5 × 105 years
	has poisson's ratio 0.25 to 0.33 GPa
	has pronunciation anti-moni
	has registry number 7440-36-0 for Chemical Abstracts System database
has relative atomic mass 112.760 in units of 12C = 12.000
has reserves 2.5 × 106 tonnes
has rigidity modulus 20.7 GPa
has space group P63/mmc for metal form
has space group Pm3m for grey cubic form
has space group R3m for grey rhombohedral form
has specimen available as pieces, powder or shot. Care !
has stable isotope antimony 121
has stable isotope antimony 123
has symbol Sb
has symbol origin stibium from latin
has term symbol 4S3/2 in ground state
has thermal conductivity 24.3 W m-1 K-1 at 300 K
has thermal neutron capture cross section 4.91 barns
has toxic intake 100 mg
has uses hardenning other metals, stotage batteries, bearings
has van der Waals radii 220 pm
has world production 53000 tonnes per year
has young's modulus 54.7 GPa
is a kind of accumulating oceanic element
is a kind of group V element
is a kind of metallic metalloid
reacts with stable in dry air, not attacked by dilute acids or alkalis
antimony 119	has atomic mass 118.903948
	has decay mode γ
	has decay mode EC (0.59 Mev)
	has half life 38.1 hours
	has nuclear magnetic moment μ = +3.45
	has nuclear spin I = 5/2+
	has number of neutrons 68
	has number of nucleons 119
	has symbol 119Sb
	is an instance of antimony
antimony 120	has atomic mass 119.903821
	has decay mode β+ (2.68 Mev) 41%
	has decay mode γ
	has decay mode EC 59%
	has half life 15.89 minutes
	has nuclear magnetic moment μ = +2.3
	has nuclear spin I = 1+
	has number of neutrons 69
	has number of nucleons 120
	has symbol 120Sb
	is an instance of antimony
antimony 120m	has atomic mass 119.903821
	has decay mode γ
	has decay mode EC
	has half life 5.76 days
	has nuclear magnetic moment μ = 2.34
	has nuclear spin I = 8-
	has number of neutrons 69
	has number of nucleons 120
	has symbol 120mSb
	is an instance of antimony
antimony 121	has atomic mass 120.9038212
	has magnetogyric ratio 6.4016 × 107 rad T-1 s-1
	has natural abundance 57.3 %
	has NMR frequency 23.930 MHz where 1H = 100 MHz; 2.3488 T
	has NMR receptivity 520 where 13C = 1.00
	has nuclear magnetic moment μ = +3.3634
	has nuclear quadrupole moment -0.360 × 10-28 m2
	has nuclear spin I = 5/2+
	has number of neutrons 70
	has number of nucleons 121
	has relative NMR sensitivity 0.16 where 1H = 1.00
	has symbol 121Sb
	has uses experimental, Nuclear Magnetic Resonance
	is an instance of antimony
antimony 122	has atomic mass 121.905179
	has decay mode β+ (0.619 Mev) 2%
	has decay mode β- (1.9820 Mev) 98%
	has decay mode γ
	has half life 2.73 days
	has nuclear magnetic moment μ = -1.90
	has nuclear spin I = 2-
	has number of neutrons 71
	has number of nucleons 122
	has symbol 122Sb
	is an instance of antimony
antimony 123	has atomic mass 122.9042160
	has magnetogyric ratio 3.4668 × 107 rad T-1 s-1
	has natural abundance 42.7 %
	has NMR frequency 12.959 MHz where 1H = 100 MHz; 2.3488 T
	has NMR receptivity 111 where 13C = 1.00
	has nuclear magnetic moment μ = 2.5498
	has nuclear quadrupole moment -0.490 × 10-28 m2
	has nuclear spin I = 7/2+
	has number of neutrons 72
	has number of nucleons 123
	has relative NMR sensitivity 4.57 × 10-2 where 1H = 1.00
	has symbol 123Sb
	has uses experimental, Nuclear Magnetic Resonance
	is an instance of antimony
antimony 124	has atomic mass 123.905038
	has decay mode β- (2.905 Mev)
	has decay mode γ
	has half life 60.30 days
	has nuclear magnetic moment μ = 1.2
	has nuclear spin I = 3-
	has number of neutrons 73
	has number of nucleons 124
	has symbol 124Sb
	has uses research
	is an instance of antimony
antimony 125	has atomic mass 124.905252
	has decay mode β- (0.767 Mev)
	has decay mode γ
	has half life 2.758 years
	has nuclear magnetic moment μ = +2.63
	has nuclear spin I = 7/2+
	has number of neutrons 74
	has number of nucleons 125
	has symbol 125Sb
	is an instance of antimony
antimony 126	has atomic mass 125.907250
	has decay mode β- (3.67 Mev) %
	has decay mode γ
	has half life 12.4 days
	has nuclear magnetic moment μ = 1.3
	has nuclear spin I = 8-
	has number of neutrons 75
	has number of nucleons 126
	has symbol 126Sb
	is an instance of antimony
antimony 127	has atomic mass 126.906919
	has decay mode β- (1.58 Mev)
	has decay mode γ
	has half life 3.84 days
	has nuclear magnetic moment μ = 2.6
	has nuclear spin I = 7/2+
	has number of neutrons 76
	has number of nucleons 127
	has symbol 127Sb
	is an instance of antimony
antineutrino	has antiparticle neutrino
	has definition The antiparticle of a neutrino.
	is a kind of antiparticle
	is a kind of lepton
	is a kind of neutral particle
antineutron	has antiparticle neutron
	has definition The antiparticle of a neutron. A neutron and antineutron both have the same mass and zero electric charge, but can be differentiated by their interactions: a neutron and an antineutron can annihilate into gamma rays, while two neutrons cannot.
	is an instance of antiparticle
	is an instance of baryon
	is an instance of neutral particle
	is an instance of radioactive particle
antiparticle	has definition An elementary particle of opposite charge but otherwise identical to its partner. Most of the observable universe consists of particles and matter, as opposed to antiparticles and antimatter.
	has definition Atomic particles that have the same mass as, but opposite charge and orbital direction to, an ordinary particle. Thus, instead of negatively charged electrons, atoms of antimatter have positrons. A quantity of antimatter coming into contact with matter would "cancel out" - annihilate, with total conversion of mass to energy - an exact proportion of matter corresponding to the original quantity of antimatter, provided that the elements in the matter also corresponded with the "elements" in the antimatter, i.e., that the atoms were equivalent but opposite.
	has definition For every known type of particle, there exists an antiparticle with exactly the same mass, but with the opposite electric charge. When a particle and its antiparticle come together, they can always annihilate to form gamma rays. The antiparticle of an electrically neutral particle is sometimes the same as the original particle (e.g., photons) and sometimes it is distinct (e.g., neutrons).
	has definition Particles predicted by combining the theories of special relativity and quantum mechanics. For each particle, there must exist an antiparticle with the opposite charge, magnetic moment and other internal quantum numbers (e.g., lepton number, baryon number, strangeness, charm, etc.), but with the same mass, spin and lifetime. Note that certain neutral particles (such as the photon and π0) are their own antiparticles.
	has definition Particles with identical mass and spin as those of ordinary matter, but with opposite charge. Antimatter has been produced experimentally, but little of it is found in nature. Why this should be so is one of the questions that must be answered by any adequate theory of the early universe.
	is a kind of particle
antiproton	has antiparticle proton
	has charge -1
	has definition The antiparticle of a proton, identical in mass and spin but of opposite (negative) charge.
	is an instance of antiparticle
	is an instance of baryon
	is an instance of charged particle
	is an instance of radioactive particle
antiquark	has antiparticle quark
	has definition The antiparticle of the quark.
	is a kind of antiparticle
	is a kind of charged particle
	is a kind of parton
Antlia	has acronym Ant
	has boundary
	has genitive Antliae
	has synonym Air Pump
	is a part of celestial sphere
	is an instance of constellation named by de Lacaille
Antonia Maury	has career Havard College Observatory
	is an instance of 19th century scientist
	is an instance of astronomer
Ap star	has definition Peculiar A-type stars ("magnetic" A stars) that show abnormally strong lines, sometimes of varying intensity, of certain ionized metals. Recent evidence indicates that all Ap stars are slow rotators compared with normal A stars.
	has definition Star with spectral type A in which the lines of one or several elements are abnormally enhanced. Traditionally the most important subgroups are Si λ4200, Hg-Mn and Cr-Eu-Sr stars. The latest objects of the latter group correspond to early F-type.
	has definition Stars can be classified according to their surface temperatures, which determine, in large part, the spectrum of radiation they emit. A stars have surface temperatures between about 7,500 and 11,000 degrees centigrade. Peculiar A stars are A stars whose emitted radiation spectra have many of the characteristics of A stars but are peculiar in certain ways.
	is a kind of A star
	is a kind of peculiar star
apastron	has antonym periastron
	has definition The point in the orbit of one component of a binary system where it is farthest from the other.
	has participants binary star
	is an instance of binary star orbital event
aphelion	has antonym perihelion
	has definition The point in a planetary orbit that is at the greatest distance from the Sun.
	is an instance of Sun orbital event
aplanatic system	has definition A system of three lenses which, taken together, correct for spherical aberration, chromatic aberration, and coma.
	is a kind of lens
apocenter	has antonym pericenter
	has definition The point in the orbit of one component of a binary system which is farthest from the center of mass of the system.
	has participants binary star
	is an instance of binary star orbital event
apogalacticon	has antonym perigalacticon
	has definition The point in a star's orbit farthest from the Galactic center.
	is an instance of galaxy orbital event
apogee	has antonym perigee
	has definition The point at which a body in orbit around the Earth reaches its farthest distance from the Earth.
	is an instance of Earth orbital event
Apollo	has definition Prototype of a small group of asteroids whose orbits intersect that of Earth.
	has eccentricity e = 0.57
	has inclination i = 6°.4
	has orbital period P = 622d
	has semi-major axis a = 1.486 AU
	is a part of Apollo asteroid
	is an instance of Apollo asteroid
Apollo asteroid	has definition One of a small group of asteroids whose orbits intersect that of Earth. They are named for the prototype, Apollo.
	is a kind of Earth-crossing asteroid
	is a part of Earth-crossing asteroid
Apollo space program	has definition Successful US lunar exploration program in which the Apollo spacecraft 1 to 6 were unmanned; 7 to 10 were manned but did not land; and 11, 12 and 14 to 17 landed and returned safely. (Apollo 13 was an aborted mission.) The first men to land on the Moon were Neil Armstrong and Edwin Aldrin, from Apollo 11, on 20 July 1969. The final Apollo flight (17) lasted from 7 to 19 December 1972, and left a considerable quantity of exploratory devices on the lunar surface.
	is an instance of space science institution
apparent magnitude	has definition A measure of how bright a star looks in the sky. The brighter the star, the smaller the apparent magnitude. A star that is one magnitude brighter than another (e.g., +1 versus +2) looks 2.5 times brighter. The brightest star of all, of course, is the Sun, whose apparent magnitude is -26.74, followed by Sirius, whose apparent magnitude is -1.46, Canopus (-0.72), Alpha Centauri (-0.27), Arcturus (-0.04), and Vega (+0.03). Stars of the Big Dipper are fainter, most of them around magnitude +2. On a clear, dark night, the unaided eye can see stars as faint as apparent magnitude +6, and the largest telescopes penetrate to apparent magnitude +30.
	has definition Measure of the observed brightness of a celestial object as seen from the Earth. It is a function of the star's intrinsic brightness, its distance from the observer, and the amount of absorption by interstellar matter between the star and the observer. The mv, of Sun, -26.5 mag. A sixth-magnitude star is just barely visible to the naked eye.
	is a kind of magnitude
apsidal motion	has definition Rotation of the line of apsides in the plane of the orbit; (in a binary) precession of the line of apsides due to mutual tidal distortion.
	is an instance of precession
Apus	has acronym Aps
	has boundary
	has genitive Apodis
	has synonym Bird of Paradise
	is a part of celestial sphere
	is an instance of constellation named by Bayer
Aquarius	has acronym Aqr
	has boundary
	has genitive Aquarii
	has historical image
	has synonym Water Bearer
	is a part of Zodiac
	is an instance of zodiacal constellation
Aquila	has acronym Aql
	has boundary
	has genitive Aquilae
	has historical image
	has historical origin the eagle that belonged to Zeus. Aquila's most famous task was carrying the mortal Ganymede to the heavens to serve as Zeus' cup bearer
	has synonym Eagle
	is a part of celestial sphere
	is an instance of constellation
Ara	has acronym Ara
	has boundary
	has genitive Arae
	has historical origin the altar of the centaur Chiron or the altar that Noah built after the great flood
	has synonym Altar
	is a part of celestial sphere
	is an instance of constellation
arcminute	has definition A unit of angle equal to 1/60 of a degree.
	has equivalent 1/60 degree
	is an instance of angle unit
arcsecond	has definition One sixtieth of an arcminute, or 1/3600 of a degree.
	has equivalent 1/3600 degree
	has equivalent 1/60 arcminute
	is an instance of angle unit
Arcturus	has B-V magnitude 1.23
	has declination +19 10 57
	has definition A beautiful orange star that is the brightest in the constellation Bootes and the fourth brightest in the night sky. It lies 34 light-years away and is a member of the thick-disk population. Historically, Arcturus is famous because it was one of the first stars to have its proper motion measured.
	has definition An old subgiant disk star (K2 IIIp, mv = 0.06) about 11 pc distant.
	has right ascension 14 15 39.6
	has spectral type K1IIIbCN-1
	has synonym Alpha Boötis
	has synonym HR 5340
	has V magnitude -0.04
	is a part of Boötes
	is an instance of bright giant
	is an instance of K star
	is an instance of naked eye star
area	is a kind of quantity
area unit	has definition square meter
	has symbol m2
	is a kind of derived SI unit
Arecibo	has angular resolving power 5 arcminute
	has effective diameter 300 meters
	has position Puerto Rico
	has reference NAIC Arecibo Observatory
	is an instance of radio telescope
argon	has abundance 1 × 106 in Sun relative to H = 1 × 1012
	has abundance 1.2 p.p.m. in Earth's crust
	has abundance 9300 p.p.m. in atmopshere
	has atomic emission line 695.5431 nm for Ar I
	has atomic emission line 706.7218 nm for Ar I
	has atomic emission line 750.3869 nm for Ar I
	has atomic emission line 801.4786 nm for Ar I
	has atomic emission line 912.2967 nm for Ar I
	has atomic emission line 965.7786 nm for Ar I
	has atomic emission line 811.5311 nm for Ar I (strong)
	has atomic number 18
	has atomic radii 174 pm
	has biological role none
	has boiling point 87.29 K
	has chief source lquid air
	has critical pressure 4862 kPa
	has critical temperature 150.87 K
	has crystal cell dimension a = 531.088 pm
	has crystal type f.c.c
	has daily dietary intake very low
	has definition colourless, odourless gas comprising 1% of the atmosphere
	has density 1.784 kg m-3 for gas at 273 K
	has density 1380 kg m-3 for liquid at 87.29 K boiling point
	has density 1656 kg m-3 for solid at 40 K
	has discoverer Lord Rayleigh and Sir William Ramsay
	has discovery date 1894
	has discovery location London and Bristol, England
	has electron affinity -35 kJ mol-1 from Ar to Ar-
	has electron configuration [Ne]3s23p6 in ground state
	has hazard can asphyxiate if it excludes oxygen from the lungs
	has heat capacity 20.786 J K-1 mol-1 for gas at constant pressure 0.1 MPa at 298.15 K
	has heat of fusion 2.21 kJ mol-1
	has heat of vaporization 6.53 kJ mol-1
	has image
	has isotope mass range 32 to 46
	has lethal intake non-toxic
	has level in humans trace amounts in blood
	has mass absorption coefficient 123 cm2 g-1 for CuKα X-ray diffraction
	has mass absorption coefficient 13.5 cm2 g-1 for MoKα X-ray diffraction
	has mass magnetic susceptibility -6.16 × 10-9 kg-1 m3 for gas
	has mass of element in person very small for a 70 kg average person
	has melting point 83.78 K
	has mineral none
	has molar volume 24.12 cm3 at 40 K
	has name origin argos = inactive from Greek
	has neutron scattering length 0.1909 × 10-12 cm
	has number of isotopes 15
	has number of protons 18
	has ocean oxidation state 0
	has ocean residence time 28000 years
	has pronunciation ar-gon
	has registry number 7440-37-1 for Chemical Abstracts System database
	has relative atomic mass 39.948 in units of 12C = 12.000
	has reserves 6.6 × 1013 tonnes in atmosphere
	has space group Fm3m
	has specimen available in small pressurized canisters. Safe.
	has stable isotope argon 36
	has stable isotope argon 38
	has stable isotope argon 40
	has symbol Ar
	has synthesis mechanism extracted from the atmosphere by liquefaction
	has term symbol 1S0 in ground state
	has thermal conductivity 0.0177 W m-1 K-1 at 300 K for gas
	has thermal neutron capture cross section 0.675 barns
has toxic intake non-toxic
has uses inert atmosphere in lamps and high temperature metallurgy
has van der Waals radii 191 pm
has world production 700000 tonnes per year
is a kind of inert gas
is a kind of unclassified oceanic element
reacts with nothing
argon 36	has atomic mass 35.96754552
	has natural abundance 0.337 %
	has nuclear magnetic moment μ = 0
	has nuclear spin I = 0+
	has number of neutrons 18
	has number of nucleons 36
	has symbol 36Ar
	is an instance of argon
argon 37	has atomic mass 36.966776
	has decay mode EC (0.814 Mev)
	has decay mode no γ
	has half life 35.0 days
	has nuclear magnetic moment μ = +1.15
	has nuclear spin I = 3/2+
	has number of neutrons 19
	has number of nucleons 37
	has symbol 37Ar
	has uses research
	is an instance of argon
argon 38	has atomic mass 37.9627325
	has natural abundance 0.063 %
	has nuclear magnetic moment μ = 0
	has nuclear spin I = 0+
	has number of neutrons 20
	has number of nucleons 38
	has symbol 38Ar
	is an instance of argon
argon 39	has atomic mass 38.962
	has decay mode β- (0.565 Mev)
	has decay mode no γ
	has half life 268 years
	has nuclear magnetic moment μ = -1.3
	has nuclear spin I = 7/2-
	has number of neutrons 21
	has number of nucleons 39
	has symbol 39Ar
	is an instance of argon
argon 40	has atomic mass 39.9623837
	has natural abundance 99.600 %
	has nuclear magnetic moment μ = 0
	has nuclear spin I = 0+
	has number of neutrons 22
	has number of nucleons 40
	has symbol 40Ar
	is an instance of argon
argon 41	has atomic mass 40.964501
	has decay mode β- (2.492 Mev)
	has decay mode γ
	has half life 1.82 hours
	has nuclear spin I = 7/2-
	has number of neutrons 23
	has number of nucleons 41
	has symbol 41Ar
	is an instance of argon
argon 42	has atomic mass 41.963050
	has decay mode β- (0.60 Mev) %
	has decay mode no γ
	has half life 33 years
	has nuclear spin I = 0+
	has number of neutrons 24
	has number of nucleons 42
	has symbol 42Ar
	is an instance of argon
argon 44	has atomic mass 43.963650
	has decay mode β- (3.54 Mev)
	has decay mode γ
	has half life 11.87 minutes
	has nuclear spin I = 0+
	has number of neutrons 26
	has number of nucleons 44
	has symbol 44Ar
	is an instance of argon
argument of the perihelion	has definition An orbital element representing the longitude of the ascending node plus the angle along the orbit from the ascending node to the perihelion point.
	has definition Angular distance (measured in the plane of the object's orbit and in the direction of its motion) from the ascending node to the perihelion point.
	has symbol ω
	has synonym longitude of the perihelion
	is an instance of angle
	is an instance of orbital element
Ariel	has definition Second satellite of Uranus.
	has diameter 1600 km
	has discoverer Lassell
	has discovery date 1851
	has period 2.52 days
	is a part of Uranus
	is an instance of natural satellite
Aries	has acronym Ari
	has genitive Arietis
	has historical image
	has historical origin the ram sent by Hermes to carry the abused children of the King of Thessaly to safety
	has synonym Ram
	is a part of Zodiac
	is an instance of zodiacal constellation
Arietids	is an instance of meteor shower
Aristotelian physics	has definition Physics as promulgated by Aristotle; includes the hypothesis that our world is comprised of four elements, and that the universe beyond the moon is made of a fifth element and so is fundamentally different from the mundane realm.
	is a kind of outdated belief
array telescope	has definition a telescope composed of an array of separete individual elements acting in concert
	has element aperture
	has element separation
	has number of elements
	is a kind of electromagnetic telescope
arsenic	has abundance 1.45 × 10-3 p.p.m. in Atlantic surface seawater
	has abundance 1.45 × 10-3 p.p.m. in Pacific surface seawater
	has abundance 1.5 p.p.m. in Earth's crust
	has abundance 1.53 × 10-3 p.p.m. in deep Atlantic seawater
	has abundance 1.75 × 10-3 p.p.m. in deep Pacific seawater
	has abundance see list in Sun relative to H = 1 × 1012
	has appearance metallic for α form
	has atomic emission line 419.008 nm for As II
	has atomic emission line 445.847 nm for As II
	has atomic emission line 446.635 nm for As II
	has atomic emission line 449.423 nm for As II
	has atomic emission line 450.766 nm for As II
	has atomic emission line 454.348 nm for As II
	has atomic emission line 193.759 nm for As I (Strong, used in atom absorption spectrometry)
	has atomic number 33
	has atomic radii 125 pm
	has biological role essential to some species including humans
	has boiling point 889 K sublimes
	has chief source arsenopyrite, realgar and orpiment
	has covalent radii 121 pm
	has crystal cell dimension a = 376.0 pm for β form
	has crystal cell dimension a = 413.18 pm for α form
	has crystal cell dimension alpha angle = 54 deg 10' for α form
	has crystal cell dimension c = 10.548 pm for β form
	has crystal type hexagonal for β form
	has crystal type rhombohedral for α form
	has daily dietary intake 0.04 to 1.4 mg
	has definition a metalloid element with two main forms, grey α arsenic and β arsenic
	has density 4700 kg m-3 for solid at 293 K (β form)
	has density 5780 kg m-3 for solid at 293 K (α form)
	has discoverer Albertus Magnus
	has discovery date 1250
	has discovery location Germany
	has electrical resistivity 26 × 10-8 Ω m at 273 K
	has electron affinity 78 kJ mol-1 from As to As-
	has electron configuration [Ar]3d104s24p3 in ground state
	has electronegativity 2.18 Pauling
	has hardness brittle for α form
	has hazard salts and arsine gases are very poisonous. Stimulates metabolism in small doses, but it is carcinogenic and possibly teratogenic
	has heat capacity 20.786 J K-1 mol-1 for gas at constant pressure of 0.1 MPa at 298.15 K
	has heat capacity 24.64 J K-1 mol-1 for solid at constant pressure of 0.1 MPa at 298.15 K
	has heat of fusion 27.7 kJ mol-1
	has heat of vaporization 31.9 kJ mol-1
	has image
	has ionic radii 46 pm for As5+
	has ionic radii 69 pm for As3+
	has isotope mass range 67 to 87
	has lethal intake 50 to 300 mg
	has level in humans 0.0017 to 0.09 mg dm-3 in blood
	has level in humans 0.009 to 0.65 p.p.m. in muscle
	has level in humans 0.023 to 1.61 p.p.m. in liver
	has level in humans 0.08 to 1.6 p.p.m. in bone
	has linear expansion coefficient 4.6 × 10-6 K-1
	has main mining area not much mined as such because more than required is produced as a by-product of refining certain sulfide ores
	has mass absorption coefficient 69.7 cm2 g-1 for MoKα X-ray diffraction
	has mass absorption coefficient 83.4 cm2 g-1 for CuKα X-ray diffraction
	has mass magnetic susceptibility -3.97 × 10-9 kg-1 m3 for β form
	has mass magnetic susceptibility -9.17 × 10-10 kg-1 m3 for α form
	has mass of element in person 0.5 to 15 mg for a 70 kg average person depending on diet
	has melting point 1090 K for α form under pressure
	has mineral arsenopyrite, conichalcite, enargite, lollingite, olivenite, orpiment and realgar
	has molar volume 12.95 cm3 for α form
	has molar volume 15.9 cm3 for β form
	has name origin arsenikon = yellow orpiment from Greek
has neutron scattering length 0.658 × 10-12 cm
has number of isotopes 22
has number of protons 33
has ocean oxidation state V
has ocean residence time 90000 years
has pronunciation ahrs-nik
has registry number 7440-38-2 for Chemical Abstracts System database
has relative atomic mass 74.92159 in units of 12C = 12.000
has space group R3m for α form
has specimen available as powder. Danger !
has symbol As
has term symbol 4S3/2 in ground state
has thermal conductivity 50.0 W m-1 K-1 at 300 K for α form
has thermal neutron capture cross section 4.30 barns
has toxic intake 5 to 50 mg
has uses alloys, semiconductors, pesticides, wood preservatives and glass
has van der Waals radii 200 pm
has world production 47000 tonnes per year for As2O3
is a kind of group V element
is a kind of nonmetallic metalloid
is a kind of recycled oceanic element
reacts with hot acids and molten NaOH, tarnishes burns in oxygen
arsenic 71	has atomic mass 70.927114
	has decay mode β+ (2.013 Mev) 32%
	has decay mode γ
	has decay mode EC 68%
	has half life 2.72 days
	has nuclear magnetic moment μ = +1.6735
	has nuclear spin I = 5/2-
	has number of neutrons 38
	has number of nucleons 71
	has symbol 71As
	is an instance of arsenic
arsenic 72	has atomic mass 71.926755
	has decay mode β+ (4.355 Mev) 77%
	has decay mode γ
	has half life 26.0 hours
	has nuclear magnetic moment μ = -2.1566
	has nuclear spin I = 2-
	has number of neutrons 39
	has number of nucleons 72
	has symbol 72As
	is an instance of arsenic
arsenic 73	has atomic mass 72.923827
	has decay mode γ
	has decay mode EC (0.346 Mev)
	has half life 80.3 days
	has nuclear spin I = 3/2-
	has number of neutrons 40
	has number of nucleons 73
	has symbol 73As
	has uses research
	is an instance of arsenic
arsenic 74	has atomic mass 73.923827
	has decay mode β- (1.354 Mev)
	has decay mode β- (2.562 Mev) 31%
	has decay mode γ
	has decay mode EC 37%
	has nuclear magnetic moment μ = -1.597
	has nuclear spin I = 2-
	has number of neutrons 41
	has number of nucleons 74
	has symbol 74As
	is an instance of arsenic
arsenic 75	has atomic mass 74.9215942
	has magnetogyric ratio 4.5804 × 107 rad T-1 s-1
	has natural abundance 100%
	has NMR frequency 17.126 MHz where 1H = 100 MHz; 2.3488 T
	has NMR receptivity 143 where 13C = 1.00
	has nuclear magnetic moment μ = +1.43947
	has nuclear quadrupole moment 0.314 × 10-28 m2
	has nuclear spin I = 3/2-
	has number of neutrons 42
	has number of nucleons 75
	has relative NMR sensitivity 0.0251 where 1H = 1.00
	has symbol 75As
	has uses Nuclear Magnetic Resonance
	is an instance of arsenic
arsenic 76	has atomic mass 75.922393
	has decay mode β- (2.97 Mev)
	has decay mode γ
	has half life 26.3 hours
	has nuclear magnetic moment μ = -0.906
	has nuclear spin I = 2-
	has number of neutrons 43
	has number of nucleons 76
	has symbol 76As
	is an instance of arsenic
arsenic 77	has atomic mass 76.920646
	has decay mode β- (0.6904 Mev)
	has decay mode γ
	has half life 38.8h
	has nuclear spin I = 3/2-
	has number of neutrons 44
	has number of nucleons 77
	has symbol 77As
	is an instance of arsenic
artifact	has creation date
	has creator
	has definition an object made by humans
	has owner
	has purpose
	is a kind of physical object
artificial satellite	has attitude control
	has definition a satellite made by humans which is gravitationaly bound and in orbit of a larger physical object
	has ground communication station
	has launch date
	has launch location
	has launch vehicle
	is a kind of instrument
	is a kind of satellite
	can have orbit decay date
AS Eri	has definition An eclipsing binary whose secondary is close to its Roche limit.
	is an instance of eclipsing binary
ascending node	has definition In the orbit of a solar-system body, the point where the body crosses the ecliptic from south to north: for a star, out of the plane of the sky toward the observer.
	is a kind of Sun orbital event
aspheric	has definition An optical surface with departures in shape from a perfect sphere in order to cancel optical imperfections or aberrations.
	has wavefront modification shape
	is a kind of wavefront modifier
association	has definition A sparsely populated grouping (mass range 102-103 Msun) of very young, massive stars lying along a spiral arm of the Milky Way, whose spectral types or motions in the sky indicate a common origin. The star density is insufficient for gravitation to hold the group together against shear by differential galactic rotation, but the stars have not yet had time to disperse completely. OB associations are composed of stars of spectral types O-B2; T associations have many young T Tauri stars. The internationally approved designation for associations is the name of the constellation followed by an arabic numeral - e.g., Perseus OB2.
	has most luminous member
	is a kind of collection of stars
astatine	has image
	is a kind of halogen
asterism	has definition a group of bright stars which form a conspicuous pattern on the celestial sphere
	has purpose to easily recognize a part of the sky
	is a part of constellation
	is an instance of equatorial sky area
asteroid	has asteroid number
	has definition A small planet-like body of the solar system, <e> ≃ 0.15, <i> ≃ 9°.7 . More than 1800 have been catalogued, and probably millions of smaller ones exist, but their total mass would probably be less than 3 percent that of the Moon. Their densities are poorly known (about 2.6 g cm-3), but they suggest a composition similar to carbonaceous chondrite. The bright asteroids are presumably original condensations and those fainter than about 14-15 mag are collision fragments. Asteroids and short-period comets have some orbital similarities. Also called minor planet
	has definition A small rocky body that orbits a star. In the solar system, most asteroids lie between the orbits of Mars and Jupiter. The largest asteroid is Ceres, about 900 kilometers in diameter.
	has definition Also called planetoids or minor planets, the asteroids are tiny planets most of which orbit the Sun between Mars and Jupiter. The largest - and the first discovered - is Ceres, with a diameter of 1,003 km. It is estimated that there may altogether be no fewer than 40000. A few have very elliptical orbits and cross the orbits of several other (major) planets. One or two even have their own satellites (moons).
	has synonym minor planet
	has synonym planetoid
	is a kind of celestial body
	is a part of our solar system
asteroid belt	has definition A region of space lying between Mars (1.5 AU) and Jupiter (5.2 AU), where the great majority of the asteroids are found. None of the belt asteroids have retrograde motion.
	has mean distance from Sun 1.5 to 5.2 AU
	is a kind of celestial body
	is a part of our solar system
asteroid impact	is a kind of impact event
astrology	has definition Divination using the positions of the planets, the Sun and the Moon as seen against the stars in the constellations of the zodiac - a "science" almost as old as homo sapiens. Although at one stage in history astrology and astronomy were almost synonymous- the latter has advanced so far during the last three centuries that the two now bear little relation to each other.
	has definition The belief that human affairs and people's personalities and characters are influenced by (or encoded in) the positions of the planets.
	is a kind of outdated belief
astrometric binary	is a kind of binary star
astrometry	has definition The branch of astronomy that deals with measuring the positions of celestial objects, especially stars. Astrometrists measure parallaxes and proper motions, which allow astronomers to determine the distances and velocities of the stars.
	is a kind of galactic astronomy
astronomer	has definition a scientist specializing in astronomy
	has domain astronomy
	is a kind of scientist
astronomical catalog	is a kind of data collection
astronomical constant	is a kind of constant
astronomical horizon event	has definition The time and azimuth at which a celestial body crosses the astronomical horizon of an oberver.
	is a kind of location dependent periodic celestial event
Astronomical Image Processing System	has acronym AIPS
	has definition Developed by the National Radio Astronomy Observatory.
	is a kind of astronomical software
astronomical institution	is a kind of institution
astronomical refraction	has definition The change in direction of travel (bending) of a light ray as it passes obliquely through the atmosphere. As a result of refraction, the observed altitude of a celestial object is greater than its geometric altitude. The amount of refraction depends on the altitude of the object and on atmospheric conditions.
	is a kind of refraction
astronomical software	is a kind of abstraction
astronomical unit	has definition Mean distance between the Earth and the Sun: 1.495985 × 1011 m.
	has definition The AU is the preferred unit for distances within the solar system. Mercury, the innermost planet, lies on average 0.39 AU from the Sun; Pluto, normally the farthest planet, lies on average 39.5 AU from the Sun.
	has definition The mean distance between the Earth and the Sun. The astronomical unit is defined as the length of the radius of the unperturbed circular orbit of a body of negligible mass moving around the Sun with a sidereal angular velocity of 0.017202098950 radian per day of 86400 ephemeris seconds. AU = 1.496 × 1013 cm ≈ 500 lt-sec.
	has definition The mean distance from the earth to the sun, equal to 92.81 million miles or 499.012 light-seconds.
	has definition The radius of a circular orbit in which a body of negligible mass, and free of perturbations, would revolve around the Sun in 2π / k days, where k is the Gaussian gravitational constant. This is slightly less than the semi-major axis of the Earth's orbit.
	has symbol AU
	has value in SI unit 1 au = 1.49598 × 1011 m, approximately
	is an instance of length unit
	is an instance of non SI unit
astronomy	has definition The science that studies the natural world beyond the earth.
	is a kind of science
astronomy theory	has domain astronomy
	is a kind of theory
Astrophysical Data Facility	has acronym ADF
	has definition Is responsible for designing, developing, and operating data systems that support the processing, management, archiving and distribution of NASA mission data. The ADF serves three broadly-defined astrophysics disciplines: high-energy astrophysics, UV/optical astrophysics, and infrared/submillimeter/radio astrophysics. The ADF collaborates with the GSFC Laboratory for High Energy Astrophysics (LHEA) and the Laboratory for Astronomy and Solar Physics (LASP) in managing data for specific missions. The ADF staff also support the astrophysics community's access to multi-mission and multi-spectral data archives in the National Space Science Data Center (NSSDC).
	has location Goddard Space Flight Center
	is an instance of database
Astrophysical Research Consortium Telescope	has altitude 2800 m
	has aperture 3.5 m
	has creation date 1994
	has focal ratio f/1.75
	has latitude 32° 47' N
	has location New Mexico, US
	has longitude 105° 49' W
	has mirror maker R. Angel, B. Martin
	has mirror type spin-cast borosilicate honey-comb
	has owner Apache Point
	has synonym ARC 3.5 m
	is an instance of altazimuth telescope
	is an instance of optical telescope
	is an instance of reflector
astrophysics	has definition The science that studies the physics and chemistry of extraterrestrial objects. The alliance of physics and astronomy, which began with the advent of spectroscopy, made it possible to investigate what celestial objects are and not just where they are.
	is a kind of physics
Astroweb	is an instance of database
asymptotic branch star	has definition Globular cluster stars, which are found in that part of the HR diagram that connects the top pf the giant tip with the horizontal branch stars.
	is a kind of star
atmophile element	has definition volatile element that tends to be found in the atmosphere of a planet or asteroid
	has occurrence atmosphere of a planet or asteroid
	is a kind of planetary element
atmosphere	has composition 78% nitrogen and 21% oxygen (with 1% of other gases) by volume
	has definition A unit of pressure.
	has definition Mantle of gases round a star planet or moon, sometimes even forming the apparent surface of the body. For a body to retain an atmosphere depends on the body's gravity, and the temperature and composition of the gases. Mean atmospheric pressure at the surface is 10330 kg/m2, and is also referred to as atmosphere.
	has equivalent 1.013 bars
	has equivalent 101325 pascals
	has equivalent 14.07 lb in-2
	has equivalent 760 torr
	is a part of Earth
	is an instance of pressure unit
atmospheric dispersion corrector	has definition An optical device usually comprising two thin prisms which can rotate to compensate for the elongation of a star image caused by the wavelength dependence of the refractive index of air.
	has wavefront modification wavelength
	is a kind of wavefront modifier
atmospheric extinction	has definition Decrease in the intensity of light from a celestial body due to absorption and scattering by the Earth's atmosphere. The extinction increases from the zenith to the horizon and affects short wavelengths more than long wavelengths, so that objects near the horizon appear redder than they are at the zenith.
	is a kind of selective absorption
atomic constant	is a kind of fundamental physical constant
atomic mass constant	has approval date 1961
	has definition One-twelfth the mean mass of an atom of carbon 12 (including the orbital electrons).
	has equation
	has symbol mu
	has uncertainty 0.00000013 × 10-27 kg
	has unit kg
	has value 1.66053873 × 10-27 kg
	is an instance of mass
	is an instance of physico chemical constant
atomic mass number	has definition The total number of protons and neutrons in an atom's nucleus. For example, oxygen-16 has a mass number of sixteen, because it has eight protons and eight neutrons.
	is an instance of integer
	is an instance of quantum quantity
atomic mass unit	has approval agency International Union of Pure and Applied Physics
	has approval date 1960
	has definition The masses of atoms and molecules are generally given in atomic mass units. These units are based on a scale in which the mass of carbon 12 is taken to be 12. Atomic masses were originally given as atomic weights on a scale where the mass of the hydrogen atom was unity, later they were based on oxygen or oxygen 16; these scales have all been replaced by the carbon 12 scale.
	has equivalent 1.6605402 × 10-27 kg
	has proposal date 1959
	has symbol u
	has synonym dalton
	is a unit of mass
	is an instance of mass unit
atomic number	has definition The number of protons in an atom's nucleus. This determines the type of element.
	has proposal date 1865
	has range 1 to 103
	has synonym charge number
	is an instance of integer
	is an instance of quantum quantity
	was proposed by J. A. R. Newlands
atomic process	has domain atomic physics
	is a kind of event
	is a kind of physical process
attenuation	has definition The falling off of the energy density of radiation with distance from the source, or with passage through an absorbing or scattering medium.
	is a kind of absorption
atto	has symbol a
	has value 10-18
	is an instance of unit prefix
Auger transition	has definition A radiationless quantum jump that occurs in the X-ray region. When a K-electron is removed from an atom and an L-electron drops into the vacancy in the K-shell, the energy released in the latter transition goes not into radiation, but into the liberation of one of the remaining L-electrons.
	is a kind of ionization
Auriga	has acronym Aur
	has boundary
	has genitive Aurigae
	has historical image
	has historical origin Erechtheus, the charioteer (son of Hephaestus) is carying two to three children on his arm
	has synonym Charioteer
	is a part of celestial sphere
	is an instance of constellation
aurora	has definition Light radiated by ions in the Earth's atmosphere, mainly near the geomagnetic poles, stimulated by bombardment by energetic particles ejected from the Sun (see solar wind). Aurorae appear about 2 days after a solar flare and reach their peak about 2 years after sunspot maximum.
	has definition Spectacular array of light in the night sky, caused by charged particles from the Sun hitting the Earth's upper atmosphere. The aurora borealis is seen in the north of the Northern hemisphere; the aurora australis in the south of the Southern.
	has definition the light produced by excited atoms and ions in the upper atmosphere of a planet
	has location the ionosphere in the polar regions of a planet
	is a kind of celestial event
	is a part of ionosphere
aurora australis	has definition an aurora ocurring in the southern hemisphere of the Earth
	has location southern hemisphere
	is a kind of aurora
aurora borealis	has definition an aurora ocurring in the northern hemisphere of the Earth
	has location northern hemisphere
	is a kind of aurora
autoionization	has definition A phenomenon occurring when a discrete double-excitation state of an atom lies in the ground-state continuum. In the autoionization process one of the excited electrons is ejected, leaving the ion in an excited state (see dielectronic recombination; see also Auger effect). (also called pre-ionization)
	has inverse process dielectronic recombination
	is a kind of ionization
autumnal equinox	has definition See equinox
	is an instance of equinox
Avogadro number	has definition The number of atoms in 12 grams of carbon 12. The number of atoms in a gram-atom (mass in grams numerically equal to the atomic weight) or the number of molecules in a gram-molecule (mass in grams numerically equal to the molecular weight).
	has symbol NA
	has uncertainty 0.00000047 × 1023 mol-1
	has value 6.02214199 × 1023 mol-1
	is an instance of physico chemical constant
	is named after Amadeo Avogadro
axion	has definition A hypothetical spin-0 particle with a very small mass of 10-5-10-3 eV. It was postulated in order to provide a natural solution to the "strong CP problem".
	has mass 10-5 to 10-3 eV
	has spin 0
	is a kind of boson
	is a kind of hypothetical particle
axis	has definition Theoretical straight line through a celestial body, around which it rotates.
	is a kind of line
axis motion	has definition the motion of the axis of a system
	is a kind of motion
axisymmetric collapse	has definition Collapse of mass in such a way that the mass maintains the symmetry of a cylinder.
	has symmetry cylindrical
	is a kind of collapse
azimuth	has definition Angular distance from the north point eastward to the intersection of the celestial horizon with the vertical circle passing through the object and the zenith.
	has definition Directional bearing around the horizon, measured in degrees from north (0°).
	is an instance of angle
	is an instance of local coordinate component
azimuthal quantum number	has definition A measure of the minor axis of an elliptic orbital of an electron according to the Bohr-Sommerfeld theory.
	has symbol k
	is an instance of quantum quantity
B band	has definition Telluric lines due to O2 absorption in Earth's atmosphere, but originally thought to originate in the Sun by Fraunhofer.
	has species O2
	has wavelength 7100 Å
	is a kind of Fraunhofer line
	is a kind of molecular band
	occurs in Earth's atmosphere
B ring	is a kind of ring
	is a part of Saturn ring system
B star	has absorption line He I
	has color blue-white
	has definition Blue-white star of spectral type B whose spectra are characterized by absorption lines of neutral helium which reach their maximum intensity at B2. The Balmer lines of hydrogen are strong, and lines of singly ionized oxygen and other gases are also present.
	has surface temperature 11000 to 28000 K
	is a kind of early star
b-lines	has definition A triplet of spectral lines of Mg I.
	has wavelength λλ 5167-5184 Å
	is a kind of absorption line
B[e] star	has definition Be stars exhibiting forbidden lines in emission. [JJ95]
	has emission line forbidden lines
	is a kind of Be star
Baade's Window	has definition A clearing in the dust clouds of the constellation Sagittarius where astronomers can view stars in the Galactic bulge. Baade's window lies four degrees south of the Galactic center, so an observer's line of sight passes within 1800 light-years of the Milky Way's center.
	is a part of celestial sphere
	is an instance of equatorial sky area
backscatter	has definition Scattering of radiation (or particles) through angles greater than 90° with respect to the original direction of motion.
	is a kind of scattering
Bahcall-Soneira Model	has definition A model for the Galaxy first published by John Bahcall and Raymond Soneira in 1980. In its original form, it sought to reproduce star counts in different parts of the sky by employing only a (thin) disk and a halo; it had no thick disk.
	is a kind of galaxy theory
Bailey type	has definition A classification of RR Lyrae stars according to the shape and amplitude of their light variation
	is a kind of RR Lyrae star
Baily's beads	has definition Small "beads" of sunlight which shine through the valleys on the limb of the Moon in the instant before (or after) totality in a total solar eclipse.
	has discoverer Francis Baily
	has discovery date 1836
	has synonym diamond ring effect
	is an instance of solar eclipse
Baldet-Johnson band	has species CO+ radical
	is a kind of molecular band
Ballik-Ramsay band	has wavelength 1.7625 μ (transition 0-0)
	is a kind of C2 band
Balmer jump	has definition The sudden decrease in the intensity of the continuous spectrum at the limit of the Balmer series of hydrogen at 3646 Å, representing the energy absorbed when electrons originally in the second energy level are ionized.
	has species hydrogen (H I)
	has synonym Balmer discontinuity
	has wavelength 3646 Å
	is a kind of absorption edge
Balmer line	has definition The series lies in the visible portion of the spectrum. (Deuterium Hα is 1.785 Å short-ward of hydrogen Hα)
	has lower energy level 2
	has series limit 3647 Å
	is a kind of hydrogen line
	is a kind of spectral series line
Bamberga	has absolute magnitude +8.14
	has albedo < 0.05
	has asteroid number 324
	has definition Asteroid with the darkest known surfaces in the solar system, the only minor planet known to have such a low albedo.
	has eccentricity e = 0.36,
	has inclination i = 11°.2
	has mean opposition magnitude +11.41
	has meteorite class carbonaceous chondrite
	has radius larger than Pallas
	has rotation period 8h< sup> -has source: [H76]
	has semi-major axis a = 2.80 AU,
	is a part of asteroid belt
	is an instance of asteroid
band head	has definition The conspicuous sharp boundary which usually occurs at the head of a molecular band and which fades gradually toward either longer or shorter wavelengths, depending on the quadratic relation between frequency and rotational quantum number.
	is a kind of set of molecular lines
bar	has definition An absolute unit of pressure equal to 106 dyn cm-2.
	is an instance of CGS unit
	is an instance of pressure unit
barium	has image
	has ocean oxidation state II
	has ocean residence time 10000 years
	is a kind of alkali earth metal
	is a kind of recycled oceanic element
barium star	has definition Late type giants (G2 to K4) with a very strong BaII 4554 line. Main sequence stars with strong BaII lines have also been discovered recently.
	has definition Peculiar low-velocity, strong lined red-giant stars of spectral types G, K, and M, with abnormally large abundances of heavy s-process (but not r-process) elements. They are usually regarded as old disk stars of ~ 1-2 Msun.
	has synonym Ba II star
	has synonym Ba star
	is a kind of heavy-metal star
Barnard's star	has definition A faint binary star with the second largest proper motion known. Long-term observations of its light curve suggest a possible third component with a mass about 1.2 that of Jupiter, although this observation has been challenged.
	has discoverer Edward Emerson Barnard
	has discovery date 1916
	has distance 1.83 pc
	has orbital period 25 years
	has proper motion 10.25 arcseconds per year
	has radial velocity 100 km/sec
	has spectral type M5 V
	is a part of Ophiuchus
	is an instance of binary star
	is an instance of M star
	is an instance of red dwarf
barotropic gas	has definition A gas in which the pressure is a function of the density only.
	is a kind of gas
barred spiral	has abundance 20% of spiral galaxies are barred spiral
	has definition (in Hubble's (1936) classification, SB: in Morgan's classification, B) A spiral galaxy whose nucleus is in the shape of a bar, at the ends of which the spiral arms start.
	is a kind of spiral galaxy
baryon	has definition Heavy subatomic particle composed of 3 quarks.
	has number of quarks 3
	has spin 1/2 or 3/2
	is a kind of fermion
	is a kind of fermion
	is a kind of hadron
base SI unit	has definition a unit assumed to be mutually independent from which all other units are derived
	is a kind of SI unit
Be star	has definition Irregular variables of spectral type B (or occasionally O or A) with hydrogen emission lines in their spectra. The Be phenomenon involves rapid stellar rotation, circumstellar shells, and mass loss.
	has definition Non-supergiant B-type stars, which have shown emission in at least one of the Balmer lines at some time.
	has emission line at least one of the Balmer lines at some time
	is a kind of B star
	is a kind of emission line star
	is a kind of variable
Beacon explorer A	is an instance of inflatable spacecraft
Beacon satellite	is a kind of inflatable spacecraft
beat Cepheid	has definition Dwarf Cepheid in which two or more almost identical periods exist which cause periodic amplitude fluctuations in their light curves. The "beat" period averages about 2 to 21/2 hours.
	is a kind of dwarf Cepheid
becquerel	has approval agency 15th CGPM
	has approval date May 1975
	has base unit s-1
	has definition Unit of radioactivity; 1 becquerel represents one disintegration, or other nuclear transformation, per second.
	has equivalent 2.703 × 10-11 curies
	has symbol Bq
	is an instance of named derived SI unit
	is an instance of radioactivity unit
	is named after A. H. Becquerel
bel	has symbol B
	has value in SI unit (1/2) ln 10 Np
	is an instance of non SI unit
belief	is a kind of abstraction
Bellatrix	has definition A B2 III star.
	has distance 80 pc
	has spectral type B2 III
	has synonym gamma Orionis
	is an instance of B star
	is an instance of giant
Bent-yoke equatorial telescope	has mounting bent-yoke equatorial
	is a kind of equatorial telescope
berkelium	has image
	is a kind of transuranium element
beryllium	has abundance 14 in Sun relative to H = 1 × 1012
	has abundance 17.5 × 10-8 p.p.m. in deep Atlantic seawater
	has abundance 2.6 p.p.m. in Earth's crust
	has abundance 22 × 10-8 p.p.m. in deep Pacific seawater
	has abundance 3.5 × 10-8 p.p.m. in Pacific surface seawater
	has abundance 8.8 × 10-8 p.p.m. in Atlantic surface seawater
	has atomic emission line 234.861 nm for Be I (used in atom absorption spectrometry)
	has atomic emission line 381.345 nm for Be I
	has atomic emission line 436.099 nm for Be II
	has atomic emission line 467.333 nm for Be II
	has atomic emission line 527.081 nm for Be II
	has atomic emission line 467.342 nm for Be II (strong)
	has atomic number 4
	has atomic radii 113 pm
	has biological role none
	has boiling point 3243 K under pressure
	has bulk modulus 110 GPa
	has chief source beryl, bertrandite
	has covalent radii 89 pm
	has crystal cell dimension a = 228.55, c = 358.32 pm for α-Be
	has crystal cell dimension a = 255.15 pm for β-Be
	has crystal type b.c.c. for β-Be
	has crystal type h.c.p. for α-Be
	has daily dietary intake 0.01 mg
	has definition Silvery-white, lustrous, relatively soft metal which is unaffected by air or water, even at red heat. Rare and fragile element. Nuclear reactions in stars destroy it. Most and possibly all beryllium originated when cosmic rays smashed into heavier atoms in space and split them into lighter ones, such as beryllium.
	has density 1847.7 kg m-3 for solid at 293 K
	has discoverer Nicholas Louis Vauquelin
	has discovery date 1797
	has discovery location Paris, France
	has electrical resistivity 4.0 × 10-8 Ω m at 293 K
	has electron affinity -18 kJ mol-1 from Be to Be-
	has electron configuration [He]2s2 in ground state
	has electronegativity 1.57 Pauling
	has hazard deadly poison, carcinogenic for lab animals and maybe for humans
	has hazard inhalation of dust causes severe and irreparable lung damage
	has heat capacity 16.44 J K-1 mol-1 for solid at constant pressure 0.1 MPa at 298.15 K
	has heat capacity 20.786 J K-1 mol-1 for gas at constant pressure 0.1 MPa at 298.15 K
	has heat of fusion 9.80 kJ mol-1
	has heat of vaporization 308.8 kJ mol-1
	has image
	has ionic radii 34 pm for Be+
	has isotope mass range 6 to 11
	has lethal intake 317 mg kg-1 acetate in rat
	has level in humans < 1 × 10-5 mg dm-3 in blood
	has level in humans 0.00075 p.p.m. in muscle
	has level in humans 0.0016 p.p.m. in liver
	has level in humans 0.003 p.p.m. in bone
	has linear expansion coefficient 11.5 × 10-6 K-1
	has longest lived isotope beryllium 9
	has main mining area Brazil, USA, Madagascar, Germany, Czech Republic, Russia, India
	has mass absorption coefficient cm2 g-1 0.298 for MoKalpha X-ray diffraction
	has mass absorption coefficient cm2 g-1 1.50 for CuKalpha X-ray diffraction
	has mass magnetic susceptibility -1.3 × 10-8 kg-1 m3 for solid
	has mass of element in person 0.036 mg for a 70 kg average person
	has melting point 1551 ± 5 K
	has mineral beryl, bertrandite, chrysoberyl, gadolinite, herderite
	has molar volume 4.88 cm3
	has name origin beryllos = beryl from Greek
	has neutron scattering length 0.779 × 10-12 cm
	has number of isotopes 6 including nuclear isomers
	has number of protons 4
	has ocean oxidation state II
	has ocean residence time 4000 years
	has poisson's ratio 0.02 GPa
has pronunciation be-ril-iuhm
has registry number 7440-41-7 for Chemical Abstracts System database
has relative atomic mass 9.012182 in units of 12C = 12.000
has reserves 400000 tonnes
has rigidity modulus 156 GPa
has space group Im3m for β-Be
has space group P63/mmc for α-Be
has specimen lumps, powder. Danger !
has symbol Be
has synthesis mechanism electrolysis of fused BeCl2
has term symbol 1S0 in ground state
has thermal conductivity 200 W m-1 K-1 at 300 K
has thermal neutron capture cross section 0.0092 barns
has toxic intake 13 mg kg-1 in rat
has uses alloys with copper and nickel increases electrical and thermal conductivities
has uses copper alloys used to make spark proof tools
has world production 364 tonnes year-1
has young's modulus 318 GPa
is a kind of alkali earth metal
is a kind of light element
is a kind of recycled oceanic element
beryllium 10	has atomic mass 10.013534
	has decay mode β- (0.5561 Mev)
	has decay mode no γ
	has half life 1.52 × 106 years
	has nuclear spin I = 0+ h/2π
	has number of neutrons 6
	has number of nucleons 10
	has symbol 10Be
	has uses research
	is an instance of beryllium
beryllium 11	has atomic mass 11.021658
	has decay mode α (11.48 Mev)
	has decay mode β-
	has decay mode γ
	has half life 13.8 seconds
	has nuclear spin I = 1/2+ h/2π
	has number of neutrons 7
	has number of nucleons 11
	has symbol 11Be
	is an instance of beryllium
beryllium 12	has atomic mass 12.02692
	has decay mode β- (11.71 Mev)
	has decay mode n
	has half life 2.4 × 10-2 seconds
	has nuclear spin I = 0+ h/2π
	has number of neutrons 8
	has number of nucleons 12
	has symbol 12Be
	is an instance of beryllium
beryllium 14	has atomic mass 13.0375
	has decay mode β- (16.4 Mev)
	has half life 4 × 10-3 seconds
	has nuclear spin I = 0+ h/2π
	has number of neutrons 10
	has number of nucleons 14
	has symbol 14Be
	is an instance of beryllium
beryllium 6	has atomic mass 6.019725
	has decay mode 2p.α
	has half life 5.9 × 10-21 seconds
	has nuclear spin I = 0+ h/2π
	has number of neutrons 2
	has number of nucleons 6
	has symbol 6Be
	is an instance of beryllium
beryllium 7	has atomic mass 7.016928
	has decay mode γ
	has decay mode EC (0.862 Mev)
	has half life 53.82 days
	has nuclear spin I = 3/2+ h/2π
	has number of neutrons 3
	has number of nucleons 7
	has symbol 7Be
	has uses research
	is an instance of beryllium
beryllium 8	has atomic mass 8.005305
	has decay mode 2α (0.046 Mev)
	has decay mode no γ
	has half life 7 × 10-17 seconds
	has nuclear spin I = 0+ h/2π
	has number of neutrons 4
	has number of nucleons 8
	has symbol 8Be
	is an instance of beryllium
beryllium 9	has atomic mass 9.0121822
	has magnetogyric ratio 3.7589 × 107 rad T-1 s-1
	has natural abundance 100%
	has NMR frequency 14.053 MHz where 1H = 100 MHz; 2.3488 T
	has NMR receptivity 78.8 where 13C = 1.00
	has nuclear magnetic moment μ = -1.1779 nuclear magnetons with diamagnetic correction
	has nuclear quadrupole moment 0.05288 × 10-28 m2
	has nuclear spin I = 3/2- h/2π
	has number of neutrons 5
	has number of nucleons 9
	has relative NMR sensitivity 1.39 × 10-2 where 1H = 1.00
	has symbol 9Be
	has uses Nuclear Magnetic Resonance
	is an instance of beryllium
Besselian element	has definition One of several quantities tabulated for the calculation of accurate predictions of an eclipse or occultation for any point on or above the surface of the Earth.
	is a kind of orbital quantity
Besselian year	has definition The period of one complete revolution in right ascension of the fictitious mean sun, as defined by Newcomb. The beginning of a Besselian year, traditionally used as as standard epoch, is denoted by the suffix ".0". Since 1984 standard epochs have been defined by the Julian year rather that the Besselian year. For distinction, the beginning of the Besselian year is now identified by the prefix B (e.g., B1950.0).
	is an instance of year
Beta Aquarii	has B-V magnitude 0.83
	has declination -05 34 16
	has right ascension 21 31 33.4
	has spectral type G0Ib
	has synonym HR 8232
	has V magnitude 2.91
	is a part of Aquarius
	is an instance of naked eye star
	is an instance of yellow supergiant
Beta Arae	has B-V magnitude 1.46
	has declination -55 31 47
	has right ascension 17 25 17.9
	has spectral type K3Ib-IIa
	has synonym HR 6461
	has V magnitude 2.85
	is a part of Ara
	is an instance of K star
	is an instance of naked eye star
	is an instance of supergiant
Beta Camelopardalis	has B-V magnitude 0.92
	has declination +60 26 32
	has right ascension 05 03 25.1
	has spectral type G0Ib
	has synonym HR 1603
	has V magnitude 4.03
	is a part of Camelopardalis
	is an instance of naked eye star
	is an instance of yellow supergiant
Beta Cancri	has B-V magnitude 1.48
	has declination + 9 11 8
	has right ascension 8 16 30.9
	has spectral type K4IIIBa0.5
	has synonym HR 3249
	has V magnitude 3.52
	is a part of Cancer
	is an instance of giant
	is an instance of K star
	is an instance of naked eye star
Beta Cephei	is a part of Cepheus
	is an instance of beta Cephei star
beta Cephei star	has definition A small group of short-period pulsating variables lying slightly above the upper main sequence. They have a doubly periodic light curve, and are confined within a narrow band of the H-R diagram which lies near the end of core hydrogen-burning stars of roughly 10-20 Msun. beta Cephei itself has at least three components.
	has period P = 3.5 to 6 hr
	has prototype beta Cephei
	has spectral type O9-B3
	has synonym beta Canis Majoris star
	is a kind of periodic variable
Beta Ceti	has B-V magnitude 1.02
	has declination -17 59 12
	has right ascension 0 43 35.3
	has spectral type K0IIICH-1H,K-0
	has synonym HR 188
	has V magnitude 2.04
	is a part of Cetus
	is an instance of giant
	is an instance of K star
	is an instance of naked eye star
Beta Crucis	has B-V magnitude -0.23
	has declination -59 41 19
	has right ascension 12 47 43.3
	has spectral type B0.5III
	has synonym HR 4853
	has V magnitude 1.25
	is a part of Crux
	is an instance of B star
	is an instance of giant
	is an instance of naked eye star
Beta Delphini	has B-V magnitude 0.44
	has declination +14 35 43
	has right ascension 20 37 32.9
	has spectral type F5IV
	has synonym HR 7882
	has V magnitude 3.63
	is a part of Delphinus
	is an instance of F star
	is an instance of naked eye star
	is an instance of subgiant
Beta Herculis	has B-V magnitude 0.94
	has declination +21 29 22
	has right ascension 16 30 13.1
	has spectral type G7IIIa
	has synonym HR 6148
	has V magnitude 2.77
	is a part of Hercules
	is an instance of G star
	is an instance of giant
	is an instance of naked eye star
Beta Hydri	has B-V magnitude 0.62
	has declination -77 15 16
	has right ascension 0 25 45.3
	has spectral type G2IV
	has synonym HR 98
	has V magnitude 2.80
	is a part of Hydrus
	is an instance of G star
	is an instance of naked eye star
	is an instance of subgiant
Beta Librae	has B-V magnitude -0.11
	has declination -9 22 59
	has right ascension 15 17 0.3
	has spectral type B8V
	has synonym HR 5685
	has V magnitude 2.61
	is a part of Libra
	is an instance of B star
	is an instance of dwarf
	is an instance of naked eye star
beta Lyrae star	has definition A class of eclipsing binary whose secondary minima are intermediate between those of Algol and those of W UMa. The prototype beta Lyr (B8.5 II, F V) is a complex eclipsing system and is presently in a state of rapid mass transfer. The spectrum of one companion is invisible; it may be a black hole. Beta Lyrae is also a weak radio source.
	is a kind of eclipsing binary
beta Persei star	has definition A class of eclipsing binary (see Algol) with periods of from 2 to 5 days, the depth of whose secondary minimum is almost negligible.
	has prototype Algol
	is a kind of eclipsing binary
Beta Pictoris	has distance 50 light years
	has image
	is a part of Pictor
	is an instance of star
Beta Trianguli	has B-V magnitude 0.14
	has declination +34 59 14
	has right ascension 2 9 32.5
	has spectral type A5II
	has synonym HR 622
	has V magnitude 3.00
	is a part of Triangulum
	is an instance of A star
	is an instance of bright giant
	is an instance of naked eye star
Beta Volantis	has B-V magnitude 1.13
	has declination -66 8 13
	has right ascension 8 25 44.3
	has spectral type K1III
	has synonym HR 3347
	has V magnitude 3.77
	is a part of Volans
	is an instance of giant
	is an instance of K star
	is an instance of naked eye star
beta-decay	has definition Emission of an electron and an antineutrino (or a positron and a neutrino) by a radioactive nucleus by any one of several processes. e.g., the spontaneous β-decay of a free neutron (n → p + e- + ν bar). The A-number is unchanged, but the Z-number is increased (or decreased) by 1. Beta-decay is a so-called weak interaction. Since electrons of all energies (up to a certain maximum) are emitted in β-decay, this process exhibits a continuous spectrum (unlike α-particle emission, which exhibits a line spectrum).
	has definition Spontaneous emission by a heavier element (such as uranium) of negatively charged electrons - beta particles. The result of this radioactive decay is that the original element is very gradually converted into another element. Beta particle emission may be simultaneous with alpha particle decay.
	has definition The disintegration of an atomic nucleus, in which an electron (which was historically called a beta particle) and an antineutrino are emitted. Since the electron carries away one unit of negative charge, the final nucleus has a charge one greater than the initial nucleus.
	has definition The process in which a neutron disintegrates into a proton, an electron, and an antineutrino. The escaping electron is sometimes called a beta ray. (See neutrino; neutron; proton.)
	is a kind of nuclear decay
beta-particle	has definition An electron or a positron emitted from an excited nucleus when it returns to its ground state via β-decay.
	has definition Particles first discovered in radioactive β decay - later identified as electrons.
	is a kind of electron
Betelgeuse	has definition A red semiregular variable supergiant. Betelgeuse is also a strong infrared emitter - at 2 µ the brightest in the sky.
	has definition A red supergiant star in the constellation Orion and the brightest red supergiant in Earth's sky.
	has distance 500 pc
	has image
	has spectral type M2 Iab
	has synonym alpha Ori
	is an instance of red supergiant
	is an instance of semiregular variable
biased galaxy formation	has definition The theory that bright galaxies form preferentially from anomalously overdense perturbations in the early universe.
	is a kind of galaxy theory
big bang	has antonym big crunch
	has definition A general class of cosmological models that assume the universe is homogeneous and isotropic on large scales and that allow the universe to evolve in time. Most calculation in the standard big bang model assume a Friedmann cosmology. (See Friedmann equation; homogeneity; isotropy.) A cosmological model that has the same properties as a Friedmann model under some conditions is said to have a Friedmann limit.
	has definition A homogeneous, isotropic model of the Universe involving nonstatic (i.e., expanding or contracting) solutions to Einstein's field equations (with zero cosmological constant) calculated by the Russian mathematician A. Friedmann in 1922.
	has definition A model of the Universe which started with an initial singularity. The Friedmann model of a homogeneous, isotropic universe (composed of adiabatically expanding matter and radiation, as a result of a primeval explosion) is the standard example.
	has definition According to standard cosmology, the explosion that started the universe expanding 10 to 15 billion years ago.
	has definition An evolutionary model of cosmology in which the universe began about 10 billion years ago, in a state of extremely high density and temperature. According to this model, the universe has been expanding, thinning out, and cooling since its beginning. It is an observational fact that distant galaxies are all moving away from our own galaxy, as predicted by the big bang model. (See closed universe; flat universe; Friedmann models; open universe.)
	has definition Model of cosmic history in which the universe begins in a state of high density and temperature, both of which decrease as the universe expands. Less a theory than a school of theories that attempt to trace how the universe evolved.
	has definition One of three standard Big Bang models that were formulated by Friedmann and Lemaitre of an isotropic and homogeneous universe composed of expanding matter and radiation. In these models space is unbounded.
	has definition The initial point of creation.
	has definition The most widely accepted theory of the origin of the Universe. It asserts that the Universe began some 1010 years ago from a space-time point of infinite energy density (a singularity). The expansion of the Universe since that time is akin to the expansion of the surface of an inflating balloon: every point on the balloon's surface is moving away from every other point. So, microbes living on the surface see their two-dimensional world expanding, yet there is no center to the expansion which is everywhere uniform.
	has definition The state of extremely high (classically, infinite) density and temperature from which the universe began expanding.
	has definition Theory originally proposed by Georges Lemaitre but elaborated by George Gamow and the α-β-hypothesis-γ that the Universe began with the Big Bang, the superexplosion of all the matter now dispersing in the Universe. Since the nuclear physics involved has been explained, and various supporting evidence - notably helium abundance and the sources of radio emission - has been discovered, the theory is almost universally accepted (although at one time the steady state theory rivaled it in popularity).
	has synonym Friedmann-Lemaitre universe
	is a kind of cosmology theory
Big bang crossover effect	has definition The epoch during the radiation era when the universe switched from being radiation-dominated to being matter-dominated.
	has start time 1012 s
	is a kind of Big Bang era
Big Bang era	has definition A theoretical era postulated to have occurred after the Big Bang. They are in order : Planck era hadron era inflation era lepton era nucleosynthetic era radiation era matter era
	has start time relative to the Big Bang
	has temperature
	is a kind of era
	is followed by
	is preceded by
big crunch	has antonym big bang
	has definition If the universe has a mass density exceeding the critical mass density, then gravity will eventually reverse the expansion, causing the universe to recollapse into what is often called the big crunch. See also closed universe.
	has definition One hypothesized future for the universe in which the current expansion stops, reverses, and results in all space and all matter collapsing together; a reversal of the big bang.
	has definition The state of extremely high density and temperature into which a closed universe will recollapse in the distant future.
	is a kind of cosmology theory
binary galaxy	has definition Two galaxies orbiting each other owing to their mutual gravitational attraction.
	has number of galaxies 2
	has synonym double galaxy
	is a kind of collection of galaxies
binary pulsar	is a kind of pulsar
binary star	has definition A system of two stars orbiting around a common center of gravity. Visual binaries are those whose components can be resolved telescopically (i.e., angular separation > 0'.5) and which have detectable orbital motion. Astrometric binaries are those whose dual nature can be deduced from their variable proper motion; spectroscopic binaries, those whose dual nature can be deduced from their variable radial velocity. At least half of the stars in the solar neighborhood are members of binary (or multiple) systems. (See photometric binaries; optical pairs.)
	has number of stars 2
	has orbital period
	is a kind of star
	is a kind of star system
binary star orbital event	is a kind of orbital event
binding energy	has definition The energy required to break up a system. In particular, the binding energy of an atomic nucleus is the energy released in the formation of the nucleus. The most strongly bound nuclei are those with atomic weights between about 50 and 65 (the iron group). Lighter nuclei are less strongly bound because of their larger surface-to-volume ratios; heavier nuclei, because the effects of Coulomb repulsion increase with the nuclear charge.
	is a kind of energy
biochemical process	is a kind of biological process
	is a kind of chemical process
biological process	has domain biology
	is a kind of process
biology	has definition The scientific study of life and living matter.
	is a kind of science
bismuth	has image
	has ocean oxidation state III
	is a kind of group V element
	is a kind of metallic element
	is a kind of scavenged oceanic element
BL Lacertae	has definition A highly variable object (the most rapid radio variable known, also an optically violent variable - mv = 12 to 15 mag - and an infrared source). Probably an exceedingly compact nonthermal object, and undoubtedly extragalactic. Its optical spectrum is characterized by an absence of lines, so its redshift cannot be measured. (In 1974 Oke and Gunn infer z = 0.07 from an Hβ absorption line in the surrounding halo and conclude that it lies at the center of a bright (Mv = - 23) elliptical. If true, this would make BL Lac the nearest known quasar.)
	has definition A member of a class of astronomical objects with the following characteristics: (1) rapid variations in intensity at radio, infrared, and optical wavelengths; (2) energy distributions such that most of the energy is emitted at infrared wavelengths; (3) absence of discrete features in low-dispersion spectra; and (4) strong and rapidly varying polarization at visual and radio wavelengths.
	has synonym BL Lac
	is a kind of blazar
black dwarf	has definition The final stage in the evolution of a star of roughly 1 Msun. It is a mass of cold, electron-degenerate gas, and can no longer radiate energy, because the whole star is in its lowest energy state. No black dwarfs have ever been observed. Also, an object (M < 0.085 Msun) that is not massive enough to achieve nuclear chain reactions.
	is a kind of dwarf
	is a kind of massive compact halo object
black hole	has antonym white hole
	has definition A gravitationally collapsed mass inside the Schwarzschild radius (q.v.), from which no light, matter, or signal of any kind can escape. A black hole occurs when the escape velocity of a body becomes the velocity of light (2GM / R = c2). If an object with the mass of the Sun had a radius of 2.5 km, it would be a black hole. Black holes represent one of the possible endpoints of stellar evolution for stars very much more massive than the Chandrasekhar limit.
	has definition A mass that is sufficiently compact that not even light can escape its intense gravity. Thus it appears black from the outside. If the sun were compressed to a sphere about four miles in diameter, it would become a black hole. It is believed that some massive stars, after exhausting their nuclear fuel, collapse under their own weight to form black holes.
	has definition A singularity in space, surrounded by an event horizon, caused by the collapse of a small but massively dense star through the effects of its own increasing gravity. By the time the state of singularity is reached, the remnants of the star may be minimal, but the gravitational force is so strong it prevents even light from escaping. Black holes may form the "power centers" of galaxies, thus explaining infrared radiation detected in several galactic centres. The properties of matter entering a black hole are the theme of John Wheeler's no hair theorem.
	has definition An object that is maximally gravitationally collapsed, and from which not even light can escape.
	has definition An object with such a strong gravitational field that even light cannot escape. Matter can fall into a black hole, but according to classical physics no matter or energy can leave it. (Hawking has used quantum theory to show that black holes emit blackbody radiation, but the effect is significant only for black holes much smaller than those that are expected to form by the collapse of stars, which have masses of several solar masses or more.)
	is a kind of massive compact halo object
	is a kind of singularity
blazar	has definition A highly variable active galaxy which, in general, displays no emission lines in its spectrum.
	has definition A term collectively used to refer to Optically Violent Variables (OVVs) and BL Lac objects.
	is a kind of active galaxy
	is a kind of radio galaxy
blaze angle	has definition The tilt of the facets or grooves of a diffraction grating.
	is a kind of angle
blazed grating	has definition Diffraction grating so ruled that the reflected light is concentrated into only a few orders, or even a single order, of the spectrum.
	is a kind of diffraction grating
BLS1	has definition Broad Line Seyfert 1
	is a kind of Seyfert galaxy
blue	is a kind of optical
blue compact dwarf galaxy	has acronym BCD
	is a kind of dwarf galaxy
blue compact galaxy	has acronym BCG
	is a kind of galaxy
blue dwarf	has definition High-temperature star (as opposed to red stars). Blue dwarfs represent the very dense, but very small, near-final form of what was once a red giant.
	is a kind of dwarf
blue giant	has definition A giant star with spectral type O or B. [C95]
	has definition High-temperature star (as opposed to red stars). Blue giants are generally on or near the main sequence of the Hertzsprung-Russell diagram.
	is a kind of giant
blue halo star	has definition Hot star in the horizontal-branch, post-horizontal-branch, and post-asymptotic branch phases of evolution.
	is a kind of Population II star
blue haze	has definition A condition in the Martian atmosphere which sometimes makes it opaque to radiation in the blue-violet end of the visible spectrum.
	is a part of Mars
blue horizontal-branch star	has definition Blue horizontal-branch star, the first catalog of which was compiled by Humason and Zwicky.
	has definition Population II star (B3-A0) in the galactic halo, characterized by strong, sharp hydrogen lines and large Balmer jump, and very weak lines of all other elements.
	has synonym HZ star
	is a kind of Population II star
blue supergiant	has definition A supergiant star with spectral type O or B. All blue supergiants are hot and young. Rigel, in the constellation Orion, is the best example.
	is a kind of supergiant
blueshift	has definition The shift of spectral lines toward shorter wavelengths in the spectrum of an approaching source of radiation.
	is a kind of Doppler shift
BM Orionis	has definition A peculiar eclipsing binary (B2-B3) in the Trapezium, with a flat-bottomed light curve suggesting a total eclipse. The spectrum of the secondary has never been seen.
	is a part of Orion
	is an instance of eclipsing binary
Bohr atom	has definition The model of an atom whose electrons are pictured as describing "Keplerian" orbits about the central nucleus.
	is a kind of outdated belief
Bohr magneton	has definition Magnetic moment of an electron in the first Bohr orbit.
	has equation
	has symbol μB
	has uncertainty 0.000037 × 10-26 J T-1
	has value 927.400899 × 10-26 J T-1
	is an instance of electromagnetic constant
Bohr radius	has definition A unit of length based on the radius of the first Bohr orbit of hydrogen 1.
	has equation
	has proposal date 1928
	has proposer Hartree
	has symbol a0
	has symbol a0
	has uncertainty 0.0000000019 × 10-10 m
	has value 0.5291772083 × 10-10 m
	is an instance of atomic constant
	is an instance of length unit
	is an instance of radius
	is named after Niels Bohr
bohrium	is a kind of transactinide
Bok globule	has definition A compact, spherical dark nebula that absorbs radiation. Estimates of their mass suggest that their density is too low for gravitational collapse. They tend to lie in regions of much dust but less gas than would be expected for star-forming regions.
	has radius 103 to 105 AU
	is a kind of dark nebula
	is a part of dust
Bol'shoi Teleskop Azimutal'nyi	has altitude 2100 m
	has aperture 6.00 m
	has comment Primary mirror replaced in about 1984, and a third made of Sitall glass was figured in 1992
	has creation date 1975
	has focal ratio f/4, 30
	has latitude 43° 39' N
	has location Mount Pastukhov, Russia
	has longitude 41° 26'E
	has mirror maker LOMO
	has mirror type Sitall glass
	has mounting manufacturer LOMO
	has owner Special Astrophysical Obs.
	has synonym 6 m
	is an instance of altazimuth telescope
	is an instance of optical telescope
	is an instance of Ritchey-Chrétien
bolometer	has definition A device for measuring the total amount of radiant energy received from a celestial object.
	is a kind of detector
bolometric absolute magnitude	has definition A measure of the total amount of energy radiated by a star at all wavelengths. Mbol of Sun = 4.72 mag. The fraction of total energy emitted by a very blue or very red star that lies in the visible range may differ from the total energy by 4 or 5 mag - i.e., only a few percent of the energy lies in the visible.
	has symbol Mbol
	is a kind of absolute magnitude
Boltzmann constant	has definition The constant of proportionality relating the mean kinetic energy of an atom to its absolute temperature.
	has equation
	has equivalent to molar gas constant divided by the Avogadro number
	has symbol k
	has uncertainty 0.0000024 × 10-23 J K-1
	has value 1.3806503 × 10-23 J K-1
	is an instance of physico chemical constant
boron	has abundance 2.63 × 105 in Sun relative to H = 1 × 1012
	has abundance 4.41 p.p.m. in seawater
	has abundance 950 p.p.m. in Earth's crust
	has atomic emission line 1166.004 nm for B I
	has atomic emission line 1166.247 nm for B I
	has atomic emission line 208.891 nm for B I
	has atomic emission line 208.957 nm for B I
	has atomic emission line 249.667 nm for B I
	has atomic emission line 345.129 nm for B I
	has atomic emission line 249.773 nm for B I (strong)
	has atomic number 5
	has atomic radii 83 pm
	has biological role essential to plants; toxic in excess
	has boiling point 3931 K
	has chief source kernite, borax, ulexite, colemanite
	has covalent radii 88 pm
	has crystal cell dimension a = 506.7, α = 58deg4' pm for α-B
	has crystal type rhombohedral for α-B
	has daily dietary intake 1 - 3 mg
	has definition Non-metal with several forms - the most common form is a dark amorphous powder, unreactive to water, acids and alkalis. Rare and fragile element. Nuclear reactions in stars destroy it. Most boron is created in space, by cosmic rays that smash into heavier atoms and split them.
	has density 2340 kg m-3 for β-rhombohedral solid at 293 K
	has discoverer L.J. Lussac and L.J. Thenard
	has discovery date 1808
	has discovery location Paris, France and London, England
	has electrical resistivity 18000 Ω m at 293 K
	has electron affinity 26.7 kJ mol-1 from B to B-
	has electron configuration [He]2s22p1 in ground state
	has electronegativity 2.04 Pauling
	has hazard boric acid and borates are poisonous, although once used in medicines
	has heat capacity 11.09 J K-1 mol-1 for α solid at constant pressure 0.1 MPa at 298.15 K
	has heat capacity 20.799 J K-1 mol-1 for gas at constant pressure 0.1 MPa at 298.15 K
	has heat of fusion 22.2 kJ mol-1
	has heat of vaporization 538.9 kJ mol-1
	has image
	has ionic radii 23 pm for B3+
	has isotope mass range 8 to 13
	has lethal intake 10 - 20 g as boric acid
	has level in humans 0.13 mg dm-3 in blood
	has level in humans 0.33 - 1 p.p.m. in muscle
	has level in humans 0.4 - 3.3 p.p.m. in liver
	has level in humans 1.1 - 3.3 p.p.m. in bone
	has linear expansion coefficient 5 × 10-6 K-1
	has longest lived isotope boron 11
	has main mining area ulexite in USA, Tibet, Chile; colemanite in USA, Turkey
	has mass absorption coefficient 0.392 cm2 g-1 for MoKalpha X-ray diffraction
	has mass absorption coefficient 2.39 cm2 g-1 for CuKalpha X-ray diffraction
	has mass magnetic susceptibility -7.8 × 10-9 kg-1 m3 for solid
	has mass of element in person 18 mg for a 70 kg average person
	has melting point 2573 K
	has mineral borax, colemanite, datolite, kernite, ulexite
	has molar volume 4.62 cm3
	has name origin buraq from Arabic
	has neutron scattering length 0.535 × 10-12 cm
	has number of isotopes 6 including nuclear isomers
	has number of protons 5
	has ocean oxidation state III
	has ocean residence time 1 × 107 years
	has pronunciation bohr-on
	has registry number 7440-42-8 for Chemical Abstracts System database
	has relative atomic mass 10.811 in units of 12C = 12.000
	has reserves 270 × 106 tonnes as B2O3
	has space group R3m for α-B
	has specimen crystals, pieces or powder. Safe.
	has symbol B
has synthesis mechanism
has term symbol 2P1/2 in ground state
has thermal conductivity 27.0 W m-1 K-1 at 300 K
has thermal neutron capture cross section 767 barns
has toxic intake 5 g as boric acid
has uses borosilicate glass, detergents and fire-retardants
has van der Waals radii 208 pm
has world production 1 × 106 tonnes year-1 as B2O3
is a kind of accumulating oceanic element
is a kind of group III element
is a kind of light element
is a kind of nonmetallic metalloid
reacts with metals to form borides
boron 10	has atomic mass 10.0129369
	has magnetogyric ratio 2.8740 × 107 rad T-1 s-1
	has natural abundance 19.9%
	has NMR frequency 10.746 MHz where 1H = 100 MHz; 2.3488 T
	has NMR receptivity 22.1 where 13C = 1.00
	has nuclear magnetic moment μ = +1.80065 nuclear magnetons with diamagnetic correction
	has nuclear quadrupole moment 0.08459 × 10-28 m2
	has nuclear spin I = 3+ h/2π
	has number of neutrons 5
	has number of nucleons 10
	has relative NMR sensitivity 0.0199 where 1H = 1.00 using (C2H5)2O/BF3 reference
	has symbol 10B
	has uses Nuclear Magnetic Resonance
	is an instance of boron
boron 11	has atomic mass 11.0093054
	has magnetogyric ratio 8.5794 × 107 rad T-1 s-1
	has natural abundance 80.1%
	has NMR frequency 32.084 MHz where 1H = 100 MHz; 2.3488 T
	has NMR receptivity 754 where 13C = 1.00
	has nuclear magnetic moment μ = +2.688637 nuclear magnetons with diamagnetic correction
	has nuclear quadrupole moment 0.04059 × 10-28 m2
	has nuclear spin I = 3/2- h/2π
	has number of neutrons 6
	has number of nucleons 11
	has relative NMR sensitivity 0.17 where 1H = 1.00 using (C2H5)2O/BF3 reference
	has symbol 11B
	has uses Nuclear Magnetic Resonance
	is an instance of boron
boron 12	has atomic mass 12.014352
	has decay mode β- (13.369 Mev)
	has decay mode β-α 1.6%
	has decay mode γ
	has half life 0.0202 seconds
	has nuclear magnetic moment μ = +1.0031 nuclear magnetons with diamagnetic correction
	has nuclear spin I = 1+ h/2π
	has number of neutrons 7
	has number of nucleons 12
	has symbol 12B
	is an instance of boron
boron 13	has atomic mass 13.01778
	has decay mode β- (13.436 Mev)
	has decay mode β-n 0.25%
	has decay mode γ
	has half life 0.0174 seconds
	has nuclear magnetic moment μ = +3.17778 nuclear magnetons with diamagnetic correction
	has nuclear spin I = 3/2+ h/2π
	has number of neutrons 8
	has number of nucleons 13
	has symbol 13B
	is an instance of boron
boron 8	has atomic mass 8.024605
	has decay mode β+ (17.979 Mev)
	has decay mode 2 α
	has half life 0.770 seconds
	has nuclear magnetic moment μ = 1.0355 nuclear magnetons with diamagnetic correction
	has nuclear spin I = 2+ h/2π
	has number of neutrons 3
	has number of nucleons 8
	has symbol 8B
	is an instance of boron
boron 9	has atomic mass 9.013328
	has decay mode p2α
	has half life 8 × 10-19 seconds
	has nuclear magnetic moment μ = +1.8007 nuclear magnetons with diamagnetic correction
	has nuclear spin I = 3/2- h/2π
	has number of neutrons 4
	has number of nucleons 9
	has symbol 9B
	is an instance of boron
Borosilicate	is a kind of mirror
Bose-Einstein nucleus	has definition Nucleus of even A-number (i.e., those with integral spin) (cf. Fermi-Dirac nuclei). Bose-Einstein nuclei do not obey the exclusion principle, and their ground state has zero angular momentum.
	has spin 0
	is a kind of boson
	is a kind of nucleus
boson	has definition A class of elementary particles whose spin is an integer multiple of a fundamental quantized value. The major function of bosons is to mediate the fundamental forces. The best-known boson is the photon.
	has definition A particle which does not obey Pauli's exclusion principle. It is denoted by an integer (or zero) spin.
	has definition A particle, or pattern of string vibration, with a whole number amount of spin; typically a messenger particle.
	has definition A subatomic particle whose spin is an integral multiple of h bar (cf. fermion). Bosons include the photons, the pions, the gravitons, and all Bose-Einstein nuclei. Boson number is not conserved.
	has definition Any particle with integer spin: 0, h bar, 2h bar etc.
	has definition Elementary particles that have integral spins. Force particles such as the photon, gluon, and vector bosons are all bosons. But note that there can also exist composite particles formed out of collections of fermions such as a helium atom-which act collectively as bosons.
	has definition Elementary particles with integer spin that do not obey the Pauli exclusion principle. They include the photons and the W and Z particles, carriers of the electromagnetic and the electroweak forces respectively.
	has quantum behavior Bose-Einstein statistics
	has spin integral
	is a kind of particle
bosonic string theory	has definition First known string theory; contains vibrational patterns that are all bosons.
	is a kind of string theory
bottom	has charge -1/3
	has definition A flavor of quark. See flavor.
	is an instance of heavy quark
bottom-up scenario	has definition A galaxy-formation scenario in which small galaxies form first. Larger and larger structures are then formed in due course.
	has definition The idea that small structures, perhaps galaxies or even smaller substructures, form first in the universe, followed later by larger structures.
	is a kind of galaxy theory
bound electron	has definition Zone in which the electron in atoms reside. Its radius is determined by the quantum principle, its population by the exclusion principle.
	is a kind of electron
bound-bound transition	has definition Transition between energy levels of an electron bound to a nucleus (the electron is bound both before and after the transition).
	is a kind of atomic process
bound-free transition	has definition Transition in which a bound electron in any energy level is liberated or its reverse process.
	is a kind of atomic process
Bowen fluorescence mechanism	has definition A mechanism first discovered by Bowen which explains the anomalously strong lines of O III in the spectra of some planetary nebulae as fluorescence involving the radiative excitation of the 2p3d 3Po2 level of O2+ (54.71 eV) from the 2p2 3P2 state in the ground term by He II Lyman-α photons (54.17 eV).
	is a kind of fluorescence
Boyle's law	has definition The pressure of an ideal gas kept at constant temperature varies inversely as the volume, i.e., directly as the density.
	is an instance of gas law
Boötes	has acronym Boo
	has boundary
	has genitive Boötis
	has historical image
	has historical origin the bear hunter which circles the bears, Ursa Major and Ursa Minor, around the North Pole
	has synonym Bear Driver
	has synonym Bear Hunter
	has synonym Herdsman
	is a part of celestial sphere
	is an instance of constellation
Bp star	has definition Peculiar B stars whose spectra show a deficiency in helium and in which the lines of one or several elements are abnormally enhanced. Traditionally the most important subgroups are Si λ4200, Hg-Mn and Cr-Eu-Sr stars. The latest objects of the latter group correspond to early F-type.
	is a kind of B star
	is a kind of peculiar star
Br alpha	has wavelength 40512 Å
	is an instance of Brackett line
Brackett line	has lower energy level 4
	is a kind of hydrogen line
	is a kind of spectral series line
Bragg angle	has definition Glancing angle between an incident X-ray beam and a given set of crystal planes for which the secondary X-radiation from the planes combines to give a single reflected beam.
	is a kind of angle
brane	has definition Any of the extended objects that arise in string theory. A one-brane is a string, a two-brane is a membrane, a three-brane has three extended dimensions, etc. More generally, a p-brane has p spatial dimensions.
	is a kind of string theory
bremsstrahlung	has antonym inverse bremsstrahlung
	has definition Electromagnetic radiation given out by electrons interacting with the ions in an ionized gas.
	has definition Radiation emitted or absorbed when a free electron is accelerated in the field of an atomic nucleus but remains in a hyperbolic orbit without being captured. Since bremsstrahlung is not quantized, photons of any wavelength can be emitted or absorbed. (Also called a free-free transition because the electron is free both before and after the transition.)
	has synonym deceleration radiation
	has synonym free-free transition
	is a kind of emission
	is a kind of free-free transition
bright blue variable	has definition Early-type high-luminosity star with peculiar spectra and large-amplitude light variations over a long time scale.
	is a kind of variable
bright giant	has luminosity class II
	is a kind of star
bright point	has definition Bright region (in X-ray and XUV) observed on the Sun during Skylab missions.
	has diameter 20000 km
	has discoverer Skylab
	has distribution uniform
	has electron temperature 1 to 2 × 106 K
	has mean lifetime 8 hours
	is a part of photosphere
bright ring	is a kind of ring
	is a part of Saturn ring system
Brillouin scattering	has definition Slight changes in the frequency of radiation, caused by reflection or scattering from the high-frequency sound waves that arise from thermal vibrations of atoms in the medium.
	is a kind of scattering
Broad-line radio galaxy	has acronym BLRG
	is a kind of radio galaxy
bromine	has image
	has ocean oxidation state -I
	has ocean residence time 1 × 108 years
	is a kind of accumulating oceanic element
	is a kind of atmophile element
	is a kind of halogen
	is a kind of liquid element
brown dwarf	has definition A self-gravitating, self-luminous gaseous object which is not sufficiently massive to result in thermonuclear hydrogen fusion reactions in its core and cannot therefore be considered a star. Such objects are expected to have a mass less than 7% of the Sun's mass and represent a "missing link" between low-mass stars and gas giant planets like Jupiter (at 0.1% of the Sun's mass).
	has definition A substellar object that is below the minimum mass required for nuclear fusion reactions to occur in its core.
	has definition Star with too little mass to ignite its hydrogen 1 fuel. If brown dwarfs exist, they shine faint red for a time, as they convert gravitational energy into heat, and then fade and cool.
	has mass 1 to 8 percent of the Sun
	is a kind of dwarf
	is a kind of massive compact halo object
Bruce medalist	has definition a scientist who was awarded the Bruce Medal
	is a kind of scientist
bulge	has definition The stellar population that lies within several thousand light-years of the Galactic center. The bulge is old, dense, and metal-rich.
	is a part of galactic nucleus
Bureau des Longitudes	has acronym BDL
	is an instance of science institution
Bureau International de l'Heure	has acronym BIH
	is an instance of science institution
Bureau International des Poids et Mesures	has acronym BIPM
	is an instance of science institution
Bw star	has definition B star with weak helium lines - i.e., B stars which, if classified according to their colors, would have helium lines too weak for the classification, and which, if classified according to their helium lines, would have colors too blue for their spectral type.
	is a kind of B star
Byurakan 2.6-meter Reflector	has altitude 1500 m
	has aperture 2.64 m
	has creation date 1976
	has focal ratio f/3.6, 16, 40
	has latitude 40° 20' N
	has location Mount Aragatz, Armenia
	has longitude 44° 18' E
	has mounting manufacturer LOMO
	has owner Byurakan Observatory
	has synonym Byurakan 102 inch
	is an instance of Fork equatorial telescope
	is an instance of optical telescope
	is an instance of reflector
C star	has definition A class of carbon star, defined by Morgan and Keenan to replace the Harvard R and N spectral classes.
	has definition Late type giant with strong bands of carbonated molecules (C2, CN, CH) and no metallic oxide bands. Formerly they were called R or N types, the R types being the hotter and the N types the cooler C stars.
	is a kind of carbon star
C. Donald Shane Telescope	has altitude 1290 m
	has aperture 3.05 m
	has creation date 1959
	has focal ratio f/5, 17, 36
	has latitude 37° 21' N
	has location Mount Hamilton, Calif., US
	has longitude 121° 38' W
	has mirror maker Don O. Hendrix
	has mirror type Pyrex
	has mounting manufacturer Judson Pacific-Murphy Corp.
	has owner Lick Observatory
	has synonym 120 inch
	is an instance of Cassegrain
	is an instance of Fork equatorial telescope
	is an instance of optical telescope
C2 band	has species C2 radical
	is a kind of molecular band
Ca H line	has definition A spectral line of singly ionized calcium.
	has wavelength 3968 Å
	is a kind of HK lines
Ca K line	has definition A spectral line of singly ionized calcium.
	has wavelength 3933 Å
	is a kind of HK lines
cadmium	has image
	has ocean oxidation state II
	has ocean residence time 30 years
	is a kind of chalcophile element
	is a kind of recycled oceanic element
	is a kind of transition metal
Caelum	has acronym Cae
	has genitive Caeli
	has synonym Chisel
	is a part of celestial sphere
	is an instance of constellation named by de Lacaille
Calar-Alto-Schmidtspiegel	has altitude 2168 m
	has aperture 0.80 m
	has comment Tube and optics moved from Hamburg, Germany, where the instrument had been in use since 1955
	has creation date 1980
	has focal ratio f/3.0
	has latitude 37° 13' N
	has location Calar Alto, Spain
	has longitude 2° 32' W
	has mirror diameter 1.2 m
	has mirror maker Zeiss (Jena)
	has mounting manufacturer Grubb-Parsons
	has owner Calar Alto Observatory
	has synonym Calar Alto Schmidt
	is an instance of Fork equatorial telescope
	is an instance of Schmidt
calcium	has abundance 2.24 × 106 in Sun relative to H = 1 × 1012
	has abundance 390 p.p.m. in Atlantic surface seawater
	has abundance 390 p.p.m. in Pacific surface seawater
	has abundance 41000 p.p.m. in Earth's crust
	has abundance 430 p.p.m. in deep Atlantic seawater
	has abundance 440 p.p.m. in deep Pacific seawater
	has atomic emission line 239.856 nm for Ca I
	has atomic emission line 317.933 nm for Ca II
	has atomic emission line 373.690 nm for Ca II
	has atomic emission line 393.366 nm for Ca II (strong)
	has atomic emission line 393.847 nm for Ca II
	has atomic emission line 422.673 nm for Ca I (used in atom absorption spectrometry)
	has atomic number 20
	has atomic radii 197 pm for α-form
	has biological role essential to all species
	has boiling point 1757 K
	has bulk modulus 17.2 GPa
	has chief source calcite, dolomite, gypsum (used in cement and plaster) anhydrite (used to make H2SO4)
	has covalent radii 174 pm
	has crystal cell dimension a = pm
	has crystal type
	has daily dietary intake 600 - 1400 mg
	has definition silvery-white, relatively soft metal
	has density 1365 kg m-3 for liquid at 1112 K melting point
	has density 1550 kg m-3 for solid at 293 K
	has discoverer Sir Humphry Davy
	has discovery date 1808 (isolated)
	has discovery location London, England
	has electrical resistivity 3.43 × 10-8 Ω m at 293 K
	has electron affinity -186 kJ mol-1 from Ca to Ca-
	has electron configuration [Ar]4s2 in ground state
	has electronegativity 1.00 Pauling
	has hazard calcium compounds are only toxic via the other elements they contain
	has heat capacity 20.786 J K-1 mol-1 for gas at constant pressure 0.1 MPa at 298.15 K
	has heat capacity 25.31 J K-1 mol-1 for solid α form at constant pressure 0.1 MPa at 298.15 K
	has heat of fusion 9.33 kJ mol-1
	has heat of vaporization 149.95 kJ mol-1
	has image
	has ionic radii 106 pm for Ca2+
	has isotope mass range 36 to 51
	has lethal intake 6.45 grams kg-1 in carbonate form for rat
	has level in humans 100 - 360 p.p.m. in liver
	has level in humans 140 - 700 p.p.m. in muscle
	has level in humans 170000 p.p.m. in bone
	has level in humans 60.5 mg dm-3 in blood
	has linear expansion coefficient 22 × 10-6 K-1
	has longest lived isotope
	has main mining area common everywhere
	has mass absorption coefficient 162 cm2 g-1 for CuKα X-ray diffraction
	has mass absorption coefficient 18.3 cm2 g-1 for MoKα X-ray diffraction
	has mass magnetic susceptibility +1.4 × 10-8 kg-1 m3 for solid
	has mass of element in person 1.00 kg for a 70 kg average person
	has melting point 1112 K
	has mineral anhydrite, aragonite, calcite, dolomite, gypsum, shortite, vaterite
	has molar volume 25.86 cm3
	has name origin calx = lime from Latin
	has neutron scattering length 0.476 × 10-12 cm
	has number of isotopes 16 including nuclear isomers
	has number of protons 20
	has ocean oxidation state II
	has ocean residence time 1 × 106 years
	has poisson's ratio 0.31 GPa
	has pronunciation kal-sium
	has registry number 7440-70-2 for Chemical Abstracts System database
has relative atomic mass 40.078 in units of 12C = 12.000
has reserves almost unlimited
has rigidity modulus 7.9 GPa
has space group
has specimen granules, pieces or turings. Care !
has symbol Ca
has synthesis mechanism heat calcium oxide (CaO) with aluminium metal in vacuum
has term symbol 1S0 in ground state
has thermal conductivity 200 W m-1 K-1 at 300 K
has thermal neutron capture cross section 0.43 barns
has toxic intake non-toxic
has uses alloys and in the manufacture of zirconium, thorium, and uranium and the rare earth metals
has uses CaO used in metallurgy, water treatment, chemicals industry, cement, etc
has world production 112 × 106tonnes year-1 for lime, CaO
has world production 2000 tonnes year-1 for calcium metal
has young's modulus 19.6 GPa
is a kind of alkali earth metal
is a kind of lithophile element
is a kind of recycled oceanic element
reacts with oxygen and water but soon forms a thin protective oxide-nitride film preventing further corrosion
calcium 40	has atomic mass 39.9625906
	has natural ab39undance 96.941%
	has nuclear magnetic moment μ = 0 nuclear magnetons with diamagnetic correction
	has nuclear spin I = 0+ h/2π
	has number of neutrons 20
	has number of nucleons 40
	has symbol 40Ca
	has uses isotopically enriched samples available for experimental purposes
	is an instance of calcium
calcium 41	has atomic mass 40.962278
	has decay mode EC (0.421 Mev)
	has decay mode no γ
	has half life 1.03 × 105 years
	has nuclear magnetic moment μ = -1.595 nuclear magnetons with diamagnetic correction
	has nuclear spin I = 7/2- h/2π
	has number of neutrons 21
	has number of nucleons 41
	has symbol 41Ca
	has uses research
	is an instance of calcium
calcium 42	has atomic mass 41.9586176
	has natural abundance 0.647%
	has nuclear magnetic moment μ = 0 nuclear magnetons with diamagnetic correction
	has nuclear spin I = 0+ h/2π
	has number of neutrons 22
	has number of nucleons 42
	has symbol 42Ca
	has uses isotopically enriched samples available for experimental purposes
	is an instance of calcium
calcium 43	has atomic mass 42.9587662
	has magnetogyric ratio -1.8001 × 107 rad T-1 s-1
	has natural abundance 0.135%
	has NMR frequency 6.728 MHz where 1H = 100 MHz; 2.3488 T
	has NMR receptivity 0.0527 where 13C = 1.00
	has nuclear magnetic moment μ = -1.31727 nuclear magnetons with diamagnetic correction
	has nuclear quadrupole moment -0.0408 × 10-28 m2
	has nuclear spin I = 7/2- h/2π
	has number of neutrons 23
	has number of nucleons 43
	has relative NMR sensitivity 6.40 × 10-3 where 1H = 1.00
	has symbol 43Ca
	has uses Nuclear Magnetic Resonance, E
	is an instance of calcium
calcium 44	has atomic mass 43.9554806
	has natural abundance 2.086%
	has nuclear magnetic moment μ = 0 nuclear magnetons with diamagnetic correction
	has nuclear spin I = 0+ h/2π
	has number of neutrons 24
	has number of nucleons 44
	has symbol 44Ca
	has uses isotopically enriched samples available for experimental purposes
	is an instance of calcium
calcium 45	has atomic mass 44.956185
	has decay mode β- (0.257 Mev)
	has decay mode no γ
	has half life 162.7 days
	has nuclear magnetic moment μ = -1.327 nuclear magnetons with diamagnetic correction
	has nuclear spin I = 7/2- h/2π
	has number of neutrons 25
	has number of nucleons 45
	has symbol 45Ca
	has uses research
	is an instance of calcium
calcium 46	has atomic mass 45.953689
	has natural abundance 0.004%
	has nuclear magnetic moment μ = 0 nuclear magnetons with diamagnetic correction
	has nuclear spin I = 0+ h/2π
	has number of neutrons 26
	has number of nucleons 46
	has symbol 46Ca
	has uses isotopically enriched samples available for experimental purposes
	is an instance of calcium
calcium 47	has atomic mass 46.954543
	has decay mode β- (1.988 Mev)
	has decay mode γ
	has half life 4.536 days
	has nuclear magnetic moment μ = -1.38 nuclear magnetons with diamagnetic correction
	has nuclear spin I = 7/2- h/2π
	has number of neutrons 27
	has number of nucleons 47
	has symbol 47Ca
	has uses research, medical diagnosis
	is an instance of calcium
calcium 48	has atomic mass 47.952533
	has natural abundance 0.187%
	has nuclear magnetic moment μ = 0 nuclear magnetons with diamagnetic correction
	has nuclear spin I = 0+ h/2π
	has number of neutrons 28
	has number of nucleons 48
	has symbol 48Ca
	has uses isotopically enriched samples available for experimental purposes
	is an instance of calcium
calcium 49	has atomic mass 48.955672
	has decay mode β- (5.263 Mev)
	has decay mode γ
	has half life 8.72 minutes
	has nuclear spin I = 3/2- h/2π
	has number of neutrons 29
	has number of nucleons 49
	has symbol 49Ca
	is an instance of calcium
calendar year	has definition is an approximation to the tropical year.
	is an instance of year
	see also Gregorian calendar
California Institute of Technology	has acronym Caltech
	has location Pasadena, USA
	is an instance of science institution
californium	has image
	is a kind of transuranium element
Callisto	has albedo 0.15
	has definition A Galilean satellite of Jupiter. It has the lowest density, lowest albedo, and highest temperature of any of the four main satellites of Jupiter.
	has definition Fifth (known) moon out from jupiter, and its second largest.
	has density 1.7 g cm-3
	has diameter 5050 km
	has orbital parameters e = 0.0075; i = 0°.3
	has orbital rotation period 16.7 days
	has synonym J IV
	has synonym Jupiter IV
	has temperature 156 K
	is a part of Jupiter
	is an instance of Galilean satellite
CalTech Submillimeter Observatory	has acronym CSO
	is an instance of observatory
cambrian period	has duration 100 million years
	has start time 600 million years ago
	is a kind of paleozoic era
Cambridge catalog	has definition The results of five intensive radio-astronomical surveys (1C, 2C, 3C, 4C and 5C) under the direction of Sir Martin Ryle and Anthony Hewish, during the l950s, 1960s and 1970s, at Cambridge.
	is a kind of radio source catalog
Camelopardalis	has acronym Cam
	has genitive Camelopardalis
	has historical image
	has synonym Giraffe
	is a part of celestial sphere
	is an instance of constellation
Canada-France-Hawaii Telescope	has acronym CFHT
	has altitude 4200 m
	has aperture 3.58 m
	has creation date 1979
	has focal ratio f/3.8, 8, 20, 35
	has latitude 19° 49' N
	has location Mauna Kea, Hawaii, US
	has longitude 155° 28' W
	has mirror maker Dominion Astrophys. Obs.
	has mirror type Cer-Vit
	has mounting manufacturer SNACRP
	has owner Canada-France-Hawaii Telescope Corp.
	is an instance of Horseshoe equatorial telescope
	is an instance of optical telescope
	is an instance of reflector
Cancer	has acronym Cnc
	has boundary
	has genitive Cancri
	has historical image
	has historical origin the crab sent by Hera to distract Hercules from his battle with the Hydra
	has synonym Crab
	is a part of Zodiac
	is an instance of zodiacal constellation
candela	has approval agency 13th CGPM
	has approval date 1967
	has definition the luminous intensity in a given direction, of a source that emits monochromatic radiation of frequency 540 × 1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian
	has definition The SI unit of luminous intensity, defined as "the luminous intensity, in the perpendicular direction, of a surface of 1/600000 square meter of a blackbody at the temperature of freezing platinum under a pressure of 101325 newtons per square meter."
	has historical origin in 1933 it was based on the luminous emission of a blackbody at the freezing temperature of platinum (2045 K)
	has symbol cd
	is an instance of base SI unit
	is an instance of luminous intensity unit
Canes Venatici	has acronym CVn
	has boundary
	has genitive Canum Venaticorum
	has historical image
	has historical origin the hunting dogs held on a leash by Boötes as he hunts for the bears Ursa Major and Ursa Minor
	has synonym Hunting Dogs
	is a part of celestial sphere
	is an instance of constellation
	named by Johannes Hevelius in 1687
Canis Major	has acronym CMa
	has boundary
	has genitive Canis Majoris
	has historical origin one of Orion's hunting dogs
	has synonym Big Dog
	is a part of celestial sphere
	is an instance of constellation
Canis Minor	has acronym CMi
	has genitive Canis Minoris
	has historical origin one of Orion's hunting dogs
	has synonym Little Dog
	is a part of celestial sphere
	is an instance of constellation
Canopus	has B-V magnitude 0.15
	has declination -52 41 44
	has definition A supergiant, the second brightest star in the southern sky.
	has distance 55 pc
	has right ascension 06 23 57.2
	has spectral type F0 Ib
	has spectral type F0II
	has synonym alpha Car
	has synonym HR 2326
	has V magnitude -0.72
	is a part of Carina
	is an instance of F star
	is an instance of naked eye star
	is an instance of supergiant
Canum Venaticorum	has B-V magnitude -0.12
	has declination +38 19 6
	has right ascension 12 56 1.6
	has spectral type A0pSiEuHg
	has synonym HR 4915
	has V magnitude 2.90
	is a part of Canes Venatici
	is an instance of A star
	is an instance of naked eye star
capacitance	has unit capacitance unit
	is a kind of electromagnetic quantity
capacitance unit	is a kind of electromagnetic unit
	is a unit of capacitance
Capella	has B-V magnitude 0.80
	has declination +45 59 53
	has definition The sixth brightest star in the night sky, consists of two yellow giants. A spectroscopic triple (F8-G0 III, G5 III, M5 V) (1974 parallax 0'.079). It has a high lithium content and a nearly circular orbit. It may be an X-ray source.
	has distance 13 pc
	has orbital period 104.023 days
	has right ascension 05 16 41.3
	has spectral type G5IIIe+G0III
	has synonym Alpha Aurigae
	has synonym HR 1708
	has V magnitude 0.08
	is a part of Auriga
	is an instance of G star
	is an instance of giant
	is an instance of naked eye star
Capricornus	has acronym Cap
	has boundary
	has genitive Capricorni
	has historical image
	has historical origin a goat with a fish tail
	has synonym Goat
	is a part of Zodiac
	is an instance of zodiacal constellation
carbon	has abundance 23 p.p.m. in Atlantic surface seawater
	has abundance 26 p.p.m. in deep Atlantic seawater
	has abundance 26 p.p.m. in Pacific surface seawater
	has abundance 28 p.p.m. in deep Pacific seawater
	has abundance 350 p.p.m. by volume in Earth's atmosphere as CO2
	has abundance 4.17 × 108 in Sun relative to H = 1 × 1012
	has abundance 480 p.p.m. in Earth's crust
	has atomic emission line 247.856 nm for C I
	has atomic emission line 283.671 nm for C II (strong)
	has atomic emission line 426.726 nm for C II (strong)
	has atomic emission line 723.642 nm for C II (strong)
	has atomic number 6
	has atomic radii 77 pm
	has biological role DNA constituent, organic molecules required for life
	has boiling point 5100 K (sublimes)
	has chief source graphite
	has covalent radii 60 pm for triple bonds
	has covalent radii 67 pm for double bonds
	has covalent radii 77 pm for single bond
	has crystal cell dimension a = 1414 pm for buckminsterfullerene
	has crystal cell dimension a = 246, c = 670.78 pm for hexagonal graphite
	has crystal cell dimension a = 252, c = 412 pm for hexagonal diamond
	has crystal cell dimension a = 356.703 pm for cubic diamond
	has crystal cell dimension a = 364.2 pm, α = 39°30' for rhombohedral graphite
	has crystal cell dimension a = 894.8, c = 1408 pm for hexagonal carbon (chaoite)
	has crystal type cubic for cubic diamond
	has crystal type f.c.c. for buckminsterfullerene
	has crystal type hexagonal for hexagonal carbon (chaoite)
	has crystal type hexagonal for hexagonal diamond
	has crystal type hexagonal for hexagonal graphite
	has crystal type rhombohedral for rhombohedral graphite
	has daily dietary intake 300 g
	has definition pure forms occur as graphite, diamond and buckminsterfullerene C60
	has density 1650 kg m-3 for buckminsterfullerene at 293 K
	has density 2260 kg m-3 for graphite at 293 K
	has density 3513 kg m-3 for diamond at 293 K
	has discovery date pre-historic
	has electrical resistivity 1 × 1011 Ω m for diamond at 293 K
	has electrical resistivity 1 × 1014 Ω m for buckminsterfullerene at 293 K
	has electrical resistivity 1.375 × 10-5 Ω m for graphite at 293 K
	has electron affinity 121.9 kJ mol-1 from C to C-
	has electron configuration [He]2s22p2 in ground state
	has electronegativity 2.55 Pauling
	has hazard carbon black is a nuissance but not dangerous, although soot may harbour carcinogenic materials
	has heat capacity 20.838 J K-1 mol-1 for gas at constant pressure 0.1 MPa at 298.15 K
	has heat capacity 6.113 J K-1 mol-1 for solid diamond at constant pressure 0.1 MPa at 298.15 K
	has heat capacity 8.527 J K-1 mol-1 for solid graphite at constant pressure 0.1 MPa at 298.15 K
	has heat of fusion 105.1 kJ mol-1
	has heat of vaporization 710.9 kJ mol-1
	has image
	has ionic radii 260 pm for C4-
	has isotope mass range 9 to 16
	has level in humans 0.0016 - 0.075 mg dm-3 in blood
	has level in humans 300000 p.p.m. in bone
	has level in humans 670000 p.p.m. in liver
	has level in humans 670000 p.p.m. in muscle
	has linear expansion coefficient 1.19 × 10-6 K-1 for diamond
	has longest lived isotope carbon 12
	has main mining area diamond deposits in South Africa, USA, Russia, Brazil, Zaire, Sierra Leone, Ghana, Canada
	has main mining area graphite deposits in Sri Lanka, Madagascar, Russia, South Korea, Mexico, Czech Republic, Italy
	has mass absorption coefficient 0.625 cm2 g-1 for MoKα X-ray diffraction
	has mass absorption coefficient 4.60 cm2 g-1 for CuKα X-ray diffraction
	has mass magnetic susceptibility -6.2 × 10-9 kg-1 m3 for diamond
	has mass magnetic susceptibility -6.3 × 10-9 kg-1 m3 for graphite
has mass of element in person 16 kg for a 70 kg average person
has melting point 3800 K for graphite
has melting point 3820 K for diamond
has melting point 800 K for buckminsterfullerene (sublimes)
has mineral diamond, graphite, calcium magnesium carbonates, fossil fuel
has molar volume 3.42 cm3 for diamond
has name origin carbo = charcoal from Latin
has neutron scattering length 0.66460 × 10-12 cm
has number of isotopes 8 including nuclear isomers
has number of protons 6
has ocean oxidation state IV
has ocean residence time 800000 years
has pronunciation kar-bon
has registry number 7440-44-0 for Chemical Abstracts System database
has relative atomic mass 12.011 in units of 12C = 12.000
has reserves 1000 × 109 tonnes for coal
has reserves 127 × 109 tonnes for natural gas
has reserves 140 × 109 tonnes for oil
has reserves large for tar sands
has space group Fd3m for cubic diamond
has space group P63/mmc for hexagonal diamond
has space group P63mc for hexagonal graphite
has space group R3m for rhombohedral graphite
has specimen amorphous, fullerenes, bucky tubes, diamond, graphite and soot. Safe.
has symbol C
has synthesis mechanism
has term symbol 3P0 in ground state
has thermal conductivity 1960 W m-1 K-1 for graphite at 298 K along sheets
has thermal conductivity 5.7 W m-1 K-1 for graphite at 298 K perpendicular to sheets
has thermal conductivity 990 - 2320 W m-1 K-1 for diamond at 298 K
has thermal neutron capture cross section 0.0035 barns
has toxic intake non-toxic, but some compounds can be very toxic such as CO or CN-
has uses coke in steel, carbon black in printing, as a filler, activated charcoal for water treatement and respirators
has van der Waals radii 185 pm
has world production 8.6 ×
is a kind of atmophile element
is a kind of group IV element
is a kind of nonmetallic element
is a kind of recycled oceanic element
is a kind of siderophile element
reacts with almost everything
carbon 10	has atomic mass 10.01686
	has decay mode β+ (3.650 Mev)
	has decay mode γ
	has half life 19.3 seconds
	has nuclear spin I = 0+
	has number of neutrons 4
	has number of nucleons 10
	has symbol 10C
	is an instance of carbon
carbon 11	has atomic mass 11.01143
	has decay mode β+ EC (1.982 Mev)
	has decay mode γ
	has half life 20.3 minutes
	has nuclear magnetic moment μ = -0.964
	has nuclear spin I = 3/2-
	has number of neutrons 5
	has number of nucleons 11
	has symbol 11C
	has uses
	is an instance of carbon
carbon 12	has atomic mass 12.000000000 by definition
	has natural abundance 98.90 %
	has nuclear magnetic moment μ = 0
	has nuclear spin I = 0+
	has number of neutrons 6
	has number of nucleons 12
	has symbol 12C
	is an instance of carbon
carbon 13	has atomic mass 13.003354826
	has magnetogyric ratio × 10 rad T-1 s-1
	has natural abundance 1.10 %
	has NMR frequency MHz where 1H = 100 MHz; 2.3488 T
	has NMR receptivity where 13C = 1.00
	has nuclear magnetic moment μ = +0.702411
	has nuclear quadrupole moment × 10- m2
	has nuclear spin I = 1/2-
	has number of neutrons 7
	has number of nucleons 13
	has relative NMR sensitivity where 1H = 1.00
	has symbol 13C
	has uses Nuclear Magnetic Resonance
	is an instance of carbon
carbon 14	has atomic mass 14.003241
	has decay mode β- (0.15648 Mev)
	has decay mode no γ
	has half life 5715 years
	has natural abundance traces
	has nuclear spin I = 0+
	has number of neutrons 8
	has number of nucleons 14
	has symbol 14C
	has uses research, carbon dating
	is an instance of carbon
carbon burning	has definition The stage when a star fuses carbon into heavier elements, making neon and magnesium. Carbon burning eventually occurs in all stars born with more than eight solar masses.
	has duration 600 years
	has location center of star heavier than 4 solar masses
	has optimum density 2 × 105 g cm-13
	has optimum temperature 6 × 108 K
	has part product neon, magnesium
	has reactant carbon
	is a kind of exothermic fusion process
	requires minimum mass at star birth 8 solar masses
carbon cycle	has catalyst carbon, nitrogen and oxygen
	has definition A series of nuclear reactions in which carbon is used as a catalyst to transform hydrogen into helium. The carbon cycle can take place only if the necessary C and N nuclei are present, and it requires higher temperatures and is far more temperature-dependent than the proton-proton chain. The cycle yields 26.7 MeV of energy. (On the average, 1.7 MeV of this energy is carried away because of neutrino losses.)
	has definition An important nuclear fusion process that occurs in stars. Carbon 12 both initiates it and, following interactions with nuclei of nitrogen, hydrogen, oxygen, and other elements, reappears at its conclusion.
	has definition One way that a star converts hydrogen into helium. During the CNO cycle, carbon, nitrogen, and oxygen catalyze the nuclear reaction, so the total number of carbon, nitrogen, and oxygen nuclei remains the same. However, carbon and oxygen gradually get converted into nitrogen. The CNO cycle powers the hydrogen burning that occurs in main-sequence stars with more than 1.5 solar masses and in giants and supergiants of all masses.
	has definition Use of carbon and nitrogen as intermediates in the nuclear fusion process of the Sun. Cooler stars undergo the proton-proton cycle.
	has discovered date 1938
	has minimum mass 1.5 solar masses
	has minimum temperature 15 to 20 million Kelvin
	has part catalyst carbon
	has part catalyst nitrogen
	has part catalyst oxygen
	has part product helium
	has part reactant proton
	has reaction 12C(p, γ) 13N(p, γ) 14O(β+ν) 14N(p, γ) 15O(β+ν) 15N(p, α) 12C
	has synonym Bethe-Weizsäcker cycle
	has synonym CNO cycle
	has temperature dependence E ∝ T15
	is a kind of hydrogen burning
	produces energy 26.7 MeV
carbon detonation supernova model	has definition A supernova model involving the explosive ignition of carbon in the high-density (108 - 1010 g cm-3), electron-degenerate carbon-oxygen core of a 6±2 - 7±2 Msun star by the formation and propagation of a detonation wave. A carbon-detonation supernova seems to leave no dense remnant and converts its C-O core entirely to iron.
	is a kind of stellar theory
carbon monosulfide	has symbol CS
	is an instance of diatomic molecule
	is an instance of interstellar molecule
carbon monoxide	has definition A molecule consisting of one carbon atom and one oxygen atom. It is the most abundant interstellar molecule after molecular hydrogen and is especially useful because it radiates at radio wavelengths, so astronomers can use it to map the distribution of molecular hydrogen.
	has symbol CO
	is an instance of diatomic molecule
	is an instance of interstellar molecule
	is an instance of neutral particle
carbon star	has definition In the HD system, a rather loose category of peculiar red-giant star, usually of spectral types R and N, whose spectra show strong bands of C2, CN, or other carbon compounds and unusually high abundances of lithium. Carbon stars resemble S stars in the relative proportion of heavy and light metals, but they contain so much carbon that these bands dominate their spectra. (C2,0. The number following the comma is an abundance parameter.)
	is a kind of late star
carbonyl sulfide	has symbol OCS
	is an instance of interstellar molecule
Carina	has acronym Car
	has boundary
	has genitive Carinae
	has synonym Keel
	is a part of celestial sphere
	is an instance of constellation formerly part of Argo Navis
	was named by the IAU along with Vela, Puppis and Pyxis
Carina galaxy	has definition A dwarf galaxy that orbits the Milky Way.
	has discovery date 1977
	has distance from galaxy center 350000 light-years
	is a part of Carina
	is a part of Local Group
	is an instance of satellite galaxy
	orbits Milky Way
Carina Nebula	has image
	has synonym NGC 3372
	is an instance of gaseous nebula
Carina OB 2	has definition A rich association of OB stars near η Carinae.
	is a part of Carina
	is an instance of OB association
carrier boson	carries the force
	has definition A particle that carries one of the fundamental forces between other interacting particles. For example, the carrier boson for the electromagnetic force is the photon.
	has definition Particle that acts as the transmitters of forces. The best known example is the photon, which transmits electromagnetic forces. The gluons are the transmitters of the strong interactions, and the W+, W-, and Z0 particles are the transmitters of the weak interactions. See Table 7.1 on page 120.
	has synonym force carrier
	is a kind of boson
	is a kind of elementary particle
Cassegrain	has definition An optical arrangement in which light rays striking the parabolic concave primary mirror of a reflecting telescope are reflected to the hyperbolic convex secondary mirror, and re-reflected through a hole bored in the primary to a focus behind it.
	has definition Refers to a design of reflecting telescopes in which the light collected and focussed by the large concave primary mirror is refocussed by a smaller convex secondary mirror on the same axis as the primary. The refocussed beam passes through a central hole cut into the primary mirror and emerges behind the primary.
	has definition Telescope devised by Cassegrain in which an auxiliary convex mirror reflects the magnified image, upside down, through a hole in the center of the main objective mirror - i.e., through the end of the telescope itself. It was, however, no improvement on the gregorian telescope invented probably slightly earlier.
	has optical design Cassegrain
	has primary mirror shape concave paraboloid
	has secondary mirror shape convex hyperboloid
	is a kind of reflector
Cassini's division	has definition A gap between the outermost rings of Saturn. The period of a particle in Cassini's division is about two-thirds that of Janus, one-half that of Mimas, one-third that of Enceladus, and one-quarter that of Tethys.
	has discoverer Cassini
	has discovery date 1675
	has width 1800 km
	is a kind of ring gap
	is a part of Saturn ring system
Cassiopeia	has acronym Cas
	has boundary
	has genitive Cassiopeiae
	has historical image
	has historical origin Cassiopeia is chained to her throne as punishment for her boastfulness
	has synonym Andromeda's Mother
	has synonym Queen of Ethiopia
	is a part of celestial sphere
	is an instance of constellation
Cassiopeia A	has definition A radio source in Cassiopeia, the strongest extrasolar source in the sky, believed to be the remnant of a Type II supernova whose light reached Earth about 1667. Optically it is a faint nebula. It has a mass of a few solar masses. It is also an extended source of soft X-rays.
	has distance 3 kpc
	has expansion velocity 800 km s-1
	has synonym 3C 461
	has synonym 3U 2321+58
	is a part of Cassiopeia
	is an instance of radio source
	is an instance of X-ray source
Cat's Eye	has distance 3000 light years
	has image
	has synonym NGC 6543
	is an instance of planetary nebula
	is part of Draco
cataclysmic variable	has acronym CV
	has definition A collective name for stars in which the brightness increases suddenly because of an explosive event. The class comprises supernovae, novae, recurrent novae, dwarf novae and flare stars.
	has definition A type of variable including flare stars and novae (common, recurrent, and dwarf), all of which are believed to be very close binary systems in which hydrogen-rich matter flows from a late-type star onto a hot white-dwarf primary.
	has outburst start time
	has peak brightness
	has recovery time
	has synonym eruptive variable
	has synonym explosive variable
	is a kind of irregular variable
catalog about star systems	has object type collection of stars
	is a kind of astronomical catalog
catastrophism	has antonym uniformitarianism
	has definition Nineteenth-century hypothesis that depicted the many changes evinced by the geological record as having resulted from cataclysms occurring during a relatively brief period of history.
	is a kind of doctrine
causality	has definition The doctrine that every new situation must have resulted from a previous state. Causation underlay the original atomic hypothesis of the Greeks, and was popular in classical physics. It is eroded in quantum mechanics and has, in any case, never been proved essential to the scientific world view. See chance, determinism.
	has synonym causation
	is a kind of doctrine
CDA	has definition Centre de Donnees Astronomiques (Strasbourg, France).
	is an instance of database
Cecilia Payne Gaposhkin	has career Havard College Observatory
	is an instance of 19th century scientist
	is an instance of astronomer
celestial body	has definition natural object visible in the sky
	has spectra
	is a kind of natural object
celestial ephemeris pole	has definition The reference pole for nutation and polar motion; the axis of figure for the mean surface of a model Earth in which the free motion has zero amplitude. This pole has no nearly diurnal nutation with respect to a space-fixed or Earth-fixed coordinate system.
	is an instance of equatorial coordinate
celestial equator	has definition Projection of the Earth's equator as a line across the sky (so that to an observer actually on the equator, such a line would pass through the zenith). The directional bearing of a star is given in terms of its right ascension round the celestial equator.
	is a kind of equator
celestial event	has definition Event involving one or more celestial objects
	is a kind of event
celestial latitude	has definition Angular distance on the celestial sphere measured north or south of the ecliptic along the great circle passing through the poles of the ecliptic and the celestial object.
	is an instance of angle
	is an instance of ecliptic coordinate component
celestial longitude	has definition Angular distance along the ecliptic from the vernal equinox eastward.
	has definition Angular distance on the celestial sphere measured eastward along the ecliptic from the dynamical equinox to the great circle passing through the poles of the ecliptic and the celestial object.
	is an instance of angle
	is an instance of ecliptic coordinate component
celestial mechanics	has definition Study of the movements and physical interactions of objects in space; astrophysical mathematics.
	is a kind of astrophysics
celestial pole	has definition One of two points at which the Earth's axis of rotation, if extended, would intersect the celestial sphere.
	has right ascension 0
	is a kind of equatorial coordinate
celestial sphere	has definition An imaginary sphere of arbitrary radius upon which celestial bodies may be considered to be located. As circumstances require, the celestial sphere may be centered at the observer, at the Earth's center or at any other location.
	is an instance of equatorial sky area
	is an instance of sphere
Celsius	has absolute zero -273.15
	has base unit K
	has boiling point of water 100
	has definition A mercury-in-glass temperature scale. The zero of the scale represents the melting point of ice and the boiling point of water is taken to be 100 degrees.
	has proposition date 1710
	has symbol °C
	has synonym Centigrade
	has triple point of water 0.01
	is an instance of named derived SI unit
	is an instance of temperature unit
	is named after Anders Celsius (1701-1744)
	to convert to Kelvin add 273.15
Cen X-1	is a part of Centaurus
	is an instance of transient X-ray source
Cen X-2	has definition sporadic X-ray source
	is a part of Centaurus
	is an instance of transient X-ray source
Cen X-4	has definition sporadic X-ray source
	is a part of Centaurus
	is an instance of X-ray source
cenozoic era	has duration 63 million years
	has start time 63 million years ago
	is a kind of geological era
Centaurus	has acronym Cen
	has genitive Centauri
	has historical origin the centaur Chiron which tutored Hercules and Jason
	has synonym Centaur
	is a part of celestial sphere
	is an instance of constellation
Centaurus A	has definition A strong radio source. Optically, it is an elliptical galaxy with a dark obscuring lane. It is the nearest known violent galaxy.
	has distance 4 Mpc
	has synonym 3U 1322-42
	has synonym NGC 5128
	is a part of Centaurus
	is an instance of radio source
	is an instance of X-ray source
Centaurus cluster	has definition A cluster of galaxies. Its radio counterpart is compact and located inside NGC 4696.
	has distance 200 Mpc
	has synonym 3U 1247-41
	is a part of Centaurus
	is a part of Local Supercluster
	is an instance of galaxy cluster
	is an instance of X-ray source
Centaurus X-3	has definition A pulsating binary X-ray source in the galactic plane. Optical component is Krzeminski's star, a B0 giant or supergiant. The X-ray component is probably a rotating neutron star of about 0.65-0.83 Msun. Cen X-3 is speeding up at a rate of about 1 part in 103-105 per year and will at this rate fall into its companion in about 1000 years.
	has distance 5-10 kpc
	has eclipse duration 0.488 days in X-ray
	has orbital eccentricity e < 0.002
	has orbital period 2.087 days
	has period 4.8 s
	has synonym 3U 1118-60
	has synonym Krzeminski's star
	is a part of Centaurus
	is an instance of eclipsing binary
	is an instance of X-ray pulsar
centi	has symbol c
	has value 10-2
	is an instance of unit prefix
central star of planetary nebula	has acronym CPN
	has synonym planerary nuclei
	is a kind of early star
Centre National d'Etudes Spatiales	has acronym CNES
	has location Paris, France
	is an instance of space science institution
Cep X-4	is a part of Cepheus
	is an instance of transient X-ray source
Cepheid	has definition A class of stars named after Delta Cephei which vary in brightness over a regular period of time (typically a few days). The period of change is directly related to the true, average brightness or luminosity of the star. Once the period is known the true brightness can be calculated and the distance estimated by observing the "apparent" brightness of the object as seen from Earth.
	has definition A type of variable star whose period of variation is tightly related to its intrinsic luminosity.
	has definition A yellow supergiant that pulsates, alternately brightening and dimming. Cepheids allow astronomers to measure distances, because the longer a Cepheid's period of variation, the greater the Cepheid's mean intrinsic brightness. To determine a Cepheid's distance, all an astronomer has to do is measure the Cepheid's period; comparing the star's mean intrinsic brightness with the star's mean apparent brightness then yields the distance. Cepheids are so bright that we can see them in other galaxies, allowing us to establish distances to entire galaxies beyond the Milky Way.
	has definition One of a group of very luminous supergiant pulsating stars. The luminosities of a Cepheid is proportional to its period, but a different P-L relation applies to each type. No Cepheid is near enough for an accurate trigonometric parallax (Polaris is the nearest). Cepheids are useful distance indicators to about 3 Mpc.
	has definition Strictly periodic variables with periods 1-50 days, of spectral types F, G and K.
	has period 1 to 50 days
	has prototype delta Cephei
	has spectral type F star, G star and K star
	is a kind of periodic variable
Cepheus	has acronym Cep
	has boundary
	has genitive Cephei
	has synonym Andromeda's Father
	has synonym King of Ethiopia
	is a part of celestial sphere
	is an instance of constellation
Cer-Vit	is a kind of mirror
Ceres	has definition Largest asteroid, and the first to be discovered.
	has discoverer Piazzi
	has discovery date 1801
	has eccentricity e = 0.079
	has inclination i = 10°.6
	has mass 1.17 × 1024 g
	has mean distance from Sun 2.7673 AU
	has mean orbital speed 17.9 km s-1
	has photographic albedo 0.06
	has radius R ≈ 510 km
	has rotation period 0.38 days
	has sidereal period 1682 days
	has surface spectra carbonaceous chondrite
	has synodic period 466.6 days
	is a part of asteroid belt
	is an instance of asteroid
cerium	has image
	has ocean oxidation state III
	has ocean residence time 100 years
	is a kind of rare Earth
	is a kind of scavenged oceanic element
CERN	has definition The European Laboratory for Particle Physics (formerly the Conseil Europeen pour la Recherche Nucleaire). Here, the resources of the European member nations are pooled to construct the large particle accelerators needed for high-energy experiments. The major facilities at CERN include the Super Proton Synchrotron (SPS) and the Large Electron-Positron (LEP) collider.
	has location near Geneva, Switzerland
	is an instance of high energy physics institution
cesium	has image
	has ocean oxidation state I
	has ocean residence time 600000 years
	is a kind of accumulating oceanic element
	is a kind of alkali metal
Cetus	has acronym Cet
	has genitive Ceti
	has historical image
	has historical origin the sea monster slain by Perseus to save Andromeda. Also known as the biblically famous whale who swallowed Jonah.
	has synonym Jonah's Whale
	has synonym Sea Monster
	is a part of celestial sphere
	is an instance of constellation
CGS unit	is a kind of unit
CH star	has definition G-type giant (G5 to K5) in which the molecular bands of CH are very strong.
	is a kind of G star
chalcophile element	has definition element that tends to concentrate in sulfides
	has occurrence in sulfide minerals
	is a kind of planetary element
Chamaeleon	has acronym Cha
	has genitive Chamaeleontis
	has synonym Chameleon
	is a part of celestial sphere
	is an instance of constellation named by Bayer
Chandra	has acronym AXAF
	has definition NASA's premier x-ray observatory
	is an instance of grazing-incidence telescope
	is named after Subrahmanyan Chandrasekhar
Chandrasekhar limit	has definition A limiting mass for white dwarfs. If the mass exceeds this critical mass (1.44 solar masses, for the expected mean molecular weight of 2), the load of the overlying layers will be so great that degeneracy pressure will be unable to support it, and no configuration will be stable.
	has definition The maximum mass, approximately 1.4 Msun, above which an object cannot support itself by electron degeneracy pressure; hence, the maximum mass of a white dwarf.
	is an instance of mass
Chandrasekhar-Schonberg limit	has definition Mass above which the helium core of a star begins to contract (eventually to collapse altogether). The limit is now reckoned as 10 to 15 per cent of the star's total mass.
	has definition The mass limit for an isothermal core. In order to maintain its luminosity by hydrogen burning just outside the isothermal core, the star must keep a high temperature and a high pressure at the surface of the core. When the helium core exceeds about 12% of the star's total mass, the star can no longer adjust by small changes, but must drastically increase in radius and move rapidly from the main sequence.
	is an instance of mass
characteristic impedance of vacuum	has symbol Z0
	has uncertainty 0
	has unit Ω
	has value 376.730313461 Ω
	is an instance of resistance
	is an instance of universal constant
charge	has definition The fundamental property of a particle that causes it to be affected by the electromagnetic force.
	has unit charge unit
	has unit charge unit
	is a kind of electromagnetic quantity
	is a kind of electromagnetic quantity
charge multiplet	has definition A group of particles which differ in electrical charge but which are nearly identical in mass and other respects (such as lifetime and angular momentum) and which seem to experience identical nuclear forces.
	has lifetime -has source: [H76]
	has mass -has source: [H76]
	has spin -has source: [H76]
	is a kind of hadron
charge unit	is a kind of electromagnetic unit
	is a unit of charge
charge-coupled device	has acronym CCD
	has definition A small photoelectronic imaging device (typically 1.5 cm square) made from a crystal of semiconductor silicon in which numerous (at least 250000) individual light-sensitive picture elements (pixels) have been constructed. Each tiny pixel (less than 0.03 mm in size) is capable of storing electronic charges created by the absorption of light. The name derives from the method of extracting the locally stored charges from each pixel which is done by transferring or "coupling" charges from one pixel to the next by the controlled collapse and growth of adjacent storage sites or "potential wells". Each "well" is formed inside the silicon crystal by the electric field generated by voltages applied to tiny, semi-transparent metallic electrodes on the CCD surface.
	has definition Highly sensitive photoelectric devices that can electronically record the intensity and point of arrival of tiny amounts of light. CCDs are placed at the receiving end of telescopes, to "take pictures" of very faint astronomical objects; they have almost completely replaced photographic plates.
	is a kind of integrating detector
charged particle	has charge non-zero
	has definition A particle with non-zero charge
	is a kind of particle
	is accelerated by electric or magnetic fields
Charles's law	has definition The pressure of an ideal gas at constant volume varies directly as the absolute temperature.
	is an instance of gas law
charm	has charge 2/3
	has definition The fourth flavor (i.e. type) of quark, the discovery of which in 1974 contributed both to the acceptance of the reality of quarks and to our understanding of their dynamics. The charmed quark exhibits a property called "charm" which is conserved in strong interactions.
	has definition The fourth flavor of quarks. Predicted by theory, charmed quarks were discovered in 1974.
	has definition The property that distinguishes one of the types of quarks. At present, there are six types of quarks known, one of which is the "charmed" quark. (See quark.)
	is an instance of heavy quark
chemical process	has domain chemistry
	is a kind of process
chemistry	has definition The scientific study of chemicals and chemical reactions.
	is a kind of science
Cherenkov detector	has definition Apparatus through which it is possible to observe the existence and velocity of high-speed particles important in experimental nuclear physics and in the study of cosmic radiation. It was originally built to investigate the Cherenkov radiation effect, in which charged particles travel through a medium at a speed greater than that of light in that medium.
	is a kind of high energy detector
Chi squared test	has definition A least-squares statistical test that measures the probability of randomness in a distribution.
	has symbol χ2
	is an instance of statistical test
chirality	has definition An expression of the basic handedness of nature. Fundamental theories of the elementary particles and of superstrings must possess chirality.
	has definition Feature of fundamental particle physics that distinguishes left- from right-handed, showing that the universe is not fully left-right symmetric.
	has synonym chiral
	is an instance of quantum quantity
chlorine	has abundance 130 p.p.m. in Earth's crust
	has abundance 18000 p.p.m. in seawater
	has abundance 3.2 × 105 in Sun relative to H = 1 × 1012
	has atomic emission line 479.455 nm for Cl II
	has atomic emission line 489.677 nm for Cl II
	has atomic emission line 542.323 nm for Cl II
	has atomic emission line 858.597 nm for Cl I
	has atomic emission line 837.574 nm for Cl I (strong)
	has atomic number 17
	has biological role chloride, Cl-, is essential to many species, including humans
	has boiling point 239.18 K
	has chief source halite (rock salt)
	has covalent radii 99 pm
	has critical pressure 7700 kPa
	has critical temperature 417 K
	has crystal cell dimension a = 856, c = 612 pm
	has crystal type tetragonal
	has daily dietary intake 3.00 - 6.50 g
	has definition yellow-green, dense, sharp-smelling gas (Cl2) which is a key industrial chemical
	has density 1507 kg m-3 for liquid at 239.18 K boiling point
	has density 2030 kg m-3 for solid at 113 K
	has density 3.214 kg m-3 for gas at 273 K
	has discoverer C.W. Scheele
	has discovery date 1774
	has discovery location Uppsala, Sweden
	has electron affinity 349.0 kJ mol-1 from Cl to Cl-
	has electron configuration [Ne]3s23p5 in ground state
	has electronegativity 3.16 Pauling
	has hazard Cl2 0.5 p.p.m. time-weighted average above which there is a threat to health
	has hazard Cl2 15 p.p.m. produces throat irritation
	has hazard Cl2 50 p.p.m. is dangerous even in short doses
	has hazard corrosive, vapours attack eyes and lungs
	has heat capacity 21.840 J K-1 mol-1 for atomic gas at constant pressure 0.1 MPa at 298.15 K
	has heat capacity 33.907 J K-1 mol-1 for molecular gas (Cl2) at constant pressure 0.1 MPa at 298.15 K
	has heat of fusion 6.41 kJ mol-1
	has heat of vaporization 20.4033 kJ mol-1
	has image
	has ionic radii 181 pm for Cl-
	has isotope mass range 31 to 41
	has lethal intake Cl2 inhalation 500 p.p.m. for 5 minutes for humans
	has level in humans 2000 - 5200 p.p.m. chloride in muscle
	has level in humans 2890 mg dm-3 chloride in blood
	has level in humans 3000 - 7200 p.p.m. chloride in liver
	has level in humans 900 p.p.m. chloride in bone
	has longest lived isotope chlorine 35
	has main mining area vast deposits in USA, Poland, Russia, Germany, China, India, Australia
	has mass absorption coefficient 106 cm2 g-1 for CuKα X-ray diffraction
	has mass absorption coefficient 11.4 cm2 g-1 for MoKα X-ray diffraction
	has mass magnetic susceptibility -7.2 × 10-9 kg-1 m3 for gas
	has mass of element in person 95 g for a 70 kg average person
	has melting point 172.17 K
	has mineral halite, carnallite, sylvite
	has molar volume 17.46 cm3 for solid at 113 K
	has name origin chloros = pale green from Greek
	has neutron scattering length 0.95770 × 10-12 cm
	has number of isotopes 13 including nuclear isomers
	has number of protons 17
	has ocean oxidation state -I
	has ocean residence time 4 × 108 years
	has pronunciation klor-een
	has registry number 7782-50-5 for Chemical Abstracts System database
	has relative atomic mass 35.4527 in units of 12C = 12.000
	has reserves > 1 × 1013 tonnes
	has space group P4/ncm
has specimen Cl2 in small pressurized canisters. Danger!
has symbol Cl
has synthesis mechanism electrolysis of sodium chloride solution
has term symbol 2P3/2 in ground state
has thermal conductivity 0.0089 W m-1 K-1 for gas at 300 K
has thermal neutron capture cross section 35.5 barns
has toxic intake Cl2 is very toxic affecting the eyes and lungs at 3 p.p.m. in air; chloride is non-toxic
has uses bleaching agent
has uses manufacture of organochlorine solvents
has uses PVC
has uses sterilising agent for water supplies
has van der Waals radii 181 pm
has world production 168 × 106
is a kind of accumulating oceanic element
is a kind of gaseous element
is a kind of halogen
is a kind of lithophile element
chlorine 34m	has atomic mass 33.973763
	has decay mode β+
	has decay mode γ
	has decay mode IT
	has half life 32.2 minutes
	has nuclear spin I = 3+ h/2π
	has number of neutrons 17
	has number of nucleons 34
	has symbol 34Cl
	is an instance of chlorine
chlorine 35	has atomic mass 34.968852721
	has magnetogyric ratio 2.6210 × 107 rad T-1 s-1
	has natural abundance 75.77%
	has NMR frequency 9.798 MHz where 1H = 100 MHz; 2.3488 T
	has NMR receptivity 20.2 where 13C = 1.00
	has nuclear magnetic moment μ = +0.8218736 nuclear magnetons with diamagnetic correction
	has nuclear quadrupole moment -0.08165 × 10-28 m2
	has nuclear spin I = 3/2+ h/2π
	has number of neutrons 18
	has number of nucleons 35
	has relative NMR sensitivity 4.70 × 10-3 where 1H = 1.00
	has symbol 35Cl
	has uses isotopically enriched samples available for experimental purposes
	has uses Nuclear Magnetic Resonance
	is an instance of chlorine
chlorine 36	has atomic mass 35.968306
	has decay mode β+
	has decay mode β- (0.709 Mev)
	has decay mode EC (1.142 Mev)
	has decay mode no γ
	has half life 3.01 × 107 years
	has nuclear magnetic moment μ = +1.28547 nuclear magnetons with diamagnetic correction
	has nuclear spin I = 2+ h/2π
	has number of neutrons 19
	has number of nucleons 36
	has symbol 36Cl
	has uses research
	is an instance of chlorine
chlorine 37	has atomic mass 36.96590262
	has magnetogyric ratio 2.1718 × 107 rad T-1 s-1
	has natural abundance 24.23%
	has NMR frequency 8.156 MHz where 1H = 100 MHz; 2.3488 T
	has NMR receptivity 3.8 where 13C = 1.00
	has nuclear magnetic moment μ = +0.6841230 nuclear magnetons with diamagnetic correction
	has nuclear quadrupole moment -0.06435 × 10-28 m2
	has nuclear spin I = 3/2+ h/2π
	has number of neutrons 37
	has number of nucleons 20
	has relative NMR sensitivity 2.71 × 10-3 where 1H = 1.00
	has symbol 37Cl
	has uses isotopically enriched samples available for experimental purposes
	has uses Nuclear Magnetic Resonance
	is an instance of chlorine
chlorine 38	has atomic mass 37.968010
	has decay mode β- (4.917 Mev)
	has decay mode γ
	has half life 37.2 minutes
	has nuclear magnetic moment μ = 2.05 nuclear magnetons with diamagnetic correction
	has nuclear spin I = 2- h/2π
	has number of neutrons 21
	has number of nucleons 38
	has symbol 38Cl
	is an instance of chlorine
chlorine 39	has atomic mass 38.968005
	has decay mode β- (3.44 Mev)
	has decay mode γ
	has half life 55.6 minutes
	has nuclear spin I = 3/2- h/2π
	has number of neutrons 22
	has number of nucleons 39
	has symbol 39Cl
	is an instance of chlorine
chlorine 40	has atomic mass 39.970440
	has decay mode β- (7.5 Mev)
	has decay mode γ
	has half life 1.38 minutes
	has nuclear spin I = 2- h/2π
	has number of neutrons 23
	has number of nucleons 40
	has symbol 40Cl
	is an instance of chlorine
chromium	has abundance 1.5 × 10-4 p.p.m. in Pacific surface seawater
	has abundance 1.8 × 10-4 p.p.m. in Atlantic surface seawater
	has abundance 100 p.p.m. in Earth's crust
	has abundance 2.3 × 10-4 p.p.m. in deep Atlantic seawater
	has abundance 2.5 × 10-4 p.p.m. in deep Pacific seawater
	has abundance 5.13 × 105 in Sun relative to H = 1 × 1012
	has atomic emission line 357.869 nm for Cr I (used in atom absorption spectrometry)
	has atomic emission line 359.349 nm for Cr I
	has atomic emission line 360.533 nm for Cr I
	has atomic emission line 427.480 nm for Cr I
	has atomic emission line 428.972 nm for Cr I
	has atomic emission line 520.844 nm for Cr I
	has atomic emission line 425.435 nm for Cr I (strong)
	has atomic number 24
	has atomic radii 125 pm
	has biological role essential to some species, including humans, stimulates metabolism
	has boiling point 2945 K
	has bulk modulus 160.2 GPa
	has chief source chromite
	has crystal cell dimension a = 288.46 pm
	has crystal type b.c.c.
	has daily dietary intake 0.01 - 1.2 mg
	has definition hard, blue-white metal which resists oxidation in air, can be polished to a high shine
	has density 6460 kg m-3 for liquid at 2130 K melting point
	has density 7190 kg m-3 for solid at 293 K
	has discoverer Nicholas Louis Vauquelin
	has discovery date 1780
	has discovery location Paris, France
	has electrical resistivity 12.7 × 10-8 Ω m at 293 K
	has electron affinity 64.3 kJ mol-1 from Cr to Cr-
	has electron configuration [Ar]3d54s1 in ground state
	has electronegativity 1.66 Pauling
	has hazard poison by ingestion, suspected carcinogen, chromates are corrosive to tissue
	has heat capacity 20.79 J K-1 mol-1 for gas at constant pressure 0.1 MPa at 298.15 K
	has heat capacity 23.35 J K-1 mol-1 for solid at constant pressure 0.1 MPa at 298.15 K
	has heat of fusion 15.3 kJ mol-1
	has heat of vaporization 348.78 kJ mol-1
	has image
	has ionic radii 56 pm for Cr4+
	has ionic radii 64 pm for Cr3+
	has ionic radii 84 pm for Cr2+
	has isotope mass range 45 to 57
	has lethal intake 70 mg kg-1 metal taken oraly in humans
	has level in humans 0.006 - 0.11 mg dm-3 in blood
	has level in humans 0.02 - 3.3 p.p.m. in liver
	has level in humans 0.024 - 0.84 p.p.m. in muscle
	has level in humans 0.1 - 0.33 p.p.m. in bone
	has linear expansion coefficient 6.2 × 10-6 K-1
	has longest lived isotope chromium 52
	has main mining area Turkey, South Africa, Zimbabwe, Russia, Philipipines
	has mass absorption coefficient 260 cm2 g-1 for CuKalpha X-ray diffraction
	has mass absorption coefficient 31.1 cm2 g-1 for MoKalpha X-ray diffraction
	has mass magnetic susceptibility +4.45 × 10-8 kg-1 m3 for solid
	has mass of element in person 14 mg for a 70 kg average person
	has melting point 2130 ± 20 K
	has mineral chromite, crocoite
	has molar volume 7.23 cm3
	has name origin chroma = colour from Greek
	has neutron scattering length 0.3635 × 10-12 cm
	has number of isotopes 13 including nuclear isomers
	has number of protons 24
	has ocean oxidation state VI
	has ocean residence time 10000 years
	has poisson's ratio 0.21 GPa
has pronunciation kroh-mi-uhm
has registry number 7440-47-3 for Chemical Abstracts System database
has relative atomic mass 51.9961 in units of 12C = 12.000
has reserves 1 × 109 tonnes
has rigidity modulus 115.3 GPa
has space group Im3m
has specimen chips, chunks, crystallites or powder. Safe.
has symbol Cr
has term symbol 7S3 in ground state
has thermal conductivity 93.7 W m-1 K-1 at 300 K
has thermal neutron capture cross section 3.1 barns
has toxic intake 200 mg
has uses alloys, plating and metal ceramics
has world production 20000 tonnes year-1 for chromium metal
has world production 9.6 × 106 tonnes year-1 for chromite ore
has young's modulus 279 GPa
is a kind of chalcophile element
is a kind of recycled oceanic element
is a kind of transition metal
reacts with HCL and H2SO4 by disolving
chromium 48	has atomic mass 47.954033
	has decay mode γ
	has decay mode EC (1.65 Mev)
	has half life 21.6 hours
	has number of neutrons 24
	has number of nucleons 48
	has symbol 48Cr
	is an instance of chromium
chromium 49	has atomic mass 48.951338
	has decay mode β+
	has decay mode γ
	has decay mode EC (2.627 Mev)
	has half life 42.3 minutes
	has nuclear magnetic moment μ = 0.476 nuclear magnetons with diamagnetic correction
	has nuclear spin I = 5/2- h/2π
	has number of neutrons 25
	has number of nucleons 49
	has symbol 49Cr
	is an instance of chromium
chromium 50	has atomic mass 49.9460464
	has natural abundance 4.345%
	has nuclear spin I = 0+ h/2π
	has number of neutrons 26
	has number of nucleons 50
	has symbol 50Cr
	has uses isotopically enriched samples available for experimental purposes
	is an instance of chromium
chromium 51	has atomic mass 50.944768
	has decay mode γ
	has decay mode EC (0.751 Mev)
	has half life 27.70 days
	has nuclear magnetic moment μ = -0.934 nuclear magnetons with diamagnetic correction
	has nuclear spin I = 7/2- h/2π
	has number of neutrons 27
	has number of nucleons 51
	has symbol 51Cr
	has uses medical diagnostic
	has uses research
	is an instance of chromium
chromium 52	has atomic mass 51.9405098
	has natural abundance 83.789%
	has nuclear spin I = 0+ h/2π
	has number of neutrons 28
	has number of nucleons 52
	has symbol 52Cr
	has uses isotopically enriched samples available for experimental purposes
	is an instance of chromium
chromium 53	has atomic mass 52.9406513
	has magnetogyric ratio -1.5120 × 107 rad T-1 s-1
	has natural abundance 9.501%
	has NMR frequency 5.652 MHz where 1H = 100 MHz; 2.3488 T
	has NMR receptivity 0.49 where 13C = 1.00
	has nuclear magnetic moment μ = -0.47454 nuclear magnetons with diamagnetic correction
	has nuclear quadrupole moment -0.150 × 10-28 m2
	has nuclear spin I = 3/2- h/2π
	has number of neutrons 29
	has number of nucleons 53
	has relative NMR sensitivity 9.03 × 10-4 where 1H = 1.00
	has symbol 53Cr
	has uses isotopically enriched samples available for experimental purposes
	has uses Nuclear Magnetic Resonance
	is an instance of chromium
chromium 54	has atomic mass 53.9388825
	has natural abundance 2.365%
	has nuclear spin I = 0+ h/2π
	has number of neutrons 30
	has number of nucleons 54
	has symbol 54Cr
	has uses isotopically enriched samples available for experimental purposes
	is an instance of chromium
chromium 55	has atomic mass 54.940842
	has decay mode β- (2.603 Mev)
	has decay mode γ
	has half life 3.497 minutes
	has nuclear spin I = 3/2- h/2π
	has number of neutrons 31
	has number of nucleons 55
	has symbol 55Cr
	is an instance of chromium
chromium 56	has atomic mass 55.940643
	has decay mode β- (1.62 Mev)
	has decay mode γ
	has half life 5.9 minutes
	has nuclear spin I = 0+ h/2π
	has number of neutrons 32
	has number of nucleons 56
	has symbol 56Cr
	is an instance of chromium
chromosphere	has definition The part of the solar atmosphere between the photosphere and the corona. It consists of two rather well defined zones: the lower chromosphere and the upper chromosphere.
	has definition The part of the Sun's atmosphere immediately above the surface (the photosphere) and beneath the corona.
	has has density ρ ≈ 10-16 to 10-8 g cm-3
	is a part of solar atmosphere
chromospheric network	has definition A large-scale cellular pattern along the boundaries of which lie bright and dark mottles seen in Hα and other regions.
	is a part of chromosphere
Circinus	has acronym Cir
	has genitive Circini
	has synonym Compass
	is a part of celestial sphere
	is an instance of constellation named by de Lacaille
Circinus X-1	has definition A highly variable X-ray source. Many of its properties are similar to those of Cygnus X-1.
	has synonym 3U 1516-56
	is a part of Circinus
	is an instance of X-ray source
circle	has definition An ellipse possessing but one focus.
	is a kind of ellipse
circumpolar region	has coordinates whose equatorial latitude is greater than the Earth latitude of the observer minus 90 degrees
	has definition a region defined by an equatorial latitude above which stars are visible all year (depends on observers location on Earth)
	has purpose to define a part of the sky which can be observed all year at the observers location
	is a part of celestial sphere
	is an instance of equatorial sky area
circumpolar star	has definition Star which never sets, from the viewpoint of an observer on Earth.
	is a kind of star
class I globular cluster	is a kind of globular cluster
class II globular cluster	is a kind of globular cluster
class III globular cluster	is a kind of globular cluster
class IV globular cluster	is a kind of globular cluster
class IX globular cluster	is a kind of globular cluster
class V globular cluster	is a kind of globular cluster
class VI globular cluster	is a kind of globular cluster
class VII globular cluster	is a kind of globular cluster
class VIII globular cluster	is a kind of globular cluster
class X globular cluster	is a kind of globular cluster
class XI globular cluster	is a kind of globular cluster
classical electron radius	has equation
	has symbol re
	has uncertainty 0.00000031 × 10-15 m
	has value 2.817940285 × 10-15 m
	is an instance of atomic constant
	is an instance of radius
classical nova	has initial rise of light curve typically 12 magnitudes
	has rate of decline of light curve which is nearly exponential
	has rate of decline of light curve which is proportional to the absolute magnitude at maximum (can be used as a distance measureing technique)
	has ultraviolet flux light curve which increases after optical maximum
	is a kind of nova
classical physics	has definition Physics prior to the introduction of the quantum principle. Classical physics incorporates Newtonian mechanics, views energy as a continuum, and is strictly causal.
	is a kind of physics
close binary	is a kind of binary star
closed string	has definition A type of string that is in the shape of a loop.
	is a kind of string theory
closed universe	has definition A homogeneous, isotropic universe is said to be temporally closed if gravity is strong enough to eventually reverse the expansion, causing the universe to recollapse. It is said to be spatially closed if gravity is strong enough to curve the space back on itself, forming a finite volume with no boundary. Triangles would contain more than 180°, the circumference of a circle would be less than π times the diameter, and a traveler intending to travel in a straight line would eventually find herself back at her starting point. If Einstein's cosmological constant is zero, as is frequently assumed, then a universe which is temporally closed is also spatially closed, and vice versa.
	has definition A standard universe with a spherical three-dimensional spatial geometry. Such a universe is finite in both space and time, and recollapses.
	has definition A universe that has a finite size. Closed universes expand for a finite time, reach a maximum size, and then collapse. In closed universes, the inward pull of gravity dominates and eventually reverses the outward flying apart of matter; that is, gravitational energy dominates the kinetic energy of expansion. The value of omega is greater than 1 for a closed universe. If a universe begins closed, it remains closed; if it begins open, it remains open; if it begins flat, it remains flat. In the big bang model of the universe, the question of whether the universe is closed, open, or flat is determined by the initial conditions, just as the fate of a rocket launched from earth is determined by its initial upward velocity relative to the strength of earth's gravitational pull. If the initial rate of expansion of the universe was lower than a critical value, determined by the mass density, the universe will expand only for a certain period of time and then collapse, just as a rocket launched with a velocity below a critical value, dependent on the strength of earth's gravity, will reach a maximum height and then fall back to earth. This is the behavior of a closed universe. If the initial rate of expansion of the universe was larger than a critical value, the universe is open and will keep expanding forever. If the initial rate of expansion was precisely the critical value, the universe is flat and will expand forever, but with a rate of expansion that approaches zero. (See flat universe; omega; open universe.)
	has definition Any model of the Universe in which the gravity of the matter content can reverse the expansion and cause a collapse.
	has definition Big Bang model that was formulated by Friedmann and Lemaitre which has a positive curvature, like the surface of a sphere, in which case the universe is finite, closed, and will eventually recollapse. This space is unbounded.
	has definition Cosmological model in which the universe eventually stops expanding and begins to collapse, presumably to end in a fireball like that of the big bang. Compare open universe.
	has geometry of space positive curvature
	is a kind of big bang
CN-strong star	has definition Late type giant with strong CN bands. Metallic lines are also stronger than in normal giants.
	is a kind of giant
	is a kind of late star
CN-weak star	has definition High-velocity star with both weak metallic lines and weak CN bands.
	is a kind of weak line star
CNO bi-cycle	has definition Similar to the CNO cycle, except that it also includes a cycle in which the next-to-last step becomes 15N(p, γ) 16O(p, γ) 17F(β+ν) 17O(p, α) 14N. This reaction occurs once in about 2000 CN cycles. For main-sequence stars greater than a few solar masses, hydrogen burning by the CNO bi-cycle is the main source of energy.
	has probability 2% of the energy of our Sun
	is an instance of carbon cycle
CNO tri-cycle	has definition Similar to the CNO bi-cycle, with the addition of the cycle 17O(p, γ) 18F(β+ν) 18O(p, α) 15N.
	is an instance of carbon cycle
co-moving coordinate	has definition A set of coordinates which do not change in an expanding (or otherwise moving) medium. i.e. the coordinates of a distant galaxy do not change just because of the expansion of space.
	has definition Coordinates fixed with respect to the overall Hubble flow of the universe, so that they do not change as the universe expands.
	is a kind of coordinate
Coalsack	has definition A prominent dark nebula readily visible to the naked eye, located on the galactic plane.
	has distance 170 pc
	is a part of Crux
	is a part of dust
	is an instance of dark nebula
coating	is a kind of optical device
cobalt	has abundance 1.1 × 10-6 p.p.m. in deep Pacific seawater
	has abundance 20 p.p.m. in Earth's crust
	has abundance 6.9 × 10-6 p.p.m. in Pacific surface seawater
	has abundance 7.94 × 104 in Sun relative to H = 1 × 1012
	has atomic emission line 240.725 nm for Co I (used in atom absorption spectrometry)
	has atomic emission line 242.493 nm for Co I
	has atomic emission line 340.512 nm for Co I
	has atomic emission line 344.364 nm for Co I
	has atomic emission line 350.228 nm for Co I
	has atomic emission line 356.938 nm for Co I
	has atomic emission line 345.350 nm for Co I (strong)
	has atomic number 27
	has atomic radii 125 pm
	has biological role essential to most species, including humans
	has boiling point 3143 K
	has bulk modulus 181.5 GPa
	has chief source cobaltite, skutterudite
	has covalent radii 116 pm
	has crystal cell dimension a = 250.7, c = 406.9 pm for ε-Co
	has crystal cell dimension a = 354.41 pm for α-Co
	has crystal type f.c.c. for α-Co
	has crystal type h.c.p. for ε-Co
	has daily dietary intake 0.005 - 1.8 mg
	has definition lustrous, silvery-blue, hard metal which is also ferromagnetic. Cobalt 60 is an important radioisotope
	has density 7670 kg m-3 for liquid at 293 K at melting point
	has density 8900 kg m-3 for solid at 293 K
	has discoverer Georg Brandt
	has discovery date 1735
	has discovery location Stockholm, Sweden
	has electrical resistivity 6.24 × 10-8 Ω m at 293 K
	has electron affinity 63.8 kJ mol-1 from Co to Co-
	has electron configuration [Ar]3d74s2 in ground state
	has electronegativity 1.88 Pauling
	has hazard compounds have low toxicity when ingested, but produce vomiting. Suspected carcinogen.
	has heat capacity 23.020 J K-1 mol-1 for gas at constant pressure 0.1 MPa at 298.15 K
	has heat capacity 24.81 J K-1 mol-1 for solid at constant pressure 0.1 MPa at 298.15 K
	has heat of fusion 15.2 kJ mol-1
	has heat of vaporization 382.4 kJ mol-1
	has image
	has ionic radii 64 pm for Co3+
	has ionic radii 82 pm for Co2+
	has isotope mass range 35m to 64
	has lethal intake 80 mg kg-1 chloride ingested by rat
	has level in humans 0.0002 - 0.04 mg dm-3 in blood
	has level in humans 0.01 - 0.04 p.p.m. in bone
	has level in humans 0.028 - 0.65 p.p.m. in muscle
	has level in humans 0.06 - 1.1 p.p.m. in liver
	has linear expansion coefficient 13.36 × 10-6 K-1
	has longest lived isotope cobalt 59
	has main mining area Zaire, Morocco, Sweden, Canada
	has mass absorption coefficient 313 cm2 g-1 for CuKalpha X-ray diffraction
	has mass absorption coefficient 42.5 cm2 g-1 for MoKalpha X-ray diffraction
	has mass magnetic susceptibility ferromagnetic
	has mass of element in person 3 mg for a 70 kg average person
	has melting point 1768 K
	has mineral cobaltite, erythrite, glaucodot, linnaerite, skutterudite (smaltite)
	has molar volume 6.62 cm3
	has name origin kobald = goblin from German
	has neutron scattering length 0.278 × 10-12 cm
	has number of isotopes 17 including nuclear isomers
	has number of protons 27
	has ocean oxidation state II
	has ocean residence time 40 years
	has poisson's ratio 0.32 GPa
has pronunciation koh-bolt
has registry number 7440-48-4 for Chemical Abstracts System database
has relative atomic mass 58.93320 in units of 12C = 12.000
has rigidity modulus 82 GPa
has space group Fm3m for α-Co
has space group P63/mmc for ε-Co
has specimen foil, pieces, powder, rod and wire. Care !
has symbol Co
has term symbol 4F9/2 in ground state
has thermal conductivity 100 W m-1 K-1 at 300 K
has thermal neutron capture cross section 37.2 barns
has toxic intake 500 mg
has uses magnet alloys, ceramics, catalysts and paints
has world production 17000 tonnes year-1
has young's modulus 211 GPa
is a kind of scavenged oceanic element
is a kind of siderophile element
is a kind of transition metal
reacts with dilute acids
cobalt 55	has atomic mass 54.942001
	has decay mode β+ (3.452 Mev)
	has decay mode γ
	has decay mode EC
	has half life 17.53 hours
	has nuclear magnetic moment μ = +4.822 nuclear magnetons with diamagnetic correction
	has nuclear spin I = 7/2- h/2π
	has number of neutrons 28
	has number of nucleons 55
	has symbol 55Co
	is an instance of cobalt
cobalt 56	has atomic mass 55.939841
	has decay mode β+ (4.566 Mev)
	has decay mode γ
	has decay mode EC
	has half life 77.3 days
	has nuclear magnetic moment μ = +3.85 nuclear magnetons with diamagnetic correction
	has nuclear spin I = 4+ h/2π
	has number of neutrons 29
	has number of nucleons 56
	has symbol 56Co
	is an instance of cobalt
cobalt 57	has atomic mass 56.936294
	has decay mode γ
	has decay mode EC (0.836 MeV)
	has half life 271.8 days
	has nuclear magnetic moment μ = +7.72 nuclear magnetons with diamagnetic correction
	has nuclear spin I = 7/2- h/2π
	has number of neutrons 30
	has number of nucleons 57
	has symbol 57Co
	has uses medical diagnosis
	has uses research
	is an instance of cobalt
cobalt 58	has atomic mass 57.935755
	has decay mode β+ (2.30 Mev)
	has decay mode γ
	has decay mode EC
	has half life 70.88 days
	has nuclear magnetic moment μ = +4.04 nuclear magnetons with diamagnetic correction
	has nuclear spin I = 2+ h/2π
	has number of neutrons 31
	has number of nucleons 58
	has symbol 58Co
	has uses medical diagnosis
	has uses research
	is an instance of cobalt
cobalt 59	has atomic mass 58.9331976
	has magnetogyric ratio 6.3472 × 107 rad T-1 s-1
	has natural abundance 100%
	has NMR frequency 23.614 MHz where 1H = 100 MHz; 2.3488 T
	has NMR receptivity 1570 where 13C = 1.00
	has nuclear magnetic moment μ = +4.627 nuclear magnetons with diamagnetic correction
	has nuclear quadrupole moment +0.420 × 10-28 m2
	has nuclear spin I = 7/2- h/2π
	has number of neutrons 32
	has number of nucleons 59
	has relative NMR sensitivity 0.28 where 1H = 1.00
	has symbol 59Co
	has uses Nuclear Magnetic Resonance
	is an instance of cobalt
cobalt 60	has atomic mass 59.933819
	has decay mode β- (2.824 Mev)
	has decay mode γ
	has half life 5.27 years
	has nuclear magnetic moment μ = +3.799 nuclear magnetons with diamagnetic correction
	has nuclear spin I = 5+ h/2π
	has number of neutrons 33
	has number of nucleons 60
	has symbol 60Co
	has uses medical diagnosis
	has uses medical therapy
	has uses research
	is an instance of cobalt
cobalt 61	has atomic mass 60.932478
	has decay mode β- (1.322 Mev)
	has decay mode γ
	has half life 1.650 hours
	has nuclear spin I = 7/2- h/2π
	has number of neutrons 34
	has number of nucleons 61
	has symbol 61Co
	is an instance of cobalt
coherent scattering	has definition A scattering process that leaves atoms in the same energy state after the scattered photon departs in a direction different from that of the incident photon. The energy of the scattered photon is the same (in the rest frame of the atom) as that of the incident photon.
	is a kind of scattering
cold dark matter	has acronym CDM
	has definition A model of structure formation in which an exotic particle whose energy is low at the time it decouples from other matter is responsible for structure formation.
	has definition Any dark matter candidate which was non-relativistic at the point of decoupling.
	has definition Hypothetical subatomic particles that move slowly compared with the speed of light.
	is a kind of dark matter
collapse	has definition Sudden contraction of a celestial body
	is a kind of contraction
collection of galaxies	has number of galaxies
	is a kind of collection of stars
collection of particles	is a kind of natural object
collection of stars	has catalog catalog about star systems
	has definition Two or more stars forming a gravitationally bound system
	has number of stars
	is a kind of celestial body
collider	has definition A particle accelerator in which beams of particles with equal but opposite momentum are made to collide head on.
	is a kind of particle accelerator
collision	is a kind of atomic process
color	is a kind of radiation measurement
color temperature	has definition A stellar temperature determined by comparison of the spectral distribution of the star's radiation with that of a blackbody.
	is a kind of temperature
Columba	has acronym Col
	has genitive Columbae
	has historical image
	has synonym Dove
	is a part of celestial sphere
	is an instance of constellation formerly part of Argo Navis
column grouped element	has group a column number in the table of the elements
	is a kind of element
coma	has definition The spherical region of diffuse gas, about 150000 km in diameter, which surrounds the nucleus (q.v.) of a comet. Together, the coma and the nucleus form the comet's head.
	has synonym comet head
	is a part of comet
Coma Berenices	has acronym Com
	has boundary
	has genitive Comae Berenices
	has historical image
	has synonym Berenice's Hair
	is a part of celestial sphere
	is an instance of constellation
Coma cluster	has definition The nearest massive cluster of galaxies. A symmetric cluster with primarily E and S0 galaxies. Luminous mass 4 × 1014 Msun = 8 × 1047 g; virial theorem mass about 5 × 1048 g; mass needed to bind the cluster about 4 × 1049 g. R ≈ 9 × 1024 cm.
	has diameter 10 million light years
	has distance 300 million light years
	has mass 4 × 1014 Msun (luminous matter)
	has number of galaxies 1000
	has redshift z = 0.023
	has synonym Abell 1656
	has synonym Coma X-1
	is a part of Coma Berenices
	is a part of Local Supercluster
	is an instance of rich cluster
	is an instance of X-ray source
Coma open cluster	has definition Open cluster similar to the Hyades in overall binary frequency.
	has distance 80 pc
	has number of stars 100
	is a part of Coma Berenices
	is a part of disk
	is an instance of open cluster
Coma X-1	has definition An extended X-ray source in the Coma cluster of galaxies.
	has synonym 3U 1257+28
	is a part of Coma Berenices
	is a part of Coma cluster
	is an instance of X-ray source
comet	has definition A diffuse body of gas and solid particles (such as CN, C2, NH3, and OH), which orbits the Sun. The orbit is usually highly elliptical or even parabolic (average perihelion distance less than 1 AU; average aphelion distance, roughly 104 AU). Comets are unstable bodies with masses on the order of 1018 g whose average lifetime is about 100 perihelion passages. Periodic comets comprise only about 4% of all known comets. Comets are obviously related in some manner to meteors, but no meteorites from a comet have ever been recovered. Observations of comets Bennett and Kohoutek have established that a comet is surrounded by a vast hydrogen halo.
	is a kind of celestial body
	is a part of our solar system
comet impact	is a kind of impact event
comet nucleus	has definition The stellar-appearing frozen core, containing almost the entire cometary mass, in the head of a comet.
	is a part of comet
comet tail	has definition The long streamer behind the comet head which does not usually appear until the comet is inside the orbit of Mars.
	has density 10-18 atm
	has length 107 km
	has mean distance from Sun inside the orbit of Mars
	is a part of comet
cometary nebula	has definition A reflection nebula with a fan shape that bears a superficial resemblance to a comet. Classical examples of the heads of cometary nebulae are R Mon, R CrA, and RY Tau. All have A0-G0 type spectra that resemble the spectrum of a T Tauri star, and their brightness varies from year to year.
	is a kind of reflection nebula
commensurate orbit	has definition A term applied to two bodies orbiting around a common barycenter when the period of one is an integral multiple of that of the other.
	has period an integral multiple of that of the other body
	is a kind of elliptical orbit
communication satellite	is a kind of artificial satellite
compact galaxy	has definition A galaxy similar to an N galaxy but with no disk or nebulous background. It is an object of high surface brightness which appears slightly nonstellar on photographs and which has a larger redshift than normal stars in our Galaxy. Nearest "compact" galaxy is M32.
	is a kind of galaxy
compact group	has definition Galaxy group with a few galaxies separated by a few galaxy diameters.
	has number of galaxies < 10
	is a kind of galaxy group
compact H II region	has definition A dense (ne ≥ 103 cm-3) H II region of small linear dimensions (≤ 1 pc).
	is a kind of H II region
composite spectrum star	has definition Object with a spectrum due to superposition of the spectra of two different stars.
	is a kind of star
Compton	is an instance of X-ray space telescope
Compton scattering	has antonym inverse Compton effect
	has definition Decrease in the frequency of high-energy radiation (such as X-rays) caused when a photon loses some of its energy to a free electron by collision.
	has definition Scattering of a photon due to the Compton effect (see also noncoherent scattering).
	has definition The scattering of photons by free electrons in an ionized medium.
	is a kind of scattering
condensation	has definition from gas to liquid or from gas to solid
	has final phase liquid
	has initial phase gas
	has inverse process evaporation
	is a kind of first order phase transition
conductance	has unit conductance unit
	is a kind of electromagnetic quantity
conductance quantum	has equation
	has symbol G0
	has uncertainty 0.000000028 × 10-5 S
	has value 7.748091696 × 10-5 S
	is an instance of conductance
	is an instance of electromagnetic constant
conductance unit	is a kind of electromagnetic unit
	is a unit of conductance
conic section	has dimensions 2
	is a kind of geometrical object
conjunction	has definition The phenomenon in which two bodies have the same apparent celestial longitude (see longitude, celestial) or right ascension as viewed from a third body. Conjunctions are usually tabulated as geocentric phenomena. For Mercury and Venus, geocentric inferior conjunction occurs when the planet is between the Earth and Sun, and superior conjunction occurs when the Sun is between the planet and Earth.
	has elongation 0°
	is a kind of planetary elongation event
conservation law	has definition A quantity that remains unchanged in the course of the evolution of a dynamical system. There are seven known quantities that are conserved: energy (including mass), momentum, angular momentum (including spin), charge, electron-family number, muon-family number, and baryon-family number.
	has definition Law that identifies a quantity, such as energy, that remains unchanged throughout a transformation. All conservation laws are thought to involve symmetries.
	is a kind of law
conservation of angular momentum	has conserved quantity angular momentum
	has definition The principle that the angular momentum of a system (the momentum of rotation about a point) remains the same as long as no external torque acts.
	has definition The total angular momentum of an isolated dynamical system does not change during the course of its evolution.
	is a kind of conservation law
conservation of charge	has conserved quantity charge
	is a kind of conservation law
conservation of energy	has conserved quantity energy
	has definition The principle that the total energy of a closed system never changes, that energy is only converted from one form to another. This principle must be enlarged under special relativity to include mass-energy.
	has definition The total energy of a system (including kinetic energy and gravitational energy) is conserved and does not vary. Thus, kinetic energy can only increase at the expense of gravitational potential energy. Modern physics has modified the law of conservation of energy, since matter can be created or annihilated; a more general law is the conservation of mass and energy.
	is a kind of conservation law
conservation of mass and energy	has conserved quantity mass and energy
	has definition Important physical principle and one of the basic laws of physics stating that matter is neither created nor destroyed (although mass may become energy, the energy quantitatively represents the mass). One exception to this principle is a singularity; another follows from the theory of virtual particles.
	is a kind of conservation law
conservation of matter	has conserved quantity matter
	has definition The principle that matter is neither created nor destroyed. This principle is only approximately true, since special relativity shows that matter and energy are equivalent and interconvertible.
	is a kind of conservation law
conservation of momentum	has conserved quantity momentum
	has definition The principle that the linear momentum of a system (in Newtonian mechanics, mass times velocity) remains the same as long as no external force acts.
	is a kind of conservation law
conservative scattering	has definition Scattering that occurs in the absence of absorption.
	is a kind of scattering
consistent	has definition The property possessed by a scientific theory when it contains and extends an earlier well-supported theory; for example, general relativity is consistent with Newtonian gravity.
	is a kind of theory related concept
constant	is a kind of number
constellation	has boundary defined in equatorial coordinates
	has component asterism
	has definition Precisely defined area of the celestial sphere, associated with a grouping of stars, that the International Astronomical Union has designated as a constellation.
	has genitive which is used when referring to a star in the constellation
	has historical origin asterism with pictorial or mythical significance
	has purpose to divide the celestial sphere into small visually recognizable star classification regions
	is a kind of equatorial sky area
	is a part of celestial sphere
constellation formerly part of Argo Navis	has definition a constellation created by the international astronomical union by breaking up the constellation Argo Navis
	has historical origin part of the large constellation Argo Navis
	is a kind of constellation
constellation named by Bayer	has definition a constellation named by Johann Bayer the author of the Uranometria star atlas 1607
	is a kind of constellation
	is named by Johann Bayer
constellation named by de Lacaille	has definition a constellation named by Abbé Nicolas Louis de Lacaille
	is a kind of constellation
	is named by Abbé Nicolas Louis de Lacaille
contraction	has definition A mass motion towards the center of gravity of a celestial body
	is a kind of mass motion
convection	has definition A mass motion in a circulatory pattern. Process in the Sun (and possibly other stars) perhaps caused by solar rotation, which produces the immensely powerful electrical and magnetic fields associated with sunspots.
	is a kind of mass motion
convective zone	has definition Region in a stellar interior where convection dominates the heat flow
	is a part of solar interior
coordinate	has component coordinate component
	has definition Quantities that provide references for locations in space and time.
	is a kind of point
coordinate component	has definition A component of a coordinate system, components are usually an angle or a length
	is a component of coordinate
	is a kind of quantity
coordinate transformation	has definition Method of relating observations from one frame of reference to another.
	is a kind of transformation
Copenhagen interpretation	has competing theory Copenhagen interpretation
	has definition The view of quantum mechanics holding that prior to the measurement, a system has no physical existence and is describable only in terms of the probability of each possible result of a measurement. After a measurement the physical system exists in one and only one of its possible states.
	is an instance of quantum mechanics interpretation
Copernicus	has contribution the Copernican model of the solar system with the sun at its center
	is an instance of scientist
copper	has image
	has ocean oxidation state II
	has ocean residence time 3000 years
	is a kind of recycled oceanic element
	is a kind of transition metal
core-halo galaxy	has abundance 20% of the known extended radio sources
	has definition A class of radio source characterized by an emission "halo" surrounding a more intense "core". About 20% of the known extended radio sources are of the core-halo type.
	is a kind of radio galaxy
corona	has definition Outermost atmosphere of the Sun immediately above the chromosphere, consisting of hot (1-2 × 106 K), low-density (about 10-16 g cm-3) gas that extends for millions of miles from the Suns's surface. Ordinarily it can be seen only during a total solar eclipse. Its shape varies from almost spherical at sunspot maximum to unsymmetrical at minimum. Its high temperature is probably caused by MHD shock waves generated below the photosphere. The corona, together with solar flares, is the source of solar X-rays. It is the corona, not the photosphere, that is studied by radio astronomers, except at very short wavelengths.
	is a part of solar atmosphere
Corona Australis	has acronym CrA
	has boundary
	has genitive Coronae Austrinae
	has synonym Southern Crown
	is a part of celestial sphere
	is an instance of constellation
Corona Borealis	has acronym CrB
	has boundary
	has genitive Coronae Borealis
	has synonym Northern Crown
	is a part of celestial sphere
	is an instance of constellation
coronal green line	has definition An emission line of Fe XIV, the strongest line in the solar corona.
	has species Fe XIV
	has transition 2P3/2 - 2P1/2
	has wavelength 5303 Å
	is a kind of emission line
coronal hole	has definition An area where the extreme-ultraviolet and X-ray coronal emission is abnormally low or absent; a coronal region apparently associated with diverging magnetic fields. A great part, if not all, of the solar wind starts from coronal holes.
	is a part of corona
coronene	has definition The first ultraviolet phosphor to be tried on the surface of a CCD.
	is a kind of coating
corrector plate	has definition Thin lens-like optical piece which removes certain optical aberrations.
	is a kind of aspheric
Corvus	has acronym Crv
	has boundary
	has genitive Corvi
	has synonym Crow
	is a part of celestial sphere
	is an instance of constellation
Cosmic Background Explore	has acronym COBE
	has definition satellite that studies the microwave background.
	is an instance of NASA satellite
cosmic matter density	has definition The average number of fermions per unit volume of space throughout the universe. Since matter is depicted in general relativity as bending space, the value of the cosmic matter density, if known, could reveal the overall curvature of cosmic space. See critical density, omega.
	is a kind of density
cosmic rays	has composition 85% protons, 14% alpha-particles, 1% electrons, << 1% heavy nuclei
	has definition High-energy charged particles which stream at relativistic velocities down to Earth from space. The Sun ejects low-energy (107 - 1010 eV) cosmic rays during solar flares (those of lower energy than this are unobservable from Earth because of solar system magnetic fields). Those of intermediate energy (1010 - 1016 eV) have an isotropic distribution, and are apparently produced in the Galaxy. Possible sources of acceleration are shock waves accompanying supernovae (although cosmic rays have a higher hydrogen content than would be expected from a star that has processed material to iron), and the rotating magnetic fields of pulsars. The light elements Li, Be, and B have a higher abundance ratio in cosmic rays than in the solar system.
	has energy 2 GeV (average)
	has synonym corpuscular radiation
	is a kind of charged particle
	is a kind of radioactive particle
cosmic string	has definition Long, stringlike concentrations of matter-energy that may have formed during symmetry breaking in the first moments of the big bang. If they exist, they would be candidates for the seed perturbations of structure formation.
	has definition Microscopically thin, spaghetti-like objects which, according to some theories of elementary particles, could form randomly during a phase transition in the early universe. Cosmic strings could provide the seeds for structure formation in the universe, as an alternative to the possibility that the seeds originated as quantum fluctuations during inflation.
	has definition Some contemporary cosmological theories suggest that boundaries were formed between different regions of the universe at the moment of creation. These boundaries survive today as "cosmic strings", incredibly thin but very massive strings many light years in length.
	is a kind of cosmology theory
	is a kind of theoretical celestial body
cosmic year	has definition Time the Sun takes to "orbit" in galactic rotation: about 225 million years.
	has value 225 million years
	is an instance of time unit
cosmogony	has definition The study of the origin of celestial systems, especially the solar system.
	is a kind of cosmology
cosmological constant	has definition A constant introduced into Einstein's field equations of general relativity in order to provide a supplement to gravity. If positive (repulsive), it counteracts gravity, while if negative (attractive), it augments gravity. It can be interpreted physically as an energy density associated with space itself.
	has definition A contribution to gravity that results from the effective mass density, or energy density, in the vacuum. A positive cosmological constant acts as if it were negative gravity - it makes two masses repel each other instead of attract each other. Einstein's first cosmological model contained a cosmological constant, which appeared as an additional term in the equations of general relativity. (See false vacuum; vacuum.)
	has definition A parameter that determines the strength of the cosmological term in the equations of general relativity. This term was added by Einstein because he thought the universe was static, and the term provided a repulsive gravitational force that was needed to prevent the universe from collapsing under the force of ordinary gravity. The false vacuum of inflationary models creates a similar repulsive gravitational force, except that it prevails for only a brief period in the early universe. The cosmological constant is often assumed to be zero, but it might make a significant contribution to the evolution equations of our universe.
	has definition A possible third parameter in cosmology, in addition to the Hubble constant and omega (Ω). Most cosmologists believe the cosmological constant is zero, but if it is not, it would make the universe older than astronomers calculate from the Hubble constant and Ω. The size of the cosmological constant is designated by the Greek letter lambda (λ).
	has definition A term added by Einstein to the gravitational field equations of his theory of general relativity. Such a term would produce a repulsive antigravity force at very large distances and would correspond to energy locked up in the curvature of space-time itself. There is, at present, no evidence for the existence of a cosmological constant (although one may have existed in the past).
	has definition A term introduced by Einstein into his field equations of gravitation to permit a static model of the universe. It corresponded, as introduced originally, to a cosmic repulsion force that could withstand the attractive tendency of gravity.
	has definition A term sometimes employed in cosmology to express a force of "cosmic repulsion", such as the energy released by the false vacuum thought to power exponential expansion of the universe in the inflationary universe models. Whether any such thing as cosmic repulsion exists or ever played a role in cosmic history remains an open question.
	has definition Einstein's general theory of relativity allows for space-time curvature even in an empty universe. The amount of this curvature is given by the cosmological constant. Current indications are that this constant must be zero, but the reason for its vanishing remains a mystery.
	is an instance of astronomical constant
cosmological constant problem	has definition The puzzle of why the cosmological constant has a value which is either zero, or in any case roughly 120 orders of magnitude or more smaller than the value that particle theorists would expect. Particle theorists interpret the cosmological constant as a measure of the energy density of the vacuum, which they expect to be large because of the complexity of the vacuum. See vacuum.
	is an instance of problem
cosmological redshift	has anomaly A few galaxies show blueshifts, the most famous being Andromeda galaxy, but most show redshifts, due to the expansion of the universe.
	has definition The redshift due to the expansion of the Universe. Contrary to popular belief, this is not a Doppler shift. Most galaxies move away from us, but this is not the cause of their redshifts. Instead, as a light wave travels through the fabric of space, the universe expands and the light wave gets stretched and therefore redshifted.
	has discoverer Slipher
	has discovery date 1926
	has property wavelength shift is directly proportional to distance
	has symbol z
	has value the cosmological redshift as observed is the ratio of the radius of the Universe at the present epoch to the radius of the Universe at the time the radiation left the distant object.
	is a kind of wavelength shift
cosmology	has definition The study of the origin, structure, and evolution of the Universe on the largest possible scale. In present usage, it frequently includes cosmogony.
	is a kind of extragalactic astronomy
cosmology theory	has domain cosmology
	is a kind of astronomy theory
Coster-Kronig transition	has definition An Auger transition in which the vacancy is filled by an electron from a higher subshell of the same shell.
	is a kind of Auger transition
Cotton Candy nebula	has image
	has synonym IRAS 17150-3224
	is an instance of planetary nebula
coudé focus	has definition A focus used primarily for spectroscopy. In this arrangement light from the primary mirror is reflected along the polar axis to focus at a fixed place separate from the moving parts of the telescope, where large pieces of equipment can be fitted without interfering with the telescope's balance. (The word comes from a French word meaning "bent like an elbow", not from a man's name)
	has definition A stationary focal point in an equatorial mounted telescope obtained by an arrangement of small auxiliary mirrors in the converging beam which eventually directs the light down the hollow polar axle of the telescope.
	is a kind of focus
coulomb	has base unit s·A
	has definition The practical and the SI unit of charge. It is the quantity of electricity transported in one second by a current of one ampere. From 1908 to 1948 the international coulomb, derived from the international ampere, was in use. Like the other international units it was replaced by the absolute unit on 1 January 1948. The name coulomb was given to the unit at the first meeting of the IEC in Paris in 1881. At this meeting two of the five units which were given definitions were named after French scientists. These were the ampere (A. M. Ampère 1775-1836) and the coulomb (C. A. Coulomb 1736-1806). (1 international coulomb = 0.99985 absolute coulomb.)
	has definition Unit of charge.
	has equivalent 2.998 × 109 esu
	has symbol C
	is an instance of charge unit
	is an instance of named derived SI unit
	represents electric charge, quantity of electricity
Coulomb collision	has definition The collision between two charged particles.
	is a kind of collision
Coulomb's law	has definition The force between two charged particles varies directly as the size of the charges and inversely as the square of the distance between them.
	is an instance of electromagnetic law
CP violation	breaks CPT invariance
	has definition A reaction between subatomic particles is said to be a "CP violating" reaction if the reaction produces a different result when the electrical charges of the particles are changed to their opposites and the mirror image of the particle trajectories is used.
	is a kind of symmetry breaking
CPT invariance	has definition A symmetry which is believed to hold true for all particles throughout the course of universal history. It states that matter and antimatter would only react in the same way if the spins of the antimatter particles were reversed and the reaction was caused to run backwards in time.
	has definition A theory has "CPT invariance" if for every possible reaction between subatomic particles, a reaction can also occur in which the electrical charges of the particles changed to their opposites, the mirror image of the particle trajectories is used, and the directions of motion are reversed. Assuming general notions of modern physics, all conceivable theories of nature are CPT invariant.
	is a kind of symmetry
Crab Nebula	has age 947 years
	has definition A chaotic, expanding mass of gas in Taurus, the remnant of a Type I supernova whose light reached Earth in 1054. It is an intense radio source, and its visible light is strongly polarized. It is also a source of X-rays and gamma-rays. Its total mass is about 1 Msun, but the total energy radiated by the Crab is 1037-1038 ergs s-1. It is periodically occulted by the Moon, and every June its radio spectrum is occulted by the solar corona.
	has definition A supernova remnant.
	has distance 2 kpc
	has explosion date 1054 A.D.
	has image
	has Messier number 1
	has synonym 2U 0531+22
	has synonym M 1
	has synonym Taurus A
	is a part of Taurus
	is an instance of Messier object
	is an instance of supernova remnant
Crab pulsar	has definition A pulsar associated with the Crab Nebula. It has the shortest period of any known pulsar.
	has period 0.0331 seconds
	has synonym NP 0532
	is a part of Crab Nebula
	is a part of Milky Way
	is a part of Taurus
	is an instance of pulsar
Crater	has acronym Crt
	has boundary
	has genitive Crateris
	has synonym Cup
	is a part of celestial sphere
	is an instance of constellation
creationism	has definition Belief that the universe was created by God in the relatively recent past, as implied by literal interpretations of biblical chronology, and that the species of terrestrial life did not arise through Darwinian evolution but, rather, all came into existence at once.
	is a kind of religious belief
Crepe Ring	has definition Rather transparent inner ring of the saturn ring system.
	has definition The second innermost ring of Saturn, it has fewer particles and is less dense than the outer rings; therefore, it is harder to observe.
	has diameter 149300 km
	has discoverer Bond
	has discovery date 1850
	has synonym C ring
	has width 18000 to 20000 km
	is a kind of ring
	is a part of Saturn ring system
cretacious period	has duration 72 million years
	has start time 135 million years ago
	is a kind of mesozoic era
critical density	has definition If the cosmological constant is assumed to vanish, then the critical mass density is that density which puts the universe just on the border between eternal expansion (open universe) and eventual collapse (closed universe).
	has definition The cosmic density of matter required to "close" the universe and so, eventually to halt cosmic expansion. Its value amounts to about ten hydrogen atoms per cubic meter of space. The observed density is so close to the critical value that the question of whether the universe is open or closed has not yet been resolved by observation. See open universe, closed universe.
	has definition The density that just stops the expansion of space, after infinite cosmic time has elapsed. In the standard models, the critical density requires that the spatial geometry be flat.
	has definition The value of average cosmic mass density above which the universe is closed. The average mass density of the universe is obtained by measuring the mass in a very large volume of space, including many galaxies, and dividing by the size of the volume. The critical mass density is determined by the current rate of expansion of the universe. According to estimates of the current rate of expansion, the current critical mass density is about 10-29 grams per cubic centimeter. According to the best measurements, the average mass density of our universe appears to be about one tenth the critical mass density. (See closed universe; omega; open universe.)
	is a kind of density
critical equatorial velocity	has definition In rotating early-type stars, that velocity at which the ratio of centrifugal force to gravity at the equator is unity.
	is a kind of equatorial velocity
Cross-axis equatorial telescope	has mounting equatorial with cross-axis
	is a kind of equatorial telescope
crossing time	has definition The time it takes a particle to travel from one point in its orbit to another point 180° away.
	is an instance of time
crucial experiment	has definition An experiment that has the power to decide between two competing theories.
	is a kind of experiment
Crux	has acronym Cru
	has genitive Crucis
	has historical image
	has synonym Southern Cross
	is a part of celestial sphere
	is an instance of constellation
CS star	has definition Group characteristics are: strong bands of CN, outstandingly strong absorption near the Na D lines, usually sufficient structure in the 6400-6500 Å region to suggest ZrO.
	has definition Star exhibiting combined characteristics of C- and S-type stars - i.e. the presence of both C2 and ZrO bands.
	is a kind of carbon star
culmination	has definition Passage of a celestial object across the observer's meridian. More precisely, culmination is the passage through the point of greatest altitude in the diurnal path.
	has definition The instant at which a celestial object crosses the meridian.
	has period 1 day
	has synonym meridian passage
	is a kind of location dependent periodic celestial event
curie	has definition A unit of radioactivity which is now defined as the quantity of any radioactive nuclide undergoing 37.00 × 109 disintegrations per second. The unit was adopted at a Radiography Conference in Brussels in 1910 when it was defined as the radioactivity associated with the quantity of radon in equilibrium with one gram of radium. The present definition, which refers to a unit of the same size but described in terms independent of the disintegration of radon, was agreed at the Copenhagen meeting of the International Commission on Radiological Units in July 1953. The unit is named after Pierre Curie (1859-1906), one of the discoverers of radium. The curie is too large for normal laboratory work where the radioactivity is of the order of millicuries. The number of disintegrations occurring per second is called the activity of a sample and a unit for this was originally the reciprocal second but this has been superseded by the curie.
	has definition Unit of radioactivity.
	has equivalent 3.7 × 1010 becquerel
	is an instance of radioactivity unit
Curie temperature scale	has definition This is sometimes used for indicating temperature in the vicinity of absolute zero. It is based on Curie's law, which states that the susceptibility of a paramagnetic material is approximately proportional to its absolute temperature.
	has synonym magnetic temperature
	is an instance of temperature unit
curium	has image
	is a kind of transuranium element
current	has unit current unit
	is a kind of electromagnetic quantity
current density	has definition Amount of charge passing through a unit area per unit time.
	is a kind of electromagnetic quantity
current density unit	has definition ampere per square meter
	has symbol A·m-2
	is a kind of derived SI unit
current unit	is a kind of electromagnetic unit
	is a unit of current
Curtis	has birth date 1872
	has career 1893-1900 : Taught Latin & Greek, later math 1902-1920 : Lick 1920-1930 : Director, Allegheny 1931-1942 : Director, U. Michigan Observatories
	has death date January 9, 1942
	has degree 1893 : classics, U. Michigan 1902 : PhD. U. Virginia
	has name Heber Doust Curtis
	has orbituary 1942, PASP 54, 54.
	is an instance of 19th century scientist
	is an instance of astronomer
CV Serpentis	has definition A sometimes-eclipsing binary composed of a Wolf-Rayet star and a B0 star.
	has orbital period 29.6 days
	is a part of Serpens
	is an instance of eclipsing binary
	is an instance of Wolf-Rayet
cyan	is a kind of optical
cyanoacetylene	has symbol HC3N
	is an instance of interstellar molecule
cyanogen band	has definition Molecular absorption band found in the spectra of stars of type G0 and later.
	has species CN
	has use luminosity criterion; it is more pronounced in giants than in dwarfs of the same spectral type
	is a kind of interstellar band
	occurs in G0 star or later
cyanogen radical	has symbol CN
	is an instance of diatomic molecule
	is an instance of interstellar molecule
Cygnus	has acronym Cyg
	has boundary
	has genitive Cygni
	has synonym Northern Cross
	has synonym Swan
	is a part of celestial sphere
	is an instance of constellation
Cygnus A	has definition A double radio source, the third strongest radio source in the sky (after the Sun and Cas A), at one time believed to be caused by the collision of two galaxies. It has now been identified with a distant peculiar cD galaxy (z ≈ 0.056).
	has synonym 2U 1957+40
	has synonym 3C 405
	is an instance of double radio source
	is an instance of X-ray source
Cygnus Loop	has age 20000 years
	has definition A supernova remnant, consisting of a large loop of gas ejected from a star. It is 100 pc above the galactic plane. (X-ray observations give a distance of 2-3 kpc.) It is a thermal bremsstrahlung source of soft X-rays with a spectral temperature of 2 × 106 K.
	has distance 770 pc
	has image
	has synonym Cygnus X-5
	has synonym NG6 6992
	has synonym Veil Nebula
	is a part of Cygnus
	is an instance of supernova remnant
	is an instance of X-ray source
Cygnus X-1	has definition A black hole candidate in the constellation Cygnus and a source of X-rays.
	has definition An X-ray source. The visible component is the ninth-magnitude supergiant HDE 226868 (O9.7 Iab). It has rapid night-to-night variations in spectral features.
	has distance 2.5 kpc
	has eccentricity e ≈ 0.06
	has inclination i ≈ 27°
	has period 5.5998 days
	has primary mass 20 Msun
	has secondary mass > 6 Msun
	has synonym 3U 1956+35
	is a part of Cygnus
	is an instance of X-ray source
Cygnus X-2	has definition An X-ray source optically identified with an irregular variable star.
	has synonym 3U 2142+38
	is a part of Cygnus
	is an instance of X-ray source
Cygnus X-3	has definition An X-ray binary. It is also an infrared source, a cosmic ray source, and a strongly variable radio source (interstellar extinction is too high for visible light observations). It is best fitted by a model of an expanding cloud of relativistic electrons emitting synchrotron radiation around a neutron star.
	has discovery date 1966
	has distance 10 kpc
	has orbital period 4.8 hour
	has synonym 2U 2030+40
	is a part of Cygnus
	is an instance of binary star
	is an instance of X-ray source
Cyrillids	is an instance of meteor shower
D galaxy	has definition A supergiant radio galaxy (the most common type of radio galaxy) which has an elliptical nucleus surrounded by an extended envelope. Or, an optical galaxy with a very bright nucleus. In the Morgan classification, a galaxy with rotational symmetry but without pronounced spiral or elliptical structure (a dustless galaxy). In the Yerkes 1974 system a galaxy with an elliptical-like nucleus surrounded by an extensive envelope (see also R galaxy).
	is a kind of radio galaxy
D layer	has altitude > 100 km or higher
	has definition The lowest part of Earth's ionosphere.
	has radio reflectivity "broadcast" radio waves
	is a part of ionosphere
D line	has definition Spectral line of neutral sodium, also the strongest interstellar line.
	has species Na I
	is a kind of Fraunhofer line
	is a kind of interstellar line
	is a kind of interstellar line
D ring	has definition The innermost ring of Saturn.
	has discovery date 1969
	is a kind of ring
	is a part of Saturn ring system
d-electron	has definition An orbital electron whose l quantum number is 2.
	has orbital quantum number 2
	is a kind of bound electron
D1 line	has definition Spectral line of neutral sodium.
	has wavelength 5896 Å
	is a kind of D line
D2 line	has definition Spectral line of neutral sodium.
	has wavelength 5890 Å
	is a kind of D line
DA white dwarf	has definition only strong hydrogen lines present
	is a kind of white dwarf
dark cloud gravitational contraction	is a kind of gravitational contraction
dark flight	has definition phase in the flight of a meteor after the ratardation point when the meteoroid is no longer incandescent
	has duration from the point of intial contact with atmosphere until the retardation point
	has start time the point of intial contact with the atmosphere
	is a kind of meteor event
dark halo	has definition Massive, nonluminous matter of unknown kind that surrounds and envelope of a galaxy
	has definition The massive outer region of the Milky Way that surrounds the disk and stellar halo. The dark halo consists mostly of dark matter, whose form is unknown. Though it emits almost no light, the dark halo outweighs the rest of the Galaxy.
	has definition The unseen mass that is believed to surround each galaxy and whose gravitational effects are believed to hold the galaxy together.
	is a part of Milky Way
dark matter	has definition Any form of matter which exists in the Universe in a non-luminous form.
	has definition Material astronomers cannot see but whose presence they believe in either because they detect its gravitational influence or because certain theories predict its existence. For example, astronomers believe that the outer part of the Galaxy harbors dark matter, because they notice its gravitational influence on the stars they can see; and inflationary cosmologists believe that the universe is full of dark matter, because inflation predicts that the universe has a large density.
	has definition Matter in the universe that we detect by its gravitational influences, yet do not see. Dark matter that has small random speed and is easily concentrated by gravity is called cold dark matter. Dark matter that has large random speed and is thus able to resist gravitational clumping is called hot dark matter. Recent models to explain the observed pattern of galaxy clustering can be characterized, in part, as to whether they invoke hot dark matter or cold dark matter. However, since we do not know what the dark matter is, we do not have any direct evidence of whether it is cold or hot.
	has definition Matter that is detected only by its gravitational pull on visible matter. At least 90%, and possibly 99% of the matter in the universe is dark. The composition is unknown; it might consist of very low mass stars or supermassive black holes, but big-bang nucleosynthesis calculations limit the amount of such baryonic matter to a small fraction of the critical mass density. If the mass density is critical, as predicted by the simplest versions of inflation, then the bulk of the dark matter must be a gas of weakly interacting non-baryonic particles, sometimes called WIMPS (Weakly Interacting Massive Particles). Various extensions of the standard model of particle physics suggest specific candidates for the WIMPs.
	has definition Matter that is invisible to us because it emits little or no light. As much as 90%-99% of the mass of the universe may be dark.
	has definition Matter whose existence is inferred on the basis of dynamical studies - e.g., the orbits of stars, in galaxies - but which does not show up as bright objects such as stars and nebulae. Its composition is unknown: It might consist of subatomic particles, or of dim dwarf stars or black holes, or a combination of various sorts of objects.
	has definition Matter whose presence is inferred from dynamical measurements but which has no optical counterpart. The luminous regions of galaxies have mass-luminosity ratios of about 10. However, the mass-luminosity ratio in the outer halos of many spiral galaxies is 100 or more; one sees the brightness fall off with distance from the center of the galaxy but considerable mass is present. A similar situation prevails in galaxy clusters, where nonluminous matter must provide most of the self-gravitational attraction that holds the clusters together. The missing mass is not really missing; it is present but invisible (at least to current detectors). It is generally believed to consist either of the remnants of massive stars or of planetary-sized objects comparable in mass to Jupiter.
	has synonym hidden mass
	is a kind of hypothetical particle
	is a part of dark halo
dark nebula	has definition A relatively dense cloud of interstellar matter whose dust particles obscure the light from stars beyond it and give the cloud the appearance of a region devoid of stars.
	has density up to 104 particles per cm3
	is a kind of nebula
	is a part of dust
data collection	is a kind of abstraction
data standard	is a kind of abstraction
database	is a kind of data collection
day	has definition An interval of 86400.51 seconds, unless otherwise indicated.
	has symbol d
	has value 86400.51 seconds
	has value in SI unit 24 h = 86400 s
	is a kind of non SI unit
	is a kind of time unit
db galaxy	has definition One of a small number of dumbbell-shaped radio galaxies. They might be called D systems with double nuclei, in which two elliptical nuclei share a common extended envelope.
	is a kind of D galaxy
DB white dwarf	has definition only strong helium lines present
	is a kind of white dwarf
DC white dwarf	has definition continuous spectrum, no lines clearly visible
	is a kind of white dwarf
de Sitter model	has definition A geometrical model (1917) of an empty universe, based on Einstein's field equations.
	has definition A model of the universe that contains no matter but only a positive cosmological constant. It expands exponentially forever.
	has definition A particular solution to Einstein's cosmological equations, found by Wilhelm de Sitter in 1917, in which space expands at a rapid, exponential rate. This solution was very different from the solutions of Friedmann and of Lemaitre, in which the universe expands at a much slower rate (a rate with the distance between any two points increasing as something between the square root of time and linearly with time). The Friedmann and Lemaitre type solutions became incorporated in the standard big bang model. Recent modifications of the big bang model, such as the inflationary universe model, propose that the universe went through a period of exponential growth, or a de Sitter phase, early in its evolution.
	is a kind of cosmology theory
dead person	has death date
	has definition a person that has died
	has orbituary
	is a kind of person
decay process	is a kind of physical process
deceleration parameter	has definition A dimensionless quantity describing the rate at which the expansion of the Universe is slowing down because of self-gravitation: it gives a measure of the matter density. In Friedmann's equation (which describes many cosmological models) q0 = - 1 indicates a steady-state universe, q0 < +1/2 indicates an open universe, q0 = +1/2 indicates a flat Euclidean universe, and q0 > 1/2 indicates a universe that is decelerating and will eventually contract. Sandage and Tammann (1975) obtain q0 = 0.10 for H0 = 55 km s-1 Mpc-1.
	has definition A parameter (that denotes the rate of change with time of the Hubble constant.
	has definition A parameter that measures the rate of slowing down of the expansion of the universe. Gravity causes the slowing down. The deceleration parameter equals omega (another cosmological parameter) when the universe is dominated by radiation, approximately the first 100000 years after the big bang, and 1/2 omega when the universe is dominated by matter. Since the deceleration parameter is equivalent to omega (assuming a cosmological constant of zero, as often done), it determines the ultimate fate and spatial geometry of the universe. The deceleration parameter is often denoted by the symbol q0. (See omega.)
	has definition Quantity designating the rate at which the expansion of the universe is slowing down, owing to the braking effect of the galaxies' gravitational tug on one another. It is a function of the cosmic matter density.
	has symbol q0
	is an instance of astronomical constant
deci	has symbol d
	has value 10-1
	is an instance of unit prefix
declination	has definition Angular distance above (positive) or below (negative) the celestial equator. One of the co-ordinates, with right ascension, that defines the position of a heavenly body.
	has definition Angular distance north (+) or south (-) of the celestial equator to some object, measured in degrees, minutes, and seconds of arc along an hour circle passing through the object. Declination is analogous to latitude on the Earth's surface.
	has definition Angular distance on the celestial sphere north or south of the celestial equator. It is measured along the hour circle passing through the celestial object. Declination is usually given in combination with right ascension or hour angle.
	has definition Astronomical coordinate. Equivalent to latitude. The angle in degrees above or below the Celestial Equator, i.e. the projection onto the sky of the Earth's equator. Range of declination is from from zero to ± 90°.
	has definition Location on the sky in a north-south direction. Lines of declination are the celestial equivalent of latitude on Earth. Compare right ascension.
	has symbol δ
	is an instance of angle
	is an instance of equatorial coordinate component
deduction	has definition Process of reasoning in which a conclusion is derived from a given premise or premises, without a need for additional information. Compare induction.
	is a kind of reasoning process
DEep Near Infrared Survey	has acronym DENIS
	is an instance of sky survey
deformable mirror	has definition A very thin mirror whose shape can be changed by the force applied by many small pistons behind the mirror.
	is a kind of mirror
degeneracy pressure	has definition Pressure in a degenerate electron or neutron gas.
	has definition Pressure in a degenerate electron or neutron gas.
	is a kind of pressure
degenerate gas	has definition A gas of electrons (or, more generally, fermions) in which all the lowest quantum states are occupied. For such a gas, the pressure in the nonrelativistic limit is proportional to the 5/3 power of the density.
	has definition A state of matter found in white dwarfs and other extremely dense objects, in which strong deviations from classical laws of physics occur. As the density increases at a given temperature, the pressure rises more and more rapidly, until it becomes independent of temperature and dependent on density alone. At this point, the gas is said to be degenerate.
	has definition A state of matter found in white dwarfs and other ultrahigh-density objects, in which the electrons follow Fermi-Dirac statistics. According to the classical laws of physics, the pressure of a gas is proportional to the temperature and the density. However, in 1926 Fermi and Dirac showed that if the density were high enough, departures from classical laws would occur, in that if at a given temperature the density is increased, the pressure increases more and more rapidly until it becomes independent of the temperature and is a function of the density only. When this point is reached, the gas is said to be degenerate.
	has synonym Fermi gas
	is a kind of plasma
degree	has definition An angle subtended in the sky: From the zenith to the horizon is 90 degrees.
	is an instance of angle unit
degree (angle)	has symbol °
	has value in SI unit (π/180) rad
	is an instance of non SI unit
Deimos	has albedo 0.06
	has definition The outer satellite of Mars. Mariner 9 has shown that both Phobos and Deimos are locked in synchronous rotation with Mars.
	has discoverer A. Hall
	has discovery date 1897
	has eccentricity e = 0.003
	has inclination 1°.6
	has orbital period P = 1.26 days
	has radius 12 × 13 km
	is a part of Mars
	is an instance of natural satellite
deka	has symbol da
	has value 101
	is an instance of unit prefix
delay time	has definition Time lapse between the time a signal (e.g., a radar beam) is propagated out to a distant object and the time it is received after the object bounces it back.
	is an instance of time
Delphinus	has acronym Del
	has boundary
	has genitive Delphini
	has synonym Dolphin
	is a part of celestial sphere
	is an instance of constellation
delta	has definition Short-lived baryon.
	is an instance of hyperon
Delta Aquarid	has duration 8 days
	has radiant Delta Aquarii
	has rate 30 per hour
	has start time 29 July
	is an instance of meteor shower
Delta Capricorni	has B-V magnitude 0.29
	has declination -16 07 38
	has right ascension 21 47 02.3
	has spectral type Amv
	has synonym HR 8322
	has V magnitude 2.87
	is a part of Capricornus
	is an instance of A star
	is an instance of naked eye star
Delta Cephei	has definition A pulsating star in the constellation Cepheus. It was the second Cepheid discovered and lent its name to the entire class of stars.
	is a part of Cepheus
	is an instance of Population I Cepheid
Delta Crateris	has B-V magnitude 1.12
	has declination -14 46 43
	has right ascension 11 19 20.4
	has spectral type G8III-IV
	has synonym HR 4382
	has V magnitude 3.56
	is a part of Crater
	is an instance of G star
	is an instance of giant
	is an instance of naked eye star
Delta Del star	has definition A type of late A-type and early F-type star with very weak Ca II HK lines.
	is a kind of star
Delta Scuti	has B-V magnitude 0.35
	has declination -09 03 09
	has right ascension 18 42 16.3
	has spectral type F2IIIp<04>Del
	has synonym HR 7020
	has V magnitude 4.72
	is a part of Scutum
	is an instance of F star
	is an instance of giant
	is an instance of naked eye star
delta-ray	has definition A recoil electron ejected from an atom by an energetic charged particle. Delta-rays appear as branches in the main track of a cloud chamber.
	is a kind of free electron
Demeter	has definition Unofficial name for Jupiter X.
	has discoverer Nicholson
	has discovery date 1938
	has eccentricity e = 0.12
	has inclination i = 29°
	has orbital period P = 259.2 days
	is a part of Jupiter
	is an instance of natural satellite
Deneb	has B-V magnitude 0.09
	has declination +45 16 49
	has definition An A2 Ia supergiant at the head of the Northern Cross. Most distant first-magnitude star.
	has distance 430 pc
	has right ascension 20 41 25.8
	has spectral type A2Iae
	has synonym alpha Cyg
	has synonym HR 7924
	has V magnitude 1.25
	is a part of Cygnus
	is an instance of naked eye star
	is an instance of white supergiant
density	has definition An object's mass divided by its volume. Cotton has a low density; lead has a high density. Red giants have a low density; white dwarfs have a high density.
	has definition The amount per unit volume
	has definition The mean density of a celestial body is generally reckoned as its mass divided by its volume, expressed either in comparison with the density of water, in kilograms per cubic meter, or in relation to some other known density. The mean density of the Earth is thus 5.5 times that of water, i.e. 5.5 × 103 kg m-3 and is just less than four times that of the Sun. Yet the mean density of rocks at the surface is about half the overall mean value, and that of the Earth's central core is perhaps 2 1/2 times the overall value.
	is a kind of quantity
density wave theory	has author Lindblad (1925)
	has definition Spiral structure is modelled as a small-amplitude wave propagating with fixed angular velocity, as the compression wave goes through, it triggers star formation on the leading edge of the spiral arms.
	has implication the density waves rotate more slowly than the galaxy's stars and gas
	is a kind of galaxy theory
derived SI unit	has base unit defined in terms of the seven SI base units via a system of quantity equations
	has unit defined in terms of derived SI unit via a system of quantity equations
	is a kind of SI unit
descending node	has definition The point in the orbit of a solar-system body where the body crosses the ecliptic from north to south.
	is an instance of Sun orbital event
DESI	has definition The German national laboratory for high-energy physics, located near Hamburg. It is the home of the e+e- storage rings DORIS and PETRA, and the electron-proton machine, HERA.
	has location Hamburg, Germany
	is an instance of high energy physics institution
detached binary	has definition Binary star which is not in contact and in which no significant mass exchange is occurring.
	is a kind of binary star
detector	is a kind of optical device
determinism	has definition The doctrine that all events are the predictable effects of prior causes.
	is a kind of doctrine
deuterium	has atomic mass 2
	has crystal cell dimension a = 338, c = 560 pm for tetragonal
	has crystal cell dimension a = 360.0, c = 585.8 pm for h.c.p.
	has crystal cell dimension a = 509.2 pm for cubic
	has crystal type cubic for cubic
	has crystal type h.c.p. for h.c.p.
	has crystal type tetragonal for tetragonal
	has definition A rare heavy isotope of hydrogen. Believed to be the first compound nucleus formed in the infant universe. It was discovered in interstellar space in 1965. Because deuterium is quickly destroyed in nuclear reactions, one view is that most of the deuterium in the universe is primordial.
	has interstellar space D H ratio 1.4 × 10-5
	has magnetogyric ratio 4.1064 × 107 rad T-1 s-1
	has natural abundance 0.015 %
	has NMR frequency 15.351 MHz where 1H = 100 MHz; 2.3488 T
	has NMR receptivity 8.2 × 10-313C = 1.00
	has nuclear magnetic moment μ = 0.8574376
	has nuclear quadrupole moment 2.860 × 10-31 m2
	has nuclear spin I = 1
	has number of neutrons 1
	has number of nucleons 2
	has oceanic D H ratio 1.6 × 10-4
	has relative NMR sensitivity 9.65 × 10-3 where 1H = 1.00
	has solar D H ratio < 10-6
	has space group Fm3m for cubic
	has space group I4 for tetragonal
	has space group P63/mmc for h.c.p.
	has symbol 2H
	has symbol D
	has synonym hydrogen 2
	has synonym hydrogen 2
	has uses Nuclear Magnetic Resonance, water containing deuterium instead of ordinary hydrogen is used as a moderator in nuclear reactors
	is an instance of hydrogen
deuteron	has charge 1
	has definition The nucleus of a deuterium atom. md = 2.01355 amu.
	has definition The nucleus of deuterium, an isotope of hydrogen. It consists of one proton and one neutron bound together.
	has symbol d
	is an instance of charged particle
	is an instance of nucleus
deuteron magnetic moment	applies to particle deuteron
	has symbol μd
	has uncertainty 0.000000018 × 10-26 J T-1
	has value 0.433073457 × 10-26 J T-1
	is an instance of particle magnetic moment
deuteron mass	applies to particle deuteron
	has symbol md
	has uncertainty 0.00000026 × 10-27 kg
	has value 3.34358309 × 10-27 kg
	is an instance of particle mass
device	has definition an instrumentality invented for a particular purpose
	is a kind of artifact
devonian period	has duration 60 million years
	has start time 405 million years ago
	is a kind of paleozoic era
diatomic molecule	has decompostion temperature < 10000 K
	has definition a molecule containing only two atoms which can be of different elements
	is a kind of molecule
dielectronic recombination	has inverse process autoionization
	is a kind of recombination
differential rotation	has definition As an object rotates, different parts of it may move at different rates. The Galaxy rotates differentially.
	has definition Of a stellar cluster or galaxy, the "orbiting" of stars nearer the center faster than those at the edge. Of a single body (such as the Sun or a gaseous planet), the axial rotation of equatorial latitudes faster than polar latitudes.
	is a kind of rotation
diffraction	has definition A property which distinguishes wave-like motions. When a wave is incident upon a barrier which is broken by a narrow slit (of comparable size to the wavelength), then the slit will act as a new isotopic source of secondary waves.
	has definition A wave-like property of light which allows it to curl around obstacles whose size is about that of the wavelength of the light. The disturbed waves then interfere to produce ripple-patterns.
	has definition The bending of light in passing a sharp edge or tiny aperture.
	is a kind of radiation direction modification
diffraction grating	has definition A system of parallel slits, where the slit width is of the same order as the wavelength of the incident radiation, which is capable of dispersing light into its spectrum.
	has definition An optical device containing thousands of very fine parallel grooves which produce interference patterns in a way which separates out all the components of the light into a spectrum.
	has definition Polished metallic surface (usually a metallic mirror on a block of glass or quartz) on which has been ruled a great number (in thousands) of parallel lines, used to split light to produce a spectrum.
	is a kind of spectrograph dispersing element
diffraction grating spectrograph	is a kind of spectrograph
dimethyl ether	has symbol (CH3)2O
	is an instance of interstellar molecule
Dione	has definition Fifth satellite of Saturn
	has discoverer Cassini
	has discovery date 1684
	has orbital period P = 4.5 days
	has radius 440 km
	is a part of Saturn
	is an instance of natural satellite
direct motion	has definition For orbital motion in the solar system, motion that is counterclockwise in the orbit as seen from the north pole of the ecliptic; for an object observed on the celestial sphere, motion that is from west to east, resulting from the relative motion of the object and the Earth.
	has definition Motion of a solar-system body from west to east across the sky.
	is a kind of motion
disk	has definition The central plane of a spiral galaxy, as distinguished from the halo or the nucleus.
	has definition The plate-shaped component of a spiral galaxy, in which the spiral arms are found.
	is a part of Milky Way
dispersion	has definition Resolution of white light into its component wavelengths, either by refraction or by diffraction.
	has definition The separation of a beam of light into the individual wavelengths of which it is composed by means of refraction or diffraction.
	is a kind of radiation direction modification
dissociative recombination	has definition Capture of an electron by a positive molecular ion, wherein part of the recombination energy dissociates the molecule into two neutral atoms.
	is a kind of recombination
distance	has definition The distance to a celestial object measured from the center of the Earth or Sun, in units of parsec or lightyear
	is an instance of equatorial coordinate component
	is an instance of length
diurnal motion	has definition The apparent daily motion of celestial bodies across the sky from east to west, caused by the Earth's rotation.
	has definition The apparent westward motion of celestial bodies, as seen from Earth, due to the Earth's axial rotation.
	is a kind of motion
dKe star	has definition K dwarf with hydrogen emission lines.
	is a kind of K star
	is a kind of UV Ceti star
dMe star	has definition M dwarf with hydrogen emission lines.
	is a kind of M star
	is a kind of UV Ceti star
DMSP satellite	is a kind of military spacecraft
DNA replication	is a kind of biochemical process
DO white dwarf	has definition both He and H lines present
	is a kind of white dwarf
doctrine	is a kind of belief
Doppler broadening	has cause thermal, turbulent, or mass motions of atoms along the line of sight
	has definition Line broadening caused by the thermal, turbulent, or mass motions of atoms along the line of sight.
	has effect small displacements of radiation absorbed or emitted by atoms toward longer and shorter wavelengths result in broadening of the lines
	is a kind of line broadening
Doppler shift	has definition Change in the apparent wavelength of radiation (e.g., light or sound) emitted by a moving body. A star moving away from the observer will appear to be radiating light at a lower frequency than if at rest; consequently, lines in the star's spectrum will be shifted toward the red (lower frequency) end of the spectrum. The existence of a direct relationship between the redshift of light from galaxies and their distances is the fundamental evidence for the expansion of the universe.
	has definition Displacement of spectral lines in the radiation received from a source due to its relative motion along the line of sight. A motion of approach results in a blueshift; a motion of recession results in a redshift.
	has definition Displacement of spectral lines in the radiation received from a source due to its relative motion in the line of sight. Sources approaching (-) the observer are shifted toward the blue; those receding (+), toward the red. The Doppler shift makes it possible to determine the radial velocity and the rotation of stars.
	has definition Effect on the wavelengths of light (or sound) emitted by a source at a distance that is increasing or decreasing in relation to the observer. If the distance is increasing, the wavelengths are "stretched" (the light received shifts towards the red end of the spectrum; sound received goes down in pitch). If the distance is increasing, the wavelengths are "squeezed" (the light received shifts towards the blue end of the spectrum; sound received goes up in pitch).
	has definition The alteration in frequency of electromagnetic radiation due to relative motion between the source and observer.
	has definition The alteration in frequency of electromagnetic radiation due to relative motion between the source and observer.
	has definition The apparent change of frequency or wavelength of radiation from an object due to its motion toward or away from us. If the object is receding the frequency is decreased and the wavelength is increased, i.e. becomes red-shifted.
	has definition The blueshift or redshift produced by an object's motion toward or away from us. If a star moves toward us, its light waves get compressed and its spectrum is blueshifted; if a star moves away from us, its light waves get stretched and its spectrum is redshifted. The Doppler shift allows astronomers to measure the radial velocities of stars. The Doppler shift is not responsible for the redshifts that most galaxies exhibit; that is a cosmological redshift.
	has definition The change in frequency of a wave (light, sound, etc.) due to the relative motion of source and receiver.
	has definition The shift in the received frequency and wavelength of a sound wave or electromagnetic wave that occurs when either the source or the observer are in motion. Approach causes a shift toward shorter wavelengths and higher frequencies, called a blueshift. Recession has the opposite effect, called a redshift.
	has value the amount of shift is directly proportional to relative velocity
	is a kind of wavelength shift
Dorado	has acronym Dor
	has genitive Doradus
	has historical image
	has synonym Goldfish
	has synonym mahi-mahi (a Hawaiian fish)
	has synonym Swordfish
	is a part of celestial sphere
	is an instance of constellation named by Bayer
double line spectroscopic binary	has definition a spectroscopic binary in which periodic Doppler shift is detected in both components of the binary star each out of phase by 180 degrees
	is a kind of spectroscopic binary
	can be used as a distance measuring technique
double radio source	has definition A radio galaxy, the bulk of whose radio emission comes from two sources on opposite sides of the visual galaxy. The radiation is presumably the result of an explosion in the nucleus of the parent galaxy, which caused the ejection at high speed of energetic particles in two opposite directions. About one-third of all known radio galaxies are double sources.
	is a kind of radio galaxy
double star	has definition A "system" of two stars that appear - because of coincidental alignment when viewed from Earth - to be close together; it is, however, an optical effect only, and therefore not the same as a binary star system (although until the twentieth century there were few means of distinguishing double and binary stars).
	is a kind of star
doublet	has definition In a spectrum, a pair of associated lines arising from transitions having a common lower energy level.
	is a kind of multiplet
down	has charge -1/3
	has definition A flavor of quark.
	is an instance of light quark
DQ Herculis	has definition A slow nova which also happens to be an eclipsing binary. It also has a regular flickering period of 71 seconds, the shortest period of regular variations known, except for pulsars and compact X-ray objects. It is probably composed of an M dwarf and a white dwarf with an accretion disk.
	has erruption date 1934
	has orbital period 4h39m
	has primary star white dwarf
	has secondary star M dwarf
	has synonym Nova Herculis 1934
	is an instance of eclipsing binary
	is an instance of slow nova
DQ white dwarf	has definition carbon lines (atomic or molecular) present.
	is a kind of white dwarf
Draco	has acronym Dra
	has boundary
	has genitive Draconis
	has synonym Dragon
	is a part of celestial sphere
	is an instance of constellation
Draco galaxy	has definition A dwarf elliptical galaxy, the intrinsically faintest (Mv = - 8.5) member of the Local Group (next to And I-III).
	has diameter 1 kpc
	has distance 80 kpc
	has distance from galaxy center 250000 light-years
	is a part of Draco
	is a part of Local Group
	is an instance of dwarf elliptical satellite galaxy
	orbits Milky Way
Draconids	is an instance of meteor shower
dubnium	is a kind of transactinide
DUMAND	is an acronym for Deep Underwater Muon and Neutrino Detector.
	is an instance of neutrino telescope
Dumbbell Nebula	has definition A planetary nebula of large apparent diameter and low surface brightness.
	has distance 220 pc
	has Messier number 27
	has synonym M 27
	has synonym NGC 6853
	is a part of Vulpecula
	is an instance of Messier object
	is an instance of planetary nebula
dust	has definition The dust component of a galaxy, includes dark cloud
	is a part of disk
dust tail	has charge little or none
	has definition Curved part of comet tail composed of dust driven by radiation pressure
	has synonym type II comet tail
	is a part of comet tail
dwarf	has definition Star that is on the main sequence - that is, a star fusing hydrogen into helium at its core.
	has definition Star with mass equal to or less than that of the sun. More generally, any star on or below the main sequence in the Hertzprung-Russell diagram.
	has definition Star, like the Sun, that fuses hydrogen into helium at its core. Ninety percent of all stars are main-sequence stars; examples are Sirius, Vega, Altair, and Alpha Centauri A, B, and C.
	has luminosity class V
	has symbol d
	has synonym main sequence star
	is a kind of star
dwarf Cepheid	has definition A group of pulsating variable stars with small variations in amplitude. They lie in the lower part of the Cepheid instability strip.
	has definition Type I Cepheids ( <Mv> +4 to +2).
	has period 1 to 3 hours
	has spectral type A star to F star
	has synonym delta Scuti star
	has synonym ultrashort-period Cepheid
	is a kind of Cepheid
	is a kind of Delta Del star
dwarf elliptical galaxy	has mass > 107 Msun (among the least massive galaxies)
	is a kind of elliptical galaxy
dwarf elliptical satellite galaxy	is a kind of satellite galaxy
dwarf galaxy	has definition A galaxy with low luminosity.
	has definition A small, faint galaxy, exemplified by those that orbit the Milky Way: Ursa Minor, Draco, Sculptor, Sextans, Carina, Fornax, Leo II, and Leo I.
	is a kind of galaxy
dwarf nova	has definition A short-period binary system consisting of a hot white dwarf (or a hot blue sdBe subdwarf) and a much cooler and slightly more massive late-type main-sequence companion which fills its Roche lobe and is ejecting mass onto the white dwarf through its inner Lagrangian point. (The light from dwarf novae comes from four sources: a white dwarf, a cool main-sequence star, a hot spot, and a disk.) The outbursts are usually assumed to be caused by the explosive nuclear burning of hydrogen-rich material accreted onto the surface of a degenerate star.
	has definition Cataclysmic variable in which the brightness increases suddenly at intervals ranging from several days to years.
	is a kind of nova
dynamical equinox	has definition The ascending node of the Earth's mean orbit on the Earth's equator; i.e., the intersection of the ecliptic with the celestial equator at which the Sun's declination is changing from south to north.
	is an instance of ascending node
	see also catalog equinox
	see also equinox
dynamical parallax	has definition The "parallax" (i.e., distance) for a binary star whose orbit is well known, derived by using the mass-luminosity relation and Newton's generalization of Kepler's third law.
	is a kind of parallax
dynamics	has definition Study, in physics, of the motion and equilibrium of systems under the influence of force.
	has definition The physics that explains how particles and systems move under the influence of forces. The dynamical laws of a theory give a quantitative statement of the response of a particle to an applied force.
	is a kind of physics
dyne	has definition A unit of force equal to the force necessary to give an acceleration of 1 cm sec-2 to a mass of 1 gram. 1 dyne of force is roughly equivalent to 1 mg of weight.
	has definition The force necessary to cause a mass of one gram to accelerate at one centimeter per second per second.
	is an instance of CGS unit
	is an instance of force unit
dysprosium	has image
	has ocean oxidation state III
	has ocean residence time 300 years
	is a kind of rare Earth
	is a kind of recycled oceanic element
DZ white dwarf	has definition no He nor H lines, but metallic lines present
	is a kind of white dwarf
E layer	has altitude 150 km
	has definition Former name for the D and E layers (q.v.).
	has definition The part of Earth's ionosphere where the temperature gradient reverses and starts to rise.
	has radio reflectivity "short-wave" radio waves
	has synonym Kennelly-Heaviside layer
	is a part of ionosphere
E line	has definition Spectral line consisting of a blend of Fe I and Ca I.
	has species Fe I and Ca I
	has wavelength 5270 Å
	is a kind of Fraunhofer line
E ring	is a kind of ring
	is a part of Saturn ring system
e-process	has definition A hypothetical group of nuclear reactions by which the iron group is assumed to be synthesized. At temperatures > 5 × 109 K and densities > 3 × 106 g cm-3 there are great numbers of collisions between high-energy photons and nuclei. These collisions break up the nuclei, the fragments of which promptly combine with other particles. Thus, there is in effect an equilibrium between formation and breakup. Since the iron group has the largest binding energies, the particles over the long run will tend to be trapped in these nuclei. The e-process (the e stands for equilibrium) is presumed to occur in a supernova explosion.
	is a kind of nucleosynthetic reaction
Eagle Nebula	has distance 7000 light years
	has image
	has synonym IC 4703
	is a part of Serpens
	is an instance of gaseous nebula
early star	has definition Hot star of spectral types O, B, A, and early F.
	has spectral type O, B, A, and early F
	is a kind of star
Earth	has age 4.6 ± 0.1 × 109 years
	has albedo 0.39 (water and land about 0.2; snow and clouds about 0.8)
	has aphelion distance 152100000 km
	has atmosphere composition 78% N2; 20.9% O2; 0.9% Ar, 0.03% C02 (by volume)
	has atmosphere density 1.3 × 10-3 g cm-3 (at sea level)
	has atmospheric pressure 1.013 × 106 dyn cm-2 (at sea level)
	has core density 10 g cm-3
	has core temperature 6400 K
	has definition Third planet from the Sun. First forms of life appeared about 3.2 to 3.5 × 109 years ago (Homo sapiens appeared as a species about 105 years ago).
	has eccentricity e = 0.0167
	has ecliptic inclination i = 0
	has effective temperature 287 K
	has equatorial radius 6378.17 km
	has escape velocity Vesc = 11.19 km s-1
	has magnetic field at surface 0.5 gauss
	has magnetic field in core 100 gauss
	has mass 5.977 × 1027 g
	has mean density 5.517 g cm-3
	has mean distance from Sun 149598500 km
	has obliquity 23°26'34" (in 1973)
	has orbital period 365.2564 days
	has orbital velocity vorb = 29.78 km s-1
	has perihelion distance 147100000 km in early January
	has polar radius 6356.9 km
	has precession 50'.256 per year
	has relativistic advance of perihelion 4'.6 per century
	has rotation period 23 hours 56 minutes 4.1 seconds
	has surface gravity 980 cm s-2
	is a part of our solar system
	is an instance of naked eye planet
	is an instance of terrestrial planet
Earth based telescope	has altitude height above sea level in meters
	has definition A telescope located on the surface of the Earth.
	has latitude in degrees, minutes, seconds (N or S)
	has longitude in degrees, minutes, seconds (E or W)
	has mounting designed to operate within a 1 g gravitational field
	has mounting manufacturer the person, company or institution that constructed the mounting
	has temperature ambient
	is a kind of electromagnetic telescope
Earth orbital event	has participants satellite, Earth
	is a kind of orbital event
Earth's lithosphere	is a part of Earth
Earth-crossing asteroid	is a kind of asteroid
	is a part of asteroid belt
east point	has azimuth 90
	has definition The point on the celestial horizon 90° clockwise from the north point. At the equinoxes the Sun rises in the east point.
	has elevation 0
	is an instance of local coordinate
EBS	has definition Electron Bombarded Silicon.
	is a kind of detector
eccentricity	has definition A measure of how round or elliptical an orbit is. A perfect circle has an eccentricity of 0 percent, and an extremely elliptical orbit has an eccentricity of just under 100 percent. The Sun has an orbital eccentricity of 6 percent, which means that at perigalacticon the Sun is 6 percent closer to the Galactic center than its mean distance and at apogalacticon the Sun is 6 percent farther from the Galactic center than its mean distance.
	has definition A parameter that specifies the shape of a conic section; one of the standard elements used to describe an elliptic orbit. (See elements, orbital.)
	has definition An orbital element representing the eccentricity of the elliptical orbit
	has definition In astronomy, the extent to which an elliptical orbit departs from a circular one. It is usually expressed as a decimal fraction, regarding a circle as having an eccentricity of 0.
	has definition The amount by which the elliptical orbit deviates from circularity: e = c/a, where c is the distance from the center to a focus and a is the semimajor axis.
	has symbol e
	is an instance of orbital element
eccentrics	has definition In Ptolemaic cosmology, displacement of the center of a rotating celestial sphere from the center of the universe.
	is a kind of outdated belief
echelle	has definition A type of diffraction grating with groove angles of 90°. With the grating at an angle of 45° the grooves resemble a staircase.
	is a kind of diffraction grating
eclipse	has definition Obscuration of one astronomical object (such as the sun) by another such object (such as the moon).
	has definition Occultation of one celestial body by another which passes between it and the observer. The solar eclipse is caused by the passing of the Moon between the Sun and the Earth in this way; such an eclipse may be complete (total) or incomplete (partial). Eclipsing binary stars also accord with this pattern. Alternatively - and exceptionally - a lunar eclipse is caused by the passage of the Earth between the Sun and the Moon, so that the Earth's shadow falls across the Moon, again either totally or partially, depending upon the position of the observer.
	has definition The obscuration of a celestial body caused by its passage through the shadow cast by another body.
	has definition The total or partial obscuration of the light from a celestial body caused by its passage into the shadow of another body (cf. occultation).
	is a kind of occultation
eclipse year	has definition The interval of time between two successive passages of the Sun through the same node of the Moon's orbit.
	has value 346.62 days
	is an instance of year
eclipsing binary	has definition A binary star of which, from the viewpoint of Earth, one of the two bodies regularly passes in front of the other. The resulting variation is perceived luminosity of some eclipsing binaries has led to their classification as variable stars.
	has definition A double star in which at least one of the two stars passes in front of and/or behind the other so that the system's total light periodically fades.
	has definition Eclipsing variables whose orbital plane lies so nearly in the line of sight that eclipses, as seen from the Earth, can occur and can be detected from their light curves.
	has eclipse duration
	is a kind of close binary
	is a kind of periodic variable
ecliptic	has definition Apparent linear path through the 12 constellations of the zodiac that the Sun seems to take during one Earth year, also representing therefore the "edge" of the plane of Earth's orbit. Because the equator of the Earth is at an angle of more than 22° to the plane of its orbit, the ecliptic is at the identical angle to the celestial equator, intersecting it at two points: the vernal and autumnal equinoxes.
	has definition Plane of the Earth's orbit. (Strictly speaking, the ecliptic is a mathematical fiction corresponding not to the actual plane of the Earth's orbit, but to one with all minor irregularities smoothed out.)
	has definition The mean plane of the Earth's orbit around the Sun.
	is a kind of plane
ecliptic coordinate	has celestial latitude
	has celestial longitude
	has component ecliptic coordinate component
	has definition A system of coordinates based on the plane defined by Earth's orbit around the Sun, which is inclined to the celestial equator.
	is a kind of coordinate
ecliptic coordinate component	is a component of ecliptic coordinate
	is a kind of coordinate component
Eddington approximation	has definition An approximation used in the study of radiative transfer. It is the assumption that the ratio of the second moment of the radiation field to the mean intensity is everywhere equal to 1/3, the value of this ratio for an isotropic field.
	is a kind of stellar theory
Eddington limit	has definition In essence, radiation pressure must not exceed gravity. It is the limit beyond which the radiation force on matter in the emitting region is greater than the gravitational forces that hold the star together. LE = 4πcGM/Ks, where Ks = Thomson and/or Compton scattering opacity. Eddington limit for a 1 Msun star, 1038 ergs s-1.
	is an instance of mass
Eddington's standard model	has definition A stellar model in which energy is transported by radiation throughout the whole star and the ratio of the radiation pressure to the gas pressure is assumed to be constant.
	is a kind of stellar theory
Eddington-Lemaitre universe	has definition A cosmological model in which the cosmological constant plays a crucial role by allowing an initial phase that is identical to the Einstein static universe. After an arbitrarily long time, the universe begins to expand. The difficulty with this model is that the initiation of galaxy formation may actually cause a collapse rather than initiate an expansion of the universe.
	is a kind of cosmology theory
effective temperature	has definition The temperature that a blackbody would have which emitted the same amount of energy per unit area as the star does: it is a temperature characteristic of the surface region. Teff of Sun, 5800 K.
	has symbol Teff
	is a kind of temperature
Egg Nebula	has distance 3000 light years
	has expansion rate 20 km/s
	has image
	has synonym CRL 2688
	is an instance of planetary nebula
EHF band	has frequency 30 to 300 GHz
	has wavelength 1 mm to 1 cm
	is a kind of microwave
Einstein	is an instance of grazing-incidence telescope
einsteinium	is a kind of transuranium element
elastic collision	has definition A collision between two particles which conserves the total kinetic energy and momentum of the system. For atomic collisions it is one involving energy less than the excitation potential of the atom.
	has definition Particle reactions in which the same particles emerge from the reaction as entered it (e.g. π- p → π- p). In inelastic scattering, where different and/or new particles emerge, energy is used to create new particles.
	is a kind of collision
electric constant	has symbol ε0
	has uncertainty 0
	has value 8.854187817 × 10-12 F m-1
	is an instance of universal constant
electric potential	has unit electric potential unit
	is a kind of electromagnetic quantity
electric potential unit	is a kind of electromagnetic unit
electrodynamics	has definition Study of the behavior of electromagnetic force in motion.
	is a kind of dynamics
electromagnetic constant	is a kind of fundamental physical constant
electromagnetic gauge symmetry	has definition Gauge symmetry underlying quantum electrodynamics.
	is a kind of gauge symmetry
electromagnetic law	has domain electromagnetism
	is a kind of law
electromagnetic quantity	is a kind of quantity
	is a kind of quantity
electromagnetic telescope	has definition telescope designed to collect and record electromagnetic radiation
	is a kind of telescope
electromagnetic unit	is a kind of unit
electromagnetism	has carrier boson photon
	has definition Force field of the electromagnetic force, consisting of electric and magnetic lines of force at each point in space.
	has definition Fundamental force of nature that acts on all electrically charged particles. Classical electromagnetics is based on Maxwell's and Faraday's equations, quantum electromagnetics on the theory of quantum electrodynamics (QED).
	has definition One of the four fundamental forces of nature, governing the electric and magnetic interaction between particles.
	has definition One of the four fundamental forces of nature. Electricity and magnetism arise from the electromagnetic force. The other three fundamental forces are the gravitational force, the weak nuclear force, and the strong nuclear force.
	has definition The force between charged particles, which accounts for electricity and magnetism. One of the four fundamental forces of nature, it is carried by photons and is responsible for all observed macroscopic forces, except for gravity.
	has definition The phenomena associated with electrical and magnetic forces. Electrical and magnetic forces are intimately related, since a changing electric field produces a magnetic field, and vice versa. Electromagnetic waves are an example of electromagnetism.
	has range infinite
	has strength 1 (relative to electromagnetism)
	is a kind of fundamental force
electron	has antiparticle positron
	has charge -1
	has definition A lepton with an electric charge of -1. An electron is also a fermion because it has a spin of one half.
	has definition A negatively charged spin-1/2 particle, which interacts via the electromagnetic, weak and gravitational forces. It has a mass of 0.511 MeV / c2, some 1800 times lighter than the proton.
	has definition A small, negatively charged particle that appears in every neutral atom, surrounding the positively charged nucleus like bees around honey.
	has definition A stable, negatively charged elementary particle - the lightest massive particle known. The classical electron radius is 2.82 × 10-13 cm; me = 9.1 × 10-28 g = 5.48597 × 10-4 amu. The electron family (see lepton) includes the electron e-, the positron e+, the electron neutrino νe, and the electron antineutrino νebar. Rest-mass energy of electron 8.186 × 10-7 ergs. Electron charge = 1.60219 × 10-19 coulombs.
	has definition Light elementary particle with a negative electrical charge. Electrons are found in shells surrounding the nuclei of atoms; their interactions with the electrons of neighboring atoms create the chemical bonds that link atoms together as molecules.
	has definition Negatively charged fundamental particle (also called a beta particle) found in the atoms of all elements, where it "orbits" (at different energy levels and with different directions of spin) round the central nucleus. The combined charge of the orbiting electrons is balanced (in a neutral atom) by the charge of an equal number of positively charged protons in the atomic nucleus. An electron is also the fundamental unit of electricity.
	is a kind of charged particle
	is a kind of lepton
	is a part of element
electron bombarded charge-coupled device	has acronym EBCCD
	has definition An imaging device containing a thin target material which emits electrons by the photoelectric effect when illuminated and then magnetically focuses these electrons to impact onto a silicon CCD where they generate a large charge.
	is a kind of charge-coupled device
electron Compton wavelength	applies to particle electron
	has equation
	has symbol λC
	has uncertainty 0.000000018 × 10-12 m
	has value 2.426310215 × 10-12 m
	is an instance of particle Compton wavelength
electron g factor	applies to particle electron
	has equation
	has symbol ge
	has uncertainty 0.0000000000082
	has value -2.0023193043737
	is an instance of particle g factor
electron gyromagnetic ratio	applies to particle electron
	has equation
	has symbol γe
	has uncertainty 0.000000071 × 1011 s-1 T-1
	has value 1.760859794 × 1011 s-1 T-1
	is an instance of particle g factor
electron magnetic moment	applies to particle electron
	has symbol μe
	has uncertainty 0.000037 × 10-26 J T-1
	has value -928.476362 × 10-26 J T-1
	is an instance of particle magnetic moment
electron magnetic moment anomaly	applies to particle electron
	has equation
	has symbol ae
	has uncertainty 0.0000000041 × 10-3
	has value 1.1596521869 × 10-3
	is an instance of particle magnetic moment anomaly
electron mass	applies to particle electron
	has symbol me
	has uncertainty 0.00000072 × 10-31 kg
	has value 9.10938188 × 10-31 kg
	is an instance of particle mass
electron temperature	has definition The temperature that appears in the Maxwell distribution of velocities for electrons.
	is a kind of temperature
electron volt	has definition Unit employed to indicate the energy of a charged particle in terms of the energy received by the charge on an electron due to a potential difference of one volt. An approximate value (1 in 104) for the energy of electromagnetic radiation expressed in electron volts is given by 1234 / λ, where λ is the wavelength in nanometres. In recent years it has become customary to write MeV and GeV for mega (106) and giga (109) electron volts. In the USA 109 electron volts are often written as BeV, the letter B being used in this case as an abbreviation for the American billion (109), but in 1948 the International Union of Pure and Applied Physics disapproved of the use of BeV and expressed a preference for GeV or 109 eV. The electron volt was called the equivalent volt when it was originally introduced in 1912.
	has definition Unit of energy used in atomic and nuclear physics; the kinetic energy acquired by one electron in passing through a potential difference of 1 volt in vacuum. Sometimes used as a unit of mass (see rest-mass energy)
	has equivalent 160.219 × 10-21 joule
	has symbol eV
	has symbol eV
	is an instance of energy unit
	is an instance of energy unit
	is an instance of non SI unit
	is equivalent to 1.6 × 10-12erg
	is equivalent to 1.6 × 10-19 joules
electrostatic unit	has definition Unit of charge defined as the charge which exerts a force of 1 dyne on a charge of equal magnitude at a distance of 1 cm.
	has symbol esu
	is an instance of charge unit
electroweak	has carrier boson vector boson
	has definition A unification of electromagnetism and the weak nuclear force.
	has definition Responsible for certain kinds of radioactivity; for example, the disintegration of a neutron into a proton, electron, and antineutrino.
	has definition The combination of the electromagnetic force and the weak nuclear force which takes place at high energy.
	has definition The theory that unifies the electromagnetic force and the weak nuclear force into a single force. This theory was developed in the 1960s by Sheldon Glashow, Steven Weinberg, and Abdus Salam and has been subsequently confirmed in the laboratory. One of the mathematical properties of this theory is called the electroweak symmetry.
	has definition The unified description of the weak interactions and electromagnetism, developed between 1967 and 1970 by Sheldon Glashow, Steven Weinberg, and Abdus Salam.
	has definition Theory demonstrating links between the electromagnetic and the weak nuclear forces. Indicates that in the high energies that characterized the very early universe, electromagnetism and the weak force functioned as a single, electroweak force. Also known as the Weinberg-Salam theory.
	has range
	has strength (relative to electromagnetism)
	has synonym weak force
	is a kind of fundamental force
element	has definition Different elements are distinguished by the number of protons in their nuclei. All hydrogen atoms have one proton; all helium atoms have two protons; all oxygen atoms have eight protons.
	has definition The building block of matter. The nucleus of an atom consists of one or more protons and may contain neutrons as well; any electrons surround the nucleus. The number of protons in the atom - the atomic number - determines the element.
	has definition The fundamental unit of a chemical element. An atom consists of a nucleus, which may contain protons and neutrons, and electrons, which occupy shells that surround the nucleus and are centered on it.
	has definition The smallest component of matter that retains its chemical properties. An atom consists of a nucleus and at least one electron.
	has synonym atom
	is a kind of particle
	is a part of Universe
element 110	is a kind of transactinide
element 111	is a kind of transactinide
element 112	is a kind of transactinide
elementary charge	has symbol e
	has uncertainty 0.000000063 × 10-19 C
	has value 1.602176462 × 10-19 C
	is an instance of charge
	is an instance of electromagnetic constant
elementary particle	has definition A particle considered to be fundamental, i.e. not composite, a particle which cannot be separated into components
	has spin
	is a kind of particle
elementary particle symmetry	has definition Abstract mathematical relationships that relate elementary particles together and allow them to be grouped into families. A particular symmetry transformation has the effect of, in a theoretical way, transforming one elementary particle into another.
	is a kind of symmetry
	is a kind of unitary transformation
elevation	has definition Angular distance above the horizon.
	has definition The angle in degrees above the horizon toward the zenith or overhead point. Sometimes loosely called the "altitude" of a star, but not to be confused with height above sea level. Elevation angle is 90° minus the zenith distance (or zenith angle).
	has definition The angular distance of a celestial body above or below the horizon, measured along the great circle passing through the body and the zenith. Elevation is 90 deg. minus zenith distance.
	has synonym altitude
	is an instance of angle
	is an instance of local coordinate component
eleven-dimensional supergravity	has definition Promising higher-dimensional supergravity theory developed in the 1970s, subsequently ignored, and more recently shown to be an important part of string theory.
	is a kind of supergravity
ELF band	has frequency 300 Hz to 3 kHz
	has wavelength 100 km to 1000 km
	is a kind of radio
ellipse	has definition A plane curve in which the sum of the distances of each point along its periphery from two points - its "foci" - are equal.
	is a kind of conic section
elliptical galaxy	has classification criteria E0 (spherical) to E7 (greatest eccentricity)
	has definition A galaxy with an ellipsoidal shape, without spiral arms. Ellipticals have little interstellar matter and no blue giants - the only giants are red, and they give ellipticals a slightly redder color than spirals. Ellipticals apparently produce only Type I supernovae.
	is a kind of galaxy
elliptical orbit	has amplitude equal to the semimajor axis
	has parameters orbital element
	has period proportional to the orbital period
	is a kind of ellipse
	is a kind of harmonic motion
	is a kind of orbit
elongation	has elongation
	is a kind of angle
emersion	has definition The reappearance of a celestial body after eclipse or occultation.
	is a kind of occultation phase
	is preceeded by transit
emission	has definition The process of transition of an electron from an outer orbit to an inner orbit around the nucleus results in a characteristic amount of energy being radiated (as line emission) that corresponds to the lost energy of the electron.
	has inverse process absorption
	is a kind of radiation intensity modification
emission line	has antonym absorption line
	has definition Bright line superposed on a continuous spectrum. Can be produced in the laboratory by a glowing gas under low pressure.
	has definition Bright lines produced in a spectrum by a luminous source, such as a star or a bright nebula.
	is a kind of spectral line
emission line galaxy	has emission lines
	is a kind of galaxy
emission line star	has emission line
	is a kind of star
empiricism	has definition An emphasis on sense data as a source of knowledge, in opposition to the rationalist belief that reasoning is superior to experience.
	is a kind of doctrine
Enceladus	has definition Third satellite of Saturn
	has diameter 500 km
	has discoverer Herschel
	has discovery date 1789
	has orbital period 1.37 days
	is a part of Saturn
	is an instance of natural satellite
Encke's comet	has definition The comet with the shortest known period (3.30 years) (a = 2.21 AU, e = 0.847, i = 12°.4). It has been observed at every apparition since its discovery in 1819. Its period is gradually decreasing. Named after J. F. Encke, who computed its orbit. (It was discovered by Pons.)
	is a part of our solar system
	is an instance of comet
Encke's division	has definition A region of decreased brightness in the outermost ring of Saturn.
	has definition Gap within Saturn's A ring.
	is a kind of ring gap
	is a part of Saturn ring system
endothermic fusion process	has definition a fusion process which requires energy
	is a kind of energy sink
	is a kind of fusion
energy	has unit energy unit
	is a kind of quantity
energy level	has definition Allowed energy state for electrons orbiting the nucleus of an atom.
	has definition Any of the several discrete states of energy in which an atom or ion can exist. For example, an orbital electron can exist only in those energy levels that correspond to an integral number of deBroglie wavelengths in a Bohr atom.
	is a kind of energy
energy sink	is a kind of physical phenomena
energy source	is a kind of physical phenomena
energy unit	has definition (1) The capacity to do work. (2) Manifestation of a particular variety of force.
	is a kind of unit
	is a unit of energy
English equatorial telescope	has mounting English equatorial
	is a kind of equatorial telescope
enthalphy	has definition The heat content of a body. H = U + pV, where U is the internal energy, p is the pressure, and V is the volume.
	has symbol H
	is a kind of energy
eocene epoch	has duration 22 million years
	has start time 58 million years ago
	is a kind of tertiary period
ephemeris second	has definition The length of a tropical second (1/31556925.97474 of the tropical year) on 1900 January 0.5 ephemeris time.
	has value 1/31556925.97474 tropical year
	is an instance of second unit
epicycle	has definition A means of accounting for the apparent motions of the planets in terms of circular motions in a geocentric cosmology. Each planet moves in a circle, the center of which moves in a circle of larger radius, and so on, the largest circles being centered on the earth.
	has definition Circular orbit of a body round a point that is itself in a circular orbit round a parent body. Such a system was formulated to explain some planetary orbits in the Solar System before they were known to be elliptical.
	has definition In Ptolemaic cosmology a circular orbit around a point that itself orbits another point.
	is a kind of outdated belief
epoch	has definition A date and time that specifies the reference system to which celestial coordinates are referred. Prior to 1984 coordinates of star catalogs were commonly referred to the mean equator and equinox of the beginning of a Besselian year. Beginning with 1984 the Julian year has been used, as denoted by the prefix J, e.g., J2000.0.
	has definition A point of time selected as a fixed reference.
	has definition An arbitrary fixed instant of time or date used as a chronological reference datum for calendars (see calendar), celestial reference systems, star catalogs, or orbital motions (see orbit).
	has definition An orbital element representing the time of perihelion passage.
	has symbol T
	is an instance of orbital element
	is an instance of time
epsilon Aurigae	has definition An eclipsing binary with an invisible supergiant companion. The primary is an extremely luminous A8 Ia supergiant of 30 Msun in a post-main-sequence stage of evolution; the secondary may be a collapsed star or black hole. It has at least six components.
	has distance 1 kpc
	has orbital period 27 years
	has primary A8 Ia
	is a part of Auriga
	is an instance of eclipsing binary
	is an instance of white supergiant
Epsilon Eri	has apparent magnitude 4
	has definition In 1973 van de Kamp announced that this star has a planet-like object in orbit around it at a distance of about 8 AU and with a period of about 25 years.
	has distance 3.30 pc
	has spectral type K2 V
	is a part of Eridanus
	is an instance of dwarf
	is an instance of K star
Epsilon Eridani	has definition A young orange dwarf star that is visible to the naked eye.
	has distance 10.7 light-years
	is a part of Eridanus
	is an instance of dwarf
Epsilon Indi	has definition An old orange dwarf star in the southern constellation Indus.
	has distance 11.2 light-years
	is an instance of dwarf
Epsilon Pegasi	has B-V magnitude 1.53
	has declination +9 52 30
	has right ascension 21 44 11.1
	has spectral type K2Ib
	has synonym HR 8308
	has V magnitude 2.39
	is a part of Pegasus
	is an instance of K star
	is an instance of naked eye star
	is an instance of supergiant
Epsilon Sagittarii	has B-V magnitude -0.03
	has declination -34 23 5
	has right ascension 18 24 10.3
	has spectral type B9.5III
	has synonym HR 6879
	has V magnitude 1.85
	is a part of Sagittarius
	is an instance of B star
	is an instance of giant
	is an instance of naked eye star
Epsilon Ursae Majoris	has B-V magnitude -0.02
	has declination +55 57 35
	has right ascension 12 54 1.7
	has spectral type A0pCr
	has synonym HR 4905
	has V magnitude 1.77
	is a part of Ursa Major
	is an instance of A star
	is an instance of naked eye star
equator	has definition The great circle on the surface of a body formed by the intersection of the surface with the plane passing through the center of the body perpendicular to the axis of rotation. (See celestial equator.)
	is a kind of plane
equatorial coordinate	has component equatorial coordinate component
	has declination
	has right ascension
	is a kind of coordinate
equatorial coordinate component	is a component of equatorial coordinate
	is a kind of coordinate component
equatorial sky area	has definition A fixed region on the sky defined using equatorial coordinates
	is a kind of quantity
	is defined using equatorial coordinates
equatorial telescope	has definition The classic type of telescope mount with one axis parallel to the Earth's polar axis (i.e. pointing at the celestial pole) and the other at right angles. Once the object is located, only the polar axis need be driven by a motor to counteract the Earth's rotation.
	has mounting equatorial
	is a kind of Earth based telescope
equatorial velocity	has definition The velocity at the surface of a rotating body, on its equator
	is a kind of velocity
equinox	has definition Either of the two points on the celestial sphere at which the ecliptic intersects the celestial equator; also the time at which the Sun passes through either of these intersection points; i.e., when the apparent longitude (see apparent place; celestial longitude) of the Sun is 0° or 180°. (See catalog equinox; dynamical equinox for precise usage.)
	has definition Either of the two points on the celestial sphere where the celestial equator intersects the ecliptic.
	has definition One of two points in the sky that represent where the Sun appears to cross the plane of the Earth's equator. From the Earth's viewpoint therefore, the Sun reaches one point at a quarter, the other at three quarters of the way through the sidereal year: the vernal (spring) equinox is thus on or around 21 March, the autumnal on or around 22 September. The actual points in the sky change slightly every year through a process called precession.
	has period 1 year
	is a kind of periodic celestial event
equivalence principle	has definition Core principle of general relativity declaring the indistinguishability of accelerated motion and immersion in a gravitational field (over small enough regions of observation). Generalizes the principle of relativity by showing that all observers, regardless of their state of motion, can claim to be at rest, so long as they acknowledge the presence of a suitable gravitational field.
	has definition In a freely falling and nonrotating laboratory the laws of physics, including their numerical content, are the same everywhere including gravity-free space.
	has definition States that inertial mass is indistinguishable from gravitational mass.
	has definition The principle that it is impossible to distinguish between gravitational and inertial forces; gravitational mass is precisely equal to inertial mass.
	has definition The statement that a gravitational force is completely equivalent in all of its physical effects to an overall acceleration in the opposite direction. For example, a person in an elevator in space accelerating upward at 32 feet per second per second would feel the floor pushing upward against her feet in exactly the same way as if the elevator were at rest on earth, where gravity pulls downward with an acceleration of 32 feet per second per second. The "weak equivalence principle," which is not as strong as the equivalence principle, states that all objects, independent of their mass or composition, fall with the same acceleration in the presence of gravity. The Eötvös experiment, and later refinements of this experiment, have proven the weak equivalence principle.
	is an instance of gravity law
Equuleus	has acronym Equ
	has boundary
	has genitive Equulei
	has synonym Little Horse
	is a part of celestial sphere
	is an instance of constellation
era	has definition A system of chronological notation reckoned from a given date.
	has duration
	has start time
	is a kind of event
erbium	has image
	has ocean oxidation state III
	has ocean residence time 400 years
	is a kind of rare Earth
	is a kind of recycled oceanic element
erg	has definition Unit of energy; the work done by a force of 1 dyne acting over a distance of 1 cm.
	has equivalent 1 g cm2 s-2
	has equivalent 10-7joules
	has name origin Greek verb 'to work'
	has proposal date 1873
	has proposer British Association
	has synonym dyne cm
	is an instance of CGS unit
	is an instance of energy unit
ergodic motion	has definition Motion by one or more particles which fills phase space with uniform density after a sufficiently long time.
	is a kind of mass motion
Eridanus	has acronym Eri
	has boundary
	has genitive Eridani
	has synonym River Eridanus
	is a part of celestial sphere
	is an instance of constellation
Eros	has asteroid number 433
	has closest approach to Earth < 0.15 AU
	has definition A small Asteroid.
	has diameter 35 × 16 × 17 km
	has discoverer G. Witt
	has discovery date 1898
	has eccentricity e = 0.223
	has inclination i = 10°.8
	has orbital period 642 days
	has perihelion distance 1.084 AU
	has rotation period 5h16m12s.913
	has semi-major axis a = 1.48 AU
	has shape elongated
	is a part of asteroid belt
	is an instance of asteroid
erruptive prominence	has definition A violent prominence which may reach heights of 2 million km
	has height < 2 million km
	is a kind of prominence
ESA satellite	is a kind of artificial satellite
escape velocity	has definition Speed a satellite must attain in order to free itself from returning to the parent body under the effects of gravity.
	has definition The speed at which an object can leave another object behind, without being recalled by its gravitational force. The escape velocity of Earth - which must, for instance, be attained by a spacecraft if it is to reach another planet - is 25,000 miles per hour.
	has definition The velocity that a body requires to achieve a parabolic orbit around its primary (Ve = sqrt(2GM/R)). Escape velocity at Earth's surface is 11.2 km s-1; of Moon, 2.4 km s-1; of Sun, 617.7 km s-1 (cf. orbital velocity).
	is a kind of velocity
ESO 1-meter Schmidt Telescope	has altitude 2318 m
	has aperture 1.00 m
	has creation date 1972
	has focal ratio f/3.06
	has latitude 29° 15' S
	has location La Silla, Chile
	has longitude 70° 44' W
	has mirror diameter 1.6 m
	has mirror maker Zeiss (Ober.)
	has mirror type Schott Duran 50
	has mounting manufacturer Heidenreich & Harbeck
	has owner European Southern Observatory
	has synonym ESO Schmidt
	is an instance of Schmidt
ESO 3.6-meter Telescope	has altitude 2387 m
	has aperture 3.57 m
	has creation date 1977
	has focal ratio f/3.0, 8.1, 32
	has latitude 29° 16' S
	has location La Silla, Chile
	has longitude 70° 44' W
	has mirror maker REOSC
	has mirror type Fused-silica
	has mounting manufacturer Creusot-Loire
	has owner European Southern Observatory
	has synonym ESO 3.6 m
	is an instance of Horseshoe equatorial telescope
	is an instance of optical telescope
	is an instance of reflector
Eta Aquarid	has duration 6 days
	has parent object Comet Halley
	has radiant Eta Aquarii
	has rate 60 per hour
	has start time 4 May
	is an instance of meteor shower
Eta Aquilae	has definition A pulsating star in the constellation Aquila. It was the first Cepheid discovered, in 1784.
	has discovery date 1784
	is a part of Aquila
	is an instance of Population I Cepheid
eta Carinae	is an instance of bright blue variable
Eta Piscium	has B-V magnitude 0.97
	has declination +15 20 45
	has right ascension 1 31 28.9
	has spectral type G7IIIa
	has synonym HR 437
	has V magnitude 3.62
	is a part of Pisces
	is an instance of G star
	is an instance of giant
	is an instance of naked eye star
etalon	has definition Essentially an optical filter that operates by multiple-beam interference of light reflected and transmitted by a pair of parallel flat reflecting plates.
	is a kind of Fabry-Perot interferometer
ethyl alcohol	has symbol C2H5OH
	is an instance of interstellar molecule
ethynyl	is an instance of interstellar molecule
EUMETSAT satellite	has definition meteorological satellite launched and maintained by Europe's Meteorological Satellite Organization
	is a kind of meteorological satellite
Europa	has definition One of the Galilean satellites of Jupiter
	has diameter 3600 km
	has eccentricity e = 0.00
	has inclination i = 0°.01
	has mean density 3.07 g cm-3
	has orbital period 3.55 days -has source: [H76]
	has synonym Jupiter II
	is a part of Jupiter
	is an instance of Galilean satellite
European Southern Observatory	has acronym ESO
	is an instance of observatory
European Space Agency	has acronym ESA
	is an instance of space science institution
European Synchrotron Research Facility	has acronym ESRF
	is an instance of high energy physics institution
europium	has image
	has ocean oxidation state III
	has ocean residence time 500 years
	is a kind of rare Earth
	is a kind of recycled oceanic element
evaporation	has final phase gas
	has initial phase liquid
	has inverse process condensation
	is a kind of first order phase transition
even-odd nucleus	has definition Nucleus that contain even numbers of protons but odd numbers of neutrons.
	is a kind of fermion
	is a kind of nucleus
event	has definition A point in four-dimensional spacetime referenced by three spatial coordinates and a complementary temporal ordinate.
	has duration
	has synonym world point
	has time of occurrence
	is a kind of physical phenomena
Evershed effect	has definition The radial motion outward (from the central umbra) of the gases in the penumbral regions of sunspots.
	is a kind of solar motion
exa	has symbol E
	has value 1018
	is an instance of unit prefix
excitation potential	has definition Amount of energy required to bring an electron from its ground state to a given excited state (measured in electron volts).
	is a kind of energy
exclusion principle	applies to fermion
	has definition Two identical fermions cannot occupy the same quantum state (i.e. cannot have the same charge, spin, momentum, quantum numbers etc. within the same region of space).
	has example No two electrons in an atom can have the same set of values for the four quantum numbers n, l, mb, ms.
	has implication solid objects cannot exist in the same physical space.
	has synonym Pauli exclusion principle
	is an instance of quantum law
exothermic fusion process	has definition A fusion reaction which linerates energy.
	is a kind of energy source
	is a kind of fusion
	liberates energy
expanding arm	has definition A spiral arm of neutral hydrogen receding from the galactic nucleus at about 135 km s-1.
	has distance from galaxy center between 2.5 and 4 kpc
	is a part of disk
expanding emission nebula	has composition enhanced with heavy elements relative to the interstellar medium
	has expansion velocity
	has explosion date
	has progenitor star
	is a kind of gaseous nebula
expansion	is a kind of mass motion
experiment	is a kind of theory related concept
Explorer 22	is an instance of inflatable spacecraft
explosive galaxy formation	has definition A theory of galaxy formation wherein the explosion of a large number of stars creates a giant shock wave that travels outward and compresses the surrounding gas. Galaxies form in the regions of high-density gas.
	is a kind of galaxy theory
explosive nucleosynthesis	has definition The nucleosynthesis processes that are believed to occur in supernovae. Explosive carbon burning occurs at a temperature of about 2 × 109 K and produces the nuclei from neon to silicon. Explosive oxygen burning occurs near 4 × 109 K and produces nuclei between silicon and calcium in atomic weight. At higher temperatures, still heavier nuclei, up to and beyond iron, are produced.
	has definition The nucleosynthetic processes which are thought to occur in supernovae. These explosive processes are thought to produce the nuclei from neon up to and including the e-process nuclei and possibly the r-process nuclei. Explosive carbon burning occurs for a temperature of about 2 × 109 K and a density of 104-107 g cm-3 and produces nuclei from neon to silicon. Explosive oxygen burning occurs for a temperature of about 4 × 109 K and produces nuclei from silicon to calcium, and the e-process occurs at a temperature greater than 5 × 109 K and produces the iron peak nuclei.
	is a kind of stellar nucleosynthesis
exponential expansion	has definition An expansion described by a fixed doubling time. The size doubles after one doubling time, quadruples after two doubling times, octuples after three doubling times, etc.
	has definition Extremely rapid expansion. "Exponential" is a mathematical term that precisely defines the rate of expansion. For example, a balloon that doubles its size every second is expanding exponentially. By contrast, a balloon whose radius is one inch after one second, two inches after two seconds, three inches after three seconds, and so on, is expanding linearly with time, rather than exponentially. According to the inflationary universe model, the early universe went through a brief period of exponential expansion, during which its size increased enormously.
	is a kind of expansion
extinction level event	has average interval time 100 million years
	has definition impact event which can cause the extinction of many species on a planet
	is a kind of impact event
extragalactic astronomy	has definition The field that deals with objects beyond the Milky Way, especially galaxies and quasars.
	is a kind of astronomy
extragalactic globular cluster	is a kind of globular cluster
extremal black hole	has definition Black hole endowed with the maximal amount of force charge possible for a given total mass.
	is a kind of black hole
extreme ultraviolet	has acronym EUV
	is a kind of ultraviolet
extremely red galaxy	has acronym ERG
	is a kind of infrared galaxy
F corona	has definition The outer part of the solar corona which emits a continuous spectrum in which absorption lines can be seen. The F corona is caused by radiation from the photosphere scattered by interplanetary dust, and it decreases slowly with distance from the Sun. (the F stands for Fraunhofer)
	has synonym F component
	is a part of corona
F layer	has definition One of two layers in the Earth's ionosphere immediately above the E layer.
	has synonym Appleton layer
	is a part of ionosphere
F region	has definition Region of the ionosphere above the F layers.
	is a part of ionosphere
F star	has color yellow-white
	has definition Star of spectral type F in which lines of hydrogen and Ca II are of about equal strength. Metal lines also become noticeable.
	has surface temperature 6000 to 7500 K
	is a kind of early star
f-electron	has definition An orbital electron whose l quantum number is 3.
	has orbital quantum number 3
	is a kind of bound electron
f-spot	is a kind of sunspot
F1 layer	has altitude 200 km
	is a part of F layer
F2 layer	has altitude 300 km
	is a part of F layer
Fabry-Perot interferometer	has acronym FP
	has definition A device that measures distances and changes of distance very accurately, using the pattern of overlap of waves of light.
	has definition A type of interferometer wherein the beam of light is passed through a series of pairs of partly reflecting surfaces set at various angles to it and spaced at certain prechosen numbers of the wavelength to be examined. It differs from the Michelson interferometer in that it has only one arm.
	is a kind of spectrograph dispersing element
faculae	has definition Bright areas on the face of the Sun, commonly in the vicinity of sunspots. Named by Johannes Hevelius, they are thought to be caused by luminous hydrogen clouds close to the photosphere.
	has definition Bright regions of the photosphere seen in white light, visible only near the limb of the Sun.
	has lifetime 15 days
	is a part of photosphere
Fahrenheit	has absolute zero -459.67
	has definition A temperature scale based on three fixed temperature points - the temperature of an ice and salt mixture, the freezing point of water and normal human temperature - which were taken to be 0, 32 and 96 respectively. It is mere coincidence that the temperature interval between the freezing (32 °F) and boiling (212 °F) points of water is 180° when expressed in the Fahrenheit scale.
	has proposal date 1710
	has triple point of water 32.018
	is an instance of temperature unit
	is named after G. D. Fahrenheit (1686-1736)
faint blue galaxy	has definition A distant, irregularly shaped galaxy in which a large amount of star formation is taking place.
	is a kind of irregular galaxy
far infrared	has acronym FIR
	has source atomic transitions, rotational transitions in molecules
	is a kind of infrared
farad	has base unit m-2·kg-1·s4·A2
	has symbol F
	has unit C·V-1
	is an instance of capacitance unit
	is an instance of named derived SI unit
Faraday constant	has equation
	has symbol F
	has uncertainty 0.0039 C mol-1
	has value 96485.3415 C mol-1
	is an instance of physico chemical constant
Fechner's law	has definition The intensity of a sensation increases as the logarithm of the stimulus. (See Pogson's ratio.)
	is an instance of law
femto	has symbol f
	has value 10-15
	is an instance of unit prefix
fermi	has approval date 1956
	has definition A unit of distance used for describing nuclear distances just as the ångström is used for atomic distances.
	has definition A unit of length.
	has equivalent 10-13 cm
	has equivalent 10-15 m
	is an instance of length unit
	is named after Enrico Fermi
Fermi coupling constant	has equation
	has symbol Gf/(hc)3
	has uncertainty 0.00001 × 10-5 GeV-2
	has value 1.16639 × 10-5 GeV-2
	is an instance of atomic constant
Fermi level	has definition The maximum energy of any particle in a group of low-temperature subatomic particles called fermions. Fermions, such as electrons, cannot occupy the same space at the same energy. Thus, if many fermions are placed close together, their energies must all be different. The energy of that particle with the largest energy is the Fermi energy of the system.
	is a kind of energy
Fermi National Accelerator Laboratory	has acronym Fermilab
	has definition Home of the Tevatron, the world's most powerful accelerator, a p p bar collider with a maximum collision energy of 1.8 Te V (= 1800 Ge V = 1.8 × 1012 eV).
	has location Batavia, Illinois, USA
	is an instance of high energy physics institution
Fermi-Dirac nuclei	has definition Nuclei of odd A-number (i.e., nuclei that do not have integral spin) (cf. Bose-Einstein nuclei). Fermi-Dirac nuclei therefore obey the exclusion principle (q.v.).
	is a kind of nucleus
Fermi-Dirac-Sommerfeld law	has acronym FDS
	has definition A law which gives the algebraic number of a quantized system of particles which have velocities within a small range.
	is an instance of quantum law
fermion	has definition A particle, or pattern of string vibration, with half a whole odd number amount of spin; typically a matter particle.
	has definition An elementary particle whose spin is a half-integral multiple of h/2π. Fermions include the baryons, the leptons and their antiparticles, and obey the Pauli exclusion principle (cf. boson).
	has definition Any particle with half-integer spin: 1/2 h bar, 3/2 h bar, 5/2 h bar, etc. All fermions obey Pauli's exclusion principle.
	has definition Elementary particle with fractional spin. The proton, electron, neutron, and other elementary particles are all fermions.
	has definition Particle with half-integral spin. Fermions obey the exclusion principle, which says that no two fermions can exist in an atom in the same quantum state; in practice this restricts the number of electrons, which are fermions, permitted in each electron shell.
	has quantum behavior Fermi-Dirac statistics
	has spin half-integral
	is a kind of particle
	obeys exclusion principle
fermium	is a kind of transuranium element
fibrille	has definition Striation or streak which is observed to form whirls in the solar chromosphere.
	is a part of chromosphere
field galaxy	has definition An isolated galaxy which does not belong to any cluster of galaxies. The ratio of galaxies in clusters to field galaxies is about 23:1.
	is a kind of galaxy
field horizontal branch star	has definition High-velocity metal-weak star of either B or A spectral type.
	has spectral type B star or A star
	is a kind of Population II star
field lens	has definition A lens placed in or near the focal plane of a telescope to create an image of the primary mirror inside the instrument.
	is a kind of lens
field star	has definition A star that is not part of any star cluster. Most stars, including the Sun, are field stars.
	has definition Star distributed at random in space and not belonging to any particular star cluster.
	is a kind of star
filament	has definition A prominence seen in projection on the solar disk.
	is a kind of prominence
fine structure	has definition Splitting of spectral lines by the spin-orbit energy - i.e., the potential energy of the inherent electron magnetic moment in the atom's own magnetic field.
	is a kind of set of lines
fine-structure constant	has definition A "coupling constant," e2 / h bar c, approximately 1/137, that measures the strength of an interaction between a charged particle and the electromagnetic field. It gives a rough measure of the relative importance of relativistic and spin effects in the spectra of atoms.
	has definition A coupling constant e2 / h bar c, approximately equal to 1/137 (where e is the electron charge, h bar is Plancks constant, and c is the speed of light), that measures the strength of the interaction between a charged particle and the electromagnetic field.
	has definition A parameter that measures the strength of the electromagnetic force. The fine-structure constant is a combination of other fundamental constants of nature - the electrical charge of the electron, the speed of light, and Planck's constant of quantum mechanics.
	has equation
	has symbol α
	has symbol α
	has uncertainty 0.000000027 × 10-3
	has value 7.297352533 × 10-3
	is an instance of atomic constant
fine-tuning	has definition A phrase meaning a highly constrained and implausible adjustment of the parameters of a theory.
	is a kind of theory related concept
fireball	is a kind of meteor event
FIRST	has definition Far-InfraRed Space Telescope.
	is an instance of infrared space telescope
first law of thermodynamics	has definition The law of conservation of energy
	is an instance of thermodynamics law
first order phase transition	has definition A phase transition which occurs in a manner similar to the way water boils. Bubbles of the new phase (steam) form in the midst of the old phase (water), so that temporarily the two distinct phases (steam and water) coexist.
	has final phase
	has initial phase
	is a kind of phase transition
first point of Aries	is an instance of ecliptic coordinate
first radiation constant	has equation
	has symbol c1
	has uncertainty 0.00000029 × 10-16 W m2
	has value 3.74177107 × 10-16 W m2
	is an instance of physico chemical constant
fission	has antonym fusion
	has definition In nuclear physics, the splitting of the atomic nucleus of a heavy element, resulting in the emission of nuclear energy and possibly causing a chain reaction (with similar results) within a mass of the element.
	has definition Interaction in which nucleons previously united in an atomic nuclei are disjoined, releasing energy. Fission powers atomic bombs.
	is a kind of nuclear process
fission of stable elements lighter than iron	is a kind of energy sink
five-minute oscillations	has definition Vertical oscillations of the solar atmosphere with a well-defined period of 5 minutes.
	is a kind of harmonic motion
	is a kind of solar motion
fixed mount telescope	has mounting fixed direction
	is a kind of Earth based telescope
fixed vertical mount telescope	has mounting fixed vertical
	is a kind of fixed mount telescope
flare star	has definition A member of a class of dwarf stars (usually dM3e-dM6e) that show sudden, intense outbursts of energy. The flares are usually rare and very short with mean amplitudes of about 0.5-0.6 mag. All known flare stars are intrinsically faint and have emission lines of H I and Ca II. It is generally believed that flares in flare stars have certain properties in common: rapid rise to peak light followed initially by a rapid decline and later by a slower phase that occasionally does not return to a preflare level within practical monitoring times (several hours). An increase in radio emission is often detected simultaneously with the optical outburst. About 30 flare stars are known, all within 20 pc. (In at least one theory, the flare star stage directly follows the T Tauri stage.)
	has definition A star that emits flares, which can outshine the entire star. Many red dwarfs are flare stars.
	has definition Dim red dwarf star that suddenly lights up with great - but brief - luminosity, corresponding to an equally powerful but short-lived burst of radio emission. The cause is thought to be a sudden and intense outburst of radiation on or above the star's surface.
	has definition Stars undergoing erratic jumps in brightness (up to a few magnitudes) on time scales of the order of minutes. During the quiescent phase the spectrum is that of an M dwarf with emissions in the CaII and Balmer lines.
	has synonym UV Ceti star
	is a kind of cataclysmic variable
	is a kind of radio star
	is a kind of red dwarf
flat universe	has definition A cosmological model in which a static (neither expanding nor collapsing) universe is maintained by introducing a cosmological repulsion force (in the form of the cosmological constant) to counterbalance the gravitational force.
	has definition A Friedmann model of the Universe in which the spacetime continuum is not curved.
	has definition A homogeneous world model (the simplest relativistic model) of finite density, zero curvature, and nonzero cosmological constant, subject to the field equations of general relativity in an expanding Euclidean space. The radius increases rapidly from zero, and, although it always increases, the rate of increase becomes less as time goes on.
	has definition A homogeneous, isotropic universe is called flat if it is just on the borderline between being spatially closed and spatially open, so the geometry is precisely Euclidean. If Einstein's cosmological constant is zero, then a flat universe will go on expanding forever, but the velocity of recession between any two objects would approach zero at large times.
	has definition A particular solution to Einstein's cosmological equations in which the universe is flat. (See flat universe.)
	has definition A Universe in which there is no curvature to the spacetime continuum. This means that the kinetic energy of the expansion is exactly balanced by the potential gravitational energy of the matter. Thus, after an infinite amount of time the Universe will stop expanding.
	has definition A universe that is at the boundary between an open and closed universe. In a flat universe, the average mass density always has precisely the critical value. A flat universe has zero total energy and an infinite size. Flat universes have the geometry of an infinite, flat surface, that is, Euclidean geometry. The value of omega is 1 for a flat universe. (See closed universe; critical mass density; Euclidean geometry; omega; open universe.)
	has definition A world model of a static universe with a positive cosmological constant, whose radius of curvature is constant and independent of time.
	has definition Big Bang model that was formulated by Friedmann and Lemaitre which has a zero curvature, or flat (Euclidian). This space is unbounded.
	has definition The flat (k = 0), pressureless standard model of the universe.
	has definition The Friedmann-Lemaitre model in which space is Euclidean was advocated by Einstein and de Sitter in 1932.
	has geometry of space flat or Euclidian
	has synonym Einstein-de Sitter universe
	is a kind of big bang
flatness problem	has definition A problem of the traditional big bang theory (without inflation) related to the precision required for the initial value of omega, the ratio of the actual mass density to the critical mass density. If the description is started at one second after the big bang, for example, omega must have been equal to one to an accuracy of fifteen decimal places, or else the resulting universe would not resemble our own. Yet the traditional big bang theory offers no explanation for this special value, which must be incorporated as an arbitrary postulate about the initial conditions. See also horizon problem.
	has definition Poses the question: why, out of an infinite number of possibilities, is our Universe so close to the one special case: the "flat" Universe?
	has definition The puzzle of why the universe today is so close to the boundary between open and closed, that is, why it is almost flat. Equivalently, why should the average mass density today be so close to the critical mass density, but not exactly equal to it? If omega begins bigger than 1, it should get bigger and bigger as time goes on; if it begins smaller than 1, it should get smaller and smaller. For omega to be near 0.1 today, about 10 billion years after the big bang, it had to be extraordinarily close to 1 when the universe was a second old. Some people consider such a fine balance to have been highly unlikely according to the standard big bang model, and thus are puzzled as to why the universe today is almost flat. (See closed universe; critical mass density; flat universe; open universe.)
	has definition The riddle of why the universe is neither dramatically open nor closed, but appears to be almost perfectly balanced between these states.
	is a kind of problem
flavor	has definition Designation of quark types - up down, strange, charmed, top, and bottom. Flavor determines how the weak nuclear force influences quarks.
	has definition In particle physics, another word for "type". For example, there are 6 flavors of quarks, meaning that there are 6 different types of quarks.
	has definition The known quarks exist in six different types, or flavors: up, down, charmed, strange, top, and bottom. The up and down quarks belong to the first generation, the charmed and strange quarks belong to the second, and the top and bottom quarks belong to the third. The up, charmed, and top quarks each have an electrical charge 2/3 that of a proton, while the down, strange, and bottom quarks have a charge -1/3 that of a proton. See Table 7.1 on page 120.
	has definition The term used to describe different quark types. There are six quark flavors: up, down, strange, charm, bottom and top.
	is an instance of quantum quantity
Flexible image transport system	has acronym FITS
	has definition A method for saving image data which has become standard in astronomy.
	is an instance of data standard
fluorescence	has definition The absorption of a photon of one wavelength and reemission of one or more photons at longer wavelengths, especially the transformation of ultraviolet radiation into visible light.
	has definition The emission of light at one wavelength, the green say, following absorption of light with a much shorter wavelength such as the ultraviolet. The UV photon parts with its energy by ejecting the electron into a high-energy level from which it cascades back down, releasing photons of lower energy and therefore longer wavelengths. A material which has this property is called a phosphor.
	is a kind of bound-bound transition
	is a kind of emission
fluorine	has abundance 0.4 × 10-4 p.p.m. in deep Pacific seawater
	has abundance 0.96 × 10-4 p.p.m. in deep Atlantic seawater
	has abundance 1.0 × 10-4 p.p.m. in Atlantic surface seawater
	has abundance 1.0 × 10-4 p.p.m. in Pacific surface seawater
	has abundance 3.63 × 104 in Sun relative to H = 1 × 1012
	has abundance 950 p.p.m. in Earth's crust
	has atomic emission line 690.248 nm for F I
	has atomic emission line 703.747 nm for F I
	has atomic emission line 712.789 nm for F I
	has atomic emission line 685.603 nm for F I (strong)
	has atomic number 9
	has atomic radii 70.9 pm
	has biological role essential in trace quantities for mammals, including humans, in the form of fluoride (F-)
	has boiling point 85.01 K
	has chief source fluorite
	has covalent radii 58 pm
	has critical pressure 5573 kPa
	has critical temperature 144.3 K
	has crystal cell dimension a = 550, b = 328, c = 728 pm, β = 102.17 deg for α-F2
	has crystal cell dimension a = 667 pm for β-F2
	has crystal type cubic for β-F2
	has crystal type monoclinic for α-F2
	has daily dietary intake 0.3 - 0.5 mg
	has definition pale yellow gas (F2) which is the most reactive of all the elements, and is the strongest oxidizing agent
	has density 1.696 kg m-3 for gas at 273 K
	has density 1516 kg m-3 for liquid at 85.01 K boiling point
	has discoverer H. Moissan
	has discovery date 1886 (isolated)
	has discovery location Paris, France
	has electron affinity 328 kJ mol-1 from F to F-
	has electron configuration [He]2s22p5 in ground state
	has electronegativity 3.98 Pauling
	has hazard gas is highly corrosive and toxic, low concentration exposure can cause eye and lung irritation
	has hazard metal florides are very toxic
	has hazard organic fluorides are often quite harmless
	has heat capacity 22.744 J K-1 mol-1 for atomic gas at constant pressure 0.1 MPa at 298.15 K
	has heat capacity 31.30 J K-1 mol-1 for molecular gas (F2) at constant pressure 0.1 MPa at 298.15 K
	has heat of fusion 5.10 kJ mol-1
	has heat of vaporization 6.548 kJ mol-1
	has image
	has ionic radii 133 pm for F-
	has isotope mass range 17 to 23
	has lethal intake 185 p.p.m. for 1 hour F2 inhalation for rat
	has lethal intake 5 - 25 g NaF
	has lethal intake 5 - 25 g NaF
	has lethal intake 52 mg kg-1 NaF ingestion for rat
	has level in humans 0.05 p.p.m. in muscle
	has level in humans 0.22 - 7 p.p.m. in liver
	has level in humans 0.5 mg dm-3 in blood
	has level in humans 2000 - 12000 p.p.m. in bone
	has longest lived isotope fluorine 19
	has main mining area Canada, USA, UK, Russia, Mexico, Italy
	has mass absorption coefficient 1.80 cm2 g-1 for MoKalpha X-ray diffraction
	has mass absorption coefficient 16.4 cm2 g-1 for CuKalpha X-ray diffraction
	has mass of element in person 2.6 g for a 70 kg average person
	has melting point 53.53 K
	has mineral apatite, cryolite, fluorite
	has molar volume 18.05 cm3
	has name origin fluere = to flow from Latin
	has neutron scattering length 0.5654 × 10-12 cm
	has number of isotopes 7 including nuclear isomers
	has number of protons 9
	has ocean oxidation state -I
	has ocean residence time 400000 years
has pronunciation floor-een
has registry number 7782-41-4 for Chemical Abstracts System database
has relative atomic mass 18.9984032 in units of 12C = 12.000
has reserves 123 × 106 tonnes
has space group C2/m for α-F2
has space group Pm3n for β-F2
has specimen not available for sale as pure gas because it is too reactive and dangerous
has symbol F
has synthesis mechanism electrolysis of molten KF·2HF
has term symbol 2P3/2 in ground state
has thermal conductivity 0.0248 W m-1 K-1 at 273 K
has thermal conductivity 0.0279 W m-1 K-1 at 300 K
has thermal neutron capture cross section 0.0096 barns
has toxic intake 250 mg NaF
has uses AlF3 in aluminium production
has uses CaF2 as a flux in metallurgy
has uses fabrication of UF6, SF6 and fluorinating agents such as ClF3
has uses polymers, pesticides and antibiotics
has van der Waals radii 135 pm
has world production 2400 tonnes year-1 for gas (F2)
has world production 4.7 × 106 tonnes year-1 for fluorite (CaF2)
is a kind of accumulating oceanic element
is a kind of gaseous element
is a kind of halogen
reacts with almost everything violently
fluorine 17	has atomic mass 17.002095
	has decay mode β+ (2.761 Mev)
	has decay mode no γ
	has half life 64.5 seconds
	has nuclear magnetic moment μ = +4.722 nuclear magnetons with diamagnetic correction
	has nuclear spin I = 5/2+ h/2π
	has number of neutrons 8
	has number of nucleons 17
	has symbol 17F
	is an instance of fluorine
fluorine 18	has atomic mass 18.000937
	has decay mode β+ EC (1.655 Mev) 97%
	has decay mode no γ
	has half life 1.83 hours
	has nuclear spin I = 1+ h/2π
	has number of neutrons 9
	has number of nucleons 18
	has symbol 18F
	has uses medical diagnosis
	has uses research
	is an instance of fluorine
fluorine 19	has atomic mass 18.99840322
	has magnetogyric ratio 25.1665 × 107 rad T-1 s-1
	has natural abundance 100%
	has NMR frequency 94.077 MHz where 1H = 100 MHz; 2.3488 T
	has NMR receptivity 4730 where 13C = 1.00
	has nuclear magnetic moment μ = +2.628867 nuclear magnetons with diamagnetic correction
	has nuclear spin I = 1/2+ h/2π
	has number of neutrons 10
	has number of nucleons 19
	has relative NMR sensitivity 0.83 with CFCl3 reference where 1H = 1.00
	has symbol 19F
	has uses Nuclear Magnetic Resonance
	is an instance of fluorine
fluorine 20	has atomic mass 19.999981
	has decay mode β- (7.029 Mev)
	has decay mode γ
	has half life 11.0 seconds
	has nuclear magnetic moment μ = +2.094 nuclear magnetons with diamagnetic correction
	has nuclear spin I = 2+ h/2π
	has number of neutrons 11
	has number of nucleons 20
	has symbol 20F
	is an instance of fluorine
FOC	has definition Faint Object Camera (Hubble).
	is a kind of integrating detector
focal reducer	has definition An optical component or system for changing the image scale of a telescope to achieve a better match between the seeing disk and the pixel size. See optical matching.
	has wavefront modification shape
	is a kind of wavefront modifier
focus	is a kind of optical device
Fomalhaut	has B-V magnitude 0.09
	has declination -29 37 20
	has definition An A star. The brightest star in the constellation Piscis Austrinus.
	has distance 7 pc
	has right ascension 22 57 39.0
	has spectral type A3 V
	has synonym alpha PsA
	has synonym HR 8728
	has V magnitude 1.16
	is a part of Pisces Australis
	is an instance of A star
	is an instance of dwarf
	is an instance of naked eye star
forbidden line	has definition Spectral line emitted from a metastable state (q.v.), or those which have a very low probability (10-9-10-10) of occurrence. They appear at particle densities ≤ 108 cm-3. All forbidden lines have low excitation potentials. Forbidden lines are designated by enclosing them in brackets, e.g., [O II].
	has synonym nebular line
	is a kind of emission line
Forbush decrease	has definition A decrease in cosmic-ray intensity during active Sun.
	has discoverer Forbush
	has discovery date 1954
	has period 11 years
	is an instance of periodic celestial event
	is an instance of solar event
force	has carrier boson
	has range
	has strength (relative to electromagnetism)
	is a kind of physical phenomena
force quantity	has definition Agency responsible for a change in a system. In Newtonian mechanics, gravitational force bends the moon away from the straight trajectory it would otherwise pursue.
	has unit force unit
	is a kind of quantity
force unit	is a kind of unit
	is a unit of force quantity
Fork equatorial telescope	has mounting equatorial fork
	is a kind of equatorial telescope
formaldehyde	has definition An organic molecule, the first polyatomic molecule to be discovered in interstellar space. In 1973 It was discovered in two external galaxies.
	has discovery in space date 1969
	has symbol H2CO
	is an instance of interstellar molecule
formamide	has definition An interstellar molecule discovered in space at a wavelength of 6.5 cm.
	has discovery in space date 1971
	has symbol HCONH2
	is an instance of interstellar molecule
formic acid	has definition The first organic acid to be detected in interstellar space at 18.3 cm. Formic acid is the "sting" of an insect.
	has discovery in space date 1970
	has symbol H2CO2
	is an instance of interstellar molecule
Fornax	has acronym For
	has boundary
	has genitive Fornacis
	has historical origin the laboratory Furnace or chemical furnace in honor of Antoine Lavoisier
	has synonym Furnace
	is a part of celestial sphere
	is an instance of constellation named by de Lacaille
Fornax A	has apparent magnitude 10
	has synonym NGC 1316
	is a part of Fornax
	is an instance of radio galaxy
	is an instance of S0 galaxy
Fornax galaxy	has definition A dwarf spheroidal galaxy, in the Local Group, that orbits the Milky Way. (Mv ≈ -12).
	has discovery date 1938
	has distance 190 kpc
	has distance from galaxy center 440000 light-years
	has mass 2 × 107 Msun
	is a part of Fornax
	is a part of Local Group
	is an instance of satellite galaxy
	orbits Milky Way
Four-shooter	has definition An astronomical instrument comprised of four highly sensitive photoelectric cells (CCDs). The four-shooter is placed at the end of a telescope and used to electronically record incoming light.
	is a kind of charge-coupled device
Fourier Transform Spectrometer	has acronym FTS
	is a kind of spectrograph
Fourier transformation	has acronym FT
	is a kind of transformation
fractal	has definition A geometric figure in which a pattern is repeated ad infinitum on smaller and smaller scales. A classic example is Von Koch's snowflake, for which the construction begins with an equilateral triangle. Trisect each side, and replace the middle section by two sides of a smaller equilateral triangle, bulging outward. The snowflake is obtained by repeating this process for each side of the resulting figure, then for each side of the subsequent figure, and continuing forever.
	has definition A pattern that repeats itself or nearly repeats itself on many different scales of magnification. For example, suppose that some ink on a piece of paper appears to form a star. If you look at the piece of paper with a magnifying glass, you see that the dark areas are not solid black, but are formed of tiny stars themselves. If you look at one of these small stars with a microscope, you see that the dark areas of each of the tiny stars is formed from an arrangement of even tinier stars. Such a repeating pattern of stars would be called a fractal.
	has dimensions fractional
	is a kind of geometrical object
frame of reference	has acceleration
	has definition A set of axes to which positions and motions in a system can be referred.
	has rotation rate
	is a kind of mathematical concept
frame transfer	has definition A CCD construction in which one half of the imaging area of the device is purposely covered with a mask opaque to light to provide a temporary charge storage section.
	is a kind of charge-coupled device
francium	has image
	is a kind of alkali metal
Fraunhofer line	has definition Absorption line in the spectrum of the Sun, studied by Fraunhofer in 1814. The nine most prominent he labeled with capital letters (from the red end) A. B. C. D. E. F. G. H. and K. The A band and B band are now known to be groups of telluric lines due to O2 absorption in Earth's atmosphere, and C and F are respectively known as Hα and Hβ.
	has dicovery date 1814
	has discoverer Joseph Fraunhofer
	is a kind of absorption line
	occurs in Sun
free electron	has definition Electron not bound to a nucleus.
	is a kind of electron
free parameter	has definition A number which is needed to define a theory well enough so that predictions can be made, but which must be determined by experiment or observation.
	is a kind of theory related concept
free-fall	has definition A collapse in which gas clouds do not hit or impede one another. According to ELS, the Galactic halo formed in a free-fall collapse.
	is a kind of collapse
free-free transition	has definition The acceleration of an unbound (or free) electron by a proton or atomic nucleus results in the emission of electromagnetic radiation.
	is a kind of atomic process
frequency	has definition Number of oscillations per second of an electromagnetic wave. The amplitude of a wave depends on the intensity; the wavelength, on the frequency.
	has definition The number of complete wave cycles a wave completes each second.
	has definition The number of cycles or complete alternations per unit time of a carrier wave, band, or oscillation.
	has definition The number of oscillations or wave cycles per second passing a given point. For electromagnetic radiation, the product of the frequency and the wavelength is the speed of light.
	has definition The number of peaks (often called crests) of a propagating wave that cross a given point in a unit of time. For example, if 1000 peaks cross a given point in one second, one says that the frequency is 1000 cycles per second or 1000 hertz.
	has symbol ν
	has unit frequency unit
	is a kind of quantity
frequency distribution	has definition A statistical arrangement of numerical data according to size or magnitude (see also distribution function).
	is a kind of statistical distribution
frequency unit	is a kind of unit
	is a unit of frequency
FU Ori star	has definition A kind of T Tau stars with considerable changes in brightness. The post-eruption spectrum is that of a late supergiant.
	has synonym fuors
	is a kind of T Tauri star
Fundamental Catalog	has acronym FK
	is a kind of star catalog
fundamental force	has definition One of four fundamental forces which mediate particle interaction.
	is a kind of force
fundamental frequency	has definition The lowest characteristic frequency of oscillation of a dynamical system.
	is a kind of frequency
fundamental physical constant	has accuracy
	has unit
	has value changes in value are due solely to an increase in measurement accuracy or convention
	is a kind of constant
fundamental star	has definition Star for which coordinates have been determined to a very high degree of accuracy.
	is a kind of star
Fused-quartz	is a kind of mirror
fusion	has antonym fission
	has catalyst
	has definition In nuclear physics, the combining of the atomic nuclei of lighter elements to form nuclei of a heavier element. Such a process involving the atomic nuclei of elements lighter than iron is accompanied by the emission of energy; for fusion of heavier elements, energy must be supplied. The process is thought to contribute to the condensation of stars from interstellar gas and dust. See also nuclear fusion.
	has definition Interaction in which nucleons are forged together, creating new atomic nuclei and releasing energy. Fusion powers hydrogen bombs.
	has definition Process by which the Sun (and other stars) radiates energy. The nucleus of an atom fuses with the nuclei of other atoms to form new, heavier atoms at the same time releasing large amounts of energy. In the Sun, hydrogen atoms are converted into helium by this process, with carbon and nitrogen as intermediates. Cooler stars undergo the proton-proton cycle with a similar result.
	has minimum temperature
	has product
	has reactant
	has reaction probability
	is a kind of nuclear process
	produces energy
fusion device	is a kind of instrument
G band	has definition A band of CH. It is conspicuous in the spectra of G-K stars.
	has species CH
	has wavelength 4303 Å
	is a kind of Fraunhofer line
	is a kind of molecular band
	occurs in G star, K star
G star	has color yellowish
	has definition Spectral type for yellow stars.
	has definition Yellowish star in which the H and K lines of Ca II have become dominant and in which a tremendous profusion of spectral lines of both neutral and ionized metals, particularly iron, begins to show. The Balmer lines of hydrogen are still recognizable. Examples are the Sun and Capella.
	has surface temperature 5000 to 6000 K
	is a kind of late star
G1	has image
	has synonym Mayall II
	is a part of Andromeda galaxy
	is an instance of extragalactic globular cluster
gadolinium	has image
	has ocean oxidation state III
	has ocean residence time 300 years
	is a kind of rare Earth
	is a kind of recycled oceanic element
galactic astronomy	has definition The study of the Milky Way.
	is a kind of astronomy
galactic center	has definition Now thought to comprise black holes - which would explain why the centre of our Galaxy appears strangely obscure, and emits only infrared radiation.
	has definition The direction of the sky toward the center of the Milky Way.
	has galactic latitude 0
	has galactic longitude 0
	is a part of galactic nucleus
	is a part of Sagittarius
	is an instance of galactic coordinate
	is opposite of anticenter
galactic coordinate	has component galactic coordinate component
	has definition A system of coordinates based on the mean plane of the Galaxy, which is inclined about 63° to the celestial equator. Galactic latitude (b) is measured from the galactic equator north (+) or south (-); galactic longitude (l) is measured eastward along the galactic plane from the galactic center. In 1958, because of increased precision in determining the location of the galactic center, a new system of galactic coordinates was adopted, with the origin at the galactic center in Sagittarius at α(1950) = 17h42m.4, δ(1950) = - 28°55'. The new system is designated by a superior roman numeral II (i.e., bII, lII) and the old system by a superior roman numeral I: lII ≈ lI + 32°.31. Galactic coordinates are independent of precession.
	has galactic latitude
	has galactic longitude
	is a kind of coordinate
galactic coordinate component	is a component of astronomical coordinate
	is a kind of coordinate component
galactic equator	has definition The primary circle defined by the central plane of the Galaxy.
	is a kind of equator
galactic latitude	has definition The angle between the line of sight to a star and the Galactic plane. Galactic latitude ranges from +90 degrees to -90 degrees; the Galactic plane has a Galactic latitude of 0 degrees. Regions north of the Galactic plane have positive Galactic latitude; regions south have negative Galactic latitude. The point with a Galactic latitude of +90 degrees is called the north Galactic pole, and the point with a Galactic latitude of -90 degrees is called the south Galactic pole.
	is an instance of angle
	is an instance of galactic coordinate component
galactic longitude	has definition A measure of a star's position with respect to the Sun and Galactic center. Galactic longitude ranges from 0 degrees to 360 degrees. Imagine the Sun at the center of a giant clock, with the Galactic center located in the direction of six o'clock. A Galactic longitude of 0 degrees would correspond to the direction of six o'clock, a Galactic longitude of 90 degrees to the direction of three o'clock, a Galactic longitude of 180 degrees to the direction of twelve o'clock, and a Galactic longitude of 270 degrees to the direction of nine o'clock.
	is an instance of angle
	is an instance of galactic coordinate component
galactic nucleus	has definition In the innermost region of a galaxy, there is often a concentration of stars and gas, sometimes extending over thousands of light-years from the center of the galaxy.
	has definition The central region of a galaxy.
	is a part of Milky Way
galactic plane	has definition The plane that contains the disk of the Milky Way. By definition, one direction perpendicular to this plane is called "above" or "north", and the opposite direction, also perpendicular to the Galactic plane, is called "below" or "south". From Earth, due Galactic north is marked by the north Galactic pole, which lies near the bright star Arcturus, and due Galactic south is marked by the south Galactic pole, which lies in the faint constellation Sculptor.
	is a kind of plane
galactic pole	has definition Either of the two points in the sky where we look perpendicular to the disk of the Milky Way. The north Galactic pole is the Galactic pole located above the disk; the south Galactic pole is the Galactic pole located below the disk.
	has definition The poles of the galactic plane. The new system puts the galactic north pole in Coma at α(1950) = 12h49m, δ(1950) = 27°.4
	has galactic longitude 0 degrees
	is a kind of galactic coordinate
galactic rotation	has definition The revolving of a galaxy round its central nucleus even as it continues its proper motion. Such rotation, however, is not uniform but differential. One revolution of the Sun within our own Galaxy takes about 225 million years, or 1 cosmic year.
	is a kind of differential rotation
	is a kind of galaxy motion
galactic velocity component	has definition One of three components of a star's motion with respect to the center of a galaxy.
	is a kind of velocity
galactic wind	has definition A hypothetical outflow of tenuous material from a galaxy, analogous to the solar wind.
	is a part of Milky Way
Galactocentric distance	has definition A star's distance from the Galactic center. The Sun's Galactocentric distance is about 27000 light-years.
	is an instance of galactic coordinate component
	is an instance of length
galaxy	has catalog galaxy catalog
	has definition A collection of matter which usually manifests itself by the production of stars.
	has definition A huge collection of millions, billions, or trillions of stars. When referring to the Milky Way, "galaxy" is capitalized, otherwise not; thus: "Andromeda is the nearest giant galaxy to the Galaxy".
	has definition A large (108-1013 Msun), gravitationally bound aggregate of stars and interstellar matter. Galaxy formation is currently believed to have occurred around z ≈ 3-4.
	has definition A large aggregation of stars, bound together gravitationally. There are three major classifications of galaxies - spiral, elliptical, and irregular - and several subclassifications. The sun belongs to a spiral galaxy, the Milky Way galaxy.
	has definition An isolated aggregation of stars and gas, held together by their mutual gravity. A typical galaxy has about 100 billion stars, has a total mass equal to about a trillion times the mass of the sun, is about 100,000 light years in diameter, and is separated from the nearest galaxy by a distance of about 100 times its own diameter. Thus, galaxies are islands of stars in space. Our galaxy is called the Milky Way. Galaxies come in two major shapes: flattened disks with a central bulge, called spirals, and amorphous, semispherical blobs, called ellipticals. If galaxies are found bunched up next to each other, they are said to lie in groups or clusters. Clusters with a particularly large number of galaxies in them are called rich clusters. Galaxies that do not lie in such groups but rather seem to be scattered uniformly and randomly through space are called field galaxies. Some galaxies are characterized by the dominant type of radiation they emit.
	has definition Vast system of celestial objects, typically consisting of between 106 and 1012 stars, plus interstellar gas and dust. There are three basic types: spiral (further subdivided into normal spirals and spirals with a "bar" at the centre, and yet further subdivided according to the "openness" of the spiral arms), elliptical (subdivided according to ellipticity) and irregular (subdivided according to whether they are made up of Population I or Population II stars). Another not uncommon type of galaxy is a lenticular form mid-way between the spiral and the elliptical.
	has number of stars 106 to 1012
	is a kind of collection of stars
galaxy catalog	has object type galaxy or nebula (some nebulae are misidentified as galaxies)
	is a kind of catalog about star systems
galaxy cluster	has definition A conglomeration of hundreds or thousands of galaxies. The nearest large galaxy cluster is the Virgo cluster.
	has definition An aggregate of galaxies. Bautz and Morgan divide them into three morphological types: type I contains a supergiant cD galaxy; Coma is type II, type III contains no members significantly brighter than the general bright population. Virgo is type III. 21 known X-ray sources are associated with clusters of galaxies.
	has number of galaxies 102 to 103 (order of magnitude)
	is a kind of collection of galaxies
galaxy containing Cepheids	has distance determined from Cepheid period-luminosity relation
	has number of Cepheids
	is a kind of galaxy
galaxy group	has definition A small gathering of galaxies, smaller than a cluster. The Milky Way is part of the Local Group, which contains about thirty galaxies.
	has number of galaxies < 100
	is a kind of galaxy cluster
galaxy merger	has definition The formation of a galaxy from the collision of two or more separate galaxies.
	is a kind of merger
galaxy motion	is a kind of mass motion
galaxy orbital event	has participants star, galaxy
	is a kind of orbital event
galaxy peculiar velocity	has definition A deviation in the velocity of a galaxy from that expected on the basis of a uniform expansion of the universe. (See Hubble law.)
	is a kind of velocity
galaxy theory	has domain extragalactic astronomy
	is a kind of astronomy theory
Galilean satellite	has definition The largest satellites of Jupiter - Io, Europa, Ganymede, and Callisto. All are locked in synchronous rotation with Jupiter.
	has discoverer Galileo
	has discovery date 1610
	is a kind of natural satellite
	is a part of Jupiter
Galilean transformation	has definition The non-relativistic method of relating observations from one frame of reference to another.
	is a kind of coordinate transformation
Galileo Galilei	is an instance of astronomer
gallium	has image
	has ocean oxidation state III
	has ocean residence time 10000 years
	is a kind of group III element
	is a kind of metallic element
	is a kind of unclassified oceanic element
Gamma Corvi	has B-V magnitude -0.11
	has declination -17 32 31
	has right ascension 12 15 48.3
	has spectral type B8IIIpHgMn
	has synonym HR 4662
	has V magnitude 2.59
	is a part of Corvus
	is an instance of B star
	is an instance of giant
	is an instance of naked eye star
Gamma Draconis	has B-V magnitude 1.52
	has declination +51 29 20
	has right ascension 17 56 36.3
	has spectral type K5III
	has synonym HR 6705
	has V magnitude 2.23
	is a part of Draco
	is an instance of giant
	is an instance of K star
	is an instance of naked eye star
Gamma Microscopii	has B-V magnitude 0.89
	has declination -32 15 28
	has right ascension 21 1 17.4
	has spectral type G6III
	has synonym HR 8039
	has V magnitude 4.67
	is a part of Microscopium
	is an instance of G star
	is an instance of giant
	is an instance of naked eye star
Gamma Normae	has B-V magnitude 1.08
	has declination -50 09 20
	has right ascension 16 19 50.3
	has spectral type G8III
	has synonym HR 6072
	has V magnitude 4.02
	is a part of Norma
	is an instance of G star
	is an instance of giant
	is an instance of naked eye star
gamma ray	has definition An electromagnetic wave with a wavelength in the range of 10-13 to 10-10 meter, corresponding to photons with energy in the range of 104 to 107 electron volts.
	has definition Electromagnetic radiation similar to X-radiation, although of shorter wavelength, emitted spontaneously by some radioactive substances from atomic nuclei during radioactive decay.
	has definition Electromagnetic radiation with a wavelength less than about 1 Å (10-10 m); blends from the "hard" X-ray region. Photons of energy greater than about 10 keV.
	has definition Extremely short-wavelength electromagnetic energy.
	has definition Photon of very high frequency (wavelength shorter than a few tenths of an angstrom); the most energetic form of electromagnetic radiation, although there is no sharp boundary between γ-ray photon and an X-ray photon. Usually γ-ray photons come from the nucleus and X-ray photons come from the inner orbital electrons. Galactic γ-ray photons seem to originate primarily in the spiral arms.
	has definition Photons of very high frequency and very short wavelength; the most energetic and penetrating form of electromagnetic radiation.
	has definition Rays first discovered in radioactive material, and later identified as very high energy photons.
	has frequency 3 EHz or more
	has wavelength 100 pm or less
	is a kind of photon
gamma ray source	has wavelength gamma ray
	is a kind of celestial body
Gamma Sagittae	has B-V magnitude 1.57
	has declination +19 29 32
	has right ascension 19 58 45.3
	has spectral type M0III
	has synonym HR 7635
	has V magnitude 3.47
	is a part of Sagitta
	is an instance of giant
	is an instance of M star
	is an instance of naked eye star
Gamma Velorum	has absolute magnitude Mv = - 5.6
	has B-V magnitude -0.22
	has declination -47 20 12
	has definition A triple system (WC8, B1 IV, O9 I) embedded in the Gum Nebula. It is the brightest Wolf-Rayet star in the sky.
	has distance 400 pc
	has orbital period 78.5 days
	has right ascension 8 9 31.9
	has spectral type WC8+O7.5e
	has synonym HR 3207
	has V magnitude 1.78
	is a part of Vela
	is a part of Vela
	is an instance of naked eye star
	is an instance of triple star
	is an instance of WC star
gamma-ray burst	has definition Short, intense, low-energy bursts, first recorded by the Vela satellite system on 1967 July 2. Their isotropic distribution suggests an extragalactic origin, but a galactic disk origin cannot be ruled out: there is a large increase in γ-ray flux in the direction of the galactic center.
	has discovery date July 2, 1967
	has distribution isotropic with an increase towards the galactic center
	has duration 0.1 - 4 s
	has energy spectrum 0.1-1.2 MeV
	has frequency of occurrence 5 per year
	is a kind of gamma ray source
	is a part of celestial sphere
Ganymede	has definition The largest satellite of Jupiter.
	has eccentricity e = 0.0015
	has mass 1.65 × 1026 g
	has orbital period 7.155 days
	has radius 2635 km
	has synonym Jupiter III
	is a part of Jupiter
	is an instance of Galilean satellite
gas	has definition The gaseous component of a galaxy, includes nebula
	has definition A phase of matter consisting of a low density collection of particles which have no short range order
	is a kind of phase of matter
	is a part of disk
gas giant	has composition Mostly made of various light elements such as gases and simple compounds
	has definition Giant planet that has a gaseous surface.
	has synonym Jovian planet
	is a kind of planet
gas law	is a kind of thermodynamics law
gaseous element	has state gas at standard temperature and pressure
	is a kind of element
gaseous fragmentation	has definition The systematic breakup of a gas cloud into smaller and smaller subunits as the gas cools and continues to collapse. The gravitational forces continually overtake the opposing pressure gradients as long as the cloud is able to radiate freely; consequently, the Jeans mass decreases, and fragments divide into smaller subfragments. The process stops only when opacity intervenes to inhibit the cooling and radiation.
	is a kind of contraction
gaseous nebula	has catalog gaseous nebula catalog
	has definition An H II region, a supernova remnant, or a planetary nebula. H II regions have an emission-line optical spectrum, and a thermal continuous spectrum declining in intensity as the wavelength increases (from maximum in the ultraviolet) through infrared and radio. Supernova remnants have an emission-line optical spectrum and a nonthermal radio spectrum. Temperatures of planetary nebulae are much higher than those of H II regions.
	has synonym emission nebula
	is a kind of H II region
gaseous nebula catalog	has object type gaseous nebula
	is a kind of nebula catalog
gauge symmetry	has definition A property of modern theories of physics, formulated in the 1960s and confirmed by experiment. A symmetry, in general, is a property that allows a system to behave in the same way even though it has undergone some change. For instance, a snowflake has a 6-sided "rotational symmetry" - a snowflake appears identical after every 60 degree rotation. A gauge symmetry is something like a rotation, in which the amount of rotation can vary randomly from one point of space to the next. (See field theory.)
	has definition Symmetry principle underlying the quantum-mechanical description of the three nongravitational forces; the symmetry involves the invariance of a physical system under various shifts in the values of force charges, shifts that can change from place to place and from moment to moment.
	is a kind of symmetry
Gauge theory	has author Chen Ning Yang and Robert Mills (1954)
	has definition A theory that treats force in a geometrical way in terms of global and local symmetries.
	has definition A theory whose dynamics originate from a symmetry. That is, the formulae describing the theory (in particular, the Lagrangian) are unchanged under certain symmetry transformations, called "gauge" transformations. For example, the equations of classical electrodynamics are invariant under local redefinitions of the electrostatic potential. This symmetry is ultimately responsible for the conservation of electric charge. However, in quantum electrodynamics this gauge symmetry is reinterpreted as invariance under local redefinitions of the phase of the electron wave function.
	has definition Account of forces that views them as arising from broken symmetries.
	has definition In 1973 David Gross, Frank Wilczek, and David Politzer showed that these theories possess a property called asymptotic freedom, just what was needed for a theory of how quarks bind to form protons and neutrons. The new theory, dubbed quantum chromodynamics or QCD, proposed that the color of the quarks acts as the charge of the Yang-Mills interactions.
gauge theory	has gauge symmetry
Gauge theory	has name origin the term "gauge theory" is an archaic one, coming from earlier theories which were based on invariance under transformation of scale (i.e. gauge)
	has synonym Yang-Mills theories
gauge theory	is a kind of quantum field theory
Gauge theory	is a kind of unified theory
gauss	has definition Unit of magnetic flux density.
	has equivalent 10-4 tesla
	is an instance of CGS unit
	is an instance of magnetic flux density unit
Gaussian distribution	has definition A random distribution of initial conditions is often referred to as a Gaussian distribution. Also, a certain kind of bell-shaped curve is called a Gaussian.
	has definition A statistical distribution defined by the equation p = c exp(-k2x2), in which x is the statistical variable. It yields the familiar bell-shaped curve. Accidental errors of measurement and similar phenomena follow this law.
	is an instance of statistical distribution
Gaussian gravitational constant	has definition The constant defining the astronomical system of units of length (astronomical unit), mass (solar mass) and time (day), by means of Kepler's third law. The dimensions of k2 are those of Newton's constant of gravitation: L3M-1T-2.
	has symbol k
	has value 0.01720209895
	is an instance of universal constant
Gaussian year	has definition The period associated with Kepler's third law with a = 1.
	is an instance of year
gegenschein	has definition A very faint glow (about 10° across) which can occasionally be seen in a part of the sky opposite the Sun.
	has definition Faint nebulous light about 20° across near the ecliptic and opposite the Sun, best seen in September and October. Also called counterglow.
	has definition Faint oval patch of light visible from Earth only at certain times of the year, opposite the Sun. Its nature and cause are still not known. It is sometimes known as "counterglow".
	has synonym counterglow
	is a kind of backscatter
Gemini	has acronym Gem
	has boundary
	has genitive Geminorum
	has historical image
	has historical origin the twins Castor and Pollux, Greek heroes Jason led on his voyages on the Argo
	has synonym Twins
	is a part of Zodiac
	is an instance of zodiacal constellation
Gemini Telescope North	has altitude 4100: m
	has aperture 8.1 m
	has comment Coming ULE meniscus primary mirror; this northem instrument is to be optimized for IR work
	has creation date (1999)
	has focal ratio f/1.8, 16IR, 19.6
	has latitude 19° 49' N
	has location Mauna Kea, Hawaii, US
	has longitude 155° 28' W
	has owner Joint Astronomy Center
	is an instance of altazimuth telescope
	is an instance of optical telescope
	is an instance of Ritchey-Chrétien
Gemini Telescope South	has altitude 2725 m
	has aperture 8.1 m
	has comment Partners in the Gemini project are the US, UK, Canada, Chile, Brazil, and Argentina
	has creation date (2000)
	has focal ratio f/1.8, 6
	has latitude 30° 21' S
	has location Cerro Pachon, Chile
	has longitude 70° 49' W
	has owner Cerro Tololo Inter-American Obs.
	is an instance of altazimuth telescope
	is an instance of optical telescope
	is an instance of Ritchey-Chrétien
Geminid	has duration 3 days
	has parent object Asteroid Phaethon
	has radiant Gemini
	has rate 90 per hour
	has start time 13 December
	is an instance of meteor shower
general precession	has definition The sum of the lunisolar and the planetary precession. It causes the ecliptic longitude to increase at a constant rate but has no effect on ecliptic latitude.
	has rate 50".27 per year
	is an instance of precession
general relativity	has author Einstein (1915)
	has competing theory twistor theory
	has definition Gravitation is a curvature in four-dimensional space-time rather than as a force existing between two masses.
	has implication four-dimensional structure of space-time
	has phenomena gravity
	has subset special relativity
	has synonym Einstein's theory of gravity
	is an instance of relativity theory
geocentric coordinate	has component geocentric coordinate component
	has coordinate origin Earth
	has definition The latitude and longitude of a point on the Earth's surface relative to the center of the Earth; also celestial coordinates given with respect to the center of the Earth.
	has definition The longitude and latitude of a point on the Earth relative to the geoid. These coordinates are influenced by local gravity anomalies.
	has height
	has terrestrial latitude
	has terrestrial longitude
	is a kind of coordinate
geocentric coordinate component	is a component of geocentric coordinate
	is a kind of coordinate component
geocentric cosmology	has definition A model of the universe in which the earth is centrally located, and the Sun, planets, and stars revolve around the Earth.
	has definition School of ancient theories that depicted the earth as standing, immobile, at the center of the universe.
	is a kind of outdated belief
geocorona	has definition The outermost part of Earth's atmosphere, a hydrogen halo extending out to perhaps 15 Earth radii, which emits Lyman-α radiation when it is bombarded by sunlight.
	is a part of atmosphere
geodesy	has definition Measurement and study of the Earth's size and shape.
	is a kind of geology
geodetic coordinate	has definition The latitude and longitude of a point on the Earth's surface determined from the geodetic vertical (normal to the specified spheroid). (See zenith; latitude, terrestrial; longitude, terrestrial.)
	is a kind of coordinate
geodetic precession	has definition A small, relativistic, direct motion of the equinox along the ecliptic.
	has rate 1".915 per century
	is an instance of precession
geological era	is a kind of era
geology	has definition Scientific study of the dynamics and history of the earth, as evidenced in its rocks, chemicals, and fossils.
	is a kind of science
geometrical object	has dimensions
	is a kind of mathematical concept
geophysical satellite	is a kind of artificial satellite
George Ellery Hale Telescope	has altitude 1706 m
	has aperture 5.08 m
	has creation date 1948
	has focal ratio f/3.3. 16, 30
	has latitude 33° 21' N
	has location Palomar Mountain, Calif., US
	has longitude 116° 52' W
	has mirror maker J.A. Anderson
	has mirror type ribbed Corning Pyrex
	has mounting Horseshoe yoke mount
	has mounting manufacturer Westinghouse
	has operator California Institute of Technology
	has owner Palomar Observatory
	has synonym 200 inch
	is an instance of Cassegrain
	is an instance of Horseshoe equatorial telescope
	is an instance of optical telescope
GEOS 10	is an instance of GEOS satellite
GEOS 11	is an instance of GEOS satellite
GEOS 8	is an instance of GEOS satellite
GEOS satellite	is a kind of geosynchronous satellite
	is a kind of NOAA satellite
	is an acronym for Geostationary Operational Environmental Satellite
geosynchronous satellite	has constant longitude
	has definition an artificial satellite which orbits Earth every 24 hours
	has location above the same position on the Earth
	has orbital inclination approximately zero degrees
	has orbital period 24 hours (one day)
	is a kind of artificial satellite
	is a part of Earth
	orbits Earth
German equatorial telescope	has mounting German equatorial
	is a kind of equatorial telescope
germanium	has image
	has ocean oxidation state IV
	has ocean residence time 20000 years
	is a kind of group IV element
	is a kind of metallic metalloid
	is a kind of recycled oceanic element
	is a kind of siderophile element
giant	has definition 1. A star that has evolved off the main sequence and is roughly a hundred times as luminous as the Sun. Giants can be of any color, but yellow, orange, and red giants are the most common. 2. A planet much more massive than Earth. The solar system has four giant planets, all far from the Sun: Jupiter, Saturn, Uranus, and Neptune.
	has definition High-luminosity star that lies above the main sequence on the Hertzsprung-Russell diagram.
	has luminosity class III
	is a kind of star
giant elliptical galaxy	has definition Massive elliptial galaxy which is too far for individual stars to be resolved.
	has mass 1013 Msun (most massive galaxies known)
	is a kind of elliptical galaxy
giga	has symbol G
	has value 109
	is an instance of unit prefix
GIS	has definition Gas Imaging Spectrometer (ASCA X-ray satellite).
	is a kind of high energy detector
global inertial frame	has definition A coordinate system or frame of reference anchored with respect to the overall distribution of matter in the universe.
	is a kind of nonaccelerating reference frame
	is a kind of nonrotating reference frame
globular cluster	has age oldest stellar formation in the Galaxy
	has catalog globular cluster catalog
	has definition A tightly packed, symmetrical group of thousands of very old (pure Population II) stars. The stellar density is so great in the center that the nucleus is usually unresolved. The stars within a globular cluster orbit each other because of their mutual gravity.
	has diameter about 100 light-years
	has location most are in the halo some are members of the thick disk
	has mass 104 to 106 Msun
	has number of stars 105 to 106
	has orbit elongated, around the galactic center
	has spectra low abundances of heavy elements
	has velocity very high
	is a kind of star cluster
	is a part of halo
globular cluster catalog	has object type globular cluster
	is a kind of catalog about star systems
glueball	has definition Theoretical particles made exclusively of gluons. Tentative evidence of the existence of glueballs had been found in accelerator experiments by the mid-1980s.
	is a kind of hypothetical particle
gluon	carries the force strong force
	has color charge
	has definition Gluons are the massless gauge bosons of QCD which mediate the strong color force between quarks. Because of the non-Abelian structure of the theory, gluons can interact with themselves, and may form particles consisting of gluons bound together. The existence of these "glueballs" has yet to be confirmed.
	has definition Quanta that carry the strong nuclear force. Like photons, vector bosons, and gravitons - the carriers respectively of electromagnetism, the weak force, and gravitation - gluons are massless bosons. Consequently, for simplicity's sake, some physicists lump together all the force-carrying quanta under the term "gluons".
	has definition Smallest bundle of the strong force field; messenger particle of the strong force.
	has definition The force-carrying particles associated with the strong interactions, the forces which bind quarks inside of protons and neutrons. For more details, see Yang-Mills theories.
	has definition The hypothetical particle that carries the force between quarks.
	is a kind of carrier boson
	is a kind of massless particle
	is a kind of parton
GMS	has launch date 14 Jul 1977
	has launch location Kenedy Space Center
	has launch vehicle Delta rocket
	has mass 315 kg
	is an instance of GMS satellite
GMS satellite	has definition Geostationary Meteorological Satellite
	has specification
	is a kind of geosynchronous satellite
	is a kind of NASDA satellite
GMS-2	has launch date 11 Aug 1981
	has launch location Tanegashima Space Center, Kagoshima, Japan
	has launch vehicle N-II rocket
	has mass 296 kg
	is an instance of GMS satellite
GMS-3	has launch date 3 Aug 1984
	has launch location Tanegashima Space Center, Kagoshima, Japan
	has launch vehicle N-II rocket
	has mass 303 kg
	is an instance of GMS satellite
GMS-4	has attitude control spin stabilized
	has constant longitude 140 deg east
	has dimension 2.146 m diameter cylinder, height 0.3451 m
	has launch date 6 Sept 1989
	has launch location Tanegashima Space Center, Kagoshima, Japan
	has launch vehicle H-I rocket
	has mass 325 kg
	is an instance of GMS satellite
GMS-5	has attitude control spin stabilized
	has constant longitude 140 deg east
	has dimension 2.146 m diameter cylinder, height 0.3451 m
	has image
	has launch date early 1995
	has launch location Tanegashima Space Center, Kagoshima, Japan
	has launch vehicle H-II rocket
	has mass 344 kg
	is an instance of GMS satellite
Goddard Space Flight Center	has acronym GSFC
	is an instance of space science institution
GOES-L	has creator NASA
	has image
	has launch date 3 May 2000
	has launch vehicle Lockheed Martin Atlas IIA rocket
	is an instance of GEOS satellite
Golay cell	has definition A gas bulb used to detect infrared radiation.
	is a kind of detector
gold	has image
	has ocean oxidation state I
	is a kind of siderophile element
	is a kind of supernova produced element
	is a kind of transition metal
	is a kind of unclassified oceanic element
Goldstone boson	has definition A massless spin-0 particle which arises whenever a (continuous) global symmetry is spontaneously broken.
	has spin 0
	is a kind of boson
	is a kind of hypothetical particle
Gould Belt	has definition The local system of stars and gas within about 300 pc of the Sun in which the greatest concentration of naked-eye O and B stars occurs.
	has distance from Sun 300 pc
	has inclination 10° to 20° to the galactic plane
	is a part of Orion arm
GPS satellite	is a kind of navigation satellite
Gran Telescopio Canarias	has acronym GTC
	has altitude 2400 m
	has aperture 10 m
	has comment Design similar to that of the Keck instruments; funded by Spain with international partners
	has creation date 2003
	has focal ratio f/1.75. 15, 25
	has latitude 28° 45' N
	has location La Palma, Canary Islands
	has longitude 17°54' W
	has owner Obs. del Roque de los Muchachos
	is an instance of altazimuth telescope
	is an instance of optical telescope
	is an instance of Ritchey-Chrétien
	is an instance of segmented mirror telescope
grand unified theory	has acronym GUT
	has definition A speculative class of theories of particle interactions, first developed in 1974, which attempt to describe electromagnetism, the weak interactions, and the strong interactions in a fully unified theory. Of the known forces, only gravitation is omitted.
	has definition An attempt to produce a unification of all the forces of nature. While some success was made in unifying the gluon force between quarks with the electroweak force, problems always arose when gravity was included. Grand unification eventually gave way to superstring theory.
	has definition Class of theories that merge all three nongravitational forces into a single theoretical framework.
	has definition Class of theories that purport to reveal identities linking the strong and electroweak forces. The differences between these forces in nature today is attributed to the breaking of symmetrical relationships among force-carrying particles as the very early universe expanded and cooled.
	has definition Theory in physics that attempt to explain the forces of nature as manifestations of a single underlying force.
	has synonym grand unification
	is a kind of unified theory
granule	has definition Convective cell in the solar photosphere. A granule represents a temperature roughly 300° higher than the surrounding dark areas. At any one time, granules cover about one-third of the solar photosphere.
	has definition The mottled appearance of the solar photosphere, caused by gases rising from the interior of the Sun.
	has diameter 1000 km in diameter
	has lifetime 5 minutes
	is a part of photosphere
gravitational collapse	has definition The sudden collapse of a massive star when the radiation pressure outward is no longer sufficient to balance the gravitational pressure inward. In gravitational collapse there is a sudden, catastrophic release of great quantities of gravitational potential energy, and this release has been postulated as the cause of supernovae, neutron stars, and black holes.
	is a kind of collapse
gravitational contraction	is a kind of energy source
gravitational lens	has definition A galaxy that intervenes between us and a distant astronomical object and that gravitationally deflects the light from that distant object. (Light, like matter, is attracted by gravity.) Gravitational lenses can focus, distort, and split light beams in the same way that ordinary glass lenses do.
	is a kind of galaxy
gravitational lensing	has definition Deflection of electromagnetic radiation from a distant background source by a strong gravitational field associated with a foreground source resulting in more than one image of the original source. Many double-quasars are produced by this phenomenon.
	has definition Gravitational effect that bends a ray of light. Such an effect was predicted within the general theory of relativity, although previously considered impossible.
	has definition The apparent path of a photon is altered from a straight line by the gravitational field of the Sun. The path is deflected radially away from the Sun by up to 1".75 at the Sun's limb. Correction for this effect, which is independent of wavelength, is included in the reduction from mean place to apparent place.
	has definition The bending of light caused by the gravity of an object lying between us and the light source. This may cause the light source to look brighter than it normally does.
	has definition The bending of the beam of light due to gravity. It is observable when the light from a star or planet passes a massive object such as the Sun.
	has definition The effect of matter in curved spacetime, which tends to focus any beam of radiation from a distant source. In effect, the spacetime curvature is a lens of great focal length. At z ≈ 1, the angular size of an object starts increasing with distance.
	has synonym deflection of light
	is a kind of radiation direction modification
gravitational mass	has definition That property of matter which makes it create a gravitational field and attract other particles (cf. inertial mass; equivalence principle).
	is an instance of mass
gravitational potential energy	has definition Energy that a body can acquire by falling through a gravitational field and that decreases as the kinetic energy increases. There is no general reference level (analogous to the state of rest of a body in defining kinetic energy), and so we customarily define the change in gravitational potential energy as the negative of the work done by the gravitational forces during the bodys change of position.
	has definition When we lift a weight from the floor to a tabletop, we clearly put energy into it. The energy is not lost, however, because we can retrieve it by allowing the weight to fall back to the floor. While the weight is on the table, we say that the energy is stored as gravitational potential energy. The energy is stored in the gravitational field.
	is a kind of potential energy
gravitational radius	has definition The radius which an object should have in order that light emitted from its surface just ceases to escape from its surface.
	is an instance of radius
gravitational redshift	has definition Displacement of spectral lines toward longer wavelengths due to the effects of gravity.
	has definition Light is emitted at a lower frequency and longer (or redder) wavelength in a gravitational field than in the absence of a gravitational field.
	has definition The rate at which a clock keeps time when it is in a gravitational field is slower than the rate at which it will keep time in the absence of a gravitational field. (The gravitational redshift was experimentally verified by Pound and Rebka in 1960.) The amount of redshift is directly proportional to the mass of the emitting body and inversely proportional to its radius.
	has example Object Gravitational Redshift Earth 10-9 white dwarf 10-4 black hole large (proportional to mass divided by radius)
	has synonym Einstein effect
	is a kind of wavelength shift
gravitino	has definition Hypothetical force-carrying particles predicted by supersymmetry theories. The gravitino's spin would be 1/2. Its mass is unknown.
	is a kind of hypothetical particle
graviton	carries the force gravity
	has definition A hypothetical elementary particle associated with the gravitational interaction. It is a stable particle with zero rest mass, zero charge, and a spin of ± 2, and travels with the speed of light.
	has definition A massless spin-2 particle which is the hypothetical quantum of the gravitational field. It mediates the force of gravity in a similar way to that in which the spin-1 gauge bosons (i.e. the photon, W±, Z0, and gluons) mediate the other forces.
	has definition A quantum of gravitational radiation.
	has definition Smallest bundle of the gravitational force field; messenger particle for the gravitational force.
	has definition The hypothetical quantum particle of the gravitational field. It could also be thought of as a quantized element of space-time curvature.
	has definition The quanta thought to convey gravitational force; analogous to the photons, gluons, and intermediate vector bosons of electromagnetism and the strong and weak nuclear forces. Predicted by quantum theory of gravity, gravitons have not yet been detected.
	has spin 2
	is a kind of carrier boson
	is a kind of hypothetical particle
	is a kind of massless particle
	is a kind of neutral particle
gravity	has carrier boson graviton
	has definition As described first by Isaac Newton, gravity is a force that exists between bodies of any mass whatever (from particles to stars) in proportion to the product of their masses, and in inverse proportion to the square of the distance between them. The weakest of the four natural forces (the other three being the electromagnetic and the two nuclear interactive forces), its real nature is still not fully understood. Einstein's General Theory of Relativity presented another viewpoint.
	has definition Fundamental force of nature, generated by all particles that possess mass. Interpreted by means of Newtonian mechanics or by the general theory of relativity.
	has definition In Aristotelian physics, an innate tendency of the elements earth and water to fall. In Newtonian physics, the universal, mutual, attraction of all massive objects for one another; its force is directly proportional to the mass of each object, and decreases by the square of the distance separating the objects involved. In Einstein's general relativity, gravity is viewed as a consequence of the curvature of space induced by the presence of a massive object. In quantum mechanics the gravitational field is said to be conveyed by quanta called gravitons.
	has definition One of the four fundamental forces of nature, and the one most different from the other three.
	has definition The mutual attraction between any two masses, as was first described accurately by Newton. Gravity appears strong because it has infinite range and it is always attractive (except for a false vacuum), but on a subatomic level gravity is the weakest of the known interactions; the gravitational force between a proton and an electron is 2 × 1039 times weaker than the electrical attraction.
	has definition The universal ability of all material objects to attract each other; F = Gm1m2 / r2.
	has definition The weakest of the four fundamental forces of nature, the gravitational force between any two masses is proportional to the product of the masses and varies inversely as the square of the distance between them. The other three fundamental forces are the electromagnetic force and two kinds of nuclear forces. (See electromagnetic force; nuclear forces.)
	has definition The weakest of the four fundamental forces of nature. Described by Newton's universal theory of gravity, and subsequently by Einstein's general relativity.
	has range infinite
	has strength (relative to electromagnetism)
	is a kind of fundamental force
gravity law	has domain gravity
	is a kind of law
gravity wave telescope	is a kind of telescope
gray	has base unit m2·s-2
	has symbol Gy
	has unit J·kg-1
	is an instance of named derived SI unit
	is an instance of radioactivity unit
	represents absorbed dose, specific energy (imparted), kerma
grazing-incidence telescope	has acronym XRT
	has definition A telescope used in X-ray and gamma-ray astronomy. It focuses these rays by making use of the fact that they behave like light rays if they strike surfaces at a shallow enough angle.
	has number of concentric surfaces
	is a kind of X-ray space telescope
great circle	has definition A circle on the surface sphere whose diameter equal to the diameter of the sphere.
	is a kind of circle
Great Rift	has definition A split in the Milky Way between Cygnus and Sagittarius caused by a succession of large, overlapping dark clouds in the equatorial plane of the Galaxy.
	has distance from Sun 100 pc
	is a part of Orion arm
Great Wall	has definition A sheet of galaxies which stretches more than 500 million light-years across the sky.
	is a kind of collection of galaxies
greatest elongation	has definition The instants when the geocentric angular distances of Mercury or Venus are at a maximum from the Sun.
	has elongation varies
	is a kind of planetary elongation event
	is an instance of planetary elongation
green	is a kind of optical
Gregorian	has definition A class of reflecting telescope which uses a concave secondary mirror placed after the prime focus is reached instead of a convex secondary placed before the prime focus.
	has definition Telescope devised - but never constructed - by James Gregory, in which an auxiliary concave mirror reflects the magnified image, the right way up, through a hole in the centre of the main objective mirror, i.e., through the end of the telescope itself. The Cassegrain telescope is similar but produces an inverted image.
	has secondary mirror shape concave
	is a kind of reflector
grism	has definition This is a right-angled glass prism with a transmission diffraction grating deposited on the hypotenuse surface. The spectrum produced by the grating is deflected by the prism to remain on the optical axis and the apex angle of the prism is chosen to get a certain wavelength in the center of the detector. Grisms can be placed in a filter wheel.
	is a kind of diffraction grating
Groombridge 1830	has definition A famous halo star whose proper motion, discovered in 1841, was then the largest known, displacing that of 61 Cygni.
	has distance 28 light-years
	is a part of Ursa Major
	is an instance of high proper motion star
Grosser Refraktor	has altitude 241 m
	has aperture 0.67 m
	has creation date 1880
	has focal ratio f/15.7
	has latitude 48° 14' N
	has lens maker Grubb
	has location Vienna, Austria
	has longitude 16° 20' E
	has mounting manufacturer Grubb
	has owner Astronomisches Inst., Univ. Obs.
	is an instance of German equatorial telescope
	is an instance of refractor
Grosser Refraktor^2	has altitude 41 m
	has aperture 0.68 m
	has creation date 1896
	has focal ratio f/30.9
	has latitude 52° 29' N
	has lens maker C.A. Steinheil
	has location Alt Treptow, Berlin, Germany
	has longitude 13° 29'E
	has mounting manufacturer Hoppe-Berlin
	has owner Arehenhold-Sternwarte
	is an instance of German equatorial telescope
	is an instance of refractor
ground state	has definition The state in which all electrons are in the lowest possible energy states.
	is a kind of energy
group III element	has group 13
	is a kind of column grouped element
group IV element	has group 14
	is a kind of column grouped element
group V element	has group 15
	has synonym pnictogen
	is a kind of column grouped element
group VI element	has group 16
	has synonym chalcogen
	is a kind of column grouped element
Grus	has acronym Gru
	has boundary
	has genitive Gruis
	has historical image
	has historical origin the crane or Phoenicopterus (Flamingo), in the past used to be part of Piscis Austrinus.
	has synonym Crane
	is a part of celestial sphere
	is an instance of constellation named by Bayer
gyrofrequency	has definition The frequency with which an electron or other charged particle executes spiral gyrations in moving across a magnetic field.
	is a kind of frequency
H alpha	has upper energy level 3
	has wavelength 6563 Å
	is an instance of Balmer line
	is an instance of Fraunhofer line
	is an instance of spectral line
h and Chi Persei	has definition A double stellar association. It contains many young O and B stars and also many M supergiants.
	has distance 2 kpc
	has synonym double cluster in Perseus
	has synonym Perseus OB1
	is a part of Perseus
	is a part of Perseus arm
	is an instance of naked eye object
	is an instance of OB association
H and K emission line star	has definition Late objects (F4 to M), which exhibit emission features in their Ca II HK lines.
	has emission line Ca II HK lines
	is a kind of emission line star
	is a kind of late star
H beta	has upper energy level 4
	has wavelength 4861 Å
	is an instance of Balmer line
	is an instance of Fraunhofer line
	is an instance of spectral line
H delta	has upper energy level 6
	has wavelength 4101 Å
	is an instance of Balmer line
	is an instance of spectral line
H gamma	has upper energy level 5
	has wavelength 4342 Å
	is an instance of Balmer line
	is an instance of spectral line
H II condensation	has definition A high-density H II region.
	is a kind of H II region
H II region	has composition ionized hydrogen
	has definition An area of ionized hydrogen. Most H II regions are red and arise from hot blue O and B stars, whose ultraviolet light can ionize all the hydrogen for dozens or even hundreds of light-years in every direction. The most famous H II region is the Orion Nebula.
	has definition Hydrogen gets ionized by hot O and B stars in H II regions. The most famous H II region is the Orion Nebula.
	has definition Region of ionized hydrogen in interstellar space. H II regions occur near stars with high luminosities and high surface temperatures. The kinetic temperature of H II regions is about 10,000-20,000 K, and the density is about 10 atoms per cm3. Ionized hydrogen, of course, having no electron, does not produce spectral lines; however, occasionally a free electron will be captured by a free proton and the resulting radiation can be studied optically (see also radio recombination lines).
	is a kind of nebula
H magnitude	has band 1.6 microns
	has definition The magnitude derived from observations at 1.6 microns.
	is an instance of infrared magnitude
h-line	has definition Singly ionized magnesium resonance line.
	has species Mg II
	has wavelength 2803 Å
	is a kind of absorption line
Hades	has definition An unofficial name for Jupiter IX, the outermost satellite of Jupiter.
	has discoverer Nicholson
	has discovery date 1914
	has eccentricity e = 0.28
	has inclination i = 156°
	has orbital period P = 758 days retrograde
	has synonym Jupiter IX
	is a part of Jupiter
	is an instance of natural satellite
hadron	has definition High mass elementary particles that are influenced by the strong nuclear force.
	is a kind of elementary particle
hadron era	has definition The Big Bang era during which quantum and general-relativistic effects are expected to modify each other in an unknown way.
	has definition The Big Bang era when the Universe was matter-dominated, containing many hadrons in equilibrium with the radiation field and when kT ≈ mπ. The hadron era ended when the characteristic photon energy fell below the rest mass of a pion or π-meson (270 electron masses), and very few hadrons remained (about one hadron for every 108 photons).
	has density ρ = 1093 g cm-3
	has duration 10-5 s
	has start time 10-43 s after Big Bang
	has synonym hadron barrier
	is a kind of Big Bang era
	is followed by lepton era
	is preceded by Planck era
hafnium	has image
	has ocean oxidation state IV
	is a kind of transition metal
	is a kind of unclassified oceanic element
Hale	has associate Shapley
	has birth date 29 June 1868
	has career 1886-1996 : Harvard & private observatory 1887-1904 : Yerkes/Chicago 1904-1923 : Mt. Wilson 1923-1938 : honorary director & private observatory
	has death date 21 February 1938
	has degree 1890 : BSc. MIT 12 honorary PhDs
	has image
	has name George Ellery Hale
	has name George Ellery Hale
	has orbituary Adams, W.S. : 1938, ApJ 87, 369. Babcock, H.D. : 1938, PASP 50, 156. Dunham, T. : 1938, MNRAS 99, 322.
	is an instance of 19th century scientist
	is an instance of astronomer
half-life	has definition Time it takes for the number of particles to halve. For a radioactive substance, the length of time required for half the atoms to disintegrate.
	has symbol τ1/2
	is a kind of time
	see also mean life
half-power beamwidth	has acronym HPBW
	has definition The angle across the main lobe of an antenna pattern between the two directions where the sensitivity of the antenna is half the value at the center of the lobe. This is the nominal resolving power of the antenna system.
	is a kind of angle
Halley's comet	has definition Probably the best known of all comets. Its orbit was computed by Edmund Halley in 1704, at which time he predicted that the bright comet of 1682 would return in 1758 (Halley died in 1742, before he could see his prediction verified). Records of Halley's comet (a = 17.8 AU, e = 0.967, i = 162°.3, P = 76.2 yr perihelion distance 0.587 AU) have been traced back to 240 B.C. Last appearance 1910, next appearance 1986.
	is a part of our solar system
	is an instance of comet
halo	has definition A spherical aggregation of stars, globular star clusters, and thin gas clouds, centered on the nucleus of the galaxy and extending beyond the known extremities of the galactic disk.
	has definition the galactic halo, however, describes the spherical collection of stars forming a surrounding "shell" for our otherwise compact, discoid Galaxy.
	has definition The somewhat round population of old, metal-poor stars in the Milky Way. Also, the huge entity that surrounds the disk and contains most of the Galaxy's dark matter. To distinguish between the two, astronomers call the former the stellar halo and the latter the dark halo. Most of the stellar halo lies closer to the Galactic center than the Sun, while most of the dark halo lies farther from the Galactic center than the Sun.
	is a part of Milky Way
halogen	has group 17
	has synonym group VII element
	is a kind of column grouped element
	is a kind of nonmetallic element
hard gamma ray	is a kind of gamma ray
hard X-ray	is a kind of X-ray
Harlan J. Smith Telescope	has altitude 2075 m
	has aperture 2.72 m
	has creation date 1969
	has focal ratio f/3.9, 8.8, 18
	has latitude 30° 4d N
	has location Mount Locke, Texas, US
	has longitude 104° 01' W
	has mirror maker Davidson Optronics
	has mirror type Fused-silica
	has mounting Cross-axis equatorial
	has mounting manufacturer Westinghouse
	has owner McDonald Observatory
	has synonym 107 inch
	is an instance of Cross-axis equatorial telescope
	is an instance of optical telescope
	is an instance of Ritchey-Chrétien
harmonic motion	has amplitude
	has definition A motion that repeats itself in equal intervals of time. An oscillating particle in harmonic motion is a harmonic oscillator.
	has period
	has synonym periodic motion
	is a kind of motion
harmonic overtone	has definition Any integral multiple of the fundamental frequency
	is a kind of frequency
Haro galaxy	has definition Blue object whose spectra show sharp emission lines.
	is a kind of emission line galaxy
Hartree energy	has equation
	has symbol Eh
	has uncertainty 0.00000034 × 10-18 J
	has value 4.35974381 × 10-18 J
	is an instance of atomic constant
	is an instance of energy
Harvard College Observatory	has owner Harvard
	is an instance of observatory
hassium	is a kind of transactinide
HD 141569	has distance 320 light years
	has image
	is a part of Libra
	is an instance of star
He II Balmer alpha	has wavelength 1640 Å
	is an instance of ionized helium Balmer line
He II Lyman alpha	has wavelength 303.78 Å
	is an instance of ionized helium Lyman line
	is an instance of resonance line
He II Paschen alpha	has wavelength 4686 Å
	is an instance of ionized helium Paschen line
He2-104	has image
	is an instance of symbiotic star
head-tail galaxy	has definition A class of relatively weak radio sources associated with clusters of galaxies and characterized by a high-brightness "head" close to the optical galaxy and a long low-brightness "tail".
	is a kind of radio galaxy
HEAO	has definition High-Energy Astronomical Observatory.
	is an instance of X-ray space telescope
heavy quark	is a kind of quark
heavy-metal star	has definition A class of peculiar giant that includes the Ba II stars and the S stars.
	is a kind of giant
hecto	has symbol h
	has value 102
	is an instance of unit prefix
height	has definition Elevation above ground or distance upwards from a given level (especially sea level) to a fixed point. (See altitude.)
	is an instance of geocentric coordinate component
	is an instance of length
Hektor	has albedo 0.28
	has asteroid number 624
	has definition Largest and brightest Trojan asteroid.
	has diameter 100 km
	has obliquity large
	has rotation period 6.9225 hours
	has shape elongated
	has visual magnitude +14.5 (at mean opposition)
	is a part of Trojan asteroid
	is an instance of Trojan asteroid
heliocentric cosmology	has definition School of models in which the sun was portrayed as standing at the center of the universe.
	is a kind of outdated belief
heliograph	has definition Device for recording the positions of sunspots.
	is a kind of integrating detector
helion mass	applies to particle helion
	has symbol mh
	has uncertainty 0.00000039 × 10-27 kg
	has value 5.00641174 × 10-27 kg
	is an instance of particle mass
helium	has abundance 0.008 p.p.m. in Earth's crust
	has abundance 4 × 10-6 p.p.m. in seawater
	has abundance 5.2 p.p.m. by volume in Earth's atmosphere
	has abundance 6.31 × 1010 in Sun relative to H = 1 × 1012
	has atomic emission line 1083.025 nm for He I
	has atomic emission line 1868.534 nm for He I
	has atomic emission line 2058.130 nm for He I
	has atomic emission line 388.865 nm for He I
	has atomic emission line 587.562 nm for He I
	has atomic emission line 1083.034 nm for He I (strong)
	has atomic number 2
	has atomic radii 128 pm
	has biological role none
	has boiling point 4.216 K
	has chief source natural gas which may contain up to 7% helium
	has critical pressure 229 kPa
	has critical temperature 5.25 K
	has crystal cell dimension a = 353.1, c = 569.3 pm for α-He at 1.15 K, 6.69 MPa
	has crystal cell dimension a = 411 pm for γ-He at 1.73 K, 2.94 MPa
	has crystal cell dimension a = 424.0 pm for β-He at 16K, 127 MPa
	has crystal type b.c.c. for γ-He at 1.73 K, 2.94 MPa
	has crystal type f.c.c. for β-He at 16K, 127 MPa
	has crystal type h.c.p. for α-He at 1.15 K, 6.69 MPa
	has daily dietary intake very low
	has definition
	has definition Colourless, odorless gas obtained mainly from gas wells. Element which, after hydrogen, is the second lightest and second most abundant in the Universe. Its atom comprises two protons and two electrons. The nucleus of helium 4 is sometimes called an alpha particle. Helium is the product of the nuclear fusion of hydrogen in most stars, but this does not explain the overall helium abundance. Most of it was produced by the big bang, with main-sequence stars making an additional contribution.
	has density 0.1785 kg m-3 for gas at 273 K
	has density 124.8 kg m-3 for liquid at 4.216 K boiling point
	has discoverer Norman Lockyer and Edward Frankland
	has discovery date 1868
	has discovery location England
	has electron affinity 0.0 kJ mol-1 from to -
	has electron configuration 1s2 = [He] in ground state
	has hazard harmless but could asphyxiate if it excludes oxygen from the lungs
	has heat capacity 20.786 J K-1 mol-1 for gas at constant pressure 0.1 MPa at 298.15 K
	has heat of fusion 0.021 kJ mol-1
	has heat of vaporization 0.082 kJ mol-1
	has image
	has isotope mass range 3 to 8 except 7
	has lethal intake non-toxic
	has level in humans traces
	has longest lived isotope helium 4
	has mass absorption coefficient 0.207 cm2 g-1 for MoKα X-ray diffraction
	has mass absorption coefficient 0.383 cm2 g-1 for CuKα X-ray diffraction
	has mass magnetic susceptibility -5.9 × 10-9 kg-1 m3 for gas
	has mass of element in person very small for a 70 kg average person
	has melting point 0.95 K under pressure
	has mineral present in some minerals
	has molar volume 32.07 cm3 at 4 K
	has name origin helios = sun from Greek
	has neutron scattering length 0.326 × 10-12 cm
	has number of isotopes 5 including nuclear isomers
	has number of protons 2
	has ocean oxidation state 0
	has pronunciation heel-iuhm
	has registry number 7440-59-7 for Chemical Abstracts System database
	has relative atomic mass 4.005602 in units of 12C = 12.000
	has reserves 3.7 × 109 tonnes in Earth's atmosphere
	has space group Fm3m for β-He at 16K, 127 MPa
	has space group Im3m for γ-He at 1.73 K, 2.94 MPa
	has space group P63/mmc for α-He at 1.15 K, 6.69 MPa
	has specimen small pressurized canisters. Safe.
	has stable isotope helium 3, helium 4
	has symbol He
has synthesis mechanism
has term symbol 1S0 in ground state
has thermal conductivity 0.152 W m-1 K-1 at 300 K
has thermal neutron capture cross section 0.007 barns
has toxic intake non-toxic
has uses deep-sea diving, weather baloons, low temperature research
has van der Waals radii 120 pm
has world production 4500 tonnes year-1
is a kind of inert gas
is a kind of light element
is a kind of unclassified oceanic element
reacts with nothing
helium 3	has atomic mass 3.01602931
	has magnetogyric ratio -20.378 × 107 rad T-1 s-1
	has natural abundance 0.000137 % stable isotope
	has NMR frequency 76.178 MHz where 1H = 100 MHz; 2.3488 T
	has NMR receptivity 0.00326 where 13C = 1.00
	has nuclear magnetic moment μ = -2.127624
	has nuclear spin I = 1/2
	has number of neutrons 1
	has number of nucleons 3
	has relative NMR sensitivity 0.44 where 1H = 1.00
	has symbol 3He
	has uses Nuclear Magnetic Resonance, tracer
	is an instance of helium
helium 4	has atomic mass 4.00260324
	has natural abundance 99.999863 %
	has nuclear magnetic moment μ = 0
	has nuclear spin I = 0+
	has number of neutrons 2
	has number of nucleons 4
	has symbol 4He
	is an instance of helium
helium 6	has atomic mass 6.018886
	has decay mode β- (3.5010 Mev) 100 %
	has decay mode γ
	has half life 0.807 seconds
	has nuclear spin I = 0+
	has number of neutrons 4
	has number of nucleons 6
	has symbol 6He
	is an instance of helium
helium 8	has atomic mass 8.03392
	has decay mode β- (14 Mev)
	has decay mode β- n
	has decay mode γ
	has half life 0.119 seconds
	has nuclear spin I = 0+
	has number of neutrons 6
	has number of nucleons 8
	has symbol 8He
	is an instance of helium
helium burning	has definition The stage when a star fuses helium into carbon and oxygen. All stars born with more than half a solar mass eventually burn helium.
	has duration 5 × 105 years
	has next higher temperature reaction carbon burning
	has optimum density 700 g/cm3
	has optimum temperature 2 × 108 K
	has part product carbon
	has product carbon, oxygen
	has reactant helium
	is a kind of exothermic fusion process
	requires minimum mass at star birth 0.5 solar masses
Helium discontinuity	has species helium (He I)
	is a kind of absorption edge
helium flash	has definition The onset of runaway helium burning under degenerate conditions. The helium flash occurs in the hydrogen-exhausted core of a star in the red-giant phase of evolution. When gravitational pressure has brought the degenerate core to a temperature of about 108 K, the helium nuclei can start to undergo thermonuclear reactions. Once the helium burning has started, the temperature builds up rapidly (without a cooling, stabilizing expansion), and the extreme sensitivity of the nuclear reaction rate to temperature causes the helium-burning process to accelerate. This in turn raises the temperature, which further accelerates the helium burning, until a point is reached where the thermal pressure expands the core and thus removes the degeneracy and limits the flash. The helium flash can only occur when the helium core is less than the 1.4 Msun Chandrasekhar mass limit and thus it is restricted to low-mass stars.
	has minimum temperature 108 K
	is a kind of helium burning
helium problem	has definition Poses the question: what physical process caused the current abundance of helium in the Universe?
	is an instance of problem
helium shell flash	has definition It has been shown that helium shell burning outside a degenerate core is unstable; the helium-burning shell does not generate energy at a constant rate but instead produces energy primarily during short flashes. During a flash, the region just outside the helium-burning shell becomes unstable to convection. The resultant mixing probably leads to the s-process as well as to the upward movement of carbon produced by helium burning.
	is a kind of helium burning
helium temperature scale	has approval date 1958
	has definition The vapour pressure of helium 4 is used as an indication of temperature in the region 1 to 5.2 K.
	is an instance of temperature unit
helium variable star	has definition Bp star in which the strength of the helium lines varies periodically. At the extreme phases the objects appear as helium-rich, whereas at other phases He can be very weak or absent.
	is a kind of Bp star
helium-strong star	has definition B star in which the helium lines are stronger than in normal stars. One distinguishes usually the extreme helium stars (also called hydrogen-deficient stars), in which no trace of hydrogen is seen, and the intermediate helium-rich stars, in which the hydrogen lines are still visible, but weaker than in normal stars. Related to these objects are the hydrogen deficient C stars.
	is a kind of B star
helium-weak star	has definition B-type star in which the helium lines are weaker than in normal stars.
	is a kind of Bp star
Helix Nebula	has definition A planetary nebula with the largest known angular diameter.
	has distance 140 pc
Helix nebula	has distance 450 light years
	has image
	has synonym NGC 7293
Helix Nebula	has synonym NGC 7293
	is a part of Aquarius
	is an instance of planetary nebula
Helix nebula	is an instance of planetary nebula
	is part of Aquarius
henry	has base unit m2·kg·s-2·A-2
	has symbol H
	has unit Wb·A-1
	is an instance of inductance unit
	is an instance of named derived SI unit
Henry Draper catalog	has number of objects 225300
	has object properties position, spectral type and magnitude
	has publication date 1924
	is a kind of star catalog
Hera	has definition Unofficial name for Jupiter VII.
	has discoverer Perrine
	has discovery date 1905
	has eccentricity e = 0.21
	has inclination i = 28°
	has orbital period P = 259.65 days
	has synonym Jupiter VII
	is a part of Jupiter
	is an instance of natural satellite
Hercules	has acronym Her
	has boundary
	has genitive Herculis
	has historical origin Hercules defeated the Nemean Lion (Leo), the many-headed beast (Hydra) and the little crab (Cancer)
	is a part of celestial sphere
	is an instance of constellation
Hercules cluster	has definition An unsymmetrical cluster. Half of the galaxies are spiral or irregular and about half elliptical or S0. It contains a rather large number of disturbed and peculiar galaxies. The "missing mass", if present, must constitute more than 95% of the total.
	has number of galaxies 75
	has redshift z = 0.036
	has synonym 3U 1551 + 15
	is a part of Hercules
	is a part of Local Supercluster
	is an instance of galaxy cluster
Hercules X-1	has definition An X-ray pulsar, a member of an occulting binary system. The visible component has been identified as the blue variable HZ Herculis, whose spectrum varies from late A or early F to B. Her X- l has a pulsation period of 1.2378 seconds, presumably its rotation period, and exhibits a 35-day quasi-periodicity in the X-ray region (but not in the optical). It is probably a rotating neutron star in a circular orbit with a mass of about 0.7 Msun, which is accreting matter from HZ Her. The orbital period is stable, but the pulsation period is speeding up at a rate of about 1 part in 105 per year. The X-ray eclipse lasts 0.24 days.
	has distance 5 kpc
	has eccentricity e < 0.1
	has eclipse duration 0.24 days (in X-rays)
	has orbital period 1.7 days
	has synonym 3U 1653+35
	is a part of Hercules
	is an instance of eclipsing binary
	is an instance of X-ray pulsar
hertz	has base unit s-1
	has definition A unit of frequency equal to one cycle (or wave) per second.
	has symbol Hz
	is a kind of frequency unit
	is an instance of named derived SI unit
Hertzsprung	is an instance of 19th century scientist
	is an instance of astronomer
Hestia	has definition Unofficial name for Jupiter VI.
	has discoverer Perrine
	has discovery date 1904
	has eccentricity e = 0.16
	has inclination i = 29°
	has orbital period P = 250 days
	has synonym Jupiter VI
	is a part of Jupiter
	is an instance of natural satellite
heterotic e-string theory	has definition One of the five superstring theories; involves closed strings whose right-moving vibrations resemble those of the Type II string and whose left-moving vibrations involve those of the bosonic string. Differs in important but subtle ways from the Heterotic-O string theory.
	has synonym E8 × E8
	is a kind of heterotic string theory
heterotic o-string theory	has definition One of the five superstring theories; involves closed strings whose right-moving vibrations resemble those of the Type II string and whose left-moving vibrations involve those of the bosonic string. Differs in important but subtle ways from the Heterotic-E string theory.
	has synonym O(32)
	is a kind of heterotic string theory
heterotic string theory	has definition Gross's version of string theory in which space-times of different dimensions are associated with the same closed loop.
	is a kind of superstring theory
HF band	has frequency 3 to 30 MHz
	has wavelength 10 to 100 m
	is a kind of radio
Hidalgo	has asteroid number 944
	has definition Asteroid with the largest known orbit with the second highest inclination to the ecliptic and second highest eccentricity of any asteroid.
	has diameter 20 km
	has discoverer Baade
	has discovery date 1920
	has eccentricity e = 0.66
	has inclination 42°.5
	has orbirtal period 13.7 years
	has semi-major axis a = 5.8 AU
	is a part of asteroid belt
	is an instance of asteroid
hierarchy problem	has definition In the context of grand unified theories, the hierarchy problem is our inability to understand theoretically why the energy scale at which the unification becomes apparent, about 1016 GeV (billion electron volts), is so much higher than other energy scales of relevance to particle physics, such as the mass/energy of a proton, which is only 1 GeV.
	is an instance of problem
Higgs boson	has definition A hypothetical, spinless particle that plays an important role in the Glashow-Weinberg-Salam electroweak theory (and in other theories involving spontaneous symmetry breaking, e.g. GUTs).
	has definition The particle or particles associated with the bundles of energy in the Higgs field. Such particles are analogous to the photons that are associated with the electromagnetic field. The standard model of particle physics predicts one electrically neutral Higgs particle which has not yet been found, but which will be sought in upcoming particle accelerator experiments. The grand unified theories predict many Higgs particles, but they are too massive to be accessible at existing or foreseeable accelerators.
	is a kind of boson
	is a kind of hypothetical particle
High Energy Astrophysics Science Archive Research Center	HEASARC -has URL: http://heasarc.gsfc.nasa.gov
	is an instance of astronomical institution
high energy detector	has definition Device for recording the presence of subatomic particles. A typical modem detector consists of an array of electronic sensors connected to a computer, capable of recording the paths of the particles as they fly out from the collision site in a particle accelerator.
	is a kind of instrument
High energy peaked BL Lac object	has acronym HBL
	is a kind of BL Lacertae
high energy physics	is a kind of physics
high energy physics institution	is a kind of institution
high proper motion star	has proper motion high
	is a kind of high-velocity star
high-velocity star	has definition A star whose U and/or V and/or W velocities are much greater or much less than zero. Such stars usually have eccentric orbits around the Galaxy.
	has definition Late type stars whose spatial velocities are greater than 100 km s-1. Other authors prefer the definition, with radial velocities greater than 60 km s-1.
	has spatial velocity > 100 km s-1
	is a kind of star
higher-dimensional supergravity	has definition Class of supergravity theories in more than four spacetime dimensions.
	is a kind of supergravity
highly polarized quasar	has acronym HPQ
	is a kind of quasar
Hiltner Telescope	has altitude 1938 m
	has aperture 2.34 m
	has comment Mirrors repolished 1991
	has creation date 1986
	has focal ratio f/2.07, 13.5
	has latitude 31° 57' N
	has location Kitt Peak, Arizona, US
	has longitude 111° 37' W
	has mirror maker Contraves (USA)
	has mirror type Cer-Vit
	has mounting Equatorial fork, friction-disk drives
	has mounting manufacturer DFM Engineering, L & F Industries
	has owner Michigan-Dartmouth-MIT Obs.
	has synonym Hiltner 2.3 m
	is an instance of Fork equatorial telescope
	is an instance of optical telescope
	is an instance of Ritchey-Chrétien
Hind's nebula	has definition A reflection nebula illuminated by the star T Tauri. It is remarkable for its changes in brightness.
	has discoverer Hind
	has discovery date 1852
	has synonym NGC 1554, NGC 1555
	is a part of dust
	is a part of Taurus
	is an instance of reflection nebula
Hiparcos catalog	is a kind of star catalog
Hipparcos	is an instance of ESA satellite
Hirayama family	has definition A group of asteroids with similar orbital elements. The members of a given family are widely believed to have resulted from collisions between larger parent bodies.
	is a kind of asteroid
	is a part of asteroid belt
HK line	is a kind of interstellar line
HK lines	has definition Two spectral lines of singly ionized calcium. The second strongest interstellar line.
	has species Ca II
	is a kind of Fraunhofer line
Hobby-Eberly Telescope	has altitude 2002 m
	has aperture 9.2 m equivalent
	has comment A project of five universities in US and Germany; 91 spherical mirrors with combined focus for spectroscopy
	has creation date 1997
	has focal ratio (f/1.4) f/4.7
	has latitude 30° 41' N
	has location Mount Fowlkes, Texas, US
	has longitude 104° 01' W
	has mirror maker Univ. of Texas, Penn. State
	has mounting altazimuth fixed in altitude but rotates in azimuth
	has optical design Spherical figure
	has owner University of Texas
	has purpose spectroscopic survey telescope
	is an instance of altazimuth telescope
	is an instance of optical telescope
	is an instance of reflector
hold-time	has definition The time taken to use up all the liquid cryogens, like LN2, in a cooled CCD cryostat.
	is an instance of time
Holmberg radius	has definition The radius of an external galaxy at which the surface brightness is 26.6 mag arcsec-2. This criterion was developed by Holmberg in 1958 to estimate the actual dimensions of the major and minor axes of a galaxy without regard to its orientation in space.
	is an instance of radius
holmium	has image
	has ocean oxidation state III
	is a kind of rare Earth
	is a kind of recycled oceanic element
hologram	has definition An interferometric method of recording information about the three-dimensional nature of an object which relies on preserving both the amplitudes and phases of the wavefronts which reach the detector, instead of merely the amplitudes. Hologram means "whole record". The basic principle was outlined by D. Gabor in 1948.
	is a kind of integrating detector
honeycomb mirror	has definition A construction method for a large mirror in which the back is hollowed-out to leave a ribbed structure that resembles a honeycomb.
	is a kind of mirror
Hooker Telescope	has altitude 1742 m
	has aperture 2.5 m
	has comment telescope out of service 1985-92
	has creation date 1917
	has focal ratio f/5, 16, 30
	has latitude 34° 13' N
	has location Mount Wilson, Calif., US
	has longitude 118° 03' W
	has mirror maker G.W. Ritchey from Saint-Gobain (Paris)
	has mirror type plate-glass
	has mounting manufacturer FG. Pease and Fore River Shipyards
	has owner Mount Wilson Observatory
	has synonym 100 inch
	is an instance of Cassegrain
	is an instance of English equatorial telescope
	is an instance of optical telescope
horizon	has definition A plane perpendicular to the line from an observer to the zenith. The great circle formed by the intersection of the celestial sphere with a plane perpendicular to the line from an observer to the zenith is called the astronomical horizon.
	has definition The maximum distance that an observer can see. In cosmology, our horizon is the distance from us that light has traveled since the beginning of the universe. Objects more distant than our horizon are invisible to us because there hasn't been enough time for light to have traveled from there to here.
	is a kind of plane
horizon problem	has definition A problem of the traditional big bang theory (without inflation) related to the large scale uniformity of the observed universe. The problem is seen most clearly in the cosmic background radiation, which is believed to have been released at about 300000 years after the big bang, and has been observed to have the same temperature in all directions to an accuracy of one part in 100,000. Calculations in the traditional big bang theory show that the sources of the background radiation arriving today from two opposite directions in the sky were separated from each other, at 300000 years after the big bang, by about 100 horizon distances. Since no energy or information can be transported further than one horizon distance, the observed uniformity can be reconciled only by postulating that the universe began in a state of near-perfect uniformity. See also flatness problem.
	has definition A quandary in standard big bang theory, which indicates that few of the particles of the early universe would have had time to be in causal contact with one another at the outset of cosmic expansion. It appears to have been resolved in the inflationary universe theory.
	has definition Cosmological puzzle associated with the fact that regions of the universe that are separated by vast distances nevertheless have nearly identical properties such as temperature. Inflationary cosmology offers a solution.
	has definition The puzzle that widely separated regions of the universe are observed to share the same physical properties, such as temperature, even though these regions were too far apart when they emitted their radiation to have exchanged heat and homogenized during the time since the beginning of the universe. In particular, we detect the same intensity of cosmic radio waves (cosmic background radiation) from all directions of space, suggesting that the regions that emitted that radiation had the same temperature at the time of emission. However, at the time of emission, when the universe was about 1 million years old, those regions were separated by roughly 100 million light years, much exceeding the distance light or heat could have traveled since the big bang. The horizon problem is also called the causality puzzle. (See horizon.)
	has synonym homogeneity problem
	is a kind of problem
horizontal branch star	has color blue to yellow
	has definition A metal-poor star after it has undergone the helium flash and begins to quietly burn helium into carbon and oxygen in its core and hydrogen in a surrounding envelope.
	has location on H-R diagram extends shortward from the asymptotic branch at an approximately constant absolute bolometric magnitude of about 0.3
	is a kind of metal-poor star
horizontal parallax	has definition The difference between the topocentric and geocentric positions of an object, when the object is on the astronomical horizon.
	is a kind of parallax
Horologium	has acronym Hor
	has genitive Horologii
	has historical image
	has historical origin the clock in honor of Christian Huygens, the inventor of the pendulum clock in 1656-57
	has synonym Clock
	has synonym Horologium Oscillitorium
	is a part of celestial sphere
	is an instance of constellation named by de Lacaille
Horsehead Nebula	has definition An absorption nebula in the middle of Orion.
	has synonym NGC 2024
	is a part of dust
	is an instance of dark nebula
Horseshoe equatorial telescope	has mounting Horseshoe equatorial
	is a kind of equatorial telescope
hot dark matter	has definition Any form of dark matter which was relativistic at its point of decoupling.
	is a kind of dark matter
hour	has symbol h
	has value in SI unit 60 min = 3600 s
	is an instance of non SI unit
hour circle	has definition A great circle passing through the celestial poles - i.e., perpendicular to the celestial equator.
	is a kind of great circle
Hourglass Nebula	has definition A compact H II region in the center of Lagoon Nebula.
	has distance 8000 light-years away
	has image
	has synonym MyCn18
	is a part of Lagoon Nebula
	is an instance of compact H II region
	is an instance of planetary nebula
Hoyle-Narlikar theory	has definition A reformulation of the general theory of relativity that incorporates and extends Mach's principle (q.v.). In this theory, the inertial mass of a particle is a function of the masses of all other particles, multiplied by a coupling constant which is a function of cosmic epoch. In cosmologies based on this theory, the gravitational constant G decreases strongly with time.
	is a kind of cosmology theory
HR 4247	has B-V magnitude 1.04
	has declination +34 12 53
	has right ascension 10 53 18.6
	has spectral type K0III-IV
	has V magnitude 3.83
	is a part of Leo Minor
	is an instance of giant
	is an instance of K star
	is an instance of naked eye star
HR 4796A	has image
	is an instance of star
HRS	has definition High-Resolution Spectrograph (Hubble).
	is a kind of diffraction grating spectrograph
Hubble	has birth date November 20, 1889
	has career 1914-1917 : Assistant, Yerkes Observatory 1919-1953 : Mt. Wilson
	has death date September 28, 1953
	has degree 1910 : BSc. U. Chicago 1913 : law, Oxford 1917 : PhD. Yerkes, ("Photographic Investigations of Faint Nebulae", Frost as supervisor)
	has image
	has name Edwin Powell Hubble
	has orbituary Adams, W.S. : 1954, Observatory 74, 32. Humason, M.L. : 1954, MNRAS 114, 291. Robertson, H.P. : 1954, PASP 66, 120. Sandage, A. : 1989, JRASC 83, 352.
	is an instance of 19th century scientist
	is an instance of astronomer
Hubble 5	has distance 2200 light years
	has image
	is an instance of planetary nebula
	is part of Sagittarius
Hubble constant	has definition According to Hubble's law, discovered by Edwin Hubble in 1929, distant galaxies are receding from us, on average, with a speed equal to the product of the Hubble constant and the distance to the galaxy. Hubble's "constant" is independent of distance, but actually decreases slowly in time as the expansion is slowed by the gravitational pull of each galaxy on all the others. The present value is somewhere between 15 and 30 kilometers per second per million light-years.
	has definition Hubble's constant in units of 100 km s-1 Mpc-1.
	has definition The constant of proportionality in the Hubble law. Its value must vary with time, so it is often referred to as the Hubble parameter. The Hubble constant is generally used to mean the value of the Hubble parameter at the current epoch, and is somewhere between 50 and 100 km/s/Mpc with possibly a value close to 75 km/s/Mpc.
	has definition The present expansion rate of the universe, in units of kilometers per second per megaparsec. The larger the Hubble constant, the younger the universe.
	has definition The rate at which the universe expands, equal to approximately fifty kilometers of velocity per megaparsec of distance.
	has definition The rate of expansion of the universe. The Hubble constant is equal to the recessional speed of a distant galaxy, divided by its distance from us. Assuming a homogeneous and isotropic universe, the recessional speed of a distant galaxy is proportional to its distance; thus the Hubble constant as determined by any receding galaxy should be the same, yielding a universal rate of expansion of the universe. According to estimates, the current value of the Hubble constant is approximately 100 km/s/Mpc, meaning that the distance between any two distant galaxies will double in about 10 billion years at the current rate of expansion.
	has rate of change non-zero because gravity is slowing down the rate of expansion of the universe
	has symbol H0
	has unit km/s/Mpc
	has value 75 km/s/Mpc
	is an instance of astronomical constant
Hubble flow	has definition The movement of the galaxies away from us caused by the expansion of the Universe.
	is a kind of galaxy motion
Hubble law	has definition A relation which states that recessional speed is proportional to distance for a homogeneous and isotropic universe. Galaxies moving away from us with a speed precisely following this relation are said to follow the Hubble flow. Because the actual universe is not precisely homogeneous, with lumpiness arising from clustering of galaxies and voids of empty space, the motions of actual galaxies deviate somewhat from the Hubble flow.
	is a kind of cosmology theory
Hubble nebula	has definition A cometary nebula whose apex star is R Mon.
	has synonym NGC 2261
	is a part of dust
	is an instance of cometary nebula
Hubble radius	has definition The radius of the observable universe ().
	has symbol c/H
	has value > 1027 cm
	is an instance of astronomical constant
	is an instance of radius
Hubble Space Telescope	has acronym HST
	has acronym HST
	has angular resolving power 0.05 arcseconds
	has aperture 2.4 m
	has creation date 1981
	has creation date 1990
	has definition Hubble Space Telescope. A space-based reflecting telescope with a primary mirror diameter of 2.4 m (94 in) capable of high-resolution imaging from the far ultraviolet to the near infrared. A joint NASA/ESA mission. Launched in 1990 with a planned lifetime of 15 years. Encountered reduced performance when the mirror was found to have spherical aberration. Solved by the installation of corrective optics (COSTAR) in 1994.
	has effective diameter 2.1 meters
	has elliptical orbit 500 km altitude, 35 degree inclination
	has focal ratio f/12.9, 30, 48, 96
	has launch date 1986
	has location Earth orbit
	has mirror maker Perkin Elmer
	has mirror type Corning ULE glass
	has mounting 3-axis-stabilized spacecraft
	has mounting manufacturer Lockheed
	has owner Space Telescope Science Institute
	has reference Hubble Space Telescope from CADC
	is an instance of naked eye object
	is an instance of optical space telescope
	is an instance of optical telescope
	is an instance of Ritchey-Chrétien
	orbits Earth
Hubble time	has definition The Hubble time is one divided by the Hubble constant, which gives a number from 10 to 20 billion years. For a flat universe with no cosmological constant, the age of the universe is two-thirds of the Hubble time.
	has definition The inverse of the Hubble constant and a crude measure of the universe's age. For a Hubble constant of 50, one can calculate that the Hubble time is 19.6 billion years; for a Hubble constant of 80, the Hubble time is 12.2 billion years. If there is no cosmological constant, the universe is younger than the Hubble time. In particular, if the mass density of the universe (designated Ω) is 0.1, the universe's age is 90 percent of the Hubble time; if Ω is 1.0, the universe's age is 67 percent of the Hubble time.
	has symbol H0-1
	has value 10 to 20 billion years
	is an instance of astronomical constant
	is an instance of time
Hulse-Taylor pulsar	has definition A binary pulsar probably consisting of a neutron star and an even more compact object in an eccentric orbit.
	has discovery date 1974
	has orbital period 0.3230 days
	has pulsation period 59 ms
	has synonym PSR 1913+16
	is an instance of binary pulsar
Humason	has name Milton La Salle Humason
	is an instance of 19th century scientist
	is an instance of astronomer
	is associated with Mayall
Hyades	has age 5 × 108 yr
	has composition spectral types A1-K
	has distance 40 pc
	has note Aldebaran is a foreground star in that region of the sky, it is not a member of the Hyades.
	has number of stars 200
	has radial velocity + 36 km s-1
	is a part of Taurus
	is an instance of is an instance of naked eye object
	is an instance of open cluster
hybrid array	has definition A device in which the roles of radiation (infrared mostly) detector and signal multiplexer are separated. The device is a sandwich of two slabs. Other names include focal plane array (FPA) and sensor chip assembly (SCA).
	is a kind of detector
Hydra	has acronym Hya
	has genitive Hydrae
	has synonym Serpent of Lerna
	has synonym Water Monster
	is a part of celestial sphere
	is an instance of constellation
hydrodynamic soliton	has definition A finite-amplitude disturbance which is propagated through a fluid without any change of shape. MHD solitons are also known.
	is a kind of mass motion
hydrodynamics	has definition The study of how gases and fluids flow under applied forces.
	is a kind of dynamics
hydrogen	has abundance 0.5 p.p.m. by volume in Earth's atmosphere
	has abundance 1 in Sun relative to H = 1 × 1012
	has abundance 1520 p.p.m. in Earth's crust
	has abundance main constituent of water; some dissolved H2 in seawater
	has atomic emission line 1875.10 nm for H I
	has atomic emission line 434.047 nm for H I
	has atomic emission line 486.133 nm for H I
	has atomic emission line 656.272 nm for H I
	has atomic emission line 656.285 nm for H I (strong)
	has atomic number 1
	has atomic radii 78 pm
	has biological role constituent element of DNA. Component of water, essential to life.
	has boiling point 20.28 K
	has chief source natural methane gas
	has covalent radii 30 pm
	has critical pressure 1297 kPa
	has critical temperature 33.35 K
	has daily dietary intake mainly as water
	has definition
	has definition Colourless, odourless gas, insuluable in water. Lightest, most abundant element in the Universe. Its atom comprises one proton and one electron. The element occurs both in stars and as interstellar clouds, in regions where it may be neutral (H I regions) or ionized (H II regions). It was produced by the big bang. Hydrogen 1 is the most common isotope; deuterium, is rarer; and tritium, is radioactive.
	has density 0.08988 for gas at 273 K
	has density 70.8 kg m-3 for liquid at 20.28 K boiling point
	has density 76.0 kg m-3 for solid at 11 K
	has discoverer H. Cavendish
	has discovery date 1766
	has discovery location London, England
	has electron affinity 72.8 kJ mol-1 from H to H-
	has electron configuration 1s1 in ground state
	has electronegativity 2.20 Pauling
	has French translation hydrogene
	has German translation Wasserstoff
	has hazard flammable and explosive when mixed with air; can asphyxiate if it excludes oxygen from lungs
	has heat capacity 20.784 J K-1 mol-1 for atomic gas at constant pressure 0.1 MPa at 298.15 K
	has heat capacity 28.824 J K-1 mol-1 for molecular gas (H2) at constant pressure 0.1 MPa at 298.15 K
	has heat of fusion 0.12 kJ mol-1
	has heat of vaporization 0.46 kJ mol-1
	has image
	has ionic radii 0.00066 pm for H+
	has ionic radii 154 pm for H-
	has isotope mass range 1 to 3
	has Italian translation idrogeno
	has lethal intake non-toxic
	has level in humans 52000 p.p.m. in bone
	has level in humans 93000 p.p.m. in liver
	has level in humans 93000 p.p.m. in muscle
	has level in humans constituent of water in blood
	has longest lived isotope hydrogen 1
	has main mining area
	has mass absorption coefficient 0.380 cm2 g-1 for MoKα X-ray diffraction
	has mass absorption coefficient 0.435 cm2 g-1 for CuKα X-ray diffraction
	has mass magnetic susceptibility -2.50 × 10-8 kg-1 m3 for gas
	has mass of element in person 7 kg for a 70 kg average person
	has melting point 14.01 K
	has mineral water
	has molar volume 13.26 cm3
	has name origin hydro genes = water forming from Greek
	has neutron scattering length -0.37390 × 10-12 cm
	has number of isotopes 3
	has number of protons 1
	has ocean oxidation state I
	has pronunciation hy-dro-jen
	has registry number 1333-74-0 for Chemical Abstracts System database
	has relative atomic mass 1.00794 in units of 12C = 12.000
	has reserves almost limitless
has Spanish translation hidrogeno
has specimen small pressurized canisters. Warning !
has stable isotope hydrogen 1, deuterium
has symbol H
has synthesis mechanism CH4 + 2H2O = 3H2 + CO (main production mechanim)
has synthesis mechanism electrolysis of brine using a mercury amalgam cell (minor production mechanism)
has synthesis mechanism steam flowing over red hot coke produces H2 and CO (minor production mechanism)
has term symbol 2S1/2 in ground state
has thermal conductivity 0.1815 W m-1 K-1 at 300 K for gas
has thermal neutron capture cross section 0.3326 barns
has toxic intake non-toxic
has triple point temperature 13.96 K at 7.2 kPa
has uses making ammonia, cyclohexane, methanol, etc.
has van der Waals radii 120 pm
has world production 350 × 109 m3 year-1 as H2 gas
is a kind of atmophile element
is a kind of gaseous element
is a kind of light element
is a kind of nonmetallic element
is a kind of unclassified oceanic element
reacts with air explosively
hydrogen 1	has atomic mass 1.0078
	has crystal cell dimension a = 377.6, c = 616.2 pm for h.c.p.
	has crystal cell dimension a = 450, c = 368 pm for tetragonal
	has crystal cell dimension a = 533.8 pm for cubic
	has crystal type cubic for cubic
	has crystal type h.c.p. for h.c.p.
	has crystal type tetragonal for tetragonal
	has magnetogyric ratio 26.7510 × 107 rad T-1 s-1
	has natural abundance 99.985 %
	has NMR frequency 100.000 MHz where 1H = 100 MHz; 2.3488 T
	has NMR receptivity 5680 where 13C = 1.00
	has nuclear magnetic moment μ = 2.7928456
	has nuclear spin I = 1/2
	has number of neutrons 0
	has number of nucleons 1
	has relative NMR sensitivity 1.0 by definition
	has space group Fm3m for cubic
	has space group I4 for tetragonal
	has space group P63/mmc for h.c.p.
	has symbol 1H
	has uses Nuclear Magnetic Resonance
	is an instance of hydrogen
hydrogen burning	has catalyst
	has definition The fusion of hydrogen into helium and the process by which all main-sequence stars generate energy. Every star born with more than 0.08 solar masses burns hydrogen.
	has minimum mass 0.08 solar masses
	has next higher temperature reaction helium burning
	has product helium
	has reactant hydrogen
	is a kind of exothermic fusion process
hydrogen cyanide	has symbol HCN
	is an instance of interstellar molecule
hydrogen isocyanide	has symbol HNC
	is an instance of interstellar molecule
hydrogen line	has definition Set of spectral features associate with neutral hydrogen (H I)
	has species hydrogen (H I)
	is a kind of spectral line
hydrogen sulfide	has symbol H2S
	is an instance of interstellar molecule
hydrogen-deficient C star	has definition A subgroup of high-luminosity C stars with weak or absent hydrogen lines, mostly of types F and G.
	has spectral type F star, G star
	is a kind of C star
	is a kind of hydrogen-deficient star
hydrogen-deficient early type star	has definition Early type star of type O, B or A in which the hydrogen lines are very weak or absent.
	has spectral type O star, B star or A star
	is a kind of hydrogen-deficient star
hydrogen-deficient star	has hydrogen line strength weak
	is a kind of star
hydroxyl radical	has definition An interstellar molecule first detected in 1963 at a wavelength of 18 cm. The four transitions that occur near 18 cm are caused by the splitting of the ground level. Galactic OH sources have been divided into three classes according to whether the OH emission is strongest in the main lines, particularly at 1665 MHz (Class 1), whether the emission and absorption are highly anomalous only in the satellite lines (Class 2) (Class 2a, 1720-line emitters; Class 2b, 1612-line emitters), or whether there is only absorption in all four lines (Class 3).
	has symbol OH
	is an instance of diatomic molecule
	is an instance of interstellar molecule
Hydrus	has acronym Hyi
	has genitive Hydri
	has historical image
	has historical origin Julius Schiller combined Hydrus and Tucana to form the Biblical figure Archangel Raphael
	has synonym Sea Serpent
	is a part of celestial sphere
	is an instance of constellation named by Bayer
hyperbola	is a kind of conic section
hyperbolic orbit	is a kind of hyperbola
	is a kind of orbit
hypergalaxy	has definition A system consisting of a dominant spiral galaxy surrounded by a cloud of dwarf satellite galaxies, often ellipticals.
	is a kind of collection of galaxies
Hyperion	has definition Eighth satellite of Saturn
	has diameter 160 km
	has discoverer Bond
	has discovery date 1848.
	has orbital period P = 21d6h38m
	is a part of Saturn
	is an instance of natural satellite
hyperon	has definition Baryons heavier than the neutron (this term is seldom used today). They have non-zero strangeness. Free hyperons are unstable and decay into end products, one of which is a proton.
	has lifetime 10-8 to 10-10 seconds
	has spin 1/2 or 3/2
	is a kind of baryon
	is a kind of radioactive particle
hypothesis	has definition A scientific proposition that purports to explain a given set of phenomena; less comprehensive and less well established than a theory.
	is a kind of abstraction
hypothetical particle	has acceptance status hypothetical
	is a kind of particle
Iapetus	has definition The ninth satellite of Saturn. It has the most extreme variation in albedo of any satellite in the solar system (0.04 for the leading side, 0.28 for the trailing side). Discovered by Cassini in 1671.
	has eccetricity e = 0.028
	has inclination inclination 14°.7 to Saturn's orbital plane
	has period 79d7h55m
	has radius 850±100 km
	is a part of Saturn
	is an instance of natural satellite
IC 1613	is a part of Local Group
	is an instance of irregular galaxy
IC 3568	has distance 9000 light years
	has extent 0.4 light years
	has image
	is an instance of planetary nebula
	is part of Camelopardalis
Icarus	has asteroid number 1566
	has definition Asteroid with the smallest orbit and highest eccentricity of any asteroid. It is the only asteroid known to come closer to the Sun than Mercury.
	has diameter 1.1 km
	has dicovery date 1948
	has discoverer Baade
	has eccetricity e = 0.827
	has inclination i = 23°
	has perihelion distance 0.19 AU
	has period P = 408d
	has rotation period 2h16m
	has semi-major axis a = 1.07 AU
	is a part of asteroid belt
	is an instance of asteroid
ideal gas	has definition A nondegenerate gas in which the individual molecules are assumed to occupy mathematical points and to have zero volume, and in which the mutual attraction of neighboring molecules is zero.
	has definition he pressure of a gas is directly proportional to the product of its temperature and density (p = CρT). The higher the temperature and the more rarefied a gas, the more closely it obeys the ideal gas laws, so the gases in most stars closely approximate ideal gases. For a degenerate gas, the pressure depends only on the density and is independent of the temperature.
	has synonym perfect gas
	is a kind of gas
image dissector scanner	has definition A specialized television camera used as a light detector (instead of a photographic plate) in the 1970's.
	is a kind of integrating detector
image intensifier	has definition An electronic device for increasing the brightness of a faint optical image. The image is first formed on a thin metallic surface called a photocathode from which electrons are then ejected. The stream of electrons is accelerated and focussed onto a phosphorescent screen which glows brightly as a result of the impact.
	is a kind of imager
image photon counting system	has acronym IPCS
	has definition A form of very low light level detector used in astronomy. By means of an image intensifier the IPCS is capable of counting individual photons of light.
	is a kind of image intensifier
Image Reduction and Analysis Facility	has acronym IRAF
	has definition An extensive suite of programs developed for astronomy applications and supported by the US National Optical Astronomy Observatories.
	is a kind of astronomical software
image tube	has definition An electronic camera in which electrons, emitted from a photocathode surface exposed to light, are focused electronically onto a phosphor or photographic plate.
	has synonym image intensifier
	is a kind of integrating detector
imager	is a kind of detector
imaging spectrometer	has definition Class of instruments which preserve the image field while also determining the spectrum. Integral Field Unit (IFU). Usually implies some kind of image slicing either with facets or fiber optics.
	is a kind of spectrograph
immersion	has definition The disappearance of a celestial body due to eclipse or occultation.
	is a kind of occultation phase
	is followed by transit
impact event	has definition The collision of two celestial bodies
	is a kind of celestial event
	can produce a crater
incandescent flight	has definition phase in the flight of a meteor before the ratardation point when the meteoroid is producing its own incandescent light
	has duration from the point of intial contact with atmosphere until the retardation point
	has start time the point of intial contact with the atmosphere
	is an instance of meteor event
inclination	has definition An orbital element representing the inclination of the object's orbital plane to the ecliptic
	has definition The angle between one plane and another. The (equatorial) inclination of a planet is the angle between the plane of its equator and that of its orbit. The inclination of the orbit of a planet in the Solar System other than Earth is the angle between the plane of that orbit and the ecliptic.
	has definition The angle between two planes or their poles; usually the angle between an orbital plane and a reference plane.
	has symbol i
	is an instance of angle
	is an instance of orbital element
indium	has image
	has ocean oxidation state III
	is a kind of group III element
	is a kind of metallic element
	is a kind of unclassified oceanic element
inductance	has unit inductance unit
	is a kind of electromagnetic quantity
inductance unit	is a kind of electromagnetic unit
	is a unit of inductance
induction	has definition System of reasoning in which the conclusion, though implied by the premises and consistent with them, does not necessarily follow from them.
	is a kind of reasoning process
Indus	has acronym Ind
	has genitive Indi
	has historical image
	has historical origin Julius Schiller combined Indus and Pavo to form the Biblical figure Job
	has synonym American Indian
	is a part of celestial sphere
	is an instance of constellation named by Bayer
inelastic collision	has definition A reaction involving a change in the kinetic energy of the system, as in ionization, excitation, or capture; or a process which changes the energy level of the system.
	is a kind of collision
inert gas	has group 18
	has synonym group VIII element
	has synonym noble gas
	is a kind of atmophile element
	is a kind of column grouped element
	is a kind of gaseous element
	is a kind of nonmetallic element
inflatable spacecraft	is a kind of artificial satellite
inflation era	has definition Big Bang era in which the infant universe went through a brief period of extremely rapid (exponential) expansion, after which it settled back into the more leisurely rate of expansion of the standard model. The period of rapid expansion began and ended when the universe was still much less than a second old, yet it provides a physical explanation for the flatness and horizon puzzles. The inflationary universe model also suggests that the universe is vastly larger than the portion of it that is visible to us. (See exponential expansion.)
	has definition Big Bang era in which the spacetime continuum underwent an intense period of exponential expansion, in response to the separation of the strong nuclear force from the electroweak force. This idea solves the flatness and horizon problems.
	has definition Big Bang era in which the universe is driven into exponential expansion by the repulsive gravitational field created by a false vacuum. The inflation would end with the decay of the false vacuum. Although the inflation would occur in far less than a second, it could account for the "bang" of the big bang theory, it could explain the origin of essentially all the matter in the observed universe, and it can solve the horizon problem and the flatness problem. It could also generate the density perturbations that would later become the seeds for galaxy formation.
	has definition Big Bang era in which universe undergoes a brief burst of enormous expansion.
	has definition The idea that, when it was a fraction of a second old, the universe expanded dramatically. If inflation is correct, then the mass density of the universe (Ω) should be 1.0, if there is no cosmological constant; if there is a cosmological constant and inflation is correct, the sum of Ω and the cosmological constant (λ) should be 1.0.
	has start time 10-35 s
	is a kind of Big Bang era
infrared	has acronym IR
	has definition That part of the electromagnetic spectrum that lies beyond the red, having wavelengths from about 7500 Å to a few millimeters (about 1011-1014 Hz). Infrared radiation is caused by atomic transitions, or by vibrational (near-IR) and rotational (far-IR) transitions in molecules. (IRe1, IRc1, IRs1: the e sources are extended; the c sources are unresolved; the s indicates an infrared nebula surrounding a visible star.)
	has definition The region of the electromagnetic spectrum from a wavelength of about 1 μm (10-6 m) to about 200 μm. The region from 1 to 5 µm is the near infrared: 5-30 is the mid infrared and 30-200 µm is the far infrared.
	has frequency 3 THz to 430 THz
	has wavelength 700 nm to 100 μm
	is a kind of photon
infrared galaxy	is a kind of galaxy
infrared magnitude	has band
	has definition The magnitude derived from the observations at an infrared wavelength
	is a kind of apparent magnitude
	is a kind of apparent magnitude
Infrared Space Observatory	is an instance of ESA satellite
infrared space telescope	has wavelength sensitivity 1 to 250 microns
	is a kind of space telescope
inner bremsstrahlung	has definition The continuous electromagnetic radiation that accompanies the β-decay of nuclei.
	is a kind of bremsstrahlung
inner Lagrangian point	has definition An unstable Lagrangian point between the two bodies on the line passing through the centers of mass of the two bodies. Mass transfer occurs through this point in a close binary star system.
	has location between the two bodies on the line passing through the centers of mass of the two bodies
	has synonym L1
	is an instance of unstable Lagrangian point
inner transition metal	is a kind of transition metal
inner Van Allen belt	has altitude 3000 km
	has composition mostly protons
	has particle energy higher than the outer Van Allen belt
	is a part of Van Allen belt
inspector satellite	is a kind of reconnaissance spacecraft
Institut de RadioAstronomie Millimetrique	has acronym IRAM
	is an instance of astronomical institution
Institute of Space and Astronautical Science	has acronym ISAS
	is an instance of space science institution
institution	is a kind of abstraction
instrument	has definition a device that requires skill for proper use
	has operator
	is a kind of device
integer	has value whole number
	is a kind of number
integrating detector	has definition Any imaging device, like a photographic emulsion or CCD, which can build up more signal and contrast by a longer exposure to light or other electromagnetic energy.
	is a kind of imager
integration time	has definition The interval of time used to collect photons of light on a detector and build-up a strong signal.
	is an instance of time
Interactive Data Language	has acronym IDL
	is a kind of astronomical software
intercombination line	has definition Spectral line emitted in transitions between two levels with different values of S.
	is a kind of spectral line
interference filter	has definition A filter used to shut out all light except the desired wavelengths.
	has definition A method of constructing an optical filter to select a particular wavelength band for transmission and reject wavelengths outside this band. Similar to a Fabry-Perot etalon. The construction relies on constructive and destructive interference effects in a multilayer stack of quarter-wave reflective layers and half-wave spacer layers.
	is a kind of spectrograph dispersing element
interline transfer	has definition A CCD construction consisting of vertical strips which are alternately opaque and light sensitive. The opaque strips conceal charge transfer registers.
	is a kind of charge-coupled device
Intermediate Frequency	has acronym IF
	has definition The beat frequency between the signal and the local oscillator in a radio detection system.
	is a kind of frequency
internal symmetry	has definition The properties of different elementary particles can be related to each other by mathematical transformations that look very much like the more familiar symmetry properties of our own physical space. Physicists have therefore hypothesized an abstract internal space in which these internal symmetries are defined. With the help of these internal symmetries, the elementary particles can be gathered into families. The relationship between space-time and these internal symmetries remains to be fully explained.
	is a kind of symmetry
International Astronomical Union	has acronym IAU
	is an instance of astronomical institution
International temperature scale	has acronym ITS90
	has definition A temperature scale defined between 0.65 K and the highest temperatures practicably measurable in terms of the Planck radiation law using monochromatic radiation. There are 17 fixed temperature points : Below 5 K are defined in terms of the vapour pressures of helium 3 and helium 4 Between 5 K and the triple point of water are marked by the triple points of certain elements, e.g. neon (24.5561 K) Above 0.01 °C are defined with reference to the freezing points of specified metallic elements Upper limit is 1064.43 °C (1337.58 K), the freezing point of gold [JN92] -has source: J. A. Hall, 1967
	has proposal date 1927
	is a kind of temperature unit
interstellar band	is a kind of molecular band
interstellar gas	has composition mainly hydrogen
	has definition Sparse, cool gas (mainly hydrogen) in interstellar space. Dust absorbs and scatters radiation; gas does not interact directly with radiation but is coupled to the dust by collisions. Interstellar gas affects only light of certain wavelengths. Temperature 10-100 K.
	has density very low
	is a kind of gas
interstellar line	has definition Sharp, distinct absorption lines superposed on stellar spectra, produced by the interstellar gas located between the source and the observer. Strongest are the D lines, followed by the H and K lines, and the K I doublet at 7699 and 7644 Å.
	has detection procedure narrow absorption lines which don't participate in the periodic Doppler shift of intrinsic absorption lines in a binary star
	has equivalent width depends on the amount of absorbing matter between the source and the observer
	has width narrow
	is a kind of absorption line
	occurs in O stars
interstellar molecule	has definition Molecule present in interstellar space. As of late 1974, at least 33 molecular species had been identified with reasonable certainty.
	has discovery in space date
	is a kind of molecule
intrinsic luminosity	has definition The amount of light an object actually emits, as opposed to how bright the object looks from Earth. An apparently bright star can be intrinsically bright and far away or intrinsically faint and nearby.
	has definition The energy per second emitted by an astronomical object, analogous to the wattage of a light bulb.
	is a kind of luminosity
invariable plane	has definition The plane through the center of mass of the solar system perpendicular to the angular momentum vector of the solar system.
	is a kind of plane
invariant plane	has definition The plane defined by the total angular momentum of the solar system. It is within about 1°.5 of the ecliptic.
	is a kind of plane
inverse beta-decay	has definition The relatively rare process p + vbar → n + e+. Free-electron capture (e + p → n + v) is sometimes called inverse β-decay in astrophysics.
	is a kind of nuclear decay
inverse bremsstrahlung	has antonym bremsstrahlung
	has definition Absorption of a photon by an electron in the field of a nucleus.
	is a kind of absorption
	is a kind of free-free transition
inverse Compton effect	has antonym Compton scattering
	has definition The collision between a photon and an energetic (cosmic-ray) electron, in which some of the energy of the electron is transferred to the photon.
	is a kind of scattering
inverse P Cygni line	has definition An emission line in which the emission is on the violet side of the absorption. It is usually interpreted to mean infall of matter.
	is a kind of emission line
Io	has albedo 0.91
	has definition The innermost Galilean satellite of Jupiter, similar in size and density to the Moon. Jupiter's decametric radiation has been linked at least partially to Io. It has the highest mean density of any of the Galilean satellites. Pioneer 10 also detected the presence of an ionosphere, and Na D emission. It also has the highest albedo in the solar system.
	has eccentricity e = 0.01
	has inclination i = 0.03
	has mean density 3.48 g (data from Pioneer 10)
	has period 1.77 days
	has radius R &asmyp; 1850 km (data from Pioneer 10)
	has synonym Jupiter I
	has synonym Jupiter I
	is a part of Jupiter
	is an instance of Galilean satellite
iodine	has image
	has ocean oxidation state V
	has ocean residence time 300000 years
	is a kind of atmophile element
	is a kind of halogen
	is a kind of recycled oceanic element
ionic tail	has charge large
	has definition Straight part of comet tail composed of ions driven by the solar wind
	has synonym type I comet tail
	is a part of comet tail
ionization	has antonym recombination
	has definition Loss or gain by an atom of one or more electrons, by which process the atom becomes an ion and instead of being neutral, has a charge: positive if it has lost an electron, negative if it has gained one. High temperature is particularly conducive to ionization.
	is a kind of atomic process
ionization potential	has definition The minimum energy required to remove an electron from an atom. It always takes a higher energy to remove a second electron from a singly ionized atom, a still higher energy to remove a third, etc. The ionization potential for hydrogen is 13.596 eV, which corresponds to a wavelength of 912 Å.
	is a kind of energy
ionized helium Balmer line	is a kind of ionized helium line
	is a kind of spectral series line
ionized helium line	has definition Set of spectral features associate with ionized helium (He II)
	has species ionized helium (He II)
	is a kind of spectral line
ionized helium Lyman line	has lower energy level 1
	has series limit 227 Å
	is a kind of ionized helium line
	is a kind of spectral series line
ionized helium Paschen line	has lower energy level 3
	has series limit 2040 Å
	is a kind of ionized helium line
	is a kind of spectral series line
ionized methylidyne	has charge 1
	has symbol CH+
	is an instance of charged particle
	is an instance of interstellar molecule
ionosphere	has definition The region of Earth's atmosphere (80-500 km), immediately above the stratosphere. The ionosphere consists of the D layer, the E layer, and the F layers (q.v.). It is strongest at the end of the day.
	is a part of atmosphere
IRAS	is an instance of infrared space telescope
	orbits Earth
IRAS galaxy	emits excessive amounts of infrared light
	has definition Any galaxy which was discovered by the Infra-Red Astronomical Satellite (IRAS) to possess an excessive amount of infrared emission.
	is a kind of infrared galaxy
iridium	has image
	is a kind of siderophile element
	is a kind of transition metal
iron	has abundance 1 × 10-4 p.p.m. in Atlantic surface seawater
	has abundance 1 × 10-4 p.p.m. in deep Pacific seawater
	has abundance 1 × 10-5 p.p.m. in Pacific surface seawater
	has abundance 3.16 × 107 in Sun relative to H = 1 × 1012
	has abundance 4 × 10-4 p.p.m. in deep Atlantic seawater
	has abundance 41000 p.p.m. in Earth's crust
	has abundance p.p.m. in seawater
	has atomic emission line 248.814 nm for Fe I
	has atomic emission line 252.285 nm for Fe I
	has atomic emission line 344.061 nm for Fe I
	has atomic emission line 371.994 nm for Fe I
	has atomic emission line 373.713 nm for Fe I
	has atomic emission line 374.556 nm for Fe I
	has atomic emission line 248.327 nm for Fe I (strong)
	has atomic emission line 385.991 nm for Fe I (strong)
	has atomic number 26
	has atomic radii 124 pm
	has biological role essential to all species
	has boiling point 3023 K
	has bulk modulus 109.5 GPa for cast iron
	has bulk modulus 160 GPa for steel
	has chief source hematite, magnetite, goethite, lepidocrocite, siderite
	has covalent radii 116 pm
	has crystal cell dimension a = 286.645 pm for α-Fe
	has crystal cell dimension a = 293.22 pm for δ-Fe
	has crystal cell dimension a = 364.68 pm for γ-Fe
	has crystal type b.c.c. for α-Fe
	has crystal type b.c.c. for δ-Fe
	has crystal type c.c.p. for γ-Fe
	has daily dietary intake 6 - 40 mg
	has definition lustrous, silvery and soft or workable metal when absolutely pure
	has density 7035 kg m-3 for liquid at 1808 K melting point
	has density 7874 kg m-3 for solid at 293 K
	has discovery date 2500 B.C.
	has electrical resistivity 9.71 × 10-8 Ω m at 293 K
	has electron affinity 15.7 kJ mol-1 from Fe to Fe-
	has electron configuration [Ar]3d64s2 in ground state
	has electronegativity 1.83 Pauling
	has hazard deficiency leads to anemia but excess causes liver and kidney damage
	has hazard fire or explosion of dust chronic exposure can cause pneumoconiosis
	has heat capacity 25.10 J K-1 mol-1 for solid at constant pressure 0.1 MPa at 298.15 K
	has heat capacity 25.677 J K-1 mol-1 for gas at constant pressure 0.1 MPa at 298.15 K
	has heat of fusion 14.9 kJ mol-1
	has heat of vaporization 351.0 kJ mol-1
	has image
	has ionic radii 67 pm for Fe2+
	has ionic radii 82 pm for Fe3+
	has isotope mass range 49 to 63
	has lethal intake 7 - 35 grams
	has level in humans 180 p.p.m. in muscle
	has level in humans 250 - 1400 p.p.m. in liver
	has level in humans 3 - 380 p.p.m. in bone
	has level in humans 447 mg dm-3 in blood
	has linear expansion coefficient 12.3 × 10-6 K-1
	has longest lived isotope iron 56
	has main mining area USA, Canada, Sweden, South Africa, Russia, India, Japan
	has mass absorption coefficient 308 cm2 g-1 for CuKα X-ray diffraction
	has mass absorption coefficient 38.5 cm2 g-1 for MoKα X-ray diffraction
	has mass magnetic susceptibility ferromagnetic
	has mass of element in person 4.2 kg for a 70 kg average person
	has melting point 1808 K
	has mineral goethite, hematite, lepidocrocite, magnetite, siderite, ...
	has molar volume 7.09 cm3
	has name origin iron from Anglo-Saxon
has neutron scattering length 0.954 × 10-12 cm
has number of isotopes 16 including nuclear isomers
has number of protons 26
has ocean oxidation state III
has ocean residence time 98 years
has poisson's ratio 0.27 GPa for cast iron
has poisson's ratio 0.27 GPa for steel
has pronunciation iy-on ?
has registry number 7439-89-6 for Chemical Abstracts System database
has relative atomic mass 55.845 in units of 12C = 12.000
has reserves 1.1 × 1011 tonnes
has rigidity modulus 60.0 GPa for cast iron
has rigidity modulus 81 GPa for steel
has space group Fm3m for γ-Fe
has space group Im3m for α-Fe
has space group Im3m for δ-Fe
has specimen chips, filings, foil, granules, powder and wire. Safe.
has symbol Fe
has symbol name origin ferrum = iron from Latin
has synthesis mechanism
has term symbol 5D4 in ground state
has thermal conductivity 80.2 W m-1 K-1 at 300 K
has thermal neutron capture cross section 2.56 barns
has toxic intake 200 mg Iron (II) compounds are more toxic than iron (III)
has uses steel etc...
has world production 7.16 × 108 tonnes year-1
has young's modulus 152.3 GPa for cast iron
has young's modulus 208 GPa for steel
is a kind of recycled oceanic element
is a kind of siderophile element
is a kind of transition metal
reacts with damp air by rusting
reacts with dilute acids by disolving
iron 52	has atomic mass 51.948114
	has decay mode β+ (2.37 Mev) 57%
	has decay mode γ
	has decay mode EC 43%
	has half life 8.28 hours
	has nuclear spin I = 0+ h/2π
	has number of neutrons 26
	has number of nucleons 52
	has symbol 52Fe
	has uses research
	is an instance of iron
iron 54	has atomic mass 53.939612
	has natural abundance 5.8
	has nuclear spin I = 0+ h/2π
	has number of neutrons 28
	has number of nucleons 54
	has symbol 54Fe
	has uses isotopically enriched samples available for experimental purposes
	is an instance of iron
iron 55	has atomic mass 54.938296
	has decay mode EC (0.2314 Mev)
	has decay mode no γ
	has half life 2.73 years
	has nuclear spin I = 3/2- h/2π
	has number of neutrons 29
	has number of nucleons 55
	has symbol 55Fe
	has uses research, medical diagnosis
	is an instance of iron
iron 56	has atomic mass 55.934939
	has natural abundance 91.72
	has nuclear spin I = 0+ h/2π
	has number of neutrons 30
	has number of nucleons 56
	has symbol 56Fe
	has uses isotopically enriched samples available for experimental purposes
	is an instance of iron
iron 57	has atomic mass 56.935395
	has magnetogyric ratio 0.8661 × 107 rad T-1 s-1
	has natural abundance 2.2
	has NMR frequency 3.231 MHz where 1H = 100 MHz; 2.3488 T
	has NMR receptivity 4.2 × 10-3 where 13C = 1.00
	has nuclear magnetic moment μ = +0.09062294 nuclear magnetons with diamagnetic correction
	has nuclear spin I = 1/2- h/2π
	has number of neutrons 31
	has number of nucleons 57
	has relative NMR sensitivity 3.37 × 10-5 where 1H = 1.00
	has symbol 57Fe
	has uses isotopically enriched samples available for experimental purposes
	has uses Nuclear Magnetic Resonance
	is an instance of iron
iron 58	has atomic mass 57.933277
	has natural abundance 0.28
	has nuclear spin I = 0+ h/2π
	has number of neutrons 32
	has number of nucleons 58
	has symbol 58Fe
	has uses isotopically enriched samples available for experimental purposes
	is an instance of iron
iron 59	has atomic mass 58.934877
	has decay mode β- (1.565 Mev)
	has decay mode γ
	has half life 44.51 days
	has nuclear magnetic moment μ = 0.29 nuclear magnetons with diamagnetic correction
	has nuclear spin I = 3/2- h/2π
	has number of neutrons 33
	has number of nucleons 59
	has symbol 59Fe
	has uses research, medical diagnosis
	is an instance of iron
iron 60	has atomic mass 59.934080
	has decay mode β- (0.243)
	has half life 1.5 × 106 years
	has nuclear spin I = 0+ h/2π
	has number of neutrons 34
	has number of nucleons 60
	has symbol 60Fe
	is an instance of iron
iron core of supernova gravitational contraction	is a kind of gravitational contraction
iron fusion	is a kind of endothermic fusion process
irregular galaxy	has definition A galaxy with amorphous structure and with relatively low mass (108-1010 Msun). Fewer than 10% of all galaxies are classified as irregular.
	is a kind of galaxy
irregular variable	is a kind of variable
Irénée du Pont Telescope	has altitude 2282 m
	has aperture 2.54 m
	has creation date 1976
	has diameter corrector plate 741 mm
	has diameter hole in primary 826 mm
	has diameter primary 2540 mm
	has diameter secondary 953 mm
	has distance between mirrors 5153 mm
	has focal length Cassegrain 19050 mm
	has focal length primary 7620 mm
	has focal point distance behind surface of primary 1013 mm
	has focal ratio f/3.0
	has image
	has latitude 29° 00' S
	has location Las Campanas, Chile
	has longitude 70° 42' W
	has mirror maker Donald A. Loomis
	has mirror type fused-silica
	has mounting manufacturer Bruce H. Rule
	has operator Carnegie Inst. of Washington
	has owner Las Campanas Observatory
	has radius of curvature of focal surface 9.042 mm
	has scale 0.0924 mm/arcsec
	has synonym du Pont 100 inch
	has unvignetted corrected field of view 1.45 degrees
	has vignetting at 63 arcmin field angle 3%
	is an instance of Fork equatorial telescope
	is an instance of optical telescope
	is an instance of Ritchey-Chrétien
Isaac Newton	is an instance of physicist
Isaac Newton Telescope	has altitude 2336 m
	has aperture 2.5 m mirror
	has comment originally set up in England in 1967
	has creation date 1984
	has focal ratio f/3, 15
	has latitude 28° 46' N
	has location La Palma, Canary Islands
	has longitude 17° 53' W
	has mirror maker Grubb-Parsons
	has mounting manufacturer Grubb-Parsons
	has operator Royal Greenwich Observatory
	has owner Obs. del Roque de Ins Muchachos
	has synonym Isaac Newton 98 inch
	is an instance of optical telescope
	is an instance of Polar-disk equatorial telescope
	is an instance of reflector
ISO	has image
	has launch date 1998
	has reference 'Infrared Space Observatory', ESA
	is an instance of infrared space telescope
isocyanic acid	has symbol HNCO
	is an instance of interstellar molecule
isomer	has definition Nucleus with the same A and Z numbers but in different energy states.
	is a kind of nucleus
isomer shift	has definition Displacement of an absorption line due to the fact that the absorbing nuclei have a different s-electron density from that of the emitting nuclei.
	is a kind of wavelength shift
isoplanatic patch	has definition The angular region on the sky over which the wavefront correction applied by an adaptive optics system remains valid. It is relatively small and therefore a nearby reference star is also required.
	is a kind of solid angle
isotone	has definition Nucleus with the same number of neutrons but with different A and Z numbers.
	is a kind of nucleus
isotope	has definition An atomic nucleus having the same number of protons as a more commonly found atomic nucleus but a different number of neutrons. For example, the hydrogen nucleus has a single proton; deuterium has one proton and one neutron and would be called an isotope of hydrogen. (see neutron; proton.)
	has definition Two nuclei with the same number of protons but different numbers of neutrons are said to represent the same element, but different isotopes. For example, helium-3, with two protons and one neutron in each nucleus, and helium-4, with two protons and two neutrons, are two different isotopes of helium. For another example, see deuterium.
	is a kind of nucleus
isotropy	has antonym anisotropy
	has definition A universe is said to be isotropic, from the point of view of a given observer, if it looks the same in all directions. The isotropy of the real universe is seen most strikingly in the cosmic background radiation, which has the same temperature in all directions to an accuracy of about one part in 100,000. See also homogeneous.
	has definition Having equal and uniform properties at all points and in all directions. In astronomy the term describes microwave background radiation.
	has definition In cosmology, the property that the universe appears the same in all directions. The uniformity of the cosmic background radiation, coming from all directions of space, suggests that on the large scale the universe is isotropic about our position. If we then assume that our position is not unique, we conclude that: the universe appears isotropic about all points. This last result requires that the universe be homogeneous. (See cosmic background radiation; homogeneity.)
	has definition Quality of being the same in all directions.
	has definition The property of being unchanged by a rotation. A sphere is rotationally invariant, but a rectangle is not. As far as we know the fundamental laws of physics are rotationally invariant.
	has definition The same in all directions.
	has synonym rotational invariance
	is a kind of symmetry
J magnitude	has band 1.3 microns
	has definition The magnitude derived from observations at 1.3 microns.
	is an instance of infrared magnitude
Janus	has definition The innermost satellite of Saturn, just outside Saturn's rings. Named Janus for the first and the last.
	has discoverer Dollfus
	has discovery date 1966
	has eccentricity e ≈ 0
	has inclination i ≈ 0
	has period P = 0.75 days
	has radius R = 175 km
	is a part of Saturn
	is an instance of natural satellite
Jeans mass	has definition The critical mass a volume of space must contain before it will collapse under the force of its own gravity.
	has definition The mass enclosed within a sphere of diameter equal to the Jeans length.
	is an instance of mass
Jet Propulsion Laboratory	has acronym JPL
	has definition Funded by NASA and operated by the California Institute of Technology (Caltech).
	has location Pasadena, California
	is an instance of space science institution
Jodrell Bank	has definition A telescope in England designed to detect radio waves emitted by astronomical objects.
	is an instance of observatory
Johann Bayer	has career named many of the constellations in the southern hemisphere
	is an instance of astronomer
Josephson constant	has equation
	has symbol KJ
	has uncertainty 0.019 × 109 Hz V-1
	has value 483597.898 × 109 Hz V-1
	is an instance of electromagnetic constant
joule	has approval agency International Conference on Weights and Measures
	has approval date 1948
	has base unit m2kg·s-2
	has definition Unit of energy, work, or quantity of heat. 1 J is equal to a force of 1 newton acting over a distance of 1 meter. 1 J = 107 ergs.
	has definition Unit of work which is equal to a force of one newton acting over a distance of one metre in the direction of the force. It was recognized by the IEC in 1889. The specific heat of water at 15 °C is 4185.5 joule (kg °C)-1, a figure previously always associated with J, the mechanical equivalent of heat.
	has equivalent 1 watt second (electrical units)
	has equivalent 107 ergs
	has equivalent 2.78 × 10-7 kW h
	has equivalent 9.47 × 10-4 Btu
	has equivalent 9.47 × 10-9 therm
	has proposal date 1888
	has proposer British Association
	has symbol J
	has unit N·m
	is an instance of energy unit
	is an instance of named derived SI unit
	is named after J. P. Joule
Jovian aurora	has definition an aurora ocurring on the planet Jupiter
	has location Jupiter
	is a kind of aurora
Julian year	has definition A period served as the basis for the Julian calendar.
	has value 365.25 days
	is an instance of year
Juno	has albedo 0.2
	has definition An asteroid with a relatively large albedo.
	has diameter 250 km
	has eccentricity e = 0.256
	has inclination i = 13°.0
	has orbital period P = 1594 days
	has rotation period 7h.21
	has semi-major axis a = 2.67 AU
	is a part of asteroid belt
	is an instance of asteroid
Jupiter	has atmosphere composition primarily H2 (85%) and He (14%), with traces of methane, ammonia
	has central density 35 megabar
	has central temperature 5 × 104 K
	has core composition metallic hydrogen in a pressure-ionized liquid phase
	has definition Fifth planet from the Sun. It is more massive than all other planets and satellites combined; if it were about 80 times more massive, it would become self-luminous due to fusion reactions. The heat flux to from the center to the surface is mainly convective. For both Jupiter and Saturn it is necessary to invoke a substantial source of internal heating (presumably gravitational contraction) to account for the surface temperature (Jupiter radiates about 2 1/2 times as much heat as it receives from the Sun). Jupiter's surface shows pronounced horizontal striations: the light layers (zones) are at a slightly higher altitude and about 15° cooler than the dark layers (belts). It is surrounded by a partial torus of atomic H in the orbit of Io. Thirteen satellites, the four outermost of which have retrograde motion, high eccentricity, and high inclination. (Jupiter XIII, discovered in 1974, has a period of 239 days; i = 26°.7, e = 0.147.)
	has eccentricity e = 0.048
	has equatorial radius 7.135 × 109 cm = 11 REarth
	has escape velocity Vesc = 61 km s-1
	has has albedo 0.51
	has has mean orbital velocity 13.06 km s-1
	has has orbital period 11.86 years
	has has surface temperature 120 K
	has has synodic period 398.9 days
	has inclination i = 1°18'18"
	has magnetic axis inclination 15° to the rotational axis (data from Pioneer 10)
	has magnetic axis offset 0.1 Jupiter radius from the center of the planet (data from Pioneer 10)
	has magnetic field 4 gauss (data from Pioneer 10)
	has mass 1.90 × 1030 g = 318 MEarth
	has mean density 1.33 g cm-3< sup>
	has oblateness 0.065 (from Pioneer 10)
	has obliquity 3°.1
	has polar radius 6.7 × 109 cm
	has rotational period 9h50m at equator; 9h55m at polar regions (see Systems I and II longitude).
	has semi-major axis 5.203 AU
	has surface gravity 2.7 that of Earth
	is a part of our solar system
	is an instance of gas giant
	is an instance of naked eye planet
	is an instance of superior planet
jurassic period	has duration 181 million years
	has start time 46 million years ago
	is a kind of mesozoic era
K alpha	has definition A spectral line in the X-ray region (α = 0.334 Å), produced by the transition between the lowest level of the K shell and the lowest level of the L shell.
	is a kind of emission line
K corona	has definition The inner part of the Solar corona (the gaseous phase) which emits a continuous spectrum without absorption lines. Physically, the K component results from Thomson scattering of photospheric radiation by free electrons in the corona. The K component is polarized and decreases rapidly with distance from the Sun. (from the German Kontinuum)
	has synonym K component
	is a part of corona
K edge	has definition The absorption edge of the K shell (see absorption edges).
	is a kind of absorption edge
K electron	has definition An electron in the K shell, the innermost shell, or energy level, of an atom. All elements heavier than hydrogen have a filled K shell, which consists of two 1s electrons orbiting the nucleus.
	has energy quantum number 1
	is a kind of bound electron
K I doublet	has species potassium (K I)
	has wavelength 7699 and 7644 Å
	is a kind of absorption line
k line	has definition Singly ionized magnesium resonance line.
	has species Mg II
	has wavelength 2795.5 Å
	is a kind of absorption line
K magnitude	has band 2.2 microns
	has definition The magnitude derived from observations at 2.2 microns.
	is an instance of infrared magnitude
K star	has color orange to red
	has definition Cool star with spectral type K with spectra resembling those of sunspots, in which the hydrogen lines have been greatly weakened. The HK lines (q.v.) reach their greatest intensity. Strongest lines are Ca I (4227 Å) and the G-band (4303 Å).
	has surface temperature 3600 to 5000 K
	is a kind of late star
Kaluza-Klein theory	has definition An early attempt to unify general relativity and electromagnetism by working in five dimensions. The electromagnetic field was obtained by curling up or compactifying the extra dimension. With the advent of higher dimensional theories such as superstrings, the Kaluza-Klein approach came back into fashion.
	has definition Class of theories incorporating extra curled-up dimensions, together with quantum mechanics.
	is a kind of unified theory
	is a kind of unified theory
Kamiokande II	has component water detector
	is an instance of neutrino telescope
kaon	has definition Meson with non-zero strangeness quantum number.
	has synonym K meson
	is a kind of meson
Kapteyn Universe	has definition An incorrect model for the Galaxy proposed by Jacobus Kapteyn in which the Milky Way was small and the Sun located at or near the Galaxy's center.
	is a kind of outdated belief
Kapteyn's star	has definition A nearby high-velocity M0 subdwarf that is both the nearest halo star to the Sun and the nearest star that orbits the Galaxy backward.
	has discover Jacobus Kapteyn
	has discovery date 1897
	has distance 13 light-years
	has radial velocity +242 km s-1
	has spectral type M0
	has synonym CD -45 1841
	has synonym HD 33793
	is an instance of M star
	is an instance of subdwarf
katal	has base unit s-1·mol
	has symbol kat
	is an instance of named derived SI unit
	represents catalytic activity
kbTop	has definition Astronomy
Keck	has comment Uses 36 hexagonal mirror segments. Plans include using both Keck I and Keck II as an optical interferometer
	has creation date 1991
	has focal ratio f/1.75, 15, 25 IR
	has location Mauna Kea, Hawaii, US
	has mirror diameter 9.82 m
	has mirror maker Itek, Tinsley
	has mirror type Zerodur
	has mounting manufacturer TIW Systems
	has number of segments 36
	has operator Univ. of California and Calif. Institute of Technology
	has owner W.M. Keck Observatory
	is a kind of altazimuth telescope
	is a kind of optical telescope
	is a kind of Ritchey-Chrétien
	is a kind of segmented mirror telescope
Keck I	has altitude 4150 m
	has creation date 1991
	has focal ratio f/1.75, 15, 25 IR
	has latitude 19° 49' N
	has longitude 155° 28' W
	is an instance of Keck
Keck II	has altitude 4150 m
	has creation date 1996
	has focal ratio f/1.75
	has latitude 19° 49' N
	has longitude 155° 28' W
	is an instance of Keck
Kelvin	has absolute zero 0
	has approval agency 16th CGPM
	has approval date 1968
	has definition The fraction 1/273.16 of the thermodynamic temperature of the triple point of water (the point at which the solid, liquid and gaseous phase of pure water are in equilibrium). Absolute zero corresponds to zero energy. Represents the same temperature interval as those on the Celsius scale.
	has definition Unit of thermodynamic temperature, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water
	has symbol K
	has synonym absolute
	has triple point of water 273.16
	is an instance of base SI unit
	is an instance of temperature unit
	is named after Lord Kelvin (1824-1907)
Kelvin contraction	has definition The contraction of a star contemplated by Kelvin and Helmholtz as a consequence of a star's radiating its thermal energy. It is currently believed that the contraction of a star occurs in this manner in its pre-main-sequence evolution.
	has synonym Kelvin-Helmholtz contraction
	is a kind of contraction
Kelvin timescale	has definition The time it takes a star to contract gravitationally from infinite radius down to its present radius by radiating its thermal energy.
	has value (gravitational binding energy) / luminosity
	is a kind of time
	see also nuclear time scale
Kelvin	to convert to Celsius subtract 273.15
Kepler's first Law	has definition Each planetary orbit is an ellipse with the Sun at one focus.
	is an instance of Kepler's Law
Kepler's Law	has definition One of three orbital laws formulated by Kepler
	has discoverer Johannes Kepler
	is a kind of gravity law
Kepler's second Law	has definition Equal areas are swept out in equal times.
	has synonym law of areas
	is an instance of Kepler's Law
Kepler's Supernova	has definition A Type I supernova (SN Oph 1604) whose light reached Earth in 1604. If H0 = 50, then Kepler's supernova is out in the galactic halo at a distance of 12.1 kpc and 1.4 kpc above the galactic plane, according to van den Bergh. Kepler's supernova is the prototype of Type I supernovae; at its brightest it reached an apparent magnitude of about -2.2. (3C 358)
	is an instance of type I supernova
Kepler's third Law	has definition The square of the period is proportional to the cube of the distance.
	has equation P2 = 4π2 a3 / [G (m1 + m2)]
	has synonym harmonic law
	is an instance of Kepler's Law
Keplerian orbit	has definition The orbit of a spherical particle of a finite mass around another spherical particle, also of finite mass, by virtue of the gravitational attraction between them. In the Bohr-Sommerfeld picture of atoms, the electrons are considered as describing Keplerian orbits in the field of the positive nucleus by virtue of the inverse square electric attraction between the electrons and the nuclei.
	has definition The path that an orbital body (e.g., a planet) would follow if it were subject only to the inverse-square attraction of the Sun or other central body. In practiced secondary bodies, such as Jupiter, produce perturbations.
	has parameters osculating element
	has synonym osculating orbit
	is a kind of conic section
	is a kind of orbit
Kerr black hole	has definition A rotating, axisymmetric black hole based on Kerr's 1963 solution to Einstein's field equations.
	is a kind of black hole
Keyhole Nebula	is a part of Carina Nebula
	is an instance of gaseous nebula
kilo	has symbol k
	has value 103
	is an instance of unit prefix
kilogram	has definition Unit of mass (not of weight or of force). Kilogram is equal to the mass of 1.000028 cubic decimeters of water at the temperature of its maximum density.
	has definition Unit of mass equal to the international prototype of the kilogram
	has historical origin at the end of the 18th century, a kilogram was the mass of a cubic decimeter of water
	has symbol kg
	is an instance of base SI unit
	is an instance of mass unit
kinematic relativity	has definition Theory proposed by Edward Milne as a viable alternative to Einstein's general theory of relativity, and based generally on kinematics (the science of pure motion, without reference to matter or force), from which Milne successfully derived new systems of dynamics and electrodynamics.
	is a kind of cosmology theory
kinematics	has definition The branch of mechanics that studies bodies undergoing continuous change of position. Whereas dynamics takes into account mass, force, distance, and time, kinematics is concerned only with distance and time.
	is a kind of mechanics
kinetic energy	has definition The energy associated with motion; the work that must be done to change a body from a state of rest to a state of motion, equal to 1/2 mv2 for a body of mass m moving at velocity v.
	is a kind of energy
kinetic temperature	has definition A measure of the average random motion of the particles in a system.
	is a kind of temperature
Kirchhoff's first law	has definition To each chemical species there corresponds a characteristic spectrum.
	is an instance of Kirchhoff's law
Kirchhoff's law	has definition (1) To each chemical species there corresponds a characteristic spectrum. (2) Every element is capable of absorbing the radiation which it is able to emit; this is the phenomenon of the reversal of the lines.
	is a kind of quantum law
Kirchhoff's second law	has definition Every element is capable of absorbing the radiation which it is able to emit; this is the phenomenon of the reversal of the lines.
	is an instance of Kirchhoff's law
Kirkwood gap	has definition Region in the asteroid belt which has been swept clear of asteroids by the perturbing effects of Jupiter.
	is a part of asteroid belt
	named after American astronomer Daniel Kirkwood
Kiso Schmidt Telescope	has altitude 1130 m
	has aperture 1.05 m
	has comment alternate secondary mirror offers an f/22.6 Cass. focus
	has creation date 1975
	has focal ratio f/3.1
	has latitude 35° 48' N
	has location Kiso, Japan
	has longitude 137° 38' E
	has mirror diameter 1.5 m
	has mirror maker Nikon
	has mirror type Cer-Vit
	has mounting manufacturer Nikon
	has operator Univ. of Tokyo
	has owner Kiso Observatory
	is an instance of Fork equatorial telescope
	is an instance of Schmidt
Klein-Alfvén cosmology	has definition A cosmological model in which the early universe is depicted as a giant collapsing spherical cloud of matter and antimatter. When a critical density is reached, the matter and antimatter begin to annihilate, the resulting release of radiation and energy causing the universe to expand. There are many difficulties with this model of the expanding universe, which is largely discredited on observational grounds.
	has definition A cosmology in which the observed expansion of the Universe results from the bounce of an originally collapsing cloud of matter and antimatter. The bounce is caused by radiation pressure generated by annihilations when the cloud reaches high density (10-2 cm-3).
	is a kind of cosmology theory
Kolmogorov-Smirnov test	has definition A nonparametric test used in statistics. The Kolmogorov statistic is simply the magnitude of the maximum deviation between the integral distribution function of a sample and the theoretical distribution one wishes to test.
	is an instance of statistical test
KREEP	has definition Lunar basaltic material rich in radioactive elements. (from K for potassium, REE for rare-earth elements, P for phosphorus)
	is a part of Moon
Kruger 60 AB	has apparent magnitude 12
	has definition A faint dM binary in the Solar neighborhood. It may be a subluminous star.
	has distance 3.93 pc
	has orbital period P = 44.5 years
	is an instance of binary star
	is an instance of dwarf
	is an instance of M star
krypton	has image
	has ocean oxidation state 0
	is a kind of inert gas
	is a kind of unclassified oceanic element
Kuiper Airborne Observatory	has acronym KAO
	is an instance of airborne telescope
Kuiper band	has species methane
	has wavelength 7500 Å
	is a kind of molecular band
	occurs in Uranus and Neptune
kurtosis	has definition The peakedness or flatness in the graphical representation of a statistical distribution.
	is a kind of statistical quantity
Kvistaberg Schmidt Telescope	has altitude 33 m
	has aperture 1.00 m
	has creation date 1963
	has focal ratio f/3.00
	has latitude 59° 30' N
	has location Kvistaberg, Sweden
	has longitude 17° 36' E
	has mirror diameter 1.35 m
	has mirror maker Uppsala Univ. Obs.
	has mounting manufacturer Various Swedish factories
	has owner Uppsala University Observatory
	has synonym Uppsala Schmidt
	is an instance of Schmidt
L corona	has definition The part of the solar corona whose spectrum consists of emission lines.
	is a part of corona
L magnitude	has band 3.5 microns
	has definition The magnitude derived from observations at 3.5 microns.
	is an instance of infrared magnitude
L2	has definition An unstable Lagrangian point beyond the most massive body on the line passing through the centers of mass of the two bodies.
	has location beyond the most massive body on the line passing through the centers of mass of the two bodies
	is an instance of unstable Lagrangian point
L3	has definition An unstable Lagrangian point beyond the less massive body on the line passing through the centers of mass of the two bodies.
	has location beyond the less massive body on the line passing through the centers of mass of the two bodies
	is an instance of unstable Lagrangian point
L4	has definition A stable Lagrangian point in the orbit of the less massive component which is equidistant from the two main components in the orbit direction
	has location point in the orbit of the less massive component which is equidistant from the two main components in the orbit direction
	is an instance of stable Lagrangian point
L5	has definition A stable Lagrangian point in in the orbit of the less massive component which is equidistant from the two main components opposite the orbit direction
	has location point in the orbit of the less massive component which is equidistant from the two main components opposite the orbit direction
	is an instance of stable Lagrangian point
Laboratory for High Energy Astrophysics	has acronym LHEA
	is an instance of astronomical institution
Lacaille	has birth date 1713
	has career Invented several southern constellations (many of which are still in use) compiled a catalog of Southern Deep-Sky objects with 42 entries, 33 of which are real. Among them are 25 original and at least two independent rediscoveries. discovered Eta Carinae Nebula NGC 3372 discovered globular cluster 47 Tucanae (NGC 104) discovered the Tarantula Nebula NGC 2070 in the Large Magellanic Cloud discovered spiral galaxy M83, the first discovered galaxy beyond the Local Group
	has contribution named many of the constellations in the southern hemisphere
	has death date 1762
	has name Abbé Nicholas Louis de la Caille
	is an instance of astronomer
Lacerta	has acronym Lac
	has genitive Lacertae
	has synonym Lizard
	is a part of celestial sphere
	is an instance of constellation
Lagoon Nebula	has definition An emission nebula in Sagittarius 2 kpc distant.
	has distance 5000 light years
	has image
	has Messier number 8
	has synonym M 8
	has synonym NGC 6523
	is a part of Sagittarius
	is an instance of gaseous nebula
	is an instance of Messier object
Lagrangian point	has definition One of five points in the orbital plane of two massive particles in circular orbits around a common center of gravity, where a third particle of negligible mass can remain in equilibrium.
	has equilibrium stable or unstable
	has location a point in the orbital plane of the system
	is a kind of rotating coordinate
Lallemand camera	has definition A type of image tube (q.v.).
	is a kind of image tube
Lamb shift	has definition A tiny change in the energy of an electron orbiting the nucleus of an atom, caused by the interaction of the electron with other particles that appear and quickly disappear in "quantum fluctuations". (See quantum fluctuations.)
	has definition The difference in energy levels of the H atom between 2S1/2 and 2P4.
	is a kind of wavelength shift
lambda	has definition Heavy short-lived baryon.
	is an instance of hyperon
lambda Bootis star	has definition A type of young (usually early A), weak-lined, metal-poor stars with low radial velocities.
	has definition A-type stars with weak metallic lines, low rotational velocity and low radial velocity.
	has radial velocity small
	is a kind of weak line star
	rotational velocity small
lambda doublet	has definition Two lines in the microwave region of the spectrum of the OH molecule caused by splitting of electronic levels.
	has species OH
	is a kind of set of lines
Landsat 5	is an instance of Landsat satellite
Landsat satellite	is a kind of artificial satellite
lanthanum	has image
	has ocean oxidation state III
	has ocean residence time 200 years
	is a kind of rare Earth
	is a kind of recycled oceanic element
Laplacian determinism	has definition Clockwork conception of the universe in which complete knowledge of the state of the universe at one moment completely determines its state at all future and past moments.
	is a kind of determinism
Laplacian plane	has definition For a system of satellites, the fixed plane relative to which the vector sum of the disturbing forces has no orthogonal component.
	is a kind of plane
Large Binocular Telescope	has acronym LBT
	has altitude 3170 m
	has aperture 11.8 m
	has comment Twin 8.4-m reflectors; a project of Univ. of Arizona, Arcetri Astrophysical Obs., and Research Corp. (Tucson)
	has creation date 2004
	has focal ratio f/1.14, 5.4, 15 (each)
	has latitude 32° 42' N
	has location Mount Graham, Arizona, US
	has longitude 109° 51' W
	has mirror maker R. Angel, B. Martin
	has mounting altazimuth "two-shooter" with telescopes 14.4 m apart (center to center)
	has owner Columbus Project
	is an instance of altazimuth telescope
	is an instance of Cassegrain
	is an instance of optical telescope
Large Magellanic Cloud	has acronym LMC
	has declination -69d45m22s
	has definition The nearest and largest of the many galaxies that orbit the Milky Way. It is 160000 light-years away.
	has diameter 645 × 550 arcminutes
	has distance from galaxy center 50 kpc
	has image
	has magnitude 0.9
	has mass 10 billion solar masses
	has NED data
	has right ascension 05h23m34.5s
	is a part of Dorado
	is a part of Local Group
	is an instance of Magellanic Cloud
	orbits Milky Way
large-scale motions	has definition Bulk motions of distant galaxies deviating from the Hubble flow.
	is a kind of galaxy motion
Larmor frequency	has definition The frequency of precession of a charged particle orbiting in a uniform magnetic field. It is equal to eH / 4πme, where e is the electron charge, me is the electron mass, and H is the magnetic field strength.
	is a kind of frequency
Larmor radius	has definition The radius of the circular orbit that a charged particle describes transverse to a magnetic field.
	is an instance of radius
late star	has definition Star with spectral type later than the sun (G2).
	has spectral type cooler than G2
	is a kind of star
law	has definition A theory of such wide and invariable application that its violation is thought to be impossible.
	has discoverer
	is a kind of theory
lawrencium	is a kind of transuranium element
lead	has image
	has ocean oxidation state II
	has ocean residence time 50 years
	is a kind of group IV element
	is a kind of metallic element
	is a kind of scavenged oceanic element
leap second	has definition A second (see second, Système International) added between 60s and 0s at announced times to keep UTC within 0s.90 of UT1. Generally, leap seconds are added at the end of June or December.
	is an instance of second unit
Leavitt	had appointment reaserch assistant, Harvard College Observatory, 1908
	has accomplishment discovered 2400 Cepheids in the Large Magellanic Cloud
	has greatest achievement the Cepheid period-luminosity relation used in Cepheid distance determination
	has name Henrietta Leavitt
	is an instance of 19th century scientist
	is an instance of astronomer
	is associated with Pickering
length	has unit length unit
	is a kind of quantity
length unit	is a kind of unit
	is a unit of length
lens	has focal length
	has focal ratio
	has index of refraction
	has wavefront modification shape
	is a kind of wavefront modifier
Lense-Thirring effect	has definition The precession of the plane of the geodesic orbit of a test particle around a rotating mass in general relativity. It arises from the coupling of the rotation of the central mass with the orbital angular momentum of the test particle. This precession is described as resulting from the dragging of inertial frames.
	is an instance of precession
Lenz's law	has definition The current induced by an electromotive force will appear in such a direction that it opposes the charge that produced it.
	is an instance of electromagnetic law
Leo	has acronym Leo
	has boundary
	has genitive Leonis
	has historical origin the Nemean Lion strangled to death by Hercules
	has synonym Lion
	is a part of Zodiac
	is an instance of zodiacal constellation
Leo dwarf	has definition Two dwarf elliptical galaxies (about 220-250 kpc distant) belonging to the Local Group. Leo I (dE3), Mv ≈ - 11, diameter 1.8 kpc; Leo II, Mv = -9.5, diameter 1.3 kpc.
	is a kind of dwarf elliptical galaxy
	is a kind of satellite galaxy
Leo I	has definition The most distant galaxy that orbits the Milky Way. It is close to the bright star Regulus, whose glare interferes with the study of the galaxy.
	has discovery date 1950
	has distance from galaxy center 890000 light-years
	is a part of Leo
	is a part of Local Group
	is an instance of Leo dwarf
	orbits Milky Way
Leo II	has definition The second most distant galaxy that orbits the Milky Way. Like Leo I, Leo II is a dwarf galaxy that was discovered in 1950.
	has discovery date 1950
	has distance from galaxy center 720000 light-years
	is a part of Leo
	is a part of Local Group
	is an instance of Leo dwarf
	orbits Milky Way
Leo Minor	has acronym LMi
	has genitive Leonis Minoris
	has synonym Little Lion
	is a part of celestial sphere
	is an instance of constellation
Leonid	has definition Shower of meteors emanating from an apparent point in Leo every 33 years; the next is due in about 1999.
	has duration 2 days
	has parent object Comet Tempel-Tuttle 1866 I
	has radiant Leo
	has rate 20 per hour
	has start time 16 November
	is an instance of meteor shower
lepton	has definition A class of non-strongly-interacting particles that includes the electron, muon, tau, and their associated neutrinos. See Table 7.1 on page 120.
	has definition A fermion which is not made of quarks.
	has definition A fundamental particle of nature, which may interact via all of the fundamental forces except the strong nuclear force. The electron is an example of a lepton.
	has definition Any fermion that does not participate in the strong interactions. Leptons include the electron family and the muon family.
	has definition Elementary particle like the electron and neutrino that do not experience the strong nuclear force. Unlike the strongly interacting hadrons, the leptons have small masses.
	has definition Elementary particle that does not participate in the strong interactions, including electrons, muons, and neutrinos.
	has definition Elementary particle that have no measurable size and are not influenced by the strong nuclear force. Electrons, muons, and neutrinos are leptons.
	has definition The generic name for any spin-1/2 particle which does not feel the strong nuclear force. The six known leptons are the electron, the muon, the tau lepton, and their respective neutrinos. The name was originally coined to refer to light particles.
	has spin 1/2
	is a kind of elementary particle
	is a kind of fermion
lepton era	has definition The Big Bang era when the temperature had dropped to about 1012 K and when the Universe consisted mainly of leptons and photons. It lasted until the temperature fell below 1010 K. At this stage, the characteristic photon energy fell below the rest mass energy of an electron, and the abundance of electron-positron pairs fell by many orders of magnitude. Only one electron survived for every 108 photons. The universe was subsequently radiation-dominated (substantial numbers of neutrinos were also present, but they did not interact directly with the matter or the radiation).
	has duration 1 s
	has start time 10-5 s after Big Bang
	has temperature 1010 K to 1012 K
	is a kind of Big Bang era
	is followed by nucleosynthetic era
	is preceded by hadron era
Lepus	has acronym Lep
	has boundary
	has genitive Leporis
	has historical origin one of the many animals hunted by Orion, the Hunter
	has synonym Hare
	is a part of celestial sphere
	is an instance of constellation
LF band	has frequency 30 to 300 kHz
	has wavelength 1 to 10 km
	is a kind of radio
LHe	has definition The symbol for liquid helium. The temperature of liquid helium is normally 4 K, that is, four degrees above absolute zero.
	is a kind of liquid
Libra	has acronym Lib
	has genitive Librae
	has synonym Scales
	is a part of Zodiac
	is an instance of zodiacal constellation
libration	has definition Any of several oscillations in the apparent aspect of the Moon as seen from Earth, which, when combined, enable Earth-based observers over a period of time to see altogether about 59 percent of the Moon's surface. Physical librations are angular motions about the center of mass due to gravitational torques on the Moon. Optical librations are the apparent rotations of the Moon, caused by our observing it from somewhat different directions at different times.
	has definition The "turning" of the Moon so that although the same face is presented to Earth at all times, the overall surface of the Moon visible is 59% of the total. Libration is described as latitudinal, longitudinal and diurnal.
	has definition Variations in the orientation of the Moon's surface with respect to an observer on the Earth. Physical librations are due to variations in the rate at which the Moon rotates on its axis. The much larger optical librations are due to variations in the rate of the Moon's orbital motion, the obliquity of the Moon's equator to its orbital plane, and the diurnal changes of geometric perspective of an observer on the Earth's surface.
	is a kind of rotation
libration orbit	has definition An elliptical orbit associated with a stable Lagrangian point.
	is a kind of elliptical orbit
life zone	has definition The region around a star where a planet can have liquid water and so may support life.
	is a part of solar system
lifetime	has definition The time it takes for a sample of identical particles to decay to 1/e of its initial population.
	has equivalent half-life / ln 2
	has symbol τ
	has synonym mean life
	is a kind of time
	see also half-life
light element	has definition In astronomy these elements are hydrogen, helium, and lithium; sometimes also beryllium and boron.
	is a kind of element
light quark	is a kind of quark
light-year	has definition Distance traveled at the speed of light for one Earth-year: 9.46 million million km.
	has definition The distance light travels in a vacuum in 1 year. 1 lt-yr = 9.4605 × 1012 km = 0.307 pc (c = 299792.46 km s-1 = 186274 miles s-1). (1 lt-min ≈ 0.13 AU.)
	has definition The distance light travels in one year, equal to 5.8 × 1012 (about six trillion) miles.
	has definition The distance light travels in one year: 5.88 trillion miles, or 9.46 trillion kilometers. The nearest star system to the Sun is 4.3 light-years away.
	has definition The distance that light traverses in a vacuum during one year.
	has definition The distance traveled in a vacuum by light in one year, equal to 9.46 × 1017cm.
	has symbol lt-yr
	has symbol ly
	is an instance of length unit
LIGO	has definition Large Interferometric Gravitational Wave Observatory.
	is an instance of gravity wave telescope
line	has definition A geometrical object with one dimension
	has dimensions 1
	is a kind of geometrical object
line broadening	changes property spectral line width
	has definition Increase in the range of wavelengths in which some characteristic emission or absorption occurs.
	is a kind of radiation modification
line of apsides	has definition The major axis of an elliptical orbit.
	is a kind of line
line of nodes	has definition The intersection between the orbital plane of the Moon or a planet and the plane of the ecliptic.
	is a kind of line
liquid	has definition a high density cohesive collection of particle which move relative to one another and which has short range order
	is a kind of phase of matter
liquid element	has definition element which is in liquid form at standard temperature and pressure
	has state liquid at standard temperature and pressure
	is a kind of element
liter	has symbol L
	has value in SI unit 1 dm3 = 10-3 m3
	is an instance of non SI unit
lithium	has abundance 0.17 p.p.m. in seawater
	has abundance 10 in Sun relative to H = 1 × 1012
	has abundance 20 p.p.m. in Earth's crust
	has atomic emission line 323.266 nm for Li I
	has atomic emission line 548.355 nm for Li II
	has atomic emission line 548.565 nm for Li II
	has atomic emission line 610.362 nm for Li I
	has atomic emission line 670.776 nm for Li I (strong)
	has atomic emission line 670.791 nm for Li I (strong, used in atom absorption spectrometry)
	has atomic number 3
	has atomic radii 152 pm
	has biological role none; but lithium acts to stimulate metabolism and can control manic-depressive disorders
	has boiling point 1620 K
	has chief source petalite, lepidolite
	has covalent radii 123 pm
	has crystal cell dimension a = 351.00 pm for α-Li
	has crystal cell dimension a = 437.9 pm for β-Li
	has crystal type b.c.c. for α-Li
	has crystal type f.c.c. for β-Li
	has daily dietary intake 0.1 - 2 mg
	has definition Soft, silvery-white, metal. Lightest of all solid elements, third in the periodic table after hydrogen and helium. Its atom comprises one proton and three electrons. One of the electrons is at a higher energy level than the other two. Some lithium formed in the big bang, along with huge amounts of hydrogen and helium.
	has density 515 kg m-3 for liquid at 453.69 K melting point
	has density 534 kg m-3 for solid at 293 K
	has discoverer J.A. Arfvedson
	has discovery date 1817
	has discovery location Stockholm, Sweden
	has electrical resistivity 8.55 × 10-8 Ω m at 273 K
	has electron affinity 59.6 kJ mol-1 from Li to Li-
	has electron configuration [He]2s1 in ground state
	has electronegativity 0.98 Pauling
	has hazard moderately toxic by ingestion but there are wide variations of tolerances.
	has heat capacity 20.786 J K-1 mol-1 for gas at constant pressure 0.1 MPa at 298.15 K
	has heat capacity 24.77 J K-1 mol-1 for solid at constant pressure 0.1 MPa at 298.15 K
	has heat of fusion 4.60 kJ mol-1
	has heat of vaporization 134.7 kJ mol-1
	has image
	has ionic radii 78 pm for Li+
	has isotope mass range 5 to 9
	has lethal intake 525 mg kg-1 carbonate ingested by rat
	has level in humans 0.004 mg dm-3 in blood
	has level in humans 0.023 p.p.m. in muscle
	has level in humans 0.025 p.p.m. in liver
	has level in humans 1.3 p.p.m. in bone
	has linear expansion coefficient 56 × 10-6 K-1
	has longest lived isotope lithium 7
	has main mining area USA, brines of Searles Lake, California
	has mass absorption coefficient 0.217 cm2 g-1 for MoKalpha X-ray diffraction
	has mass absorption coefficient 0.716 cm2 g-1 for CuKalpha X-ray diffraction
	has mass magnetic susceptibility +2.56 × 10-8 kg-1 m3 for solid
	has mass of element in person 7 mg for a 70 kg average person
	has melting point 453.69 K
	has mineral amblygonite, lepidolite, petalite, spodumene
	has molar volume 13.00 cm3
	has name origin lithos = stone from Greek
	has neutron scattering length -0.190 × 10-12 cm
	has number of isotopes 5 including nuclear isomers
	has number of protons 3
	has ocean oxidation state I
	has ocean residence time 2 × 106 years
	has poisson's ratio 0.36 GPa
	has pronunciation lith-iuhm
	has registry number 7439-93-2 for Chemical Abstracts System database
	has relative atomic mass 6.941 in units of 12C = 12.000
	has reserves 7.3 × 106 tonnes
has rigidity modulus 4.24 GPa
has space group Fm3m for β-Li
has space group Im3m for α-Li
has specimen chunks, ingot, powder, ribbon, rod, shot or wire. Care !
has stable isotope lithium 6, lithium 7
has symbol Li
has term symbol 2S1/2 in ground state
has thermal conductivity 84.7 W m-1 K-1 at 300 K
has thermal neutron capture cross section 70.5 barns
has toxic intake 20 - 200 g
has uses light-weight alloys, especially with aluminium and magnesium, greases, batteries, glass, medicine and nuclear bombs
has world production 39000 tonnes year-1
has young's modulus 4.91 GPa
is a kind of accumulating oceanic element
is a kind of alkali metal
is a kind of light element
reacts with oxygen and water slowly
lithium 6	has atomic mass 6.015121
	has magnetogyric ratio 3.9366 × 107 rad T-1 s-1
	has natural abundance 7.5%
	has NMR frequency 14.716 MHz where 1H = 100 MHz; 2.3488 T
	has NMR receptivity 3.58 where 13C = 1.00
	has nuclear magnetic moment μ = +0.8220467 nuclear magnetons with diamagnetic correction
	has nuclear quadrupole moment -0.00082 × 10-28 m2
	has nuclear spin I = 1+ h/2π
	has number of neutrons 3
	has number of nucleons 6
	has relative NMR sensitivity 8.50 × 10-3 where 1H = 1.00
	has symbol 6Li
	has uses isotopically enriched samples available for experimental purposes
	has uses Nuclear Magnetic Resonance
	is an instance of lithium
lithium 7	has atomic mass 7.016003
	has magnetogyric ratio 10.3964 × 107 rad T-1 s-1
	has natural abundance 92.5%
	has NMR frequency 38.863 MHz where 1H = 100 MHz; 2.3488 T
	has NMR receptivity 1540 where 13C = 1.00
	has nuclear magnetic moment μ = +3.256424 nuclear magnetons with diamagnetic correction
	has nuclear quadrupole moment -0.041 × 10-28 m2
	has nuclear spin I = 3/2- h/2π
	has number of neutrons 4
	has number of nucleons 7
	has relative NMR sensitivity 0.29 where 1H = 1.00
	has symbol 7Li
	has uses isotopically enriched samples available for experimental purposes
	has uses Nuclear Magnetic Resonance
	is an instance of lithium
lithium 8	has atomic mass 8.022485
	has decay mode β- α (16.005 Mev)
	has decay mode γ
	has half life 0.84 seconds
	has nuclear magnetic moment μ = +1.6536 nuclear magnetons with diamagnetic correction
	has nuclear spin I = 2+ h/2π
	has number of neutrons 5
	has number of nucleons 8
	has symbol 8Li
	is an instance of lithium
lithium 9	has atomic mass 9.026789
	has decay mode β- (13.6068 Mev) 65%
	has decay mode γ
	has decay mode n2α 35%
	has half life 0.177 seconds
	has nuclear magnetic moment μ = +3.439 nuclear magnetons with diamagnetic correction
	has nuclear spin I = 3/2- h/2π
	has number of neutrons 6
	has number of nucleons 9
	has symbol 9Li
	is an instance of lithium
lithium star	has definition C star with a very strong Li I 6078 line.
	has definition Peculiar giant star (spectral types G-M) whose spectra show high abundances of lithium. They are primarily S stars and carbon stars, although Li is also found in T Tauri stars, and is sometimes observed in normal late-type giants. Interstellar Li / H ≈ 2 × 10-10. (Solar system Li / H ≈ 10-9.) Lithium is destroyed rapidly (in about 7500 years) at typical nuclear burning temperatures. Spallation is the only production mechanism known for 6Li, but 7Li can be transported from the core in the form of 7Be and converted in the envelope to 7Li by electron capture. Observed 7Li / 6Li > 10.
	has spectral type G star, M star
	is a kind of C star
lithophile element	has definition element that tends to concentrate in silicate materials such as igneous rocks
	has occurrence in silicate materials such as igneous rocks
	is a kind of planetary element
Littrow	has definition The configuration of a diffraction grating spectrograph in which the diffracted ray returns along the same direction as the incident ray.
	is a kind of diffraction grating spectrograph
LN2	has definition The symbol for liquid nitrogen. The temperature of liquid nitrogen is normally 77 K, that is, 77 degrees above absolute zero.
	is a kind of liquid
local bubble	has definition The region of the Galaxy near the Sun which has little neutral hydrogen gas.
	has radius 100 light-years (up to 10000 light-years in certain directions)
	is a part of Orion spur
	is produced by supernovae
local coordinate	has azimuth
	has component local coordinate component
	has coordinate origin observer location on the surface of the Earth
	has elevation
	is a kind of coordinate
local coordinate component	is a component of local coordinate
	is a kind of coordinate component
Local Group	has definition A conglomeration of about twenty galaxies including our own Milky Way.
	has definition The association of galaxies to which the Milky Way galaxy belongs.
	has definition The cluster of galaxies of which our own, the Milky Way, is one. Its radius is estimated at 106 parsecs. Largest of the Group is the Andromeda galaxy.
	has definition The cluster of galaxies to which our Galaxy belongs. It is a poor, irregular cluster with some 20 certain members: three spirals (the Galaxy, M31, and M33); four irregulars (LMC, SMC, IC 1613, and NGC 6822); and about 13 intermediate or dwarf ellipticals (NGC 147; NGC 185; NGC 205; M32; the Sculptor, Fornax, Leo I and II, UMi, and Draco systems; and three companions to M31 discovered by van den Bergh in 1972). It may also include several other dwarf galaxies as well as the giant elliptical Maffei 1. The total mass of the Local Group is less than 1.5 times the combined masses of the Galaxy and M31.
	has definition The gravitationally bound collection of nearby galaxies ruled by the Andromeda galaxy and the Milky Way, which are its largest members. The Local Group has about 30 known galaxies.
	has definition The system of galaxies to which our Milky Way galaxy belongs is a small group, containing only two large spirals (our galaxy and the Andromeda galaxy, Messier 31) and twenty or more smaller systems.
	has diameter 2 Mpc
	has members Milky Way, M31, M33, LMC, SMC, IC 1613, NGC 6822, NGC 147, NGC 185, NGC 205, M32, Sculptor galaxy, Fornax galaxy, Leo I, Leo II, UMi galaxy, Draco system and three companions to M31
	has number of galaxies 30
	is a part of Local Supercluster
	is an instance of loose group
local hypothesis of quasars	has definition The hypothesis that quasars are not at the distances inferred from their redshifts.
	is a kind of galaxy theory
local inertial frame	has definition A coordinate system or frame of reference defined in the vicinity of the earth in which Newtons first law of motion is valid; that is, a nonrotating and nonaccelerating reference frame.
	is a kind of nonaccelerating reference frame
	is a kind of nonrotating reference frame
local standard of rest	has acronym LSR
	has definition A frame of reference in which the mean motion of stars in the immediate neighborhood is zero. In such a reference system, the motions of stars in the solar neighborhood (a volume of space about 100 pc in diameter) average out to zero (cf. solar motion). It is a coordinate system in which the origin is a point in the galactic plane moving in a circular orbit around the galactic center, and in which the three velocity components are Π, in the direction from the galactic center to the origin; θ, in the direction of galactic rotation; and Z, perpendicular to the galactic plane.
	has definition An imaginary point, located at the Sun's distance from the Galactic center, that revolves clockwise around the Galaxy on a circular orbit. Astronomers measure a star's U, V, and W velocities with respect to the local standard of rest rather than with respect to the Sun, because the Sun has a slightly noncircular orbit. The orbital velocity of the local standard of rest around the Galaxy is about 220 kilometers per second.
	has rotation rate equal to one complete orbit of the Sun around the Milky Way
	is a kind of rotating reference frame
Local Supercluster	has definition A loosely knit assemblage of some 100 clusters of galaxies, including the Local Group.
	has definition An aggregation of galaxies - roughly ten thousand of them - to which the Virgo Cluster and our own galaxy belong.
	has definition The supercluster of galaxies to which the Local Group may belong (see Virgo Supercluster). De Vaucouleurs lists 54 groups of galaxies in the LSC (R ≈ 17 Mpc).
	has definition The supercluster to which the Local Group belongs. It is shaped like a cigar, with the Virgo cluster of galaxies at its center and the Local Group near one end.
	has number of clusters 100
	has synonym Virgo supercluster
	is a part of Universe
	is an instance of supercluster
location dependent periodic celestial event	has definition Periodic celestial event which depends on the observers location on the planet
	has observer location
	is a kind of periodic celestial event
long-period variable	has amplitude 9 magnitudes in the visible, but only 2 or 3 magnitudes in the integrated spectrum
	has definition Cyclic variable with cycles 100-500 days, and of spectral types K, M, S and C.
	has definition Pulsating red giant or supergiant. Population I typically have periods greater than 200 days; Population II, periods less than 200 days. Long-period variables emit most of their radiation in the infrared.
	has period 100 to 1000 days
	has prototype Mira
	has spectral type M star, R star, or N star
	has synonym Mira variable
	has synonym red variable
	is a kind of periodic variable
	is a part of disk
longitude of the ascending node	has definition An orbital element representing the point where the body crosses the ecliptic from south to north
	has symbol Ω
	is an instance of angle
	is an instance of orbital element
longitudinal wave	has definition A wave vibrating along the direction of propagation.
	is a kind of wave
lookback time	has definition Phenomenon that, owing to the finite velocity of light, the more distant an object being observed, the older is the information received from it. A galaxy one billion light-years away, for instance, is seen as it looked one billion years ago.
	has definition The time in the past at which the light we now receive from a distant object was emitted. Galaxies of a certain type (redshift and luminosity) can be seen only at a certain distance.
	is an instance of time
loose group	has definition Galaxy group with on the order of 10 galaxies separated by 10-100 galaxy diameters.
	has number of galaxies 10 to 100
	is a kind of galaxy group
Lorentz contraction	has definition A moving object appears shortened along its direction of motion.
	is a consequence of special relativity
	is an instance of relativity process
Lorentz invariance	has definition A law which states that laws of physics are identical for all observers moving at constant velocity relative to each other.
	has generalization equivalence principle
	has synonym principle of relativity
	has transformation Lorentz transformation
	is a principle of special relativity
	is an instance of law
Lorentz transformation	has definition A transformation which enables one to relate the physical parameters describing an object when viewed in one frame of reference to those which are appropriate to an observer moving with a uniform velocity in that frame.
	has definition The transformation which keeps the speed of light invariant between relativistic frames of reference.
	is a kind of coordinate transformation
Loschmidt constant	has definition Number of molecules of an ideal gas per unit volume (2.687 × 1019 molecules per cm3).
	has definition This is the number of molecules in a cubic metre of an ideal gas at s.t.p. It is equal to the Avogadro constant divided by the molar volume. In Germany it is sometimes called Avogadro's constant.
	has equation
	has pressure 101.325 kPa
	has proposal date 1865
	has symbol n0
	has temperature 273.15 K
	has uncertainty 0.0000047 × 1025 m-3
	has value 2.6867775 × 1025 m-3
	is an instance of physico chemical constant
	is named after J. Loschmidt (1821-1895)
low surface brightness galaxy	has acronym LSBG
	has definition A galaxy which is very faint because it contains a very limited number of stars.
	is a kind of galaxy
Low-excitation radio galaxy	has acronym LERG
	is a kind of radio galaxy
low-velocity star	has definition A star whose U, V, and W velocities are all near zero. Such stars have nearly circular orbits around the Galaxy.
	is a kind of star
Lowell	19th century scientist
	has death date 1916
	has degree 1876 Harvard (distinction in mathematics)
	has greatest achievement Lowell Observatory (financed with his own money)
	has greatest blunder believes he observed a network of linear canals on the planet Mars built by extraterrestrial beings
	has name Percival Lowell
	is an instance of astronomer
Lowell Observatory	has address 1400 Mars Hill Road, Flagstaff, Arizona 86001
	has altitude 2100 m
	has creation date 1894
	has creator Lowell
	has location Flagstaff, Arizona, USA.
	has telescope 0.5 m (1894)
	is an instance of observatory
Lowell's band	has definition A dark border sometimes found on the Martian polar cap.
	is a part of Mars
lower chromosphere	has composition neutral hydrogen
	has definition The lower part of the chromosphere and consists of cool neutral hydrogen in which radiation at certain wavelengths is absorbed from the continuous spectrum emitted from the Sun's photosphere.
	has density ρ ≈ 10-13 to 10-8 g cm-3
	has location 0 to 4000 km
	has synonym reversing layer
	has temperature 7500 K
	is a part of chromosphere
lower culmination	has definition Passage of a celestial object across the observer's meridian. The crossing farther from the zenith.
	has synonym culmination below pole
	is an instance of culmination
lower precambrian period	has duration million years
	has start time million years ago
	is a kind of precambrian era
lumen	has base unit m2·m-2·cd = cd
	has symbol lm
	has unit cd·sr
	is an instance of named derived SI unit
	represents luminous flux
luminance unit	has definition candela per square meter
	has symbol cd·m-2
	is a kind of derived SI unit
luminosity	has definition Brightness of a celestial body, measured in terms of (apparent) magnitude, absolute magnitude, or using the Sun's brightness as 1.0 on a solar scale. The luminosity of a star corresponds with its internal radiation pressure, which in turn depends on its mass.
	has definition The intrinsic brightness of a star. Usually defined in terms of absolute magnitude.
	has definition The total amount of energy radiated by a star - that is, its true, or intrinsic, brightness.
	has definition Total radiant energy output per second (absolute brightness, usually expressed in ergs s-1 or in magnitudes). L = 4πR2σT4 (see Stefan's law).
	is a kind of radiation measurement
luminous blue variable	has definition A variable-star designation for the high-luminosity early type objects.
	has synonym Hubble-Sandage variable
	has synonym S Dor variable
	is a kind of early star
	is a kind of variable
luminous intensity unit	is a kind of unit
	is a unit of luminous intensity
luminous mass	has definition The mass contributed by luminous matter in galaxies (see missing mass). Luminous mass density, 5 × 10-32 g cm-3 for H0 = 50 km s-1 Mpc-1.
	is an instance of mass
Lumogen	has definition Fluorescent UV coating which improves the blue sensitivity of a CCD by emitting light at approximately 540 to 580 nm when excited with light of wavelengths shorter than 450 nm.
	is a kind of coating
lunar eclipse	has definition An eclipse in which the Moon passes through the shadow cast by the Earth.
	is a kind of eclipse
lunar fines	has definition Small particles of rock or powdered rock on the Moon.
	is a part of Moon
lunar spacecraft	is a kind of artificial satellite
Lundmark	has country of origin Sweden
	has name Knut Lundmark
	is an instance of 19th century scientist
	is an instance of astronomer
Lunette Bischoffscheim	has altitude 372 m
	has aperture 0.74 m
	has creation date 1886
	has focal ratio f/24.2
	has latitude 43° 43' N
	has lens maker Henry Brothers
	has location Mont Gros, France
	has longitude 7° 18' E
	has mounting manufacturer P. Gautier
	has owner Observatoire de Nice
	is an instance of German equatorial telescope
	is an instance of refractor
Lupus	has acronym Lup
	has genitive Lupi
	has synonym Wolf
	is a part of celestial sphere
	is an instance of constellation
Lupus Loop	has definition A radio source, a large broken shell 4°.5 in diameter, identified as a prehistoric supernova remnant.
	is a part of Lupus
	is an instance of radio source
lutetium	has image
	has ocean oxidation state III
	has ocean residence time 4000 years
	is a kind of rare Earth
	is a kind of recycled oceanic element
lux	has base unit m2·m-4·cd = m-2·cd
	has symbol lx
	has unit lm·m-2
	is an instance of named derived SI unit
	represents illuminance
Luyten 726-8	has definition A binary (M5.5e V, M6e V) (component B is UV Cet) with a very small mass (total mass of system (1974) 0.3 Msun ).
	has distance 2.7 pc
	is an instance of binary star
Lyman alpha	has definition The resonance line of hydrogen. It can only be studied from spacecraft or in the spectra of highly redshifted quasars.
	has upper energy level 2
	has wavelength 1215.67 Å
	is an instance of Lyman line
	is an instance of resonance line
Lyman beta	has upper energy level 3
	has wavelength 1026 Å
	is an instance of Lyman line
lyman break galaxy	has acronym LBG
	is a kind of galaxy
Lyman line	has lower energy level 1
	has series limit 912 Å
	is a kind of hydrogen line
	is a kind of spectral series line
Lynx	has acronym Lyn
	has boundary
	has genitive Lyncis
	has synonym Lynx
	is a part of celestial sphere
	is an instance of constellation
Lyon Meudon Extragalactic Database	has acronym LEDA
	is an instance of database
Lyot division	has definition A gap between the B ring and the C ring.
	is a kind of ring gap
	is a part of Saturn ring system
Lyra	has acronym Lyr
	has boundary
	has genitive Lyrae
	has historical origin invented by Hermes as a gift to his half-brother Apollo, who gave it to Orpheus, the musician of the Argonauts
	has synonym Lyre
	is a part of celestial sphere
	is an instance of constellation
Lyrid	has duration 1 day
	has parent object Comet Thatcher
	has radiant Lyra
	has rate 15 per hour
	has start time 22 April
	is an instance of meteor shower
M 10	has discoverer Charles Messier
	has discovery date 1764
	has image
	has Messier number 10
	has reference NED, SIMBAD
	has synonym NGC 6254
	is a part of halo
	is a part of Ophiuchus
	is an instance of class VII globular cluster
	is an instance of Messier object
M 100	has discoverer Pierre Mechain
	has discovery date 1781
	has image
	has Messier number 100
	has synonym NGC 4321
	is a part of Coma Berenices
	is a part of Local Group
	is an instance of Messier object
	is an instance of Sc spiral
M 107	has discoverer Pierre Mechain
	has discovery date 1782
	has image
	has Messier number 107
	has reference NED, SIMBAD
	has synonym NGC 6171
	is a part of halo
	is a part of Ophiuchus
	is an instance of class X globular cluster
	is an instance of Messier object
M 12	has discoverer Charles Messier
	has discovery date 1764
	has image
	has Messier number 12
	has reference NED, SIMBAD
	has synonym NGC 6218
	is a part of halo
	is a part of Ophiuchus
	is an instance of class IX globular cluster
	is an instance of Messier object
M 13	has discoverer Edmond Halley
	has discovery date 1714.
	has image
	has Messier number 13
	has reference NED, SIMBAD
	has synonym NGC 6205
	is a part of halo
	is a part of Hercules
	is an instance of class V globular cluster
	is an instance of Messier object
M 14	has discoverer Charles Messier
	has discovery date 1764
	has image
	has Messier number 14
	has reference NED, SIMBAD
	has synonym NGC 6402
	is a part of halo
	is a part of Ophiuchus
	is an instance of class VIII globular cluster
	is an instance of Messier object
M 15	has discoverer Jean-Dominique Maraldi
	has discovery date 1746
	has image
	has Messier number 15
	has reference NED, SIMBAD
	has synonym NGC 7078
	is a part of halo
	is a part of Pegasus
	is an instance of class IV globular cluster
	is an instance of Messier object
M 19	has discoverer Charles Messier
	has discovery date 1764
	has image
	has Messier number 19
	has reference NED, SIMBAD
	has synonym NGC 6273
	is a part of halo
	is a part of Ophiuchus
	is an instance of class VIII globular cluster
	is an instance of Messier object
M 2	has discoverer Jean-Dominique Maraldi
	has discovery date 11 September 1746
	has image
	has Messier number 2
	has reference NED, SIMBAD
	has synonym NGC 7089
	is a part of Aquarius
	is a part of Aquarius
	is a part of halo
	is an instance of class II globular cluster
	is an instance of Messier object
M 22	has discoverer Abraham Ihle
	has discovery date 1665
	has image
	has Messier number 22
	has reference NED, SIMBAD
	has synonym NGC 6656
	is a part of halo
	is a part of Sagittarius
	is an instance of class VII globular cluster
	is an instance of Messier object
M 28	has discoverer Charles Messier
	has discovery date 1764
	has image
	has Messier number 28
	has reference NED, SIMBAD
	has synonym NGC 6626
	is a part of halo
	is a part of Sagittarius
	is an instance of class IV globular cluster
	is an instance of Messier object
M 3	has discoverer Charles Messier
	has discovery date 3 May 1764
	has image
	has Messier number 3
	has reference NED, SIMBAD
	has synonym NGC 5272
	is a part of Canes Venatici
	is a part of halo
	is an instance of class VI globular cluster
	is an instance of Messier object
M 30	has discoverer Charles Messier
	has discovery date 1764
	has image
	has Messier number 30
	has reference NED, SIMBAD
	has synonym NGC 7099
	is a part of Capricornus
	is a part of halo
	is an instance of class V globular cluster
	is an instance of Messier object
M 32	has definition An elliptical galaxy that orbits the Andromeda galaxy.
	has Messier number 32
	is a part of Local Group
	is an instance of dwarf elliptical satellite galaxy
	is an instance of Messier object
	orbits Andromeda galaxy
M 4	has discoverer Philippe Loys de Cheseaux
	has discovery date 1746
	has image
	has Messier number 4
	has reference NED, SIMBAD
	has synonym NGC 6121
	is a part of halo
	is a part of Scorpius
	is an instance of class IX globular cluster
	is an instance of Messier object
M 5	has discoverer Gottfried Kirch
	has discovery date 1702
	has image
	has Messier number 5
	has reference NED, SIMBAD
	has synonym NGC 5904
	is a part of halo
	is a part of Serpens
	is an instance of class V globular cluster
	is an instance of Messier object
M 53	has discoverer Johan Elert Bode
	has discovery date 1775
	has image
	has Messier number 53
	has reference NED, SIMBAD
	has synonym NGC 5024
	is a part of Coma Berenices
	is a part of halo
	is an instance of class V globular cluster
	is an instance of Messier object
M 54	has discoverer Charles Messier
	has discovery date 1778
	has image
	has Messier number 54
	has reference NED, SIMBAD
	has synonym NGC 6715
	is a part of halo
	is a part of Sagittarius
	is an instance of class III globular cluster
	is an instance of Messier object
M 55	has discoverer Abbe Nicholas Louis de la Caille
	has discovery date 1751
	has image
	has Messier number 55
	has reference NED, SIMBAD
	has synonym NGC 6809
	is a part of halo
	is a part of Sagittarius
	is an instance of class XI globular cluster
	is an instance of Messier object
M 56	has discoverer Charles Messier
	has discovery date 1779
	has image
	has Messier number 56
	has reference NED, SIMBAD
	has synonym NGC 6779
	is a part of halo
	is a part of Lyra
	is an instance of class X globular cluster
	is an instance of Messier object
M 62	has discoverer Charles Messier
	has discovery date 1771
	has image
	has Messier number 62
	has reference NED, SIMBAD
	has synonym NGC 6266
	is a part of halo
	is a part of Ophiuchus
	is an instance of class IV globular cluster
	is an instance of Messier object
M 68	has discoverer Pierre Mechain
	has discovery date 1780
	has image
	has Messier number 68
	has reference NED, SIMBAD
	has synonym NGC 4590
	is a part of halo
	is a part of Hydra
	is an instance of class X globular cluster
	is an instance of Messier object
M 69	has discoverer Abbe Nicholas Louis de la Caille
	has discovery date 1751
	has image
	has Messier number 69
	has reference NED, SIMBAD
	has synonym NGC 6637
	is a part of halo
	is a part of Sagittarius
	is an instance of class V globular cluster
	is an instance of Messier object
M 70	has discoverer Charles Messier
	has discovery date 1780
	has image
	has Messier number 70
	has reference NED, SIMBAD
	has synonym NGC 6681
	is a part of halo
	is a part of Sagittarius
	is an instance of class V globular cluster
	is an instance of Messier object
M 71	has discoverer Philippe Loys de Cheseaux
	has discovery date 1745
	has image
	has Messier number 71
	has reference NED, SIMBAD
	has synonym NGC 6838
	is a part of halo
	is a part of Sagitta
	is an instance of class X globular cluster
	is an instance of Messier object
M 72	has discoverer Pierre Mechain
	has discovery date 1780
	has image
	has Messier number 72
	has reference NED, SIMBAD
	has synonym NGC 6981
	is a part of Aquarius
	is a part of halo
	is an instance of class IX globular cluster
	is an instance of Messier object
M 75	has apparent magnitude 8.5 (V)
	has diameter 6.0 arc min
	has discoverer Pierre Mechain
	has discovery date 1780
	has distance 57.7 kly
	has image
	has Messier number 75
	has reference NED, SIMBAD
	has synonym NGC 6864
	is a part of halo
	is a part of Sagittarius
	is an instance of class I globular cluster
	is an instance of Messier object
M 79	has discoverer Pierre Mechain
	has discovery date 1780
	has image
	has Messier number 79
	has reference NED, SIMBAD
	has synonym NGC 1904
	is a part of halo
	is a part of Lepus
	is an instance of class V globular cluster
	is an instance of Messier object
M 80	has discoverer Charles Messier
	has discovery date 1781
	has distance 28000 light years
	has image
	has Messier number 80
	has reference NED, SIMBAD
	has synonym NGC 6093
	is a part of halo
	is a part of Scorpius
	is an instance of class II globular cluster
	is an instance of Messier object
M 81	has definition A giant spiral galaxy 11 million light-years away in the constellation Ursa Major. It rules the M81 group, the second nearest galaxy group to the Local Group.
	has discoverer Johan Elert Bode
	has discovery date 1774
	has image
	has Messier number 81
	has synonym NGC 3031
	is a part of Local Group
	is a part of Ursa Major
	is an instance of Messier object
	is an instance of Sb spiral
M 81 group	is a part of Local Supercluster
	is an instance of loose group
M 87	has definition A strong radio source. Optically, it is an elliptical galaxy with a luminous blue jet about 1500 pc long. It is also one of the most powerful extragalactic sources of radiation at infrared wavelengths.
	has Messier number 87
	has synonym 2U 1228+12
	has synonym 3C 274
	has synonym Virgo A
	has synonym Virgo X-1
	is a part of Virgo
	is a part of Virgo cluster
	is an instance of giant elliptical galaxy
	is an instance of Messier object
	is an instance of radio galaxy
	is an instance of X-ray source
M 9	has discoverer Charles Messier
	has discovery date 1764
	has image
	has Messier number 9
	has reference NED, SIMBAD
	has synonym NGC 6333
	is a part of halo
	is a part of Ophiuchus
	is an instance of class VIII globular cluster
	is an instance of Messier object
M 92	has discoverer Johan Elert Bode
	has discovery date 1777
	has image
	has Messier number 92
	has reference NED, SIMBAD
	has synonym NGC 6341
	is a part of halo
	is a part of Hercules
	is an instance of class IV globular cluster
	is an instance of Messier object
M magnitude	has band 5 microns
	has definition The magnitude derived from observations at 5 microns.
	is an instance of infrared magnitude
M star	has definition Having a spectral type of M, that is, red like Betelgeuse and Antares.
	has definition Spectral type for red stars, such as Betelgeuse, Antares, and Proxima Centauri.
	has definition Star of spectral type M are cool red stars with surface temperatures of less than 3600 K whose spectra are dominated by molecular bands, especially those of TiO. M dwarfs are the most numerous type in our galaxy.
	is a kind of late star
M-Theory	has definition Theory emerging from the second superstring revolution that unites the previous five superstring theories within a single overarching framework. M-theory appears to be a theory involving eleven spacetime dimensions, although many of its detailed properties have yet to be understood.
	is a kind of superstring theory
M2-9	has distance 2100 light years
	has expansion velocity 200 miles per second
	has image
	is an instance of planetary nebula
	is part of Ophiucus
Mach's Principle	has definition A pre-relativity statement to the effect that the local inertial frame is determined by some average of the motion of all the matter in the universe. In essence, mach's principle says that space, which is the arena in which matter interacts, is itself an aspect of that matter.
	has definition The hypothesis that the inertia of bodies - that is, their resistance to acceleration by applied forces - is determined not by any absolute properties of space but by the effects of distant matter in the universe. equivalently, Mach's principle proposes that the distinction between accelerated and nonaccelerated frames of reference is determined by the effects of distant matter.
	has definition The hypothesis that the local inertial frame and the inertia of any body is determined by the distribution of all the matter in the universe.
	has definition The precept that the inertia of objects results not from their relationship to Newtonian absolute space, but to the rest of the mass and energy distributed throughout the universe. Though unproved and perhaps unprovable, Mach's principle inspired einstein, who sought with partial success to incorporate it into the general theory of relativity.
	is a kind of outdated belief
Magellan Project	has altitude 2300 m
	has aperture 6.5 m
	has comment a twin, Magellan II is to be built in 2001
	has creation date (1999)
	has focal ratio f/1.25, 11, 15
	has latitude 29° 00' S
	has location Las Campanas, Chile
	has longitude 70° 42' W
	has mirror maker R. Angel, B. Martin
	has mounting manufacturer L & F Industries
	has operator Carnegie Institution of Washington
	has optical design Cassegrain and Gregorian
	has owner Las Campanes Observatory
	has synonym Magellan I
	is an instance of altazimuth telescope
	is an instance of optical telescope
	is an instance of reflector
Magellanic Cloud	has definition The Large and Small Magellanic Clouds, the two nearest and largest of the galaxies that orbit the milky way. The Magellanic Clouds lie in the southern sky and cannot be seen from the United States.
	has definition Two galaxies that lie close to the milky way galaxy. They are visible in the southern skies of Earth.
	has definition Two relatively small, nebulous stellar systems visible only in the southern hemisphere; the larger is, however, the brightest "nebular" object in the sky. both are members of the local group of galaxies, and in fact seem to be associated, though detached, parts of the milky way system.
	has definition Two small irregular (or possibly barred spiral) galaxies (satellites of the Milky Way galaxy) about 50-60 kpc (LMC, in Dorado) and 60-70 kpc (SMC, in Toucana) distant, visible to the naked eye from the southern hemisphere. Both clouds contain mainly Population I stars. The LMC contains numerous ob stars and at least 10 stars that are an order of magnitude brighter (mv = - 9) than any supergiants known in our galaxy. It also contains several times our galaxy's concentration of interstellar matter.
	is a kind of irregular galaxy
	is a kind of naked eye object
	is a kind of satellite galaxy
Magellanic Stream	has definition A name given to the common envelope of neutral hydrogen in which the LMC and SMC are embedded.
	has definition A name given to the long H I filament that extends from the region between the Magellanic Clouds down to the south galactic pole and which appears to make a 180° arc of a great circle across the sky.
	has definition A strand of gas that the Milky Way has ripped out of the Magellanic Clouds.
	has length 300000 light-years
	is a part of halo
	is a part of Large Magellanic Cloud
	is a part of Small Magellanic Cloud
magnesium	has abundance 1200 p.p.m. in seawater
	has abundance 23000 p.p.m. in Earth's crust
	has abundance 4.0 × 107 in Sun relative to H = 1 × 1012
	has atomic emission line 279.553 nm for Mg II
	has atomic emission line 280.270 nm for Mg II
	has atomic emission line 383.829 nm for Mg I
	has atomic emission line 518.361 nm for Mg I
	has atomic emission line 285.213 nm for Mg I (strong)
	has atomic number 12
	has atomic radii 160 pm
	has biological role essential to all species
	has boiling point 1363 K
	has bulk modulus 35.6 GPa
	has chief source seawater; and the ores or dolomite, magnesite; carnallite and brucite
	has covalent radii 136 pm
	has crystal cell dimension a = 320.94; c = 521.03 pm
	has crystal type h.c.p.
	has daily dietary intake 250 - 380 mg
	has definition silvery white, lustrous and relatively soft metal
	has density 1585 kg m-3 for liquid at 922.0 K melting point
	has density 1738 kg m-3 for solid at 293 K
	has discoverer Joseph Black
	has discovery date 1755
	has discovery location Edinburgh, Scotland
	has electrical resistivity 4.38 × 10-8 Ω m at 293 K
	has electron affinity -21 kJ mol-1 from Mg to Mg-
	has electron configuration [Ne]3s2 in ground state
	has electronegativity 1.31 Pauling
	has hazard compounds vary in toxicity but no evidence that metal produces systemic poisoning
	has heat capacity 20.786 J K-1 mol-1 for gas at constant pressure 0.1 MPa at 298.15 K
	has heat capacity 24.89 J K-1 mol-1 for solid at constant pressure 0.1 MPa at 298.15 K
	has heat of fusion 9.04 kJ mol-1
	has heat of vaporization 128.7 kJ mol-1
	has image
	has ionic radii 79 pm for Mg2+
	has isotope mass range 20 to 31
	has lethal intake 8100 mg kg-1 for chloride, oral, rat
	has level in humans 37.8 mg dm-3 in blood
	has level in humans 590 p.p.m. in liver
	has level in humans 700 - 1800 p.p.m. in bone
	has level in humans 900 p.p.m. in muscle
	has linear expansion coefficient 26.1 × 10-6 K-1
	has longest lived isotope magnesium 24
	has main mining area Austria, China, Poland, Russia, USA, India, Greece, Canada
	has mass absorption coefficient 38.6 cm2 g-1 for CuKα X-ray diffraction
	has mass absorption coefficient 4.11 cm2 g-1 for MoKα X-ray diffraction
	has mass magnetic susceptibility +6.8 × 10-9 kg-1 m3 for solid
	has mass of element in person 19 g for a 70 kg average person
	has melting point 922.0 K
	has mineral brucite, carnalite, cordierite, diopside, dolomite, enstatite, epsomite, kiersite, magnesite, pyrope, spinel
	has molar volume 13.98 cm3
	has name origin Magnesia = district of Thessaly from Greek
	has neutron scattering length 0.5375 × 10-12 cm
	has number of isotopes 12 including nuclear isomers
	has number of protons 12
	has ocean oxidation state II
	has ocean residence time 1 × 107 years
	has poisson's ratio 0.291 GPa
	has pronunciation mag-neez-iuhm
	has registry number 7439-95-4 for Chemical Abstracts System database
	has relative atomic mass 24.3050 in units of 12C = 12.000
	has reserves > 2 × 1010 tonnes as ores; and > 1 × 1024 tonnes in the sea
	has rigidity modulus 17.3 GPa
	has space group P63/mmc
has specimen chips, granules, powder, ribbon, rod or turnings. Safe.
has symbol Mg
has synthesis mechanism electrolysis of fused MgCl2
has term symbol 1S0 in ground state
has thermal conductivity 156 W m-1 K-1 at 300 K
has thermal neutron capture cross section 0.063 barns
has toxic intake low toxicity
has uses as a 'sacrificial' electrode to protect other metals exposed to seawater and ground
has uses bulk metal and lightweight alloys
has world production 325000 tonnes year-1
has young's modulus 44.7 GPa
is a kind of accumulating oceanic element
is a kind of alkali earth metal
is a kind of lithophile element
reacts with air when ignited
reacts with hot water
magnesium 23	has atomic mass 22.994124
	has decay mode β+ (4.058 Mev) %
	has decay mode γ
	has half life 11.32 seconds
	has nuclear spin I = 3/2 h/2π
	has number of neutrons 11
	has number of nucleons 23
	has symbol 23Mg
	is an instance of magnesium
magnesium 24	has atomic mass 23.985042
	has natural abundance 78.99%
	has nuclear spin I = 0+ h/2π
	has number of neutrons 12
	has number of nucleons 24
	has symbol 24Mg
	has uses isotopically enriched samples available for experimental purposes
	is an instance of magnesium
magnesium 25	has atomic mass 24.985837
	has magnetogyric ratio 1.6375 × 107 rad T-1 s-1
	has natural abundance 10.00%
	has NMR frequency 6.1195 MHz where 1H = 100 MHz; 2.3488 T
	has NMR receptivity 1.54 where 13C = 1.00
	has nuclear magnetic moment μ = -0.85546 nuclear magnetons with diamagnetic correction
	has nuclear quadrupole moment 0.1994 × 10-28 m2
	has nuclear spin I = 5/2+ h/2π
	has number of neutrons 13
	has number of nucleons 25
	has relative NMR sensitivity 2.67 × 10-3 where 1H = 1.00
	has symbol 25Mg
	has uses isotopically enriched samples available for experimental purposes
	has uses Nulclear Magnetic Resonance
	is an instance of magnesium
magnesium 26	has atomic mass 25.982593
	has natural abundance 11.01%
	has nuclear spin I = 0+ h/2π
	has number of neutrons 14
	has number of nucleons 26
	has symbol 26Mg
	has uses isotopically enriched samples available for experimental purposes
	is an instance of magnesium
magnesium 27	has atomic mass 26.984341
	has decay mode β- (2.610 Mev)
	has decay mode γ
	has half life 9.45 minutes
	has nuclear spin I = 0+ h/2π
	has number of neutrons 15
	has number of nucleons 27
	has symbol 27Mg
	has uses research (radioactive nuclide)
	is an instance of magnesium
magnesium 28	has atomic mass 27.983876
	has decay mode β- (1.832 Mev)
	has decay mode γ
	has half life 21.0 hours
	has nuclear spin I = 1/2+ h/2π
	has number of neutrons 16
	has number of nucleons 28
	has symbol 28Mg
	has uses research (radioactive nuclide)
	is an instance of magnesium
magnetic constant	has symbol μ0
	has uncertainty 0
	has value 4π × 10-7 = 12.566 370 614 × 10-7 N A-2
	is an instance of universal constant
magnetic field strength unit	has definition ampere per meter
	has symbol A·m-1
	is a kind of derived SI unit
magnetic flux	has unit magnetic flux unit
	is a kind of electromagnetic quantity
magnetic flux density	has unit magnetic flux density unit
	is a kind of electromagnetic quantity
magnetic flux density unit	is a kind of electromagnetic unit
	is a unit of magnetic flux density
magnetic flux quantum	has equation
	has symbol Φ0
	has uncertainty 0.000000081 × 10-15 Wb
	has value 2.067833636 × 10-15 Wb
	is an instance of electromagnetic constant
	is an instance of magnetic flux
magnetic flux unit	is a kind of electromagnetic unit
	is a unit of magnetic flux
magnetic monopole	has definition A hypothetical particle that carries an isolated north or south magnetic pole. This is in contrast to magnets which are north-south pole pairs. If magnetic monopoles exist, they must be very massive.
	has definition A magnet with an isolated north (or south) pole, rather than a pair of equal-strength north and south poles, as in conventional magnets. Magnetic monopoles have never been observed, but they are predicted to exist by grand unified theories.
	has definition a massive particle with but one magnetic pole, the production of which is indicated in some theories of the early universe.
	has definition An hypothesized particle that would have either a magnetic north pole or a magnetic south pole but not both. all magnetic particles and magnets ever observed have both poles. Magnetic monopoles are predicted by grand unified theories of physics. that grand unified theories predict the existence of large numbers of magnetic monopoles, when none have been discovered, is called the monopole problem. (see grand unified theories.)
	has definition has definition A hypothetical quantum object being a single, isolated magnetic pole. Normally, magnetic poles, the sources of a magnetic field, occur in pairs as north and south poles.
	is a kind of hypothetical particle
magnetic monopole problem	has definition A problem, discovered by John Preskill in 1979, concerning the compatibility of grand unified theories with standard cosmology. Preskill showed that if standard cosmology were combined with grand unified theories, far too many magnetic monopoles would have been produced in the early universe.
	is an instance of problem
magnetic pressure	has definition The pressure exerted by a magnetic field on the material that contains the field. in gaussian units it is given by pm = b2 / 8π, where b is the magnetic field strength.
	is a kind of pressure
Magnetic star	has definition Star (usually of spectral type A) with strong integrated magnetic fields ranging up to 30000 gauss.
	is a kind of A star
magnetohydrodynamics	has acronym MHD
	has definition The study of the collective motions of charged particles in a magnetic field. (Sometimes called hydromagnetics)
	has synonym hydromagnetics
	is a kind of hydrodynamics
magnetopause	has definition The region in earth's ionosphere where the magnetosphere meets the solar wind. Essentially, it is the place where earth's magnetic field stops; the region above the magnetopause is no longer part of earth's atmosphere, but is part of interplanetary space.
	is a part of magnetosphere
magnetosphere	has definition The extent of a planet's magnetic field.
	has definition The region of space surrounding a rotating, magnetized sphere. Specifically, the outer region of earth's ionosphere, starting at about 1000 km above earth's surface, and extending to about 60,000 km (or considerably farther, on the side away from the sun).
	has shape teardrop-like, with the point opposite the Sun; due to the effect of the solar wind
	is a part of atmosphere
magnitude	has definition A logarithmic brightness scale for astronomical objects. See Appendix.
	has definition A measure. on a logarithmic scale, used to indicate the brightness of a celestial object. A 1-magnitude difference in brightness between two stars corresponds to a difference in luminosity by 100.4 or 2.51; 5 magnitudes corresponds to factor of 100 in luminosity.
	has definition An arbitrary number, measured on a logarithmic scale, used to indicate the brightness of an object. Two stars differing by 5 mag differ in luminosity by 100. 1 magnitude is the fifth root of 100, or about 2.512. The brighter the star, the lower the numerical value of the magnitude (see also Pogson's ratio).
	has definition The brightness of a star or planet, expressed on a scale in which lower numbers mean greater brightness. Apparent magnitude indicates the brightness of objects as we see them from earth, regardless of their distance. Absolute magnitude is defined as the apparent magnitude a star would have if viewed from a distance of ten parsecs, or 32.6 light-years. each step in magnitude equals a difference of 2.5 times in brightness: the brightest stars in the sky are apparent magnitude 1; the dimmest, 6. The magnitudes of extremely bright objects are expressed in negative values - e.g., the apparent magnitude of the Sun is about -26.
	has definition The measure of a star's brightness. apparent magnitude measures a star's apparent brightness - that is, how bright a star looks from Earth. absolute magnitude measures a star's intrinsic brightness - that is, how much light the star actually emits.
	has definition The measured brightness of a celestial body. dim objects have magnitudes of high numbers, bright objects have magnitudes of low or even negative numbers. Seen from earth, stars of (apparent) magnitude 6 or higher cannot be detected with the naked eye. The Full Moon has a magnitude of -11, and the Sun one of -26.8. in order to standardize measurements of the brightness of more distant objects, the system of absolute magnitude is used. A measure of the radiation at all wavelengths emitted by a star is known as the bolometric magnitude.
	is a kind of radiation measurement
main lines	has definition The transitions that emit radiation at 1665 and 1667 mhz of an OH source.
	is a kind of emission line
Maksutov telescope	has definition A reflector whose primary mirror is spheroidal instead of parabolic. The light initially passes through a large concave lens to remove the spherical aberration.
	has primary mirror shape concave spheroid
	is a kind of reflector
MAMA	has definition Multi-Anode Microchannel Analyzer. A detection system used with microchannel plates to detect events. Used as an imaging system in the ultraviolet. See microchannel plates.
	is a kind of microchannel plate
manganese	has abundance 0.4 × 10-4 p.p.m. in deep Pacific seawater
	has abundance 0.96 × 10-4 p.p.m. in deep Atlantic seawater
	has abundance 1.0 × 10-4 p.p.m. in Atlantic surface seawater
	has abundance 1.0 × 10-4 p.p.m. in Pacific surface seawater
	has abundance 2.63 × 105 in Sun relative to H = 1 × 1012
	has abundance 950 p.p.m. in Earth's crust
	has atomic emission line 257.610 nm for Mn I
	has atomic emission line 279.482 nm for Mn I (used in atom absorption spectrometry)
	has atomic emission line 279.827 nm for Mn I
	has atomic emission line 403.307 nm for Mn I
	has atomic emission line 403.449 nm for Mn I
	has atomic emission line 403.076 nm for Mn I (strong)
	has atomic number 25
	has atomic radii 124 pm
	has biological role essential to all species
	has boiling point 2235 K
	has bulk modulus n.a. GPa
	has chief source pyrolusite, romanechite (also known as psilomelane), manganite (useful but rare)
	has covalent radii 117 pm
	has crystal cell dimension a = 891.39 pm for α-Mn
	has crystal type b.c.c. for α-Mn
	has daily dietary intake 0.4 - 10 mg
	has definition hard, brittle, silvery metal
	has density 6430 kg m-3 for liquid at 2235 K melting point
	has density 7440 kg m-3 for α-Mn solid at 293 K
	has discoverer J.G. Gahn
	has discovery date 1774 (isolated)
	has discovery location Stockholm, Sweden
	has electrical resistivity 185.0 × 10-8 Ω m at 298 K
	has electron affinity less than 0 kJ mol-1 from Mn to Mn-
	has electron configuration [Ar]3d54s2 in ground state
	has electronegativity 1.55 Pauling
	has hazard compounds are experimental carcinogens and teratogens
	has hazard exposure to dust or fumes should not exceed 5 mg m-3
	has heat capacity 20.79 J K-1 mol-1 for gas at constant pressure 0.1 MPa at 298.15 K
	has heat capacity 26.32 J K-1 mol-1 for solid at constant pressure 0.1 MPa at 298.15 K
	has heat of fusion 14.4 kJ mol-1
	has heat of vaporization 219.7 kJ mol-1
	has image
	has ionic radii 52 pm for Mn4+
	has ionic radii 70 pm for Mn3+
	has ionic radii 91 pm for Mn2+
	has isotope mass range 49 to 62
	has lethal intake 1715 mg kg-1 for chloride in mouse
	has level in humans 0.0016 - 0.075 mg dm-3 in blood
	has level in humans 0.2 - 100 p.p.m. in bone
	has level in humans 0.2 - 2.3 p.p.m. in muscle
	has level in humans 3.6 - 9.6 p.p.m. in liver
	has linear expansion coefficient 22 × 10-6 K-1
	has longest lived isotope manganese 55
	has main mining area South Africa, Russia, Gabon, Australia, Brazil
	has mass absorption coefficient 285 cm2 g-1 for CuKalpha X-ray diffraction
	has mass absorption coefficient 34.7 cm2 g-1 for MoKalpha X-ray diffraction
	has mass magnetic susceptibility +1.21 × 10-7 kg-1 m3 for solid
	has mass of element in person 12 mg for a 70 kg average person
	has melting point 1517 K
	has mineral bixbyite, managanite, pyrolusite, rhodochrosite, rhodonite (jewelry), romanechite
	has molar volume 7.38 cm3
	has name origin magnes = magnet from Latin or magnesia nigri = black magnesia (MnO2)
	has neutron scattering length -0.373 × 10-12 cm
	has number of isotopes 15 including nuclear isomers
	has number of protons 25
	has ocean oxidation state II
	has ocean residence time 50 years
has poisson's ratio 0.24 GPa
has pronunciation man-gan-eez
has registry number 7439-96-5 for Chemical Abstracts System database
has relative atomic mass 54.93805 in units of 12C = 12.000
has reserves 3.6 × 109 tonnes (plus ocean floor nodules which are 24% Mn)
has rigidity modulus 79.5 GPa
has space group I-43m for α-Mn
has specimen chips, flake or powder. Safe.
has symbol Mn
has term symbol 6S5/2 in ground state
has thermal conductivity 7.82 W m-1 K-1 at 300 K
has thermal neutron capture cross section 13.3 barns
has toxic intake slighly toxic by ingestion
has uses steel production, ceramics, feed supplements and fertilizer additives
has world production 6.22 × 106 tonnes year-1
has young's modulus 191 GPa
is a kind of chalcophile element
is a kind of scavenged oceanic element
is a kind of transition metal
reacts with dilute acids by dissolving
reacts with oxygen by burning
reacts with water
manganese 52	has atomic mass 51.945568
	has decay mode β+ (4.712 Mev)
	has decay mode γ
	has decay mode EC
	has half life 5.591 days
	has nuclear magnetic moment μ = +3.063 nuclear magnetons with diamagnetic correction
	has nuclear spin I = 6+ h/2π
	has number of neutrons 27
	has number of nucleons 52
	has symbol 52Mn
	is an instance of manganese
manganese 53	has atomic mass 52.941291
	has decay mode EC (0.596 Mev)
	has decay mode no γ
	has half life 3.7 × 106 years
	has nuclear magnetic moment μ = 5.024 nuclear magnetons with diamagnetic correction
	has nuclear spin I = 7/2- h/2π
	has number of neutrons 28
	has number of nucleons 53
	has symbol 53Mn
	is an instance of manganese
manganese 54	has atomic mass 53.940361
	has decay mode γ
	has decay mode EC (1.377 Mev)
	has half life 312.2 days
	has nuclear magnetic moment μ = +3.282 nuclear magnetons with diamagnetic correction
	has nuclear spin I = 3+ h/2π
	has number of neutrons 29
	has number of nucleons 54
	has symbol 54Mn
	has uses research
	is an instance of manganese
manganese 55	has atomic mass 54.938047
	has magnetogyric ratio 6.6195 × 107 rad T-1 s-1
	has natural abundance 100%
	has NMR frequency 24.664 MHz where 1H = 100 MHz; 2.3488 T
	has NMR receptivity 994 where 13C = 1.00
	has nuclear magnetic moment μ = +3.4532 nuclear magnetons with diamagnetic correction
	has nuclear quadrupole moment +0.330 × 10-28 m2
	has nuclear spin I = 5/5- h/2π
	has number of neutrons 30
	has number of nucleons 55
	has relative NMR sensitivity 0.18 where 1H = 1.00 for KMnO4 (aq)
	has symbol 55Mn
	has uses Nuclear Magnetic Resonance
	is an instance of manganese
manganese 56	has atomic mass 55.938906
	has decay mode β- (3.696 Mev)
	has decay mode γ
	has half life 2.578 hours
	has nuclear magnetic moment μ = +3.2266 nuclear magnetons with diamagnetic correction
	has nuclear spin I = 3+ h/2π
	has number of neutrons 31
	has number of nucleons 56
	has symbol 56Mn
	is an instance of manganese
manganese star	has definition Star with an anomalously high Mn-Fe ratio, which show deviations from the odd-even effect for phosphorus, gallium, and yttrium.
	is a kind of star
manned spacecraft	has crew one or more
	is a kind of artificial satellite
	is disjoint from unmanned spacecraft
many-worlds interpretation	has author Everett and Wheeler
	has competing theory Copenhagen interpretation
	has definition The view of quantum mechanics holding that a physical system simultaneously exists in all of its possible states prior to and after a measurement of the system. In the many-worlds interpretation, each of these simultaneous existences is part of a separate universe. Every time we make a measurement of a physical system and find it to be in a particular one of its possible states, our universe branches off to one of the universes in which the system is in that particular state at that moment. The system, however, continues to exist in its other possible states, in parallel universes.
	is an instance of quantum mechanics interpretation
mare	has definition An area on the moon that appears darker and smoother than its surroundings. Lunar maria are scattered basaltic flows.
	has plural maria
	is a part of Moon
Markarian galaxy	has definition A galaxy in Markarian's list of galaxies with abnormally strong ultraviolet continua. They have broad emission lines arising in a bright, semi-stellar nucleus. Markarian 231 is the most luminous galaxy known if it is at its Hubble distance.
	is a kind of emission line galaxy
Mars	has albedo 0.16
	has atmosphere composition more than 90% CO2, traces of O2, CO, H2O.
	has atmospheric pressure 3.5 millibars (data from mariner 7)
	has core composition liquid Ni - Fe
	has definition Fourth major planet out from the Sun. Its tiny satellites are locked in synchronous rotation with mars.
	has eccentricity e = 0.0934
	has escape velocity 5.1 km s-1
	has inclination i = 1°.85
	has mass 6.45 × 1026< sup> g (0.11 MEarth)
	has mean density 4.0 g cm-3
	has mean distance from sun 1.5237 AU
	has mean orbital velocity 24.2 km s-1
	has number of satellites 2
	has oblateness 0.0092
	has obliquity 23°59'
	has radius (1974) 3394 km -has source: [H76]
	has rotational period 24h37m22s.6
	has sidereal period 687 days
	has surface gravity 0.38 that of Earth
	has surface temperature 248 K
	has synodic period 779.9 days
	is a part of our solar system
	is an instance of naked eye planet
	is an instance of superior planet
	is an instance of terrestrial planet
mascons	has definition Abbreviated form of mass concentrations: apparent regions on the lunar surface where gravity is somehow stronger. The effect is presumed to be due to localized areas of denser rock strata.
	has definition gravity anomalies found on the moon. As of 1971, 12 mascons were known.
	is a part of Moon
	is an acronym for mass concentrations
mass	has definition Measure of the amount of matter in an object. Inertial mass indicates the object's resistance to changes in its state of motion. Gravitational mass indicates its response to the gravitational force. In the general theory of relativity, gravitational and inertial mass are revealed to be aspects of the same quantity.
	has definition The measure of the inertia of an object, determined by observing the acceleration when a known force is applied. The gravitational force acting on an object is found to be proportional to its mass, as is the gravitational force that it exerts on other objects.
	has definition The quantitative property of an object due to the matter it contains. (Weight, in contrast, describes a force with which a body is attracted towards a gravitational focus.) Units of mass are grams and kilograms.
	has unit mass unit
	is a kind of quantity
mass density unit	has definition kilogram per cubic meter
	has symbol kg m-3
	is a kind of derived SI unit
mass fraction unit	has definition kilogram per kilogram, which may be represented by the number one
	has symbol kg·kg-1 = 1
	is a kind of derived SI unit
mass motion	has definition A non-uniform matter flow
	is a kind of motion
mass of the Galaxy	has definition The mass of the Milky Way.
	has value 1.8 × 1011 MSun
	is an instance of mass
mass unit	is a kind of unit
	is a unit of mass
massive black hole	has definition Utilized in a theoretical model for quasars and active galactic nuclei, according to which the energy source is due to infall (and resultant heating) of gas and stars onto a supermassive central black hole.
	is a kind of black hole
massive compact halo object	has acronym MACHO
	has definition These are black holes, neutron stars and brown dwarfs, none of which are luminous and all of which are postulated to exist in the halos of galaxies. They are a form of dark matter.
	is a kind of dark matter
	is a part of halo
massless black hole	has definition In string theory, a particular kind of black hole that may have large mass initially, but that becomes ever lighter as a piece of the Calabi-Yau portion of space shrinks. When the portion of space has shrunk down to a point, the initially massive black hole has no remaining mass - it is massless. In this state, it no longer manifests such usual black hole properties as an event horizon.
	is a kind of black hole
massless particle	has definition A particle with zero mass
	has mass 0
	is a kind of particle
materialism	has antonym spiritualism
	has definition Belief that material objects and their interactions constitute the complete reality of all phenomena, including such seemingly insubstantial phenomena as thoughts and dreams. Compare spiritualism.
	is a kind of doctrine
mathematical concept	is a kind of abstraction
mathematical constant	is a kind of constant
matter era	has definition A cosmic epoch during which the matter content of the Universe ceased to be ionized. This led to a decrease in the optical depth of the Universe, and the photons of radiation (which we now observe as the cosmic microwave background) became able to travel large distances without interacting with matter.
	has definition Separation of classes of particles from regular interaction with one another, as in the decoupling of photons from particles of matter that produced the cosmic background radiation.
	has definition The Big Bang era when the temperature had dropped to 3000 K, at which time the recombination of hydrogen became possible. The plasma of free electrons and nuclei condensed to form a neutral gas. Matter and radiation consequently decouple from one another because no further scattering of the radiation occurs.
	has definition The epoch at t ≈ 1013 s after the big bang (T ≈ 3000 K) when matter and radiation decoupled.
	has definition The era some 3 × 105 years after the Big Bang when the cosmic blackbody radiation was last scattered by the matter. At this era, at a redshift of about 1,000 and a temperature of about 3,000 K, the protons and electrons combined to form hydrogen atoms, which are effectively transparent to the radiation.
	has definition The rapid transition from an ionized state at a redshift of 1000, when the blackbody radiation is scattered by the free electrons, to an unionized state, when the matter is predominantly in the form of hydrogen atoms that do not scatter the radiation appreciably. Radiation subsequently does not interact with matter unless the matter becomes reionized at a later epoch by radiation from quasars or forming galaxies.
	has definition The release of photons from constant collisions with massive particles as the universe expanded and its matter density diminished. See decoupling.
	has duration > 1010 years
	has start time 1012 s
	has synonym decoupling era
	has synonym recombination era
	has temperature 3 K to 3000 K
	is a kind of Big Bang era
	is preceded by radiation era
maxwell	has definition Unit of magnetic flux through 1 cm2 normal to a field of 1 gauss.
	has equivalent 10-8 Wb
	is an instance of CGS unit
	is an instance of magnetic flux unit
Maxwell distribution	has definition An expression for the statistical distribution of velocities among the molecules of a gas at a given temperature.
	has definition The distribution function that any species of particle will have if it is in thermodynamic equilibrium. This distribution function describes both the equilibrium in velocity space or kinetic energy, and the equilibrium in potential energy.
	is an instance of statistical distribution
Mayall	has name Nick Mayall
	is an instance of 19th century scientist
	is an instance of astronomer
	is associated with Humason
McCormick Refractor	has altitude 264 m
	has aperture 0.667 m
	has creation date 1883
	has focal ratio f/14.9
	has latitude 38° 02' N
	has lens maker Alvan Clark & Sons
	has location Charlottesville, VA, US
	has longitude 78° 31' W
	has mounting manufacturer Warner and Swasey
	has owner Leander McCormick Observatory
	has synonym 26 inch
	is an instance of German equatorial telescope
	is an instance of refractor
Me star	has definition Star of spectral type M with emission lines in their spectra.
	is a kind of emission line star
	is a kind of M star
mean element	has definition One of several elements of an adopted reference orbit (see elements, orbital) that approximates the actual, perturbed orbit. Mean elements may serve as the basis for calculating perturbations.
	is a kind of orbital quantity
mean free path	has definition Mean length of the path of a particle between collisions.
	has definition The mean distance traversed by a particle before undergoing a significant deflection or collision.
	is a kind of length
mean solar day	has definition The mean length of time between two successive culminations of the Sun (i.e., the mean period from apparent noon to apparent noon).
	has value 24h00m00s
	is an instance of day
mean solar second	has definition 1/86400 of a mean solar day (cf. ephemeris second).
	has value 1/86400 mean solar day
	is an instance of second unit
mechanics	has definition The study, in physics, of the influence of forces.
	is a kind of physics
median	has definition Literally the middle value in a sequence of values arranged in increasing size order. A useful mathematical estimator of the true value from a set of values when one of these values is contaminated, i.e. known to be much larger than the average.
	is a kind of statistical quantity
mega	has symbol M
	has value 106
	is an instance of unit prefix
Meinel band	has species N2+ radical
	has wavelength 8000 Å
	is a kind of molecular band
meitnerium	is a kind of transactinide
melting	has final phase liquid
	has initial phase solid
	has inverse process solidification
	is a kind of first order phase transition
mendelevium	is a kind of transuranium element
meniscus mirror	has definition A very thin mirror with a high curvature. A method of constructing very large mirrors which assumes from the outset that the mirror is too thin to hold its shape against gravity and will require an active control system.
	is a kind of mirror
Mensa	has acronym Men
	has genitive Mensae
	has historical image
	has historical origin named after the table mountain near de Lacaille's observatory in Cape Town
	has synonym Mons Mensa
	has synonym Table Mountain
	is a part of celestial sphere
	is an instance of constellation named by de Lacaille
Mercury	has advance of perihelion 476" per century
	has albedo 0.06
	has atmosphere composition He and Ar
	has atmosphere pressure < 2 × 10-9 millibars (data from Mariner 10)
	has core composition iron rich
	has definition Innermost planet of the Solar System. Transits of the Sun occur either 7 or 13 years apart - last transit 1973 November 10.
	has eccentricity e = 0.206
	has equatorial radius 2446 km
	has escape velocity Vesc 4.2 km s-1
mercury	has image
Mercury	has inclination i = 7°.0
	has magnetic field inclination < 10° to the pole
	has magnetic field offset 0.47 RM from the core
	has magnetic field strength 200-300 gammas (data from Mariner 10)
	has mass 3.15 × 1026 g (0.054 MEarth) (data from Mariner 10)
	has maximum elongation 28°
	has mean density 5.44 g cm-3
	has mean distance from Sun 0.387 AU -has source: [H76]
	has oblateness < 0.001
mercury	has ocean oxidation state II
Mercury	has orbital period 88 days
	has orbital velocity Vorb 47.9 km s-1
	has relativistic advance of perihelion 42".6 per century
	has rotation period 58.646 days exactly 2/3 of its orbital period
	has subsolar point temperature 700 K (data from Mariner 10)
	has surface gravity 360 cm s-2< sup> -has source: [H76]
	has surface spectrum similar to lunar maria and uplands , probably lunar-like soil of low-density silicates
	has synodic period 116 days
mercury	is a kind of liquid element
	is a kind of scavenged oceanic element
	is a kind of transition metal
Mercury	is a part of our solar system
	is an instance of terrestrial planet
merger	is a kind of mass motion
meridian	has definition A great circle passing through the celestial poles and through the zenith of any location on Earth. For planetary observations a meridian is half the great circle passing through the planet's poles and through any location on the planet.
	has definition The great circle on the celestial sphere which passes through the celestial poles and the zenith of the observer.
	has definition The great circle passing through the zenith of the observer and the north and south points on his horizon.
	has definition Theoretical north-south line on the Earth's surface, or an extension of that line onto the night sky, connecting the observer's zenith with the celestial pole and the horizon. The meridian is used to state directional bearings. Devices and structures - such as meridian arcs - marking the meridian were once common in observatories.
	is a kind of great circle
meridional flow	has definition Flow between the poles, or between the equator and the poles. A positive value indicates flow away from the equator: a negative value, flow toward the equator.
	is a kind of mass motion
MERLIN	has image
	has reference MERLIN/VLBI National Facility
	is an instance of radio interferometer
meson	has definition A strongly interacting particle consisting of a quark and an antiquark. Its mass is intermediate between that of a proton and an electron, which is believed to be responsible for the strong nuclear force. In contrast to the case of baryons or leptons, meson number is not conserved: like photons, mesons can be created or destroyed in arbitrary numbers.
	has number of quarks 2
	has spin 0
	is a kind of boson
	is a kind of hadron
mesosphere	has definition The part of Earth's atmosphere immediately above the stratosphere, where the temperature drops from about 270 K to 180 K.
	has temperature 270 K to 180 K
	is a part of atmosphere
mesozoic era	has duration 167 million years
	has start time 230 million years ago
	is a kind of geological era
Messier catalog	has definition Designation of objects in the Messier Catalog of nebulae, star clusters, and galaxies, published in the Eighteenth Century.
	has definition is an instance of nebula catalog
	has definition List of the locations in the sky of more than 100 galaxies and nebulae, compiled by Charles Messier between 1760 and 1784. Some designations he originated are still used in identification; M1 is the Crab Nebula (in Taurus).
	has definition One of the earliest catalogues of nebulous-appearing astronomical objects, compiled in 1781 by the French astronomer Charles Messier. Messier's catalogue included many objects that were later realized to be galaxies.
	is a kind of nebula catalog
Messier object	has apparent dimension
	has apparent magnitude
	has catalog Messier catalog
	has definition an object assigned a number by Charles Messier
	has distance from Earth
	has Messier number
	has purpose originally to catalog all objects which could be confused with a comet
	is a kind of celestial body
	is a part of celestial sphere
metal	has definition As used in stellar spectroscopy, any element heavier than helium.
	has definition To an astronomer, a metal is any element heavier than hydrogen and helium; thus, not only are iron and copper metals, but so are elements like oxygen and neon.
	is a kind of metallic element
	is a kind of solid
metal-poor star	has metallic line strength weak
	is a kind of star
metal-rich star	has definition A small subgroup of A-type stars in which the lines of Mg II are very strong.
	has definition Star having metal-to-hydrogen ratios greater than those of the Hyades.
	has metallic line strength strong
	has synonym CN-strong star
	has synonym metal-strong star
	is a kind of star
metallic element	is a kind of element
metallic hydrogen	has definition A hypothetical form of hydrogen in which the molecules have been forced by extremely high pressures to assume the lattice structure typical of metals. It is estimated that as much as 40% of Jupiter's mass (but not more than 3% of Saturn's) may be in the form of metallic hydrogen.
	is a kind of solid
metallic metalloid	is a kind of metalloid element
metalloid element	is a kind of element
meteor	has average interval time one per hour from a given location on Earth, higher during meteorite shower
	has definition A "shooting star" - the streak of light in the sky produced by the transit of a meteoroid through the Earth's atmosphere; also the glowing meteoroid itself. The term "fireball" is sometimes used for a meteor approaching the brightness of Venus; the term "bolide" for one approaching the brightness of the full Moon.
	has definition Fragment or particle that enters the Earth's atmosphere and is then destroyed through friction, becoming visible as this occurs as a momentary streak of light. At certain times of the year, meteors apparently emanating from a single area of the sky (a radiant) form meteor showers. They are thought to originate within the Solar System. See also meteorite.
	has definition when a meteoroid collides with the atmosphere or surface of a celestial body, kinetic energy is converted to heat, light and sound
	has product meteorite (if meteoroid does not burn up)
	has synonym shooting star
	is a kind of impact event
	is a kind of naked eye object
	can produce sonic boom
meteor event	has definition The event in a sequence of the events during the passage of a meteor. They are in order : incandescent flight fireball smoke trail meteoroid break up retardation point dark flight meteorite impact
	is a kind of impact event
meteor shower	has definition A profusion of meteors that fall within a period of a few hours and that appear to radiate from a common point in the sky. Shower meteors are usually low-density material and have high eccentricities.
	has definition temporary increase in the rate at which meteors are observed when the target runs into a swarm of meteoroids that share a common orbit
	has duration a few hours
	has location which depends on the observer's location on the planet, is it facing the correct direction
	has parent object the object from which the meteroids originate
	has period one year
	has radiant a location on the sky from which meteor showers seem to originate
	has rate the number of meteors observed in a given interval from the observers location
	is a kind of periodic celestial event
meteorite impact	is a kind of meteor event
	can produce crater blinding white light like a nuclear detonation ejecta blanket tektites and impact melt glass seismic disturbances like Earthquakes
	can shatter meteoroid
meteoroid break up	is a kind of meteor event
meteorological satellite	is a kind of artificial satellite
Meteosat satellite	has creator EUMETSAT
	has ground communication station Bracknell
	has ground communication station Fucino, Italy
	has ground communication station Rome
	has ground communication station Toulouse
	is a kind of EUMETSAT satellite
	is a kind of geosynchronous satellite
Meteosat-7	has launch date 2 Sept 1997
	is an instance of Meteosat satellite
meter	has approval agency 11th CGPM
	has approval date 1960
	has consequence fixes the speed of light in vacuum at exactly 299792458 m·s-1
	has definition A unit of length. The meter is the length equal to 1650763.73 wavelengths in vacuum of the radiation corresponding to the transition between the levels 2p10 and 5d5 of the Krypton-86 atom.
	has definition length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second
	has historical origin chosen in 1791 by the French Academy of Sciences as 10-7 of the length of the meridian through Paris from pole to the equator
	has symbol m
	is an instance of base SI unit
	is an instance of length unit
methanimine	has symbol H2CNH
	is an instance of interstellar molecule
methanol	has definition More rotational lines have been observed astronomically for it than for any other molecule.
	has discovery in space date 1970
	has symbol CH3OH
	has synonym methyl alcohol
	is an instance of interstellar molecule
	is an instance of neutral particle
methyl acetylene	has symbol CH3C2H
	is an instance of interstellar molecule
methyl cyanide	has symbol CH3CN
	is an instance of interstellar molecule
methyl formate	has symbol HCOOCH3
	is an instance of interstellar molecule
methylamine	has definition A molecule discovered in interstellar apace in 1974, in Sgr B2, at a frequency of 87.77 GHz. Methylamine can react with formic acid to produce glycine, the simplest amino acid.
	has symbol CH3NH2
	is an instance of interstellar molecule
methylidyne	has symbol CH
	is an instance of interstellar molecule
metric ton	has symbol t
	has value in SI unit 103 kg
	is an instance of non SI unit
Meudon Refractor	has altitude 162 m
	has aperture 0.83 m
	has creation date 1889
	has focal ratio f/19.5
	has latitude 48° 48' N
	has lens maker Henry brothers
	has location Meudon, France
	has longitude 2° 14' E
	has mounting manufacturer P Gautier
	has owner Observatoire de Paris
	has synonym 33 inch
	is an instance of German equatorial telescope
	is an instance of refractor
MF band	has frequency 300 kHz to 3 MHz
	has wavelength 100 m to 1 km
	is a kind of radio
micro	has symbol μ
	has value 10-6
	is an instance of unit prefix
microchannel plate	has definition A compact electrostatic high-voltage electron multiplier with a very large number of narrow pores or channels. A photoelectron generated at the entrance face (photocathode) stimulates a cascade of secondary electrons down the nearest channel to produce a huge cloud of charge at the output face. The output pulse can be used in many different ways to record the event. If it impacts a phosphor screen then light emission can be detected with a CCD. Direct electrical detection can be obtained using a Multi Anode Microchannel Analyzer.
	is a kind of imager
micron	has definition A unit of length.
	has equivalent 10-4 cm
	has equivalent 104 Å
	has symbol μ
	is an instance of length unit
microphotometer	has definition A device for measuring the variations in density in a photographic emulsion.
	is a kind of detector
Microscopium	has acronym Mic
	has genitive Microscopii
	has synonym Microscope
	is a part of celestial sphere
	is an instance of constellation named by de Lacaille
microwave	has definition An electromagnetic wave (in the radio region just beyond the infrared) with a wavelength of from about 1 mm to 30 cm (about 109-1011 Hz).
	has definition An electromagnetic wave with a wavelength between one millimeter and 30 centimeters, or sometimes one meter.
	has definition Radiation in the electromagnetic spectrum between infrared and radio waves. This range has wavelengths of between about 20 cm and about 1 mm. Radiation of this type was detected as background radiation.
	has definition Radio radiation with wavelengths of about 10-4 to 1 meter, equal to 109 to 1013 hertz.
	is a kind of photon
mid infrared	has acronym MIR
	is a kind of infrared
MIDAS 1	is an instance of MIDAS satellite
MIDAS 2	is an instance of MIDAS satellite
MIDAS 3	is an instance of MIDAS satellite
MIDAS 4	is an instance of MIDAS satellite
MIDAS 5	is an instance of MIDAS satellite
MIDAS 6	is an instance of MIDAS satellite
MIDAS 7	is an instance of MIDAS satellite
MIDAS satellite	is a kind of reconnaissance spacecraft
middle precambrian period	has duration million years
	has start time million years ago
	is a kind of precambrian era
Mie diffraction	has definition The diffraction of light by small spherical particles.
	is a kind of diffraction
Mie scattering	has definition Scattering of light (without regard to wavelength) by larger particles, such as those of dust or fog in Earth's atmosphere.
	is a kind of scattering
mile	has definition The mile employed in this book is the statute mile, equal to 5280 feet.
	is an instance of length unit
military spacecraft	is a kind of artificial satellite
Milky Way	has definition A softly glowing band of light that bisects the skies of Earth, produced by light from stars and nebulae in the galactic disk.
	has definition Our own galaxy, the second largest in the local group.
	has definition System of approximately 100000 million stars, of which our Sun is one. It is a normal spiral galaxy of class Sb, with a diameter now reckoned to be probably less than 100,000 light-years, and a strong but obscure energy source at the center (emitting infrared radiation). It is undergoing galactic rotation. Possibly one tenth of the galaxy's total mass - estimated at 1.8 × 1011 solar masses - comprises interstellar gas and dust.
	has definition The galaxy to which the Sun belongs. Our Galaxy is about 1010 years old and contains about 1011 stars. Its mass is at least 1011 Msun, about 5-10 percent of which is in the form of gas and dust. Diameter ~ 30 kpc; thickness of nuclear bulge about 4 kpc; thickness of disk about 700-800 pc; distance between spiral arms about 1.4 kpc. Mv = - 20.5. Mean density about 0.1 Msun per cubic parsec. Magnetic field about 3-5 × 10-6 gauss. Total luminosity about 1044 ergs s-1
	is a part of Local Group
	is an instance of galaxy containing Cepheids
	is an instance of hypergalaxy
	is an instance of naked eye object
	is an instance of spiral galaxy
milli	has symbol m
	has value 10-3
	is an instance of unit prefix
Mimas	has albedo 0.49
	has definition The second innermost satellite of Saturn. The perturbations of Mimas and Janus produce the divisions in Saturn's rings.
	has dicovery date 1789
	has discoverer Herschel
	has orbital period P = 0d.94
	has radius R ≈ 250 km
	is a part of Saturn
	is an instance of natural satellite
mini black hole	has definition In a chaotic early universe, black holes may form at eras as early as the Planck time. The characteristic size of these mini black holes is 10-6 gram, the minimum mass of a collapsing inhomogeneity at that time. Larger mini black holes may form at later eras. Since conventional theories of stellar evolution show that only very massive stars can form black holes, the possible formation of mini black holes is a unique characteristic of the very early universe.
	is a kind of black hole
Mintaka	has definition One of the three stars in Orion's belt, and the star along whose line of sight interstellar gas was first spectroscopically detected.
	has spectral type O9.5II
	is a part of Orion
	is an instance of bright giant
	is an instance of O star
minute (angle)	has symbol '
	has synonym arc minute
	has value in SI unit (1/60)° = (π/10800) rad
	is an instance of non SI unit
minute (time)	has symbol min
	has value in SI unit 60 s
	is an instance of non SI unit
miocene epoch	has duration 12 million years
	has start time 25 million years ago
	is a kind of tertiary period
MIR space station	is an instance of space station
Mira	has amplitude 5 mag
	has definition A irregular long-period intrinsic variable. It was named Mira ("wonderful") in 1596 by Fabricius, who made the first recorded observations of its brightness fluctuations. Mira is a double star with a faint B companion which is itself variable.
	has definition A red giant that varies in brightness as it pulsates. When brightest, Mira is visible to the naked eye; when dimmest, Mira can be viewed only with optical aid. Mira is the prototype of all pulsating red giants, which are called Miras in its honor.
	has discoverer David Fabricius
	has discovery date 1596
	has distance 70 pc
	has image
	has period 331 days
	has synonym omicron Cet
	is a part of Cetus
	is an instance of binary star
	is an instance of long-period variable
	is an instance of naked eye star
	is an instance of red giant
	M6e-M9e III -has source: [H76]
Miranda	has definition The innermost satellite of Uranus.
	has diameter 500 km
	has discoverer Kuiper
	has discovery date 1948
	has orbital period P = 1d10h
	is a part of Uranus
	is an instance of natural satellite
mirror	has reflective coating
	has wavefront modification shape
	is a kind of wavefront modifier
missing mass problem	has definition Poses the question: why does the Universe seem to have much more mass in it than can be seen with a telescope? Dynamical and theoretical constraints place the proportion of missing mass to be somewhere between 90-99 per cent of the total mass of the Universe.
	has definition The cosmic mass that some scientists hypothesize so that the universe will have the critical density of matter, with an exact balance between gravitational energy and kinetic energy of expansion. Such mass is called missing because it represents about 10 times as much mass as has actually been detected. (See closed universe; critical mass density; dark matter.)
	is an instance of problem
mississippian period	has duration 35 million years
	has start time 345 million years ago
	is a kind of paleozoic era
MMT Telescope	has altitude 2608 m
	has aperture 6.5 m
	has creation date (1999)
	has focal ratio f/1.25, 5.4, 9, 15
	has latitude 31° 41' N
	has location Mount Hopkins, Arizona, US
	has longitude 110° 53' W
	has mirror maker R. Angel, B. Martin
	has mirror type Spin-cast borosilicate honey-comb
	has mounting manufacturer de Bartolomeis
	has operator Smithsonian Institution and Univ. of Arizona
	has owner MMT Observatory
	is an instance of altazimuth telescope
	is an instance of optical telescope
	is an instance of reflector
molar gas constant	has symbol R
	has uncertainty 0.000015 J mol-1 K-1
	has value 8.314472 J mol-1 K-1
	is an instance of physico chemical constant
molar volume of ideal gas	has equation
	has pressure 101.325 kPa
	has symbol Vm
	has temperature 273.15 K
	has uncertainty 0.000039 × 10-3 m3 mol-1
	has value 22.413996 × 10-3 m3 mol-1
	is an instance of physico chemical constant
mole	has approval agency 14th CGPM
	has approval date 1971
	has definition amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12
	has definition The SI unit of the amount of substance, defined as the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilograms of carbon 12. One mole, which is equal to gram multiplied by the molecular weight, contains 6.02 × 1023 molecules (see Avogadro's number). In general, 1 mole of any gas occupies a volume of 22.4 liters.
	has symbol mol
	is an instance of amount of substance unit
	is an instance of base SI unit
	requires qualification elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles
molecular band	has definition A series of closely spaced, often unresolved, emission or absorption lines found in the spectra of molecules. Each line represents an increment of energy due to a change in the rotational state of the molecule.
	is a kind of set of molecular lines
molecular hydrogen	has definition A molecule consisting of two hydrogen atoms (H2) and the most common molecule in space.
	has definition A molecule of hydrogen, discovered in interstellar space in 1970. H2 is a very hard molecule to detect. None of its transitions lie in the visible part of the spectrum. Second, being a symmetric homonuclear molecule, it does not have an electric-dipole rotation-vibration spectrum, and detection must be based on the weak quadrupole spectrum. Third, ultraviolet radiation is a very efficient dissociator of H2, so any H2 that survived would presumably be located inside very dense interstellar clouds. So far observations have borne out this supposition. Measurements of the region within about 1 kpc of the Sun suggest that H2 is about twice as abundant as atomic H.
	has symbol H2
	is a kind of interstellar molecule
	is a kind of neutral particle
	is an instance of diatomic molecule
molecular oxygen	has symbol O2
	is an instance of diatomic molecule
	is an instance of interstellar molecule
	is an instance of neutral particle
molecule	has definition The smallest unit of a chemical compound. A molecule is composed of two or more atoms, linked by interactions of their electrons.
	is a kind of particle
molybdenum	has image
	has ocean oxidation state VI
	has ocean residence time 600000 years
	is a kind of accumulating oceanic element
	is a kind of transition metal
Monoceros	has acronym Mon
	has genitive Monocerotis
	has synonym Unicorn
	is a part of celestial sphere
	is an instance of constellation
Monoceros Loop	has age 300000 years
	has definition A filamentary supernova remnant resembling a loop.
	has distance 1 kpc
	is a part of Monoceros
	is an instance of supernova remnant
month	has definition The period of one complete synodic or sidereal revolution of the Moon around the Earth; also a calendrical unit that approximates the period of revolution.
	is a kind of time unit
Moon	has V< i>esc 2.38 km s-1
	has albedo 0.07
	has center of mass location displaced about 2 km from geometrical center towards the direction of Earth
	has core temperature 1500 K
	has crust thickness 60 km
	has definition Natural satellite of Earth. Studies of lunar rocks have shown that melting and separation must have begun at least 4.5 × 109 years ago,so the crust of the Moon was beginning to form a very short time after the solar system itself. It would have taken only 107 years to slow the Moon's rotation into its present lock with its orbital period. The Moon's orbit is always concave toward the Sun.
	has eccentricity e = 0.0549
	has inclination of orbital plane to ecliptic 5°8'43"
	has mantle thickness 1000 km
	has mass 7.35 × 1025 g = 0.0123 MEarth
	has mean density 3.34 g cm-3
	has mean distance from Earth 384404.377 ± 0.001 km (1.28 lt-sec)
	has mean radius 1738 km
	has obliquity 6°41'
	has orbital velocity Vorb = 1.02 km s-1
	has sidereal period 27d7h43m11s
	has surface gravity 162.2 cm s-2 = 0.165 Earth's
	has synodic period 29d12h44m2s.9
	is a part of Earth
	is an instance of natural satellite
Moon orbital event	has participants satellite, Moon
	is a kind of orbital event
moonrise	has antonym moonset
	has definition The time at which the apparent upper limb of the Moon is on the astronomical horizon just before it rises; i.e., when the true zenith distance, referred to the center of the Earth, of the central point of the disk is 90°34' + s - π, where s is the Moon's semidiameter, π is the horizontal parallax, and 34' is the adopted value of horizontal refraction.
	is an instance of astronomical horizon event
moonset	has antonym moonrise
	has definition The times at which the apparent upper limb of the Moon is on the astronomical horizon just as it sets; i.e., when the true zenith distance, referred to the center of the Earth, of the central point of the disk is 90°34' + s - π, where s is the Moon's semidiameter, π is the horizontal parallax, and 34' is the adopted value of horizontal refraction.
	is an instance of astronomical horizon event
motion	has definition Process of passing through space or changing position
	is a kind of physical process
Mount Stromlo 2.3-meter	has altitude 1149 m
	has aperture 2.3 m
	has creation date 1984
	has focal ratio f/2.09, 18
	has latitude 31 ° 16' S
	has location Siding Spring Mtn., Australia
	has longitude 149° 03' E
	has mirror maker Norman Cole
	has mirror type Cer-Vit
	has mounting manufacturer Australian National Univ., Newcastle Dockyard
	has owner Mt. Stromlo and Siding Spring Obs.
	is an instance of altazimuth telescope
	is an instance of optical telescope
	is an instance of reflector
MS star	has definition M-type stars with ZrO bands.
	has definition Star sharing the M and S characteristics. They thus exhibit bands of both TiO and SrO.
	is a kind of M star
Mt. Wilson	has altitude 1600 m
	has definition The location, in California, of the 100-inch diameter telescope used by Edwin Hubble and others.
	has director Hale (1904-19??)
	has location 13 km from Pasadena, California, USA.
	has telescope 1.5 m (1907) - largest in the world at that time
	has telescope 2.5 m (1910) - largest in the world at that time
	has telescope solar observatory (1904)
	is an instance of observatory
Mu Columbae	has definition Runaway star which diverges from a comparatively small area in Orion.
	is a part of Columba
	is an instance of runaway star
Mulliken band	has species C2 radical
	is a kind of molecular band
multiplet	has definition A group of spectral lines arising from transitions having a common lower energy level. The group of lines have the same values of L and S but different values of J.
	is a kind of set of lines
Munich Image Data Analysis System	A suite of programs and a software environment developed at the European Southern Observatory for astronomy applications. -has source: [McL97]
	has acronym MIDAS
	is a kind of astronomical software
muon	has charge -1
	has definition A second-generation lepton. It is essentially a more massive electron.
	has definition An elementary particle, formerly called a mu-meson but now classified with the leptons because it seems to be identical with the electron except for its much greater mass (207 times that of an electron). The muon family includes the muons and their neutrinos (and their antiparticles). Muons may have a positive or a negative charge.
	has definition Elementary particles produced when cosmic rays enter the upper atmosphere.
	has definition Short-lived elementary particle with negative electrical charge. Muons are leptons. They resemble electrons, but are 207 times more massive.
	has symbol μ
	is an instance of charged particle
	is an instance of lepton
	is an instance of radioactive particle
muon Compton wavelength	applies to particle muon
	has equation
	has symbol λC,μ
	has uncertainty 0.00000035 × 10-15 m
	has value 11.73444197 × 10-15 m
	is an instance of particle Compton wavelength
muon magnetic moment anomaly	applies to particle muon
	has equation
	has symbol aμ
	has uncertainty 0.00000064 × 10-3
	has value 1.16591602 × 10-3
	is an instance of particle magnetic moment anomaly
muon mass	applies to particle muon
	has symbol mμ
	has uncertainty 0.00000016 × 10-28 kg
	has value 1.88353109 × 10-28 kg
	is an instance of particle mass
Musca	has acronym Mus
	has genitive Muscae
	has synonym Fly
	has synonym Musca Australis
	is a part of celestial sphere
	is an instance of constellation named by de Lacaille
N 159	has distance 170000 light years
	has image
	is a part of Large Magellanic Cloud
	is an instance of gaseous nebula
N galaxy	has definition A galaxy with a small, bright, blue nucleus superposed on a considerably fainter red background. (In the Yerkes 1974 system, a galaxy with a small nucleus containing a considerable fraction of the luminosity; N-, less pronounced N galaxies; N+, extreme examples of N galaxies.) Also, a type of radio galaxy having a brilliant, starlike nucleus containing most of the luminosity of the system. N galaxies are compact galaxies, and as a class are intermediate between Seyfert galaxies and quasars in properties of form, color, spectra, redshift, and optical and radio variability.
	is a kind of galaxy
N magnitude	has band 10 microns
	has definition The magnitude derived from observations at 10 microns.
	is an instance of infrared magnitude
N star	has definition In the old terminology, the cooler C-type stars.
	has definition Red star similar to M stars except that bands of C2, CN, and CH are present instead of those of TiO. N stars are strongly concentrated toward the Galactic plane.
	is a kind of C star
N1 line	has definition Green forbidden line of doubly ionized oxygen.
	has species [O III]
	has wavelength 5007 Å
	is an instance of [O III] line
N2 line	has definition Green forbidden line of doubly ionized oxygen.
	has species [O III]
	has wavelength 4959 Å
	is an instance of [O III] line
nadir	has antonym zenith
	has azimuth 0
	has definition The point on the celestial sphere diametrically opposite to the zenith in the direction of gravity.
	has elevation -90
	has synonym downward
	is an instance of local coordinate
	is opposite of zenith
naked eye object	has definition a physical object visible to the unnaided human eye
	is a kind of physical object
naked eye planet	has apparent magnitude brighter than 5
	has been observed for many centuries
	has definition a planet in our solar system visible to the unaided eye
	has motion relative to stars epicycloidal
	is a kind of planet
naked eye star	has apparent magnitude brighter than 5
	has been observed for many centuries
	has definition A star visible without visual aids
	is a kind of naked eye object
	is a kind of star
	is a part of celestial sphere
	is a part of Milky Way
	is usually part of asterism
naked singularity	has definition A singularity that will be visible and communicable to the outside world.
	is a kind of singularity
named derived SI unit	is a kind of derived SI unit
nano	has symbol n
	has value 10-9
	is an instance of unit prefix
narrow emission line galaxy	has acronym NELG
	is a kind of emission line galaxy
narrow emission line X-ray galaxy	has acronym NLXG
	is a kind of emission line galaxy
Narrow-line radio galaxy	has acronym NLRG
	is a kind of radio galaxy
NASA Extragalactic Database	has acronym NED
	is an instance of database
NASA Infrared Telescope Facility	has acronym NASA IRTF
	has altitude 4208 m
	has aperture 3.00 m
	has creation date 1979
	has focal ratio f/2.5, 35, 120 IR
	has latitude 19° 50' N
	has location Mauna Kea, Hawaii, US
	has longitude 155° 28' W
	has mirror maker KPNO Optical Shop
	has mirror type Cer-Vit
	has mounting manufacturer de Bartolomeis
	has owner Mauna Kea Observatory
	has purpose infrared work
	is an instance of Cassegrain
	is an instance of English equatorial telescope
	is an instance of optical telescope
NASA satellite	is a kind of artificial satellite
NASDA satellite	has creator NASDA
	is a kind of artificial satellite
National Aeronautics and Space Administration	has acronym NASA
	has definition US government body set up in 1958, under which the Space Center at Houston, Texas, and the Space Center at Cape Canaveral, Florida, are responsible for manned and unmanned space flights.
	is an instance of space science institution
National Air and Space Development Agency	has acronym NASDA
	has definition National Air and Space Development Agency.
	has location Japan
	is an instance of space science institution
National Bureau of Standards	has acronym NBS
	is an instance of science institution
National Optical Astronomy Observatory	has acronym NOAO
	is an instance of astronomical institution
National Radioastronomy Observatory	has acronym NRAO
	is an instance of astronomical institution
National Science Foundation	has acronym NSF
	is an instance of science institution
natural line broadening	has definition Line broadening resulting from the fact that excited levels have certain mean lives, and these mean lives, by virtue of the uncertainty principle, imply a spread in the energy values.
	is a kind of line broadening
natural logarithm e	has value 2.71828182
	is an instance of mathematical constant
natural object	has definition an object occurring naturally, not constructed by humans
	is a kind of physical object
natural satellite	has definition Body orbiting a planet. Since 1957 the term has also been applied to man-made (artificial) satellites; many astronomers make the distinction by calling natural satellites moons (and the Earth's natural satellite the Moon).
	is a kind of celestial body
	is a kind of satellite
natural unit	has definition Unit of length, time, mass, etc. in which the fundamental constants c (the speed of light), h bar (Planck's constant) and kB (Boltzmann's constant) are equal to unity. That is, c, h bar and kB have the numerical value 1. (For example, if we measure length in light-years and time in years, then c = 1 light-year per year.) The use of natural units allows these constants to be omitted from mathematical equations, leading to less-cluttered calculations. In natural units, E = mc2 becomes E = m and E = kBT becomes E = T, so that both mass and temperature can be expressed in units of energy. (Of course, the correct factors of c, h bar and kB must be inserted at the end of a calculation to obtain measurable quantities.)
	is a kind of unit
nautical mile	has definition A unit of length.
	has equivalent 1.15 statute miles
	is an instance of length unit
navigation satellite	is a kind of artificial satellite
near infrared	has acronym NIR
	has source atomic transitions, vibrational transitions in molecules
	is a kind of infrared
nebula	has catalog nebula catalog
	has definition An irregularly shaped cloud of interstellar gas or dust whose spectrum may contain emission lines (emission nebula) or absorption lines characteristic of the spectrum of nearby illuminating stars (reflection nebula).
	has definition Indistinct, nonterrestrial objects visible in the night sky. "Bright" nebulae glow with light emitted by the gas of which they are composed ("emission" nebulae) or by reflected starlight ("reflection" nebulae) or both. "Dark" nebulae consist of clouds of gas and dust that are not so illuminated. "Planetary" nebulae are shells of gas ejected by stars. Spiral nebulae are galaxies.
	has definition The term "nebula" was previously applied to all kinds of hazy patches in the sky, many of which are now recognized to be clusters or galaxies. (See also diffuse nebula, gaseous nebula, dark nebula.)
	is a kind of celestial body
	is a part of gas
nebula catalog	has object type nebula
	is a kind of astronomical catalog
nebular hypothesis	has definition Hypothesis, maintained in the nineteenth and early twentieth century, that the spiral nebulae are not galaxies but are instead whirlpools of gas from which new systems of stars and planets are condensing. Compare island universe theory.
	is a kind of outdated belief
	see also island universe hypothesis
neodymium	has image
	has ocean oxidation state III
	has ocean residence time 500 years
	is a kind of rare Earth
	is a kind of recycled oceanic element
neon	has abundance 2 × 10-4 p.p.m. in seawater
	has abundance 3.72 × 107 in Sun relative to H = 1 × 1012
	has abundance 7 × 10-5 p.p.m. in Earth's crust
	has atomic emission line 837.761 nm for Ne I
	has atomic emission line 878.062 nm for Ne I
	has atomic emission line 878.375 nm for Ne I
	has atomic emission line 885.387 nm for Be I
	has atomic emission line 865.438 nm for Ne I (strong)
	has atomic number 10
	has biological role none
	has boiling point 27.10 K
	has chief source liquid air
	has critical pressure 2721 kPa
	has critical temperature 44.5 K
	has crystal cell dimension a = 445.462 pm
	has crystal type f.c.c
	has daily dietary intake low
	has definition colourless, odourless gas
	has density 0.89994 kg m-3 for gas at 273 K
	has density 1207.3 kg m-3 for liquid at 27.10 K boiling point
	has density 1444 kg m-3 for solid at 24.48 K melting point
	has discoverer Sir William Ramsay and M.W. Travers
	has discovery date 1898
	has discovery location London, England
	has electron affinity -29 kJ mol-1 from Ne to Ne-
	has electron configuration [He]2s22p6 in ground state
	has hazard harmless gas, although it could asphyxiate if it excluded oxygen from the lungs
	has heat capacity 20.786 J K-1 mol-1 for gas at constant pressure 0.1 MPa at 298.15 K
	has heat of fusion 0.324 kJ mol-1
	has heat of vaporization 1.736 kJ mol-1
	has image
	has isotope mass range 17 to 25
	has level in humans nil in bone
	has level in humans nil in liver
	has level in humans nil in muscle
	has level in humans trace mg dm-3 in blood
	has longest lived isotope neon 20
	has mass absorption coefficient cm2 g-1 for CuKα X-ray diffraction
	has mass absorption coefficient cm2 g-1 for MoKα X-ray diffraction
	has mass magnetic susceptibility -4.2 × 10-9 kg-1 m3 for solid
	has mass of element in person tiny for a 70 kg average person
	has melting point 24.48 K
	has mineral none
	has molar volume 13.97 cm3
	has name origin neon = new from Greek
	has neutron scattering length × 10-12 cm
	has number of isotopes 9 including nuclear isomers
	has number of protons 10
	has ocean oxidation state 0
	has pronunciation nee-on
	has registry number 7440-01-9 for Chemical Abstracts System database
	has relative atomic mass 20.1797 in units of 12C = 12.000
	has reserves 6.5 × 1010 tonnes in atmosphere
	has space group Fm3m
	has specimen small pressurized canisters. Safe.
	has symbol Ne
	has synthesis mechanism liquid air
	has term symbol 1S0 in ground state
	has thermal conductivity 0.0493 W m-1 K-1 for gas at 300 K
	has thermal neutron capture cross section 0.040 barns
	has toxic intake non-toxic
	has uses ornamental lighting, neon signs
	has van der Waals radii 160 pm
	has world production 1 ton year-1
is a kind of inert gas
is a kind of unclassified oceanic element
reacts with nothing
neon 20	has atomic mass 19.992435
	has natural abundance 90.48
	has nuclear magnetic moment μ = 0 nuclear magnetons with diamagnetic correction
	has nuclear spin I = 0+ h/2π
	has number of neutrons 10
	has number of nucleons 20
	has symbol 20Ne
	is an instance of neon
neon 21	has atomic mass 20.993843
	has magnetogyric ratio -2.1118 × × 107 rad T-1 s-1
	has natural abundance 0.27
	has NMR frequency 7.894 MHz where 1H = 100 MHz; 2.3488 T
	has NMR receptivity 0.0359 where 13C = 1.00
	has nuclear magnetic moment μ = -0.661796 nuclear magnetons with diamagnetic correction
	has nuclear quadrupole moment 0.10155 × 10-28 m2
	has nuclear spin I = 3/2+ h/2π
	has number of neutrons 11
	has number of nucleons 21
	has relative NMR sensitivity 2.50 × 10-3 where 1H = 1.00
	has symbol 21Ne
	has uses Nuclear Magnetic Resonance
	is an instance of neon
neon 22	has atomic mass 21.991383
	has natural abundance 9.25
	has nuclear magnetic moment μ = 0 nuclear magnetons with diamagnetic correction
	has nuclear spin I = 0+ h/2π
	has number of neutrons 12
	has number of nucleons 22
	has symbol 22Ne
	is an instance of neon
neon 23	has atomic mass 22.994465
	has decay mode β- (4.376 Mev)
	has decay mode γ
	has half life 37.2 seconds
	has nuclear magnetic moment μ = -1.08 nuclear magnetons with diamagnetic correction
	has nuclear spin I = 3/2+ h/2π
	has number of neutrons 13
	has number of nucleons 23
	has symbol 23Ne
	has uses
	is an instance of neon
neon 24	has atomic mass 23.993613
	has decay mode β- (2.468 Mev)
	has decay mode γ
	has half life 3.38 minutes
	has nuclear spin I = 0+ h/2π
	has number of neutrons 14
	has number of nucleons 24
	has symbol 24Ne
	is an instance of neon
neon burning	has definition The stage in which a star burns neon into oxygen and magnesium.
	has duration 1 year
	has location center of star heavier than 9 solar masses
	has optimum density 4 × 106 g cm-3
	has optimum temperature 1.2 × 109 K
	has part product oxygen, magnesium
	has reactant neon
	is a kind of exothermic fusion process
neper	has symbol Np
	has value in SI unit 1
	is an instance of non SI unit
Neptune	has albedo 0.62
	has atmosphere composition hydrogen and methane
	has definition Eighth major planet out from the Sun. Discovered in by following predictions calculated by Urbain Le Verrier. Similar predictions had been made earlier by John Couch Adams but were not followed up.
	has discoverer Johann Galle and Louis d'Arrest
	has discovery date 1846
	has eccentricity e = 0.009
	has escape velocity Vesc = 25 km s-1
	has inclination 1°.8
	has mass 1.03 × 1029 g
	has maximum apparent brightness 7.6 mag
	has mean density 1.7 g cm-2
	has mean distance from Sun 30.07 AU
	has oblateness 0.02
	has obliquity 28°.8
	has orbital period 164.8 years
	has orbital velocity Vorb = 5.43 km s-1
	has radius 24500 ± 500 km
	has rotation period 15h49m30s
	has satellites Triton and Nereid
	has surface gravity 1.3 that of Earth
	has synodic period 367.49 days
	is a part of our solar system
	is an instance of gas giant
	is an instance of superior planet
neptunium	has image
	is a kind of transuranium element
Nereid	has definition The outer satellite of Neptune. It has the most eccentric orbit of any natural satellite.
	has discoverer Kuiper
	has discovery date 1950
	has eccentricity e = 0.76
	has orbital period about 360 days direct
	has radius 150-250 km
	is a part of Neptune
	is an instance of natural satellite
neutral particle	has charge 0
	has definition A particle with zero charge
	is a kind of particle
	is not accelerated by electric or magnetic fields
neutrino	has antiparticle antineutrino
	has definition A fundamental elementary particle with no electric charge and very small if any rest mass. Believed to be exceedingly abundant in the universe. The neutrino has a very low cross-section for interaction with matter and is almost impossible to detect, hence the uncertainty over its rest mass. The Sun produces neutrinos from thermonuclear reactions in its core, and a large flux of neutrinos carries away most of the energy, of a supernova. Neutrinos are one candidate for Dark Matter. Experiments to detect cosmic neutrinos involve large masses of "stopping" material and indirect detection of the effects of neutrino absorption.
	has definition A stable particle with no charge, a rest mass of zero, and a spin of 1/2, that carries away energy in the course of nuclear reactions. Its main characteristic is the weakness of its interactions with all other particles. Since the wavelengths of neutrinos at the energies at which they are normally emitted from unstable nuclei are only a few thousandths of an angstrom (compared with the wavelength of a light photon which is several thousand angstroms), they have negligible probability (10-19 that of a light photon) of interacting with matter and escape at the speed of light. Neutrinos arise only in the energy-producing regions of stars and therefore, unlike light photons, provide direct evidence of conditions in stellar cores. There are two types of neutrinos, those associated with electrons (ve) and those associated with muons (vµ).
	has definition An electrically neutral, massless particle of spin-1/2, which interacts only by the weak force and gravity. It was first postulated by Pauli in 1930 to ensure conservation of energy and angular momentum in nuclear β decay. Three different types of neutrinos are known to exist corresponding to the three massive leptons: νe, νµ and ν τ.
	has definition An electrically neutral, very weakly interacting particle, with a rest energy which is either zero or very small. The particle was predicted in 1931 as a means of reconciling the measurements of beta decays with the conservation of energy, but it was not directly detected until 1956.
	has definition Chargeless species of particle, subject only to the weak force.
	is a kind of lepton
	is a kind of massless particle
	is a kind of neutral particle
neutrino telescope	has definition telescope designed to detect neutrinos and determine their origin, type and energy
	is a kind of telescope
neutron	has composition two down quarks and one up quark
	has decay product proton, electron
	has definition A neutral baryon.
	has definition A nuclear particle with a mass slightly greater than that of a proton. A free neutron decays, after a half-life of about 10.6 minutes, into a proton, an electron, and an antineutrino. The neutron is probably made up of still more fundamental particles having both positive and negative charges. The charges balance to give a net charge of zero, but the motions of the charges are such that their magnetic contributions do not cancel and consequently the neutron is magnetic.
	has definition One of the constituents of the atomic nucleus discovered in 1932. It is bound into atomic nuclei by the strong nuclear force. Free neutrons decay slowly via the weak nuclear force. Despite being electrically neutral, the neutron possesses both an electric dipole moment (as if it were made of positive and negative charges separated a minute distance) and a magnetic moment, indicating some internal substructure.
	has definition Particle in the nucleus of all atoms except hydrogen. Through beta decay, a neutron may become a proton and an electron; the process occurs in reverse during the formation of a neutron star.
	has definition Stable within the nucleus, the neutron if isolated decays, with a ha half-life of fifteen minutes.
	has definition typically found in the nucleus of an atom.
	has lifetime 15 minutes
	has mass 1.6749 × 10-24 g
	is a part of nucleus
	is an instance of neutral particle
	is an instance of nucleon
	is an instance of radioactive particle
neutron Compton wavelength	applies to particle neutron
	has equation
	has symbol λC,n
	has uncertainty 0.000000010 × 10-15 m
	has value 1.319590898 × 10-15 m
	is an instance of particle Compton wavelength
neutron g factor	applies to particle neutron
	has equation
	has symbol gn
	has uncertainty 0.00000090
	has value -3.82608545
	is an instance of particle g factor
neutron gyromagnetic ratio	applies to particle neutron
	has equation
	has symbol γn
	has uncertainty 0.00000044 × 108 s-1 T-1
	has value 1.83247188 × 108 s-1 T-1
	is an instance of particle g factor
neutron induced fission	is a kind of fission
neutron magnetic moment	applies to particle neutron
	has symbol μn
	has uncertainty 0.00000023 × 10-26 J T-1
	has value -0.96623640 × 10-26 J T-1
	is an instance of particle magnetic moment
neutron mass	applies to particle neutron
	has symbol mn
	has uncertainty 0.00000013 × 10-27 kg
	has value 1.67492716 × 10-27 kg
	is an instance of particle mass
neutron star	has definition A dead, collapsed star that consists mostly of neutrons and is only about 20 kilometers across. Neutron stars are much denser than white dwarfs.
	has definition A star whose core is composed primarily of neutrons, as is expected to occur when the mean density is in the range 1013-1015 g cm-3. Under current theories pulsars are assumed to be rotating magnetic neutron stars. A neutron star would probably be only 10-15 km in diameter with a magnetic field of about 1012 gauss, a density of 1013-1015 g cm-3 (compared with a white dwarf's maximum density of about 108 g cm-3) and a central temperature of about 109 K and thus would be both bluer and dimmer than a white dwarf.
	has definition Remnant of a star after it has exploded as a supernova. Usually optically dim, a neutron star sends out regular or irregular radio emissions and is therefore also called a pulsar. The density of such a star may be unimaginably great although the diameter is generally around only 10 km; the gravitational and magnetic forces are correspondingly vast. It is called a neutron star because in such density, protons fuse with electrons to form neutrons, of which the star is almost entirely composed.
	is a kind of massive compact halo object
	is a kind of star
New General Catalogue	has acronym NGC
	has definition A catalogue of 7840 nebulae, star clusters, and galaxies that was published in 1888 by John Dreyer.
	is a kind of nebula catalog
New Technology Telescope	has acronym NTT
	has altitude 2353 m
	has aperture 3.50 m
	has comment mirror figure controlled by 78 active supports
	has creation date 1989
	has focal ratio U2.2, 11
	has latitude 29° 16' S
	has location La Silla, Chile
	has longitude 70° 44' W
	has mirror maker Zeiss
	has mirror type thin Zerodur
	has owner European Southern Observatory
	is an instance of altazimuth telescope
	is an instance of optical telescope
	is an instance of Ritchey-Chrétien
newton	has base unit m·kg·s-2
	has definition The SI derived unit of force, equal to the force necessary to give an acceleration of 1 meter per second2 to a mass of 1 kg.
	has equivalent 105 dynes
	has symbol N
	is an instance of force unit
	is an instance of named derived SI unit
Newton's first law	has definition Newton's law which states that a body remains in a state of rest or uniform motion when left to itself.
	is an instance of Newton's law of motion
Newton's law of motion	has definition One of three laws describing the motion of bodies based on the conception of an absolute and immutable space and time; these laws held sway until Einstein's discovery of special relativity.
	has discoverer Sir Isaac Newton
	has domain mechanics
	is a kind of law
Newton's second law	has definition Newton's law which states that the net force on a body is equal to the product of its mass and its acceleration.
	is an instance of Newton's law of motion
Newton's theory of gravity	has definition A force law that applies to the gravitational and electromagnetic forces in which the magnitude of the force decreases in proportion to the inverse of the square of the distance.
	has definition Gravitational force decreases as one over distance squared (1/r2), where r is the distance from the source.
	has definition Theory of gravity declaring that the force of attraction between two bodies is proportional to the product of their masses and inversely proportional to the square of the distance between them. Subsequently supplanted by Einstein's general relativity.
	has discoverer Sir Isaac Newton
	has synonym inverse square law
	is an instance of gravity law
Newton's third law	has definition Newton's law which states that when two bodies interact, the force on the first due to the second is equal and opposite to the force on the second due to the first.
	is an instance of Newton's law of motion
Newtonian	has definition A class of reflecting telescope developed by Sir Issac Newton with a paraboloidal primary mirror and a small, plane secondary mirror at 45°; to deflect the focus of the primary to a position outside the tube near the top of the telescope.
	has primary mirror shape concave paraboloidal
	has secondary mirror shape plane
	is a kind of reflector
Newtonian constant of gravitation	has definition The universal constant of proportionality in the attraction between two unit masses a unit distance apart.
	has effect on other constants none, increases in the accuracy of its measurement have made no difference to the values of the other physical constants.
	has equation F = G × m1 × m2/r2, where F is the gravitational force of attraction, m1 and m2 are the masses for two bodies separated by r.
	has relationship no direct connection has, as yet, been established between G and any other physical quantities
	has symbol G
	has uncertainty 0.010 × 10-11 m3 kg-1 s-2
	has value 6.673 × 10-11 m3 kg-1 s-2
	is an instance of universal constant
NGC 1275	has definition The strongest known extragalactic X-ray source. Also a strong radio source. Optically it is a Seyfert galaxy with a huge amount (about 108 Msun) of ionized gas receding from it.
	has recession velocity 5000 km s-1
	has redshift z = 0.0183
	has synonym 3C 84
	has synonym 3U 0316+41
	has synonym Abell 426
	has synonym Perseus A
	has synonym Perseus X-1
	is an instance of radio galaxy
	is an instance of Seyfert galaxy
	is an instance of X-ray source
NGC 1326A	has distance 18.7 ± 1.5 Mpc
	is an instance of galaxy containing Cepheids
NGC 1365	has distance 18.3 ± 1.7 Mpc
	is an instance of galaxy containing Cepheids
NGC 1425	has distance 22.2 ± 1.0 Mpc
	is an instance of galaxy containing Cepheids
NGC 147	is a part of Local Group
	is an instance of dwarf elliptical galaxy
NGC 1818	has distance 164000 light years
	has image
	is a part of Large Magellanic Cloud
	is an instance of open cluster
NGC 185	is a part of Local Group
	is an instance of dwarf elliptical galaxy
NGC 1850	has distance 66000 light years
	has image
	is a part of Large Magellanic Cloud
	is an instance of open cluster
NGC 1999	has image
	is a part of dust
	is an instance of reflection nebula
NGC 205	has definition An elliptical galaxy that orbits the Andromeda galaxy.
	is a part of Local Group
	is an instance of dwarf elliptical satellite galaxy
	orbits Andromeda galaxy
NGC 2346	has distance 2000 light years
	has image
	has orbit period 16 days
	is an instance of planetary nebula
	is part of Monoceros
NGC 2392	has image
	has synonym Eskimo Nebula
	is an instance of planetary nebula
NGC 3132	has distance 2000 light years
	has expansion rate 9 miles per second
	has extent 0.5 light year
	has image
	has synonym Eight-Burst Nebula
	has synonym Southern Ring Nebula
	is an instance of planetary nebula
NGC 3198	has distance 14.5 ± 1.2 Mpc
	is an instance of galaxy containing Cepheids
NGC 3319	has distance 14.3 ± 0.9 Mpc
	is an instance of galaxy containing Cepheids
NGC 3603	has image
	is an instance of gaseous nebula
NGC 3627	has distance ?
	is an instance of galaxy containing Cepheids
NGC 3918	has distance 3000 light years
	has extent 0.3 light years
	has image
	has image
	is an instance of planetary nebula
	is an instance of planetary nebula
	is part of Centaurus
NGC 4214	has distance 13 million light years
	has image
	is an instance of irregular galaxy
NGC 4535	has distance 16.0 ± 1.9 Mpc
	is an instance of galaxy containing Cepheids
NGC 4548	has distance 15.9 ± 2.0 Mpc
	is an instance of galaxy containing Cepheids
NGC 5195	has declination +47d16m05s
	has diameter 5.8 × 4.6 arcminutes
	has image
	has magnitude 10.45
	has NED data
	has right ascension 13h29m58.7s
	has synonym M51b
	is a part of Local Group
	is an instance of irregular galaxy
	is part of Canes Venatici
NGC 5307	has distance 10000 light years
	has extent 0.6 light years
	has image
	is an instance of planetary nebula
	is part of Centaurus
NGC 604	has image
	is a part of Pinwheel galaxy
	is an instance of gaseous nebula
NGC 6210	has image
	is an instance of planetary nebula
NGC 6751	has distance 6500 light years
	has extent 0.8 light years
	has image
	is an instance of planetary nebula
NGC 6818	has distance 6000 light years
	has extent 0.5 light years
	has image
	is an instance of planetary nebula
	is part of Sagittarius
NGC 6822	has declination -14d48m05s
	has diameter 2 kpc
	has distance 600 kpc from Milky Way galaxy
	has image
	has NED data
	has right ascension 19h44m56.1s
	is a part of Local Group
	is an instance of irregular galaxy
NGC 6826	has distance 2200 light years
	has image
	is an instance of planetary nebula
	is part of Cygnus
NGC 7009	has distance 1400 light years
	has image
	is an instance of planetary nebula
	is part of Aquarius
NGC 7027	has distance 3000 light years
	has image
	is an instance of planetary nebula
	is part of Cygnus
NGC 7635	has distance 7100 light years
	has extent 6 light-years
	has image
	has stellar wind speed 2000 kilometers per second
	has synonym Bubble nebula
	is an instance of planetary nebula
	is part of Cassiopeia
NGC object	has catalog New General Catalogue
	has definition an object assigned a number in the New General Catalog of non-stellar objects
	is a kind of celestial body
	is a part of celestial sphere
Nicholas U. Mayall Reflector	has altitude 2120 m
	has aperture 3.81 m
	has creation date 1973
	has focal ratio f/2.7, 8, 15.7 IR, 190
	has latitude 31° 58' N
	has location Kitt Peak, Arizona, US
	has longitude 111° 36' W
	has mirror maker KPNO Optical Shop
	has mirror type Fused-quartz
	has mounting manufacturer Western Gear Corp.
	has owner Kitt Peak National Observatory
	has synonym Kitt Peak 4 m
	is an instance of Horseshoe equatorial telescope
	is an instance of optical telescope
	is an instance of Ritchey-Chrétien
nickel	has abundance 1 × 10-4 p.p.m. in Atlantic surface seawater
	has abundance 1 × 10-4 p.p.m. in Pacific surface seawater
	has abundance 1.91 × 106 in Sun relative to H = 1 × 1012
	has abundance 4.0 × 10-4 p.p.m. in deep Atlantic seawater
	has abundance 5.7 × 10-4 p.p.m. in deep Pacific seawater
	has abundance 80 p.p.m. in Earth's crust
	has atomic emission line 232.003 nm for Ni I
	has atomic emission line 349.296 nm for Ni I
	has atomic emission line 351.505 nm for Ni I
	has atomic emission line 361.939 nm for Ni I
	has atomic emission line 341.476 nm for Ni I (strong)
	has atomic emission line 352.454 nm for Ni I (strong)
	has atomic number 28
	has atomic radii 125 pm
	has biological role essential to some species, and can act to stimulate metabolism
	has boiling point 3005 K
	has bulk modulus 177.3 GPa
	has chief source garnierite, pentlandite
	has covalent radii 115 pm
	has crystal cell dimension a = 352.38 pm
	has crystal type f.c.c.
	has Curie temperature 633 K
	has daily dietary intake 0.3 - 0.5 mg
	has definition corrosion resistant, silvery-white, lustrous, malleable and ductile metal
	has density 7780 kg m-3 for liquid at 1726 K boiling point
	has density 8902 kg m-3 for solid at 293 K
	has discoverer A.F. Cronstedt
	has discovery date 1751
	has discovery location Stockholm, Sweden
	has electrical resistivity 6.84 × 10-8 Ω m at 293 K
	has electron affinity 156 kJ mol-1 from Ni to Ni-
	has electron configuration [Ar]3d84s2 in ground state
	has electronegativity 1.91 Pauling
	has hazard element and its compounds are poisonous, carcinogenic, and teratogenic
	has hazard nickel carbonyl is extremely toxic
	has heat capacity 23.359 J K-1 mol-1 for gas at constant pressure 0.1 MPa at 298.15 K
	has heat capacity 26.07 J K-1 mol-1 for solid at constant pressure 0.1 MPa at 298.15 K
	has heat of fusion 17.6 kJ mol-1
	has heat of vaporization 371.8 kJ mol-1
	has image
	has ionic radii 62 pm for Ni3+
	has ionic radii 78 pm for Ni2+
	has isotope mass range 53 to 67
	has lethal intake 350 mg kg-1 in rat for nickel acetate
	has level in humans < 0.7 p.p.m. in bone
	has level in humans 0.01 - 0.05 mg dm-3 in blood
	has level in humans 0.02 - 1.8 p.p.m. in liver
	has level in humans 1 - 2 p.p.m. in muscle
	has linear expansion coefficient 13.3 × 10-6 K-1
	has longest lived isotope nickel 58
	has main mining area garnierite in Russia, South Africa, USA; pentlandite in Canada, South Africa
	has mass absorption coefficient 45.7 cm2 g-1 for CuKα X-ray diffraction
	has mass absorption coefficient 46.6 cm2 g-1 for MoKα X-ray diffraction
	has mass magnetic susceptibility ferromagnetic
	has mass of element in person 15 mg for a 70 kg average person
	has melting point 1726 K
	has mineral garnierite, millerite, nickeline, pentlandite, nickel-iron meteorites
	has molar volume 6.59 cm3
	has name origin kupfernickel = Devil's copper or St Nicholas's copper from German
	has neutron scattering length 1.03 × 10-12 cm
	has number of isotopes 14 including nuclear isomers
	has number of protons 28
	has ocean oxidation state II
	has ocean residence time 80000 years
has poisson's ratio 0.312 GPa
has pronunciation nik-el
has registry number 7440-02-0 for Chemical Abstracts System database
has relative atomic mass 58.6934 in units of 12C = 12.000
has reserves 70 × 106 tonnes
has rigidity modulus 76.0 GPa
has space group Fm3m
has specimen foil, powder, rod, slugs, spheres and wire. Safe.
has symbol Ni
has synthesis mechanism
has term symbol 3F4 in ground state
has thermal conductivity 90.7 W m-1 K-1 at 300 K
has thermal neutron capture cross section 4.49 barns
has toxic intake 1 - 3 mg kg-1
has uses alloys, especially stainless steel, coins, metal plating and catalysts
has world production 510000 tonnes year-1
has young's modulus 199.5 GPa
is a kind of recycled oceanic element
is a kind of siderophile element
is a kind of transition metal
reacts with acids by disolving except for concentrated HNO3
nickel 56	has atomic mass 55.943124
	has decay mode EC (2.14 Mev)
	has decay mode no γ
	has half life 6.10 days
	has nuclear spin I = 0+ h/2π
	has number of neutrons 28
	has number of nucleons 56
	has symbol 56Ni
	is an instance of nickel
nickel 57	has atomic mass 56.93799
	has decay mode β+ (3.265 Mev)
	has decay mode γ
	has decay mode EC
	has half life 35.6 hours
	has nuclear magnetic moment μ = 0.88 nuclear magnetons with diamagnetic correction
	has nuclear spin I = 3/2- h/2π
	has number of neutrons 29
	has number of nucleons 57
	has symbol 57Ni
	is an instance of nickel
nickel 58	has atomic mass 57.935346
	has natural abundance 68.27%
	has nuclear spin I = 0+ h/2π
	has number of neutrons 30
	has number of nucleons 58
	has symbol 58Ni
	has uses isotopically enriched samples available for experimental purposes
	is an instance of nickel
nickel 59	has atomic mass 58.934349
	has decay mode EC
	has decay mode no γ
	has half life 76000 years
	has nuclear spin I = 3/2- h/2π
	has number of neutrons 31
	has number of nucleons 59
	has symbol 59Ni
	is an instance of nickel
nickel 60	has atomic mass 59.930788
	has natural abundance 26.10%
	has nuclear spin I = 0+ h/2π
	has number of neutrons 32
	has number of nucleons 60
	has symbol 60Ni
	has uses isotopically enriched samples available for experimental purposes
	is an instance of nickel
nickel 61	has atomic mass 60.931057
	has magnetogyric ratio -2.3948 × 107 rad T-1 s-1
	has natural abundance 1.13%
	has NMR frequency 8.936 MHz where 1H = 100 MHz; 2.3488 T
	has NMR receptivity 0.242 where 13C = 1.00
	has nuclear magnetic moment μ = -0.7502 nuclear magnetons with diamagnetic correction
	has nuclear quadrupole moment +0.162 × 10-28 m2
	has nuclear spin I = 3/2- h/2π
	has number of neutrons 33
	has number of nucleons 61
	has relative NMR sensitivity 3.57 × 10-3 where 1H = 1.00
	has symbol 61Ni
	has uses isotopically enriched samples available for experimental purposes
	has uses Nuclear Magnetic Resonance
	is an instance of nickel
nickel 62	has atomic mass 61.928346
	has natural abundance 3.59%
	has nuclear spin I = 0+ h/2π
	has number of neutrons 34
	has number of nucleons 62
	has symbol 62Ni
	has uses isotopically enriched samples available for experimental purposes
	is an instance of nickel
nickel 63	has atomic mass 62.929669
	has decay mode β- (0.065 Mev)
	has decay mode no γ
	has half life 100 years
	has nuclear spin I = 1/2- h/2π
	has number of neutrons 35
	has number of nucleons 63
	has symbol 63Ni
	has uses research
	is an instance of nickel
nickel 64	has atomic mass 63.927967
	has natural abundance 0.91%
	has nuclear spin I = 0+ h/2π
	has number of neutrons 36
	has number of nucleons 64
	has symbol 64Ni
	has uses isotopically enriched samples available for experimental purposes
	is an instance of nickel
nickel 65	has atomic mass 64.930086
	has decay mode β- (2.134 Mev)
	has decay mode γ
	has half life 2.57 hours
	has nuclear magnetic moment μ = 0.69 nuclear magnetons with diamagnetic correction
	has nuclear spin I = 5/2- h/2π
	has number of neutrons 37
	has number of nucleons 65
	has symbol 65Ni
	is an instance of nickel
nickel 66	has atomic mass 65.929116
	has decay mode β- (0.24 Mev)
	has decay mode no γ
	has half life 54.6 hours
	has nuclear spin I = h/2π
	has number of neutrons 38
	has number of nucleons 66
	has symbol 66Ni
	is an instance of nickel
NICMOS	has definition Near Infrared Camera for Multi-Object Spectrography.
	is a kind of spectrograph
Nicol prism	has definition A device made from a split crystal of Iceland spar with which plane-polarized light can be detected.
	has wavefront modification polarization
	is a kind of wavefront modifier
Niels Bohr	is an instance of 19th century scientist
	is an instance of physicist
niobium	has image
	has ocean oxidation state V
	is a kind of transition metal
	is a kind of unclassified oceanic element
nitrogen	has abundance 25 p.p.m. in Earth's crust
	has abundance 780900 p.p.m. by volume in Earth's atmosphere
	has abundance 8.71 × 107 in Sun relative to H = 1 × 1012
	has abundance p.p.m. in seawater
	has atomic emission line 1246.962 nm for N I
	has atomic emission line 463.054 nm for N II
	has atomic emission line 500.515 nm for N II
	has atomic emission line 567.956 nm for N II
	has atomic emission line 746.831 nm for N I
	has atomic emission line 399.500 nm for N II (strong)
	has atomic number 7
	has atomic radii 71 pm
	has biological role constituent element of DNA and amino acids; nitrogen cycle in nature
	has boiling point 77.4 K
	has chief source liquified air
	has covalent radii 70 pm for single bond
	has critical pressure 3394 kPa
	has critical temperature 126.05 K
	has crystal cell dimension a = pm
	has crystal type
	has daily dietary intake high
	has definition colourless, odourless gas (N2)
	has density 1.2506 kg m-3 for gas at 273 K
	has density 1026 kg m-3 for solid at 21 K
	has density 880 kg m-3 for liquid at 77.4 K boiling point
	has discoverer D. Rutherford
	has discovery date 1772
	has discovery location Edinburgh, Scotland, UK
	has electron affinity -7 kJ mol-1 from N to N-
	has electron configuration [He]2s22p3 in ground state
	has electronegativity 3.04 Pauling
	has hazard harmless gas, but it could asphyxiate if it excluded oxygen from the lungs
	has heat capacity 20.786 J K-1 mol-1 for atomic gas at constant pressure 0.1 MPa at 298.15 K
	has heat capacity 29.125 J K-1 mol-1 for molecular gas (N2) at constant pressure 0.1 MPa at 298.15 K
	has heat of fusion 0.720 kJ mol-1
	has heat of vaporization 5.577 kJ mol-1
	has image
	has isotope mass range 12 to 18
	has level in humans 34300 mg dm-3 in blood
	has level in humans 43000 p.p.m. in bone
	has level in humans 72000 p.p.m. in liver
	has level in humans 72000 p.p.m. in muscle
	has longest lived isotope nitrogen 14
	has mass absorption coefficient 0.916 cm2 g-1 for MoKα X-ray diffraction
	has mass absorption coefficient 7.52 cm2 g-1 for CuKα X-ray diffraction
	has mass magnetic susceptibility -5.4 × 10-9 kg-1 m3 for gas
	has mass of element in person 1.8 kg for a 70 kg average person
	has melting point 63.29 K
	has mineral nitratine, nitrammite, nitrobarite, nitrocalcite and nitromagnesite
	has molar volume 12.65 cm3
	has name origin nitron genes = nitre forming (potassium nitrate) from Greek
	has neutron scattering length 0.936 × 10-12 cm
	has number of isotopes 8 including nuclear isomers
	has number of protons 7
	has ocean oxidation state V
	has ocean residence time 6000 years
	has pronunciation niy-troh-jen
	has registry number 7727-37-9 for Chemical Abstracts System database
	has relative atomic mass 14.00674 in units of 12C = 12.000
	has reserves × 10 tonnes
	has space group
	has specimen small pressurized canisters. Safe.
	has symbol N
	has synthesis mechanism liquifaction of air
has term symbol 4S3/2 in ground state
has thermal conductivity 0.02598 W m-1 K-1 at 300 K for gas
has thermal neutron capture cross section 1.91 barns
has toxic intake non-toxic as N2 gas but NO2, HCN and NH3 are toxic
has uses fertilizers, acids (HNO3), explosives, plastics and dyes
has van der Waals radii 154 pm
has world production 44 × 106 tonnes year-1
is a kind of atmophile element
is a kind of gaseous element
is a kind of group V element
is a kind of nonmetallic element
is a kind of recycled oceanic element
reacts with generaly unreactive at normal temperatures
nitrogen 13	has atomic mass 13.005738
	has decay mode β+ (2.2205 Mev) %
	has decay mode no γ
	has half life 9.97 minutes
	has nuclear magnetic moment μ = -0.3222
	has nuclear spin I = 1/2
	has number of neutrons 6
	has number of nucleons 13
	has symbol 13N
	is an instance of nitrogen
nitrogen 14	has atomic mass 14.003074002
	has magnetogyric ratio 1.9331 × 107 rad T-1 s-1
	has natural abundance 99.634%
	has NMR frequency 7.224 MHz where 1H = 100 MHz; 2.3488 T
	has NMR receptivity 5.69 where 13C = 1.00
	has nuclear magnetic moment μ = +0.4037607
	has nuclear quadrupole moment 0.0202 × 10-28 m2
	has nuclear spin I = 1+
	has number of neutrons 7
	has number of nucleons 14
	has relative NMR sensitivity 1.01 × 10-3 where 1H = 1.00
	has symbol 14N
	has uses Nuclear Magnetic Resonance
	is an instance of nitrogen
nitrogen 15	has atomic mass 15.00010897
	has magnetogyric ratio -2.7116 × 107 rad T-1 s-1
	has natural abundance 0.366
	has NMR frequency 10.133 MHz where 1H = 100 MHz; 2.3488 T
	has NMR receptivity 0.0219 where 13C = 1.00
	has nuclear magnetic moment μ = -0.2831892
	has nuclear spin I = 1/2-
	has number of neutrons 8
	has number of nucleons 15
	has relative NMR sensitivity 1.04 × 10-3 where 1H = 1.00
	has symbol 15N
	has uses Nuclear Magnetic Resonance
	is an instance of nitrogen
nitrogen 16	has atomic mass 16.006099
	has decay mode β-
	has decay mode β-α(10.4187 Mev)
	has decay mode γ
	has half life 7.13 seconds
	has nuclear spin I = 2-
	has number of neutrons 9
	has number of nucleons 16
	has symbol 16N
	is an instance of nitrogen
nitrogen 17	has atomic mass 17.008450
	has decay mode β+
	has decay mode β-n (8.680 Mev)
	has decay mode γ
	has half life 4.17 seconds
	has nuclear spin I = 1/2-
	has number of neutrons 10
	has number of nucleons 17
	has symbol 17N
	is an instance of nitrogen
NML Cygnus	has definition An infrared star. Its surface temperature is about the same as the surface temperature of Venus. It is a strong OH emitter, and CO has been identified in its spectrum.
	has discoverer Neugebauer, Martz, and Leighton
	has distance 200 pc
	has heliocentric radial velocity -43 km s-1
	has spectral type M6 III
	has surface temperature 700 K
	has synonym IRC+40448
	is a part of Cygnus
	is an instance of giant
	is an instance of M star
NOAA satellite	is a kind of meteorological satellite
nobelium	is a kind of transuranium element
nodical month	has definition The interval of time between two successive transits of the Moon through its ascending node.
	has synonym Draconic month
	has value 27.2122 days
	is an instance of month
non SI unit	has definition not an SI unit, but an important and widely used unit
	has value in SI unit
	is a kind of unit
non-coherent scattering	has definition Absorption of a photon and reemission at a different frequency (as seen by an observer) by scattering atoms. The natural width of the lines, Doppler broadening, and pressure broadening are the main processes that give rise to noncoherent scattering.
	is a kind of scattering
nonaccelerating reference frame	has acceleration 0
	is a kind of frame of reference
nonmetal	is a kind of nonmetallic element
nonmetallic element	is a kind of element
nonmetallic metalloid	is a kind of metalloid element
nonrotating reference frame	has rotation rate 0
	is a kind of frame of reference
Nordic Optical Telescope	has acronym NOT
	has altitude 2382 m
	has aperture 2.56 m
	has creation date 1989
	has focal ratio (f/2.0), 11.0
	has latitude 28° 45' N
	has location La Palma, Canary Islands
	has longitude 17° 53'W
	has mirror maker Optics Labs (Tartu)
	has mirror type Zerodur
	has mounting altazimuth in rotating building
	has owner Obs. del Roque de Ins Muchachos
	is an instance of altazimuth telescope
	is an instance of Cassegrain
	is an instance of optical telescope
Norma	has acronym Nor
	has boundary
	has genitive Normae
	has historical origin as 'Norma et Regula' (appeared as the Level and the Square to de Lacaille)
	has synonym Level (Square)
	is a part of celestial sphere
	is an instance of constellation named by de Lacaille
Norma arm	is a part of Sagittarius arm
North America Nebula	has definition An emission nebula in Cygnus.
	has synonym NGC 7000
	is a part of Cygnus
	is an instance of gaseous nebula
North American continent	is a part of Earth's lithosphere
north celestial pole	has acronym NCP
	has declination +90 degrees
	is a part of Ursa Minor
	is an instance of celestial pole
	is opposite of south celestial pole
north galactic pole	has acronym NGP
	has definition A point in the constellation Coma Berenices where we look perpendicular to and above the Galactic Plane. The nearest bright star to the North Galactic Pole is Arcturus, in the neighboring constellation Bootes.
	has galactic latitude 90 degrees
	is a part of Coma Berenices
	is an instance of galactic pole
	is opposite of south galactic pole
north point	has azimuth 0
	has definition The point at which the meridian intersects the horizon below the north celestial pole.
	has elevation 0
	is an instance of local coordinate
North Polar Spur	has definition A radio continuum feature extending from the galactic plane to the vicinity of the North Galactic Pole. It is believed to be a supernova remnant. It is also an X-ray source.
	has distance 50-200 pc
	is an instance of radio source
	is an instance of X-ray source
north pole	has definition The astronomical coordinate which coincides with the northern intersection of Earth's axis with the geoid.
	has terrestrial latitude +90 degrees
	is a part of Earth
	is an instance of pole
	is opposite of south pole
nova	has definition A star that brightens suddenly and to an unprecedented degree, creating the impression that a new star has appeared where none was before. Hence the name, from nova for "new". See supernova.
	has definition A star that exhibits a sudden surge of energy, temporarily increasing its luminosity by as much as 14 mag. (Since 1925 novae have been given variable star designations.) Novae are old disk-population stars. Unlike supernovae, novae retain their stellar form and most of their substance after the outburst. All known common novae are found in close binary systems with one component a cool red giant and the other a hot, less massive object which is the seat of the instability.
	has definition A stars that undergoes an explosion during which its brightness increases by up to ten magnitudes. Usually the following phases are distinguished (in order of time): pre-maximum, principal, diffuse enhanced, Orion, nebular and post-nova.
	has energy release 1044 ergs
	has use nova peak brightness distance determination
	is a kind of cataclysmic variable
	is a kind of close binary
	is a kind of emission line star
	can become naked eye star
Nova Cygni 1975	has initial rise of light curve 19 magnitudes
	is a part of Milky Way
	is an instance of classical nova
Nova Cygni 1978	has initial rise of light curve 11 magnitudes
	is an instance of classical nova
nova remnant	has definition an expanding emission nebula produced by a nova
	has expansion velocity typically 1000 km/s
	has lifetime a few centuries
	has mass much lower than planetary nebula
	is a kind of expanding emission nebula
	can be used to measure distance
Nu Octantis	has B-V magnitude 1.00
	has declination -77 23 24
	has right ascension 21 41 28.6
	has spectral type K0III
	has synonym HR 8254
	has V magnitude 3.76
	is a part of Octans
	is an instance of giant
	is an instance of K star
	is an instance of naked eye star
nuclear decay	is a kind of nuclear process
nuclear density	has definition The density of an atomic nucleus (about 1014 g cm-3).
	is a kind of density
nuclear disk	has definition A rotating disk of about 106 Msun of neutral hydrogen in the inner 800 pc of our Galaxy.
	is a part of disk
nuclear magneton	has equation
	has symbol μN
	has uncertainty 0.00000020 × 10-27 J T-1
	has value 5.05078317 × 10-27 J T-1
	is an instance of electromagnetic constant
nuclear physics	has definition That branch of physics that attempts to understand the physics of the nucleus.
	is a kind of high energy physics
nuclear process	is a kind of physical process
nuclear time scale	has definition Time required for a star to evolve a significant distance off the main sequence; the time it takes a star to convert all its available hydrogen into helium.
	is a kind of time
	see also Kelvin timescale
nucleon	has definition The generic name for the proton and the neutron.
	has mass 939 MeV
	has spin 1/2
	is a kind of baryon
	is a kind of charge multiplet
	is a part of nucleus
nucleosynthesis	has definition The production of heavy nuclei from the fusion of lighter ones.
	is a kind of fusion
nucleosynthetic era	has definition The Big Bang era when neutrons were produced and helium and deuterium were synthesized. At t = 200 sec, nucleosynthesis began rather abruptly and virtually all deuterium was synthesized to helium.
	has duration 1000 s
	has start time 1 s after the Big Bang
	is a kind of Big Bang era
	is followed by radiation era
	is preceded by lepton era
nucleosynthetic reaction	is a kind of nuclear process
nucleus	has definition The central part of an atom, composed of protons and neutrons (which are made of quarks) and containing nearly all of each atom's mass.
	has definition The massive, positively charged central part of an atom, composed mainly of protons and neutrons, around which the electrons revolve. The radius of an atomic nucleus is directly proportional to the cube root of its mass. Density at least 1014 g cm-3. Radius 10-12-10-13 cm.
	is a kind of charged particle
	is a part of element
nuclide	has definition A species of atomic nucleus, analogous to the word "isotope" for a species of atom. The word is also used to distinguish between atomic nuclei that are in different energy states.
	is a kind of nucleus
number	has value
	is a kind of mathematical concept
nutation	has definition A small, irregular oscillation in the precessional motion of Earth's rotational axis, caused primarily by lunar perturbations. It has a principal period of 18.6 years, and moves the equinox as much as 17" ahead of or behind its mean position.
	has definition Slight but recurrent oscillation of the axis of the Earth, caused by the Moon's minutely greater gravitational effect on the Earth's equatorial "bulge".
	is a kind of axis motion
O magnitude	has band 11 microns
	has definition The magnitude derived from observations at 11 microns.
	is an instance of infrared magnitude
O star	has absorption line He II
	has color blue-white
	has definition Very hot blue star, whose spectra is dominated by the lines of singly ionized helium (see Pickering series). (Most other lines are from at least doubly ionized elements, though H and He I lines are also present.) O stars are useful because they are found in dust clouds and virtually define the spiral arms.
	has lifetime 3 to 6 million years
	has relative abundance rare
	has rotation velocity very high
	has surface temperature 35000 K
	is a kind of early star
O((f))	has definition O-type stars in which N III is present in emission and He is strong in absorption.
	has emission line N III
	is a kind of emission line star
	is a kind of O star
O(f)	has definition O-type stars in which N III is present in emission and He II is weakly present in absorption or emission.
	has emission line N III and sometimes He II
	is a kind of emission line star
	is a kind of O star
OB association	has definition A loose gathering of O and B stars that typically stretches over hundreds of light-years and contains a few dozen OB stars.
	is a kind of association
Oberon	has definition Outermost satellite of Uranus.
	has discoverer Herschel
	has discovery date 1787
	has orbital period P = 13.46 days
	has radius R ≥ 500 km
	has rotational period 13.46 days
	is a part of Uranus
	is an instance of natural satellite
objective	has definition The primary mirror of a reflecting telescope (or the primary lens of a refractor).
	has wavefront modification shape
	is a kind of wavefront modifier
objective grating	has definition A coarse grating placed in front of the telescope objective.
	is a kind of diffraction grating
objective prism	has definition A small-angle prism placed in front of a telescope objective to transform each star image in a field of stars into an image of its spectrum.
	is a kind of spectrograph dispersing element
observatory	has address
	has altitude
	has definition a place on earth which has one or more telescopes
	has location
	has telescope
	is a kind of artifact
Occam's Razor	has author William of Ockham
	has definition Any hypothesis should be shorn of all unnecessary assumptions; if two hypotheses fit the observations equally well, the one that makes the fewest assumptions should be chosen.
	has formulation date fourteenth century
	has name origin Entia non sunt multiplicanda ("Entities are not to be multiplied")
	is a kind of doctrine
occultation	has definition The cutoff of the light from a celestial body caused by its passage behind another object.
	is a kind of conjunction
occultation phase	has definition A phase during the occultation of a celestial body
	is a kind of conjunction
oceanic element	has definition an element contained in the ocean
	has occurrence ocean
	has ocean concentration
	has ocean oxidation state
	has ocean residence time
	is a kind of planetary element
Octans	has acronym Oct
	has genitive Octantis
	has historical origin the instrument used to measure the position of Polaris
	has synonym Octant
	is a part of celestial sphere
	is an instance of constellation named by de Lacaille
Oe	has definition O star with emissions in the Balmer lines.
	has emission line Balmer lines
	is a kind of emission line star
	is a kind of O star
Oef	has definition Early O stars that show double emission lines in He II λ4686.
	has emission line He II λ4686 (double)
	is a kind of emission line star
	is a kind of O star
oersted	has definition Unit of magnetic field strength. 1 Oe corresponds to 1000/4π amperes per meter.
	has symbol Oe
	is an instance of magnetic flux density unit
Of star	has definition Peculiar O stars in which emission features at λλ4634-4641 from N III and 4686 from He II are present. They have a well-developed absorption spectrum, which implies that the excitation mechanism of the emission lines is selective, unlike that of Wolf-Rayet stars. The spectra of Of stars are usually variable, and the intensities of their emission lines vary in an irregular manner. Of stars belong to extreme Population I. All O stars earlier than 05 are Of.
	has emission line N III 4634-4641 and He II 4686
	is a kind of emission line star
	is a kind of O star
	is a kind of peculiar star
ohm	has base unit m2·kg·s-3·A-2
	has symbol Ω
	has unit V·A-1
	is an instance of named derived SI unit
	is an instance of resistance unit
Olber's paradox	has definition A paradox formulated by the German astronomer Heinrich Olbers in 1826 that can be traced back to the writings of others, such as de Cheseaux, a century or more earlier. The paradox is: Why is the sky dark at night, if the universe is infinite? We now know that several of the assumptions made by Olbers (explicitly or implicitly) are incorrect.
	has definition A paradox formulated by the German astronomer Heinrich Olbers in 1826: Why is the sky dark at night? The amount of light we receive from a star decreases as the square of the distance from us. On the other hand, if we assume a uniform distribution of stars in space, the number of stars increases as the square of their distance from us, so the two factors should cancel out. In theory, then, the night sky should be a blazing mass of light, and obviously it is not. This self-contradictory statement is Olbers' paradox. In seeking to resolve it, astronomers noted that, besides the assumption of uniformity or homogeneity, Olbers made four other assumptions: (1) space is Euclidean; (2) the laws of physics that apply on Earth apply to the Universe as a whole; (3) the Universe is static (i.e., neither expanding nor contracting); (4) the Universe is spatially and temporally infinite. It is now known that all four of these assumptions are either incorrect or inaccurate.
	has definition The puzzle of why the sky is dark at night. If the universe extends infinitely in space, as it might, then the accumulated light from an infinite number of distant stars should seemingly cause the sky to be bright at all times, whether our sun is visible or not. This paradox, first posed in the eighteenth century, has been resolved by the big bang theory. In a universe with a beginning, we can receive light only from that part of the universe close enough so that light has had time to travel from there to here since the big bang (about 10 billion years ago). Thus, even if space extends infinitely far, only a limited region, and a limited number of stars, are visible to us. And the accumulated light from this limited number of stars is not sufficient to spoil the darkness of the night sky.
	is an instance of paradox
old star	has definition Star that, according to contemporary stellar evolution theory, have an age comparable to that of the galaxy to which they belong. This is not an observational definition.
	is a kind of star
old thin disk	has definition The older part of the thin-disk population, ranging in age from about 1 to 10 billion years. The Sun and most other nearby stars belong to the old thin disk. The scale height of the old thin disk is about 1000 light-years.
	has star age 1 to about 10 billion years
	is a part of thin disk
oligocene epoch	has duration 11 million years
	has start time 36 million years ago
	is a kind of tertiary period
omega	has definition Heavy short-lived baryon.
	has definition The ratio of the average density of mass in the universe to the critical mass density, the latter being the density of mass needed to eventually halt the outward expansion of the universe. In an open universe, omega is always less than 1; in a closed universe, it is always greater than 1; in a flat universe it is always exactly equal to 1. Unless omega is exactly equal to 1, it changes in time, constantly decreasing in an open universe and constantly increasing in a closed universe. Omega has been measured to be about 0.1, although such measurements are difficult and uncertain. (See critical mass density; closed universe; flat universe; open universe.)
	is an instance of astronomical constant
	is an instance of hyperon
Omega Centauri	has definition A bright globular cluster.
	is a part of Centaurus
	is an instance of unclassified globular cluster
Omega Nebula	has definition A bright H II region. It is a double radio source.
	has discoverer Philippe Loys de Cheseaux
	has discoverery date 1745
	has image
	has Messier number 17
	has synonym M 17
	has synonym NGC 6618
	has synonym Swan Nebula
	is a part of Sagittarius
	is an instance of gaseous nebula
	is an instance of Messier object
Omicron2 Eridani	has definition A triple star that was the first white dwarf ever discovered.
	has distance 16 light-years
	is a part of Eridanus
	is an instance of white dwarf
Oort cloud	has definition H I region extending to more than 100000 AU from the Sun, barely gravitationally bound, postulated as the birthplace of comets.
	has location < 100000 AU from the Sun
	is a part of our solar system
	is an instance of dust
Oosterhoff Group globular cluster	has definition Globular cluster which differs in the period of transition between Bailey type ab and type c variables, the ratio of type c to type ab stars, in the metallicity of RR Lyrae stars, and in the mean period of the ab variables.
	is a kind of globular cluster
Oosterhoff group I globular cluster	has metal line strength slightly weak
	is a kind of Oosterhoff Group globular cluster
Oosterhoff group II globular cluster	has metal line strength very weak
	is a kind of Oosterhoff Group globular cluster
open cluster	has age varies widely
	has catalog open cluster catalog
	has definition A comparatively loose grouping (mass range 102-103 M)sun of Population I stars, strongly concentrated in the spiral arms or the disk of the Galaxy (in fact, open clusters give a good indication of where the spiral arms are). Unlike associations, open clusters are dynamically stable. Depending on their age, stars in open clusters "peel off" from the main sequence at different points (the higher the turnoff point, the younger the cluster). (Sometimes called Galactic cluster; NGC 188 is the oldest known open cluster.)
	has definition A small, loose cluster of stars that typically contains several hundred members. The best examples are the Hyades and the Pleiades, both in the constellation Taurus. Open clusters line the Galactic plane, in contrast with globular clusters, which are members of the Galaxy's halo or thick disk.
	is a kind of star cluster
	is a part of disk
open cluster catalog	has object type open cluster
	is a kind of catalog about star systems
open string	has definition A type of string with two free ends.
	is a kind of string theory
open universe	has definition A homogeneous, isotropic Universe is said to be temporally open if gravity is not strong enough to eventually reverse the expansion, so the universe goes on expanding forever. It is said to be spatially open if it curves the opposite way from a closed universe, so that triangles would contain less than 180°, the circumference of a circle would be more than π times the diameter, and the volume would be infinite. If Einstein's Cosmological Constant is zero, as is frequently assumed, then a universe which is temporally open is also spatially open, and vice versa.
	has definition Any model of the Universe which does not contain enough matter to halt its expansion.
	has definition Big Bang model that was formulated by Friedmann and Lemaitre which has a negative curvature, like a saddle-shaped surface, in which case the universe is infinite, open, and will expand forever. This space is unbounded.
	has definition Cosmological model in which the universe continues to expand forever; its space-time geometry is hyperbolic, or "open".
	has geometry of space negative curvature
	is a kind of big bang
Ophiuchus	has acronym Oph
	has boundary
	has genitive Ophiuchi
	has historical image
	has synonym Serpent Bearer
	is a part of Zodiac
	is an instance of zodiacal constellation
opposition	has definition A configuration of the Sun, Earth and a planet in which the apparent geocentric longitude of the planet differs by 180 degrees from the apparent geocentric longitude of the Sun.
	has definition Occurs when the Earth comes directly between that planet and the Sun; it can thus only happen in relation to the superior planets and the asteroids.
	has elongation 180°
	is a kind of planetary elongation event
optical	has definition Electromagnetic radiation with wavelengths of or close to those detectable by the eye.
	has frequency 430 to 750 THz
	has synonym light
	has wavelength 400 to 700 nm
	is a kind of photon
optical device	is a kind of instrument
optical fibre	has definition A long, thin strand of glass capable of excellent transmission of light over large distances.
	has definition Glass and transparent plastics can be made into a very thin wire or fiber. Typical dimensions are 10-50 µm. If a ray enters one end of a fiber at the appropriate angle, it will undergo total internal reflection and travel down the fiber without much loss through the sides.
	has wavefront modification shape
	is a kind of wavefront modifier
optical law	has domain optics
	is a kind of law
optical matching	has definition The use of lenses or other optical devices to match the size of the image of the seeing disk, as it appears in the focal plane of the telescope, to the physical size of the CCD pixels. If the telescope yields 10 arcseconds per millimeter and the seeing is 1 arcsecond then the image is 0.1 mm in size. But a typical CCD pixel is 0.022 mm, five times smaller.
	has wavefront modification shape
	is a kind of wavefront modifier
optical process	has domain optics
	is a kind of physical process
optical space telescope	has wavelength sensitivity 300 to 1000 nanometers
	is a kind of optical telescope
	is a kind of space telescope
optical telescope	has optical design
	has reference astroweb
	is a kind of electromagnetic telescope
Optical Very Large Array	has acronym OVLA
	is an instance of array telescope
Optically Violent Variable galaxy	has acronym OVV
	is a kind of blazar
optics	has definition The science of light.
	is a kind of physics
orange	is a kind of optical
orbit	has definition The path in space followed by a celestial body.
	has parameters orbital element
	is a kind of motion
orbital element	has definition One of seven quantities that must be established from observations in order to define the size, shape, and orientation of an orbit in space.
	is a kind of orbital quantity
	is required to define the size, shape, and orientation of an orbit in space
	see also mean element
	see also osculating element
orbital event	has definition Periodic celestial event in the orbit of a celestial body
	has period 1 orbital period
	is a kind of periodic celestial event
orbital quantity	has definition A quantity used for calculating orbits
	is a kind of quantity
orbital velocity	has definition Velocity required by a body to achieve a circular orbit around its primary: Vorb = (GM / r)1/2.
	is a kind of velocity
orbiting collision	has definition A collision in which an ion and an atom approach each other very closely and spend a long time (several orbits of the atomic electrons) in close proximity.
	is a kind of Coulomb collision
ordovician period	has duration 75 million years
	has start time 500 million years ago
	is a kind of paleozoic era
Orion	has acronym Ori
	has boundary
	has genitive Orionis
	has historical image
	has historical origin the hunter accompanied his dogs, Canis Major and Canis Minor hunt Lepus and Taurus near the river Eridanus
	has synonym Hunter
	is a part of celestial sphere
	is an instance of constellation
Orion A	has definition A radio continuum feature (an H II region) centered on the Trapezium, and excited by θ1 Ori C. The Orion A molecular cloud, which lies beyond it, is a rich source of molecules CO, OH, HCN, and probably NO, HCO, and H2CO have been observed.
	is a part of Orion
	is an instance of radio source
Orion arm	has definition The spiral arm of the Milky Way in which the Sun is located. It lies between the Sagittarius arm and the Perseus arm.
	has density of interstellar gas 1.5 atoms cm-3
	has distance from galaxy center 10.4 kpc
	has H I density 0.6 atoms cm-3
	has synonym local arm
	has width 600 pc
	is a part of disk
Orion B	has definition A radio continuum source.
	has synonym NGC 2024
	is a part of Orion
	is an instance of radio source
Orion Nebula	has age 20000 years
	has definition A large cloud of gas and dust giving birth to young stars in the constellation Orion and visible to the naked eye. It is an HII region 1500 light-years away.
	has definition An HII region about 500 pc distant, barely visible to the naked eye in the center of Orion's sword. It is undoubtedly a region where stars are being born; young O stars and many T Tauri variables are associated with it, and its members are extreme Population I. Probably no more than 20000 years old. It is also an X-ray source (3U 0527-05 and M42, NGC 1976).
	has definition The Orion Nebula, a star-forming region in the constellation Orion.
	has distance 500 pc
	has illuminating star(s) trapezium
	has image
	has Messier number 42
	has synonym M 42
	is a part of Orion
	is an instance of gaseous nebula
	is an instance of Messier object
	is an instance of naked eye object
Orion spur	has definition That part of the local spiral arm in which the Sun is embedded. The Sun is on an inner edge of the Orion spur.
	is a part of Orion arm
Orionid	has duration 2 days
	has parent object Comet Halley
	has radiant Orion
	has rate 30 per hour
	has start time 21 October
	is an instance of meteor shower
ortho-hydrogen	has definition Molecular hydrogen in which the two protons of the diatomic molecule have the same direction of spin. It is a higher energy state than the para form. Terrestrial H2 is 75% ortho-hydrogen, 25% para-hydrogen.
	is an instance of molecular hydrogen
ortho-spectrum	has definition Spectrum of triplet (l = 1).
	is a kind of set of lines
Oschin 48-inch Telescope	has altitude 1706 m
	has aperture 1.24 m
	has comment a new achromatic corrector plate (Grubb-Parsons) was installed in about 1984
	has creation date 1948
	has focal ratio f/2.47
	has latitude 33° 21' N
	has location Palomar Mountain, Calif., US
	has longitude 116° 51' W
	has mirror diameter 1.83 m
	has mirror maker Don O. Hendrix
	has mounting manufacturer California Institute of Technology
	has owner Palomar Observatory
	has synonym Oschin Schmidt
	is an instance of Fork equatorial telescope
	is an instance of Schmidt
Oscillating Universe	has definition Cosmological model in which the Universe is "closed" and its expansion is destined to stop, to be succeeded by collapse and "then" (if ordinary temporal terms may be said to apply) by a rebound into a new expansion phase.
	is a kind of closed universe
osculating element	has definition One of several parameters that specifies the instantaneous position and velocity of a celestial body in its perturbed orbit. Osculating elements describe the unperturbed (two-body) orbit that the body would follow if perturbations were to cease instantaneously.
	is a kind of orbital quantity
	see also orbital element
osmium	has image
	is a kind of transition metal
Otto Struve	has birth date 12 August 1897
	has career Yerkes McDonald
	has death date 6 April 1963
	has image
	is an instance of 19th century scientist
	is an instance of astronomer
our solar system	has definition The Sun and all objects gravitationally bound to it. The solar system is roughly a sphere with a radius greater than 100,000 AU, with the Sun at the center. The Sun is overwhelmingly the dominant object. Planets, satellites, and all interplanetary material together comprise only about 1/750 of the total mass. Geochemical dating methods show that the solar system chemically isolated itself from the rest of the Galaxy (4.7 ± 0.1) × 109years ago.
	is a kind of solar system
	is a part of local bubble
outdated belief	is a kind of belief
outer Van Allen belt	has altitude 18000 to 20000 km
	has composition mostly electrons
	has particle energy lower than the inner Van Allen belt
	is a part of Van Allen belt
Owl Nebula	has definition A planetary nebula.
	has distance 600 pc
	has Messier number 97
	has synonym M 97
	has synonym NGC 3587
	is a part of Ursa Major
	is an instance of Messier object
	is an instance of planetary nebula
oxygen	has abundance 474000 p.p.m. in Earth's crust (most abundant element on surface)
	has abundance 6.92 × 108 in Sun relative to H = 1 × 1012
	has abundance constituent element of water
	has atomic emission line 777.194 nm for O I
	has atomic emission line 777.417 nm for O I
	has atomic emission line 844.625 nm for O I
	has atomic emission line 844.636 nm for O I
	has atomic emission line 844.676 nm for O I
	has atomic number 8
	has atomic radii pm
	has biological role constituent element of DNA and of most other biologically important compounds
	has boiling point 90.188 K
	has chief source liquid air
	has covalent radii 66 pm for single bond
	has critical pressure 5043 kPa
	has critical temperature 154.58 K
	has crystal cell dimension a = pm
	has crystal type
	has daily dietary intake mainly as water
	has definition colourless, odourless gas (O2) which is very reactive
	has density 1.429 kg m-3 for gas at 273 K
	has density 1140 kg m-3 for liquid at 90.188 K boiling point
	has density 2000 kg m-3 for solid at 54.8 K melting point
	has discoverer J. Priestley, C.W. Sheele
	has discovery date 1774
	has discovery location Leeds, England and Uppsala, Sweeden
	has electron affinity 141 kJ mol-1 from O to O-
	has electron configuration [He]2s22p4 in ground state
	has electronegativity 3.44 Pauling
	has hazard O2 within a few percent of its natural concentration in air is harmless, but too little and it cannot sustain life, too much and it can cause pulmonary changes and teratogenic effects and is a fire hazard.
	has heat capacity 21.912 J K-1 mol-1 for atomic gas at constant pressure 0.1 MPa at 298.15 K
	has heat capacity 29.355 J K-1 mol-1 for molecular gas (O2 at constant pressure 0.1 MPa at 298.15 K
	has heat of fusion 0.444 kJ mol-1
	has heat of sublimation kJ mol-1
	has heat of vaporization 6.82 kJ mol-1
	has image
	has ionic radii 132 pm for O2-
	has ionic radii 22 pm for O+
	has isotope mass range 13 to 20
	has lethal intake 4800 p.p.m. for 4 hours inhaled ozone (O3)
	has level in humans 160000 p.p.m. in liver
	has level in humans 160000 p.p.m. in muscle
	has level in humans 285000 p.p.m. in bone
	has level in humans constitutent of water
	has longest lived isotope oxygen 16
	has mass absorption coefficient 1.31 cm2 g-1 for MoKα X-ray diffraction
	has mass absorption coefficient 11.5 cm2 g-1 for CuKα X-ray diffraction
	has mass magnetic susceptibility +1.355 × 10-6 kg-1 m3 for gas
	has mass of element in person 43 kg for a 70 kg average person, mainly as water
	has melting point 54.8 K
	has mineral oxides, silicates, carbonates, phosphates, sulfates also occurs as gas in atmosphere and as water
	has molar volume 8.00 cm3 at 54 K
	has name origin oxy genes = acid forming from Greek
	has neutron scattering length 0.5803 × 10-12 cm
	has number of isotopes 8 including nuclear isomers
	has number of protons 8
	has pronunciation oksi-jen
	has registry number 7782-44-7 for Chemical Abstracts System database
	has relative atomic mass 15.9994 in units of 12C = 12.000
	has reserves 1.2 × 1015 tonnes in atmosphere
	has space group
	has specimen small pressurized canisters. Safe, but be aware of possible dangers.
	has symbol O
	has term symbol 3P2 in ground state
has thermal conductivity 0.2674 W m-1 K-1 at 300 K
has thermal neutron capture cross section 0.00019 barns
has toxic intake non-toxic as O2, but toxic as ozone (O3)
has uses steel-making, metal-cutting, the chemical industry and in medical treatment
has van der Waals radii 140 pm
has world production 1 × 108
is a kind of atmophile element
is a kind of gaseous element
is a kind of group VI element
is a kind of nonmetallic element
is a kind of unclassified oceanic element
reacts with all other elements except He, Ne, Ar and Kr
oxygen 14	has atomic mass 14.008595
	has decay mode β+ (5.1430 Mev)
	has decay mode γ
	has half life 70.60 seconds
	has nuclear spin I = 0+
	has number of neutrons 6
	has number of nucleons 14
	has symbol 14O
	is an instance of oxygen
oxygen 15	has atomic mass 15.003065
	has decay mode β+ (2.754 Mev)
	has decay mode no γ
	has half life 122.2 seconds
	has nuclear magnetic moment μ = 0.719
	has nuclear spin I = 1/2-
	has number of neutrons 7
	has number of nucleons 15
	has symbol 15O
	is an instance of oxygen
oxygen 16	has atomic mass 15.99491463
	has natural abundance 99.762%
	has nuclear magnetic moment μ = 0
	has nuclear spin I = 0+
	has number of neutrons 8
	has number of nucleons 16
	has symbol 16O
	is an instance of oxygen
oxygen 17	has atomic mass 16.9991312
	has magnetogyric ratio -3.6264 × 107 rad T-1 s-1 using H2O reference
	has natural abundance 0.038%
	has NMR frequency 13.557 MHz where 1H = 100 MHz; 2.3488 T using H2O reference
	has NMR receptivity 0.061 where 13C = 1.00 with H2O reference
	has nuclear magnetic moment μ = -1.89380
	has nuclear quadrupole moment -0.02558 × 10-28 m2 using H2O reference
	has nuclear spin I = 5/2+
	has number of neutrons 9
	has number of nucleons 17
	has relative NMR sensitivity 0.0291 where 1H = 1.00 using H2O reference
	has symbol 17O
	has uses Nuclear Magnetic Resonance
	is an instance of oxygen
oxygen 18	has atomic mass 17.9991603
	has natural abundance 0.200%
	has nuclear magnetic moment μ = 0
	has nuclear spin I = 0+
	has number of neutrons 10
	has number of nucleons 18
	has symbol 18O
	is an instance of oxygen
oxygen burning	has definition The stage when a star fuses oxygen into silicon and sulfur. It occurs only in stars born with over eight solar masses.
	has duration 6 months
	has optimum density 107 g cm-3
	has optimum temperature 1.5 × 109 K
	has product silicon, sulfur
	has reactant oxygen
	is a kind of exothermic fusion process
	requires minimum mass at star birth 8 solar masses
oxygen-rich giant	has definition A collective designation for a giant showing metal oxide molecules - thus M, MS and S stars.
	is a kind of giant
ozone layer	has altitude 20 to 60 km
	has composition higher than normal concentration of ozone (03)
	has definition A layer in the lower part of Earth's stratosphere where the greatest concentration of ozone (03) appears. This is the layer responsible for the absorption of ultraviolet radiation.
	is a part of stratosphere
P Cygni	has distance 1200 pc
	has magnitude 5
	has spectral type B1e
	is a part of Cygnus
	is an instance of P Cygni star
P Cygni star	has definition A type of star whose spectrum shows strong emission lines, like those of the Be and Wolf-Rayet stars, with blueshifted absorption components which are presumed to come from an expanding shell of low-density matter. A P Cygni profile is taken as an indication of mass loss.
	has definition High-luminosity, early-type star, in which all lines have a P Cyg type profile (an emission component on the red side of the absorption line).
	has prototype P Cygni
	is a kind of emission line star
P-branch	has definition A set of lines in the spectra of molecules corresponding to unit increases in rotational energy.
	is a kind of set of molecular lines
p-electron	has definition An orbital electron whose l quantum number is 1.
	has orbital quantum number 1
	is a kind of bound electron
p-process	has definition The name of the hypothetical nucleosynthetic process thought to be responsible for the synthesis of the rare heavy proton-rich nuclei which are bypassed by the r-process and s-processes. It is manifestly less efficient (and therefore rarer) than the s- or r-process, since protons must overcome the Coulomb barrier, and may in fact work as a secondary process on the r-process and s-process nuclei. It seems to involve primarily (p, γ) reactions below cerium (where neutron separation energies are high) and the (γ, n) reactions above cerium (where neutron separation energies are low). The p-process is assumed to occur in supernova envelopes at a temperature greater than about 109 K and at densities less than about 104 g cm-3.
	is a kind of nucleosynthetic reaction
p-spot	is a kind of sunspot
P-strong star	has definition A small subgroup of B-type stars in which P lines are very strong.
	is a kind of B star
Pa alpha	has upper energy level 4
	has wavelength 18751 Å
	is an instance of Paschen line
Pacific ocean	is a part of Earth's lithosphere
PAH	has definition Polycyclic Aromatic Hydrocarbon
	is a kind of molecule
pairing energy	has definition A quantity which expresses the fact that nuclei with odd numbers of neutrons and protons have less energy and are less stable than those with even numbers of neutrons and protons.
	has symbol δ
	is a kind of energy
paleocene epoch	has duration 63 million years
	has start time 5 million years ago
	is a kind of tertiary period
paleozoic era	has duration 370 million years
	has start time 600 million years ago
	is a kind of geological era
palladium	has image
	has ocean oxidation state II
	has ocean residence time 50000 years
	is a kind of recycled oceanic element
	is a kind of transition metal
Pallas	has albedo 0.05
	has definition The second asteroid to be discovered.
	has diameter about 560 km -has source: [H76]
	has discoverer Olbers
	has discovery date 1802
	has eccentricity e = 0.235
	has has surface spectrum resembles meteorites of either low-grade carbonaceous chondrite or enstatite achondrite
	has inclination i = 34°.8
	has mass 2.6 × 1023 g (1972 estimate)
	has orbital period 1686 days
	has rotation period 9-12 hours
	has semi-major axis a = 2.77 AU
	is a part of asteroid belt
	is an instance of asteroid
Palomar	has definition The mountain in California upon which sits what was the largest telescope in the United States, 200 inches in diameter. The telescope itself is sometimes referred to as the Mt. Palomar telescope.
	is an instance of observatory
Palomar Sky Survey	has acronym PSS
	is a kind of sky survey
Pan	has definition Unofficial name for Jupiter XI.
	has dicoverer Nicholson
	has discovery date 1938
	has eccentricity e = 0.2
	has inclination i = 163°
	has orbital period P = 692d
	has synonym Jupiter XI
	is a part of Jupiter
	is an instance of natural satellite
pancake model	has definition A model of galaxy formation in which the first structures to condense out of the smooth background of primordial gas were very large in size. These large masses then collapsed into thin sheets (pancakes) and fragmented into many smaller pieces the size of galaxies. A competing theory, sometimes called the hierarchical clustering model, proposes that the first structures to form were the size of galaxies. As galaxies clustered together, due to gravity, larger and larger structures were formed. (See hierarchical clustering model.)
	is a kind of contraction
PAPA	has definition Precision Analogue Photon Address.
	is a kind of imager
para-hydrogen	has definition Molecular hydrogen in which the two protons of the diatomic molecule have opposite directions of spin. It is a lower energy state than ortho-hydrogen.
	is an instance of molecular hydrogen
parabola	is a kind of conic section
paradox	has definition A self-contradictory proposition. Paradoxes are most useful when they seem most likely to be true, for it is then that they best serve to expose flaws in the data or reasoning that led to their appearance.
	is a kind of problem
parallax	has definition Angle subtended by the apparent difference in a star's position when viewed from the Earth either simultaneously from opposite sides of the planet, or half such an angle, measured after a gap of six months from opposite sides of the planet's orbit; the nearer the celestial body, the greater the parallax.
	has definition The apparent displacement in the position of a star or planet occasioned by its being viewed from two different locations - e.g., by observing it from two widely separated stations on Earth, or at intervals of six months, when the earth is at either extreme of its orbit around the sun. The resulting angle can be used, by triangulation, to determine the distance of the star or planet.
	has definition The difference in apparent direction of an object as seen from two different locations; conversely, the angle at the object that is subtended by the line joining two designated points. Geocentric (diurnal) parallax is the difference in direction between a topocentric observation and a hypothetical geocentric observation. Heliocentric or annual parallax is the difference between hypothetical geocentric and heliocentric observations; it is the angle subtended at the observed object by the semi-major axis of the Earth's orbit. First trigonometric parallax was obtained in 1838. (eee also horizontal parallax.)
	has definition The tiny shift in a star's apparent position that occurs when the star is viewed from slightly different perspectives as the Earth revolves around the Sun. The larger a star's parallax, the closer the star is to Earth.
	has synonym distance
	is a kind of angle
parity	has definition The operation which reverses the signs of the coordinate axes used to describe a system, i.e. (x, y, z) -> (-x, -y, -z).
	has definition The principle of space-inversion invariance; i.e., no experiment can differentiate between the behavior of a system and that of its mirror image. Parity is conserved in strong interactions, but not in weak ones.
	is a kind of symmetry
parsec	has approval agency International Astronomical Union
	has approval date 1922
	has definition A unit used by astronomers to describe stellar distance. It is the distance from which the radius of the earth's orbit would subtend an angle of one second of arc. Alternatively one parsec is the distance at which one astronomical unit subtends one second of arc. The name was proposed by Professor H. H. Turner (1861-1930), Savilian Professor of Astronomy at Oxford.
	has definition Astronomical unit of distance, equal to 3.26 light-years.
	has definition The distance at which one astronomical unit subtends an angle of 1 second of arc. 1 pc = 206,265 AU = 3.086 × 1013 km = 3.26 light-years.
	has definition The distance at which one astronomical unit subtends an angle of one second of arc; equivalently, the distance to an object having an annual parallax of one second of arc. (abbreviation for parallax second)
	has equivalent 2.062648 × 105 astronomical units
	has equivalent 3.08572 × 1016 m
	has equivalent 3.26168 light years
	has name origin parallax second
	has proposal date 1840
	has proposer Bessel and Meadows
	has symbol pc
	is an instance of length unit
partial lunar eclipse	has definition An eclipse in which the Moon passes through the shadow cast by the Earth, the Moon passing partially through the Earth's umbra at maximum eclipse.
	is an instance of lunar eclipse
particle	has charge
	has definition Fundamental unit of matter and energy. All may be classed as fermions, which have half-integral spin and obey the exclusion principle, and bosons, which have integral spin and do not obey the exclusion principle. The term particle is metaphoric, in that all subatomic particles also evince aspects of wave-like behavior.
	has frequency inversely proportional to the wavelength
	has mass
	has wavelength inversely proportional to its momentum
	is a kind of natural object
	obeys uncertainty principle
particle accelerator	has definition A device using electric and magnetic fields to accelerate beams of particles-usually electrons, positrons, protons, or antiprotons-to high energies for experimental purposes. Modern accelerators are often very large: the main ring at Fermilab, for example, is 4 miles in circumference.
	has definition A machine for speeding subatomic particles to high velocity, then colliding them with a stationary target or with another beam of particles moving in the opposite direction. (In the latter instance, the machine may be called a collider.) At velocities approaching that of light the mass of the particles increases dramatically, adding greatly to the energy released on impact. The resulting explosion promotes the production of exotic particles, which are analyzed according to their behavior as they fly away through a particle detector.
	is a kind of instrument
particle Compton wavelength	has definition The wavelength of a photon containing the rest energy of a particular particle.
	has equation h bar / mc, where h bar is Plancks constant, c is the speed of light, and m is the particle mass.
	has unit m
	is a kind of length
	is a kind of particle constant
particle constant	applies to particle
	has definition a fundamental physical constant quantifying a particle property
	is a kind of fundamental physical constant
particle g factor	is a kind of particle constant
particle gyromagnetic ratio	has unit s-1 T-1
	is a kind of particle constant
particle magnetic moment	has unit J T-1
	is a kind of particle constant
particle magnetic moment anomaly	is a kind of particle constant
particle mass	has unit kg
	is a kind of mass
	is a kind of particle constant
particle physics	has definition That branch of physics that attempts to understand the fundamental particles and forces of nature.
	has definition The branch of science that deals with the smallest known structures of matter and energy. As their experimental investigation usually involves the application of considerable energy, particle physics overlaps with high-energy physics.
	has synonym high-energy physics
	is a kind of high energy physics
parton	has definition A generic term used to describe any particle which may be present inside nucleons. It includes quarks, antiquarks and gluons.
	has definition A hypothetical pointlike constituent of a nucleon, which contains all the charge of the nucleon.
	is a kind of elementary particle
	is a kind of hypothetical particle
pascal	has base unit m-1·kg·s-2
	has definition The derived SI unit of pressure.
	has equivalent 1 N m-2
	has equivalent 10-5 bars
	has symbol Pa
	has unit N·m-2
	is an instance of named derived SI unit
	is an instance of pressure unit
	is an instance of SI unit
Paschen line	has lower energy level 3
	has series limit 8204 Å
	is a kind of hydrogen line
	is a kind of spectral series line
Paschen-Back effect	has definition An effect on spectral lines obtained when the light source is located in a strong magnetic field, so that the magnetic splitting becomes greater than the multiplet splitting.
	is a kind of line broadening
Patroclus	has asteroid number 617
	has definition Trojan asteroid (60°) behind Jupiter.
	has eccentricity e = 0.14
	has inclination i = 22°.1
	has orbital period P = 11.82 yr
	has semi-major axis a = 5.19 AU
	is a part of Trojan asteroid
	is an instance of Trojan asteroid
Pavo	has acronym Pav
	has genitive Pavonis
	has historical image
	has historical origin the mythological peacock sacred to Hera
	has synonym Peacock
	is a part of celestial sphere
	is an instance of constellation named by Bayer
PDA	has definition Photodiode array.
	is a kind of detector
peculiar star	has definition Star with spectra that cannot be conveniently fitted into any of the standard spectral classifications. Denoted by a p after spectral type.
	is a kind of early star
peculiar velocity	has definition Velocity with respect to the Local Standard of Rest.
	is a kind of velocity
Pegasus	has acronym Peg
	has boundary
	has genitive Pegasi
	has historical origin son of Medusa, flew Bellerophon up to Mount Olympus, this impudence angered Zeus, who sent an insect to sting Pegasus
	has synonym Winged Horse
	is a part of celestial sphere
	is an instance of constellation
pennsylvanian period	has duration 30 million years
	has start time 310 million years ago
	is a kind of paleozoic era
penumbral lunar eclipse	has definition An eclipse in which the Moon passes through the shadow cast by the Earth, the Moon passing only through the Earth's penumbra.
	has synonym appulse
	is an instance of lunar eclipse
pep reaction	has definition A reaction occurring in the proton-proton chain which occurs only once in 400 p-p reactions but produces far more energetic neutrinos (1.44 MeV as against 0.42 MeV).
	has reaction probability once in 400 p-p reactions
	is an instance of proton-proton chain
periapsis	has definition The point in the orbit of a satellite where it is closest to its primary.
	is an instance of orbital event
periastron	has antonym apastron
	has definition The point in the orbit of one component of a binary system where it is nearest the other component.
	has participants binary star
	is an instance of binary star orbital event
pericenter	has antonym apocenter
	has definition The point in the orbit of one component of a binary system which is closest to the center of mass of the system.
	has participants binary star
	is an instance of binary star orbital event
pericynthion	has definition The point in the orbit of a satellite around the Moon closest to the Moon.
	is an instance of Moon orbital event
perigalacticon	has antonym apogalacticon
	has definition The point in a star's orbit around the Galaxy when the star lies closest to the Galactic center. The Sun is near perigalacticon now.
	is an instance of galaxy orbital event
perigee	has antonym apogee
	has definition The point at which a body in orbit around the Earth most closely approaches the Earth. Perigee is sometimes used with reference to the apparent orbit of the Sun around the Earth.
	is an instance of Earth orbital event
perihelion	has antonym aphelion
	has definition The point in the orbit of an object orbiting the Sun where it is closest to the Sun's center of mass. Earth's perihelion occurs early in January.
	is an instance of Sun orbital event
period	has definition An orbital element representing the time required to complete an orbit. This parameter is required when determining the orbit of a binary star system in which the mass is not known.
	has symbol P
	is an instance of orbital element
	is an instance of time
periodic celestial event	has definition Celestial event which occurs with a very regular period
	has period
	is a kind of celestial event
periodic variable	has period
	is a kind of variable
permian period	has duration 50 million years
	has start time 280 million years ago
	is a kind of paleozoic era
Perseid	has duration 3 days
	has parent object Comet Swift-Tuttle
	has radiant Perseus
	has rate 100 per hour
	has start time 12 August
	is an instance of meteor shower
Perseus	has acronym Per
	has boundary
	has genitive Persei
	has historical image
	has historical origin slew Medusa, mother of Pegasus, rescued Andromeda, daughter of Cepheus and Cassiopeia, from the sea monster Cetus
	has synonym Hero
	is a part of celestial sphere
	is an instance of constellation
Perseus arm	has definition The spiral arm that lies next out from the arm containing the Sun.
	is a part of disk
Perseus cluster	has brightest member NGC 1275
	has definition A diffuse, irregular cluster (richness class 2) dominated by and centered on the Seyfert galaxy NGC 1275 (Perseus A). Mass required to bind the cluster, greater than 1015 Msun.
	has mass 2 × 1015 Msun
	has number of galaxies 500
	has redshift z = 0.0183
	is a part of Local Supercluster
	is a part of Perseus
	is an instance of galaxy cluster
Perseus OB2	has definition A young association of OB stars.
	has distance 350 pc
	is an instance of OB association
Perseus-Pisces region	has definition A region of space containing a huge congregation of galaxies called a supercluster. The galaxies in this supercluster appear to be distributed in a long chain.
	is a part of Universe
	is an instance of supercluster
person	has birth date
	has definition a human living or dead
	is a kind of physical object
peta	has symbol P
	has value 1015
	is an instance of unit prefix
Pfund line	has lower energy level 5
	has wavelength far-infrared
	is a kind of hydrogen line
	is a kind of spectral series line
PG 1159 star	has definition Very hot star with strong O VI and C IV lines, which is an X-ray emitter. Probably these stars are the central stars of planetary nebulae that have dissipated their envelopes.
	has synonym pre-degenerate
	is a kind of central star of planetary nebula
phase angle	has definition The angle measured at the center of an illuminated body between the light source and the observer.
	is a kind of angle
phase of matter	has definition When used in reference to matter, describes its possible states: solid phase, liquid phase, gas phase. More generally, refers to the possible descriptions of a physical system as features on which it depends (temperature, string coupling constant values, form of spacetime, etc.) are varied.
	is a kind of collection of particles
phase transition	has definition An abrupt change in the equilibrium state of a system. A sudden transition between one state of matter or energy and another state. For example, when hot water turns to steam or when ice crystallizes out of a liquid that has been cooled to below freezing, a phase transition has occurred. According to the grand unified theories of particle physics, the infant universe may have undergone one or more overall phase transitions. In this case, the energy uniformly filling all space corresponded to the supercooled liquid.
	has inverse process
	is a kind of physical process
Phillips band	has wavelength 1.207 μ (transition 0-0)
	is a kind of C2 band
Phobos	has albedo 0.06
	has definition The potato-shaped inner satellite of Mars. Phobos lies just outside the Martian Roche limit.
	has discoverer A. Hall
	has discovery date 1877
	has eccentricity e = 0.021
	has inclination i = 1°.1
	has orbital period 7h39m14s
	has radius about 18 × 22 km
	has rotation period 7h39m14s
	has surface composition dust (from Infrared observations)
	is a part of Mars
	is an instance of natural satellite
Phoebe	has definition The outermost satellite of Saturn. Has retrograde orbit.
	has discoverer Pickering
	has discovery date 1898
	has orbital period 550 days
	has radius 100 km
	is a part of Saturn
	is an instance of natural satellite
Phoenix	has acronym Phe
	has boundary
	has genitive Phoenicis
	has historical image
	has historical origin a bird of incredible beauty who would live for 500 years and be reborn after being consumed fire
	has synonym Phoenix (Bird)
	is a part of celestial sphere
	is an instance of constellation named by Bayer
phosphorus	has abundance 0.0015 p.p.m. in Atlantic surface seawater
	has abundance 0.0015 p.p.m. in Pacific surface seawater
	has abundance 0.042 p.p.m. in deep Atlantic seawater
	has abundance 0.084 p.p.m. in deep Pacific seawater
	has abundance 1000 p.p.m. in Earth's crust
	has abundance 3.16 × 105 in Sun relative to H = 1 × 1012
	has atomic emission line 1648.292 nm for P I
	has atomic emission line 213.618 nm for P I (used in atom absorption spectrometry)
	has atomic emission line 979.685 nm for P I
	has atomic emission line 952.573 nm for P I (strong)
	has atomic emission line 956.344 nm for P I (strong)
	has atomic number 15
	has atomic radii 115 pm for red form
	has atomic radii 93 pm for white form
	has biological role constituent of DNA, ATP and many other biochemical molecules. Phosphate cycle.
	has boiling point 553 K for P4
	has chief source apatite, turquoise (ornamental stone)
	has covalent radii 110 pm for single bond
	has critical temperature 994 K
	has crystal cell dimension a = 1131 pm for red phosphorus
	has crystal cell dimension a = 1851 pm for white &alpha-P4
	has crystal type cubic for red phosphorus
	has crystal type cubic for white &alpha-P4
	has daily dietary intake 900 - 1900 mg
	has definition soft and flammable white solid, the red form is usually non-flammable
	has density 1820 kg m-3 for P4 solid at 293 K
	has density 2200 kg m-3 for red phosphorus solid at 293 K
	has density 2690 kg m-3 for black phosphorus solid at 293 K
	has discoverer Hennig Brandt
	has discovery date 1669
	has discovery location Hamburg, Germany
	has electrical resistivity 1 × 109 Ω m for P4 solid at 293 K
	has electron affinity 44 kJ mol-1 from to -
	has electron configuration [Ne]3s23p3 in ground state
	has electronegativity 2.19 Pauling
	has hazard white phosphorus chronic poisoning leads to necrosis of the jaw (phossy-jaw)
	has hazard white phosphorus is much more toxic than red phosphorus
	has heat capacity 20.786 J K-1 mol-1 for gas at constant pressure 0.1 MPa at 298.15 K
	has heat capacity 21.21 J K-1 mol-1 for red phosphorus solid at constant pressure 0.1 MPa at 298.15 K
	has heat capacity 23.840 J K-1 mol-1 for P4 solid at constant pressure 0.1 MPa at 298.15 K
	has heat of fusion 2.51 kJ mol-1 for P4 solid
	has heat of vaporization 51.9 kJ mol-1 for P4 solid
	has image
	has ionic radii 212 pm for P3-
	has isotope mass range 26 to 36
	has lethal intake 100 mg for white phosphorus in humans
	has level in humans 3 - 8.5 p.p.m. in liver
	has level in humans 3000 - 8500 p.p.m. in muscle
	has level in humans 345 mg dm-3 in blood
	has level in humans 67000 - 71000 p.p.m. in bone
	has linear expansion coefficient 124.5 × 10-6 K-1 for P4 solid
	has longest lived isotope phosphorus 31
	has main mining area Russia, USA, Morocco, Tunisia, Togo, Nauru
	has mass absorption coefficient 7.89 cm2 g-1 for MoKalpha X-ray diffraction
	has mass absorption coefficient 74.1 cm2 g-1 for CuKalpha X-ray diffraction
	has mass magnetic susceptibility -1.1 × 10-8 kg-1 m3 for P4 solid
	has mass magnetic susceptibility -8.4 × 10-9 kg-1 m3 for red phosphorus solid
	has mass of element in person 780 g for a 70 kg average person
	has melting point 317.3 K for P4 solid
	has melting point 683 K for red phosphorus solid under pressure
	has mineral apatite, phosphophyllite, turquoise, vivianite
	has molar volume 17.02 cm3
	has name origin phosphoros = bringer of light from Greek
	has neutron scattering length 0.513 × 10-12 cm
has number of isotopes 10 including nuclear isomers
has number of protons 15
has ocean oxidation state V
has ocean residence time 100000 years
has pronunciation fos-for-us
has registry number 7723-14-0 for Chemical Abstracts System database
has relative atomic mass 30.973762 in units of 12C = 12.000
has reserves 5.7 × 109 tonnes
has space group I-43m for white &alpha-P4
has space group Pm3m or P-43 for red phosphorus
has specimen white sticks (Danger!), red lumps or powder (Care!)
has symbol P
has term symbol 4S3/2 in ground state
has thermal conductivity 0.235 W m-1 K-1 for P4 solid at 300 K
has thermal conductivity 12.1 W m-1 K-1 for black phosphorus solid at 300 K
has thermal neutron capture cross section 0.172 barns
has toxic intake 11 μg kg-1 for white phosphorus in rat
has uses fertilizers, insecticides, metal treatment, detergents and foods
has van der Waals radii 190 pm
has world production 153 × 106 tonnes year-1
is a kind of group V element
is a kind of nonmetallic element
is a kind of recycled oceanic element
is a kind of siderophile element
reacts with air by bursting into flames (white phosphorus)
reacts with alkalis to form phosphine gas
phosphorus 30	has atomic mass 29.978307
	has decay mode β+ (4.226 Mev)
	has decay mode no γ
	has half life 2.50 minutes
	has nuclear spin I = 1+ h/2π
	has number of neutrons 15
	has number of nucleons 30
	has symbol 30P
	is an instance of phosphorus
phosphorus 31	has atomic mass 30.973762
	has magnetogyric ratio 10.8289 × 107 rad T-1 s-1
	has natural abundance 100%
	has NMR frequency 40.481 MHz where 1H = 100 MHz; 2.3488 T
	has NMR receptivity 377 where 13C = 1.00
	has nuclear magnetic moment μ = +1.13160 nuclear magnetons with diamagnetic correction
	has nuclear spin I = 1/2+ h/2π
	has number of neutrons 16
	has number of nucleons 31
	has relative NMR sensitivity 0.0663 where 1H = 1.00 for 85% H3PO4
	has symbol 31P
	has uses Nuclear Magnetic Resonance
	is an instance of phosphorus
phosphorus 32	has atomic mass 31.973907
	has decay mode β- (1.710 Mev)
	has decay mode no γ
	has half life 14.28 days
	has nuclear magnetic moment μ = -0.2524 nuclear magnetons with diamagnetic correction
	has nuclear spin I = 1+ h/2π
	has number of neutrons 17
	has number of nucleons 32
	has symbol 32P
	has uses medical diagnostic
	has uses medical therapy
	has uses research
	is an instance of phosphorus
phosphorus 33	has atomic mass 32.971725
	has decay mode β- (0.249 Mev)
	has decay mode no γ
	has half life 25.3 days
	has nuclear spin I = 1/2+ h/2π
	has number of neutrons 18
	has number of nucleons 33
	has symbol 33P
	has uses research
	is an instance of phosphorus
photo reconnaissance spacecraft	is a kind of reconnaissance spacecraft
photocathode	has definition A thin metallic plate housed inside an evacuated tube capable of releasing electrons through the "photoelectric effect" when illuminated by light. These surfaces are best for optical and ultraviolet light.
	is a kind of photoelectric device
photodiode	has definition A light-sensitive device made from the junction of two differently doped species of a semiconductor such as silicon. Also known as a pn junction. An internal electric field is generated at the junction of p and n type material. Photons absorbed in the junction create electron-hole pairs which are separated by the field and create a current.
	is a kind of detector
photoelectric device	has definition Any detector which uses the photoelectric effect to convert photons to electrons.
	is a kind of detector
photoelectric magnitude	has definition The magnitude of an object as measured with a photoelectric photometer. (mpe) (antiquated term)
	is a kind of apparent magnitude
photographic magnitude	has definition The magnitude of an object as measured on the traditional photographic emulsions, which are sensitive to a slightly bluer region of the spectrum than is the human eye. (mph)(antiquated term)
	is a kind of apparent magnitude
photoionization	has definition The ionization of an atom or molecule by the absorption of a high-energy photon by the particle. It is an important source of opacity in stars.
	is a kind of ionization
photomultiplier	has definition A vacuum encapsulated photocathode from which electrons are ejected by the photoelectric effect followed by multiple cathodes from which many additional electrons are emitted in a cascade. When finally collected, the original single electron may have generated a pulse of over one million electrons.
	has definition Device used in photometry for the amplification of light by the release and acceleration of electrons from a sensitive surface. The result is a measurable electric current that is proportional to the intensity of received radiation.
	is a kind of photoelectric device
photon	carries the force electromagnetism
	has definition A transverse wave of electric and magnetic fields which can propagate through empty space.
	has definition Smallest packet of the electromagnetic force field; messenger particle of the electromagnetic force; smallest bundle of light.
	has definition The quantum of the electromagnetic field. It is the massless spin-1 gauge boson of QED. Virtual photons mediate the electromagnetic force between charged particles. Virtual photons can also adopt a mass for a short period, in accordance with Heisenberg's uncertainty principle.
	has spin 1
	has synonym electromagnetic radiation
	is a kind of carrier boson
	is a kind of massless particle
	is a kind of neutral particle
	is a kind of transverse wave
photonics	has definition The technology of generating and harnessing light and other forms of radiant energy whose quantum unit is the photon for a range of applications ranging from detection to laser energy production to communications and information processing.
	is a kind of science
photosphere	has definition The region of a star which gives rise to the continuum radiation emitted by the star. The visible surface of the Sun (temperature about 6000K), just below the chromosphere and just above the convective zone. The photosphere ends (and the chromosphere begins) at about the place where the density of negative hydrogen ions has dropped to too low a value to result in appreciable opacity. The spectrum of the photosphere consists of absorption lines (unlike that of the chromosphere, which consists of emission lines).
	has definition The visible surface of the Sun, or more generally, the layer of a star that gives rise to the continuum (as opposed to spectral-line) radiation emitted by the star. photosphere of the Sun has definition
	is a part of solar atmosphere
photosynthesis	has definition a biochemical process operating in green plants in which carbohydrates are formed under the influence of light with chlorophyl serving as a catalyst
	is a kind of biochemical process
photovisual magnitude	has definition The magnitude of an object as measured photographically by filters and emulsions that are sensitive to the same region of the spectrum as the human eye. (antiquated)
	is a kind of apparent magnitude
physical object	has angular momentum
	has definition Anything tangible having existence (living or nonliving)
	has extent
	has location or center of gravity
	has mass
	has material
	has momentum
	has temperature
	has velocity
	has volume
	is a kind of kbTop
physical phenomena	is a kind of kbTop
physical process	has domain physics
	is a kind of process
physicist	has definition a scientist specializing in physics
	has domain physics
	is a kind of scientist
physico chemical constant	is a kind of fundamental physical constant
physics	has definition The scientific study of the interactions of matter and energy.
	is a kind of science
physics theory	is a kind of theory
Pi alpha	has wavelength 10124 Å
	is an instance of Pickering line
Pi beta	has wavelength 6560 Å
	is an instance of Pickering line
pi+	has charge 1
	has decay product muon, neutrino
	has mass 273 me
	is an instance of charged particle
	is an instance of pion
pi-	has charge -1
	has decay product muon, neutrino
	has mass 273 me
	is an instance of charged particle
	is an instance of pion
pi0	has decay product two γ-rays
	has mass 264 me
	is an instance of neutral particle
	is an instance of pion
Pickering	has career Havard College Observatory
	has greatest achievement the Henry Draper Catalog of spectral types
	has image
	has name Edward C. Pickering
	is an instance of 19th century scientist
	is an instance of astronomer
Pickering line	has lower energy level 4
	has series limit 3644 Å
	has synonym ionized helium Brackett line
	is a kind of ionized helium line
	is a kind of spectral series line
	occurs in O star
pico	has symbol p
	has value 10-12
	is an instance of unit prefix
Pictor	has acronym Pic
	has genitive Pictoris
	has historical image
	has synonym Easel
	is a part of celestial sphere
	is an instance of constellation named by de Lacaille
pinch machine	has definition A fusion device containing a plasma heated by a shock wave generated within the plasma as it is constricted by the rapidly increasing magnetic field.
	is a kind of fusion device
Pinwheel galaxy	has definition A Sc II-III spiral galaxy, a satellite of the Andromeda galaxy, about 700 kpc distant. Mv = - 18.9.
	has definition A spiral galaxy that lies 2.6 million light-years away and is the third largest member of the Local Group, after Andromeda galaxy and the Milky Way.
	has definition The third largest member of the Local Group, after Andromeda galaxy and the Milky Way. It is a spiral galaxy that lies 2.6 million light-years away.
	has discoverer Hodierna
	has discovery date 1654
	has distance 800 kpc from Milky Way galaxy
	has image
	has Messier number 33
	has synonym M 33
	has synonym NGC 598
	has synonym Triangulum galaxy
	is a part of Local Group
	is a part of Triangulum
	is an instance of Messier object
	is an instance of Sc spiral
pion	has definition An unstable nuclear particle of mass intermediate between that of a proton and an electron. The pions are believed to be the particles exchanged by nucleons, resulting in the strong nuclear force; they play a role in the strong interactions analogous to that of the photons in electromagnetic interactions.
	has mass 270 me
	has synonym π-meson
	is a kind of charge multiplet
	is a kind of meson
Pisces	has acronym Psc
	has boundary
	has genitive Piscium
	has synonym Fishes
	is a part of Zodiac
	is an instance of zodiacal constellation
Pisces Australis	has acronym PsA
	has historical image
	has synonym Southern Fish
	is a part of celestial sphere
	is an instance of constellation
pitch angle	has definition Angle specifying the direction of electron velocity; or the angle between a tangent to a spiral arm and the perpendicular to the direction of the galactic center.
	is a kind of angle
plage	has definition The bright rim of a sunspot, observed in emission in monochromatic light of some spectral line (H
	has synonym flocculus
	is a part of sunspot
Planck constant	has definition A universal constant of nature that measures the magnitude of quantum mechanical effects.
	has definition Planck's constant is a fundamental parameter in quantum mechanics. It determines the size of the discrete units of energy, mass, spin, etc. into which the microscopic world is partitioned.
	has definition The universal constant of proportionality relating the frequency of a photon to its quantum of energy.
	has symbol h
	has uncertainty 0.00000052 × 10-34 J s
	has value 6.62606876 × 10-34 J s
	is an instance of universal constant
Planck energy	has definition About 1000 kilowatt hours. The energy necessary to probe to distances as small as the Planck length. The typical energy of a vibrating string in string theory.
	has definition An energy of 1.22 × 1019 GeV (billion electron volts), at which the strength of the gravitational interactions of fundamental particles becomes comparable to that of the other interactions. It is believed that the quantum effects of gravity become important at approximately this energy.
	is a kind of energy
Planck era	has definition The Big Bang era, prior to which Einstein's theory of gravitation breaks down and a quantized theory of gravity is needed. Density was so high that gravitational force acted as strongly as the other fundamental forces on the sub-atomic scale.
	has duration 10-43 s or the time at which the size of the universe was roughly the Planck length (the time it takes light to travel the Planck length; G h bar / c5)1/2 where G is Newton's constant of gravitation, h bar is Planck's constant, and c is the speed of light
	has start time 0 s after Big Bang
	is a kind of Big Bang era
	is followed by hadron era, inflation era
	is preceded by Big Bang
Planck length	has definition The dimension at which space is predicted to become "foamlike" and at which Einstein's theory is supposed to break down.
	has definition The scale below which quantum fluctuations in the fabric of spacetime would become enormous. The size of a typical string in string theory.
	has definition The size limit at which normal notions of space-time are supposed to break down.
	has equation
	has symbol lP
	has uncertainty 0.0012 × 10-35 m
	has value 1.6160 × 10-35 m
	is an instance of length
	is an instance of universal constant
Planck mass	has definition About ten billion billion times the mass of a proton; about one-hundredth of a thousandth of a gram; about the mass of a small grain of dust. The typical mass equivalent of a vibrating string in string theory.
	has equation
	has symbol mP
	has uncertainty 0.0016 × 10-8 kg
	has value 2.1767 × 10-8 kg
	is an instance of mass
	is an instance of universal constant
Planck time	has equation
	has symbol tP
	has uncertainty 0.0040 × 10-44 s
	has value 5.3906 × 10-44 s
	is an instance of time
	is an instance of universal constant
plane	has dimensions 2
	is a kind of geometrical object
planet	has composition various elements
	has definition An object that formed in the disk surrounding a star. Unlike stars, planets do not produce light of their own but merely reflect that of the star(s) they orbit. Planets can have natural satellites.
	has mass greater than Pluto's mass and less massive than ten times Jupiter's mass
	has orbit
	is a kind of celestial body
	is a part of solar system
planetary element	has definition an element which occurs in certain parts of planets with varying concentration
	has occurrence in certain parts of planets
	is a kind of element
planetary elongation	has definition The angle planet-Earth-Sun. Eastern elongations appear east of the Sun in the evening; western elongations, west of the Sun in the morning. An elongation of 0° is called conjunction; one of 180° is called opposition: and one of 90° is called quadrature.
	has definition The geocentric angle between a planet and the Sun, measured in the plane of the planet, Earth and Sun. Planetary elongations are measured from 0° to 180°, east or west of the Sun.
	is a kind of elongation
planetary elongation event	has definition A point in the orbit of a celestial body with a specific planetary elongation such as 0°, 90° or 180°
	is a kind of celestial event
	is a kind of planetary elongation
planetary nebula	has catalog planetary nebula catalog
	has definition A bubble of gas surrounding a hot, dying star. The star is so hot that it makes the planetary nebula glow, which allows astronomers to see it. The star was once the core of a red giant, which ejected its outer atmosphere and created the planetary. A planetary nebula has nothing to do with a planet, but through a small telescope, it looks like a planet's disk, hence the misleading name.
	has definition An expanding envelope of rarefied ionized gas surrounding a hot white dwarf. The envelope receives ultraviolet radiation from the central star and reemits it as visible light by the process of fluorescence. The planetary nebula stage lasts for less than 50,000 years. During the core contraction that terminates the red-giant stage, the helium-burning shell is ejected at a velocity so high that it becomes separated from the core. Under current theories, a star with a carbon core and a mass greater than 0.6 Msun (but less than 4 Msun) will become a planetary nebula and leave behind a white dwarf. Planetary nebulae are now known to occur in stars less than 4 Msun whose envelope becomes unstable during the hydrogen shell burning stage.
	has expansion velocity typically 10 km/s
	is a kind of expanding emission nebula
planetary nebula catalog	has object type planetary nebula
	is a kind of nebula catalog
planetary precession	has definition The component of general precession caused by the gravitational coupling between the center of mass of the Earth and that of the other planets. The effect of planetary precession is to move the equinox eastward by ≈ 0".11 / year and to diminish the angle between the ecliptic and the equator by about 0".47 / year.
	has rate 0".11 per year
	is an instance of precession
planetary theory	has domain planet
	is a kind of astronomy theory
planetocentric coordinate	has definition Coordinates for general use, where the z-axis is the mean axis of rotation; the x-axis is the intersection of the planetary equator (normal to the z-axis through the center of mass) and an arbitrary prime meridian; and the y-axis completes a right-hand coordinate system. Longitude (see longitude, celestial) of a point is measured positive to the prime meridian as defined by rotational elements. Latitude (see latitude, celestial) of a point is the angle between the planetary equator and a line to the center of mass. The radius is measured from the center of mass to the surface point.
	is a kind of coordinate
planetographic coordinate	has definition Coordinates for cartographic purposes dependent on an equipotential surface as a reference surface. Longitude (see longitude, celestial) of a point is measured in the direction opposite to the rotation (positive to the west for direct rotation) from the cartographic position of the prime meridian defined by a clearly observable surface feature. Latitude (see latitude, celestial) of a point is the angle between the planetary equator (normal to the z-axis and through the center of mass) and normal to the reference surface at the point. The height of a point is specified as the distance above a point with the same longitude and latitude on the reference surface.
	is a kind of coordinate
Plaskett's star	has definition A very massive O-type giant with known anomalies in its spectrum. It is a spectroscopic binary in which mass exchange is occurring. Its spectrum can be interpreted to mean that each component has a mass of 75 Msun.
	has synonym HD 47129
	is a part of Monoceros
	is an instance of giant
	is an instance of O star
	is an instance of spectroscopic binary
plasma	has definition A completely ionized gas; the so-called fourth state of matter (besides solid, liquid, and gas) in which the temperature is too high for atoms as such to exist and which consists of free electrons and free atomic nuclei.
	is a kind of gas
plasmapause	has altitude 4 to 7 Earth radii
	has definition The region in Earth's ionosphere where the particle density (100 particles per cm3 just below the plasmapause) drops off very rapidly. It marks the transition from high to low density.
	has density 100 particles per cm3
	is a part of ionosphere
platinum	has image
	has ocean oxidation state II
	is a kind of siderophile element
	is a kind of supernova produced element
	is a kind of transition metal
	is a kind of unclassified oceanic element
Pleiades	has age very young
	has composition B star, late star
	has distance 125 pc
	has Messier number 45
	has number of stars several hundred
	has synonym M 45
	has synonym NGC 1432
	is a part of Taurus
	is an instance of Messier object
	is an instance of naked eye object
	is an instance of open cluster
Pleione	has definition A B8pe star, one of the brightest stars in the Pleiades, which developed an envelope or shell first observed in 1938. The shell increased in strength and attained its maximum intensity in 1945; thereafter it weakened and was scarcely visible by 1954. In 1972 it developed another shell. It is rotating so fast that it is unstable.
	has synonym 28 Tau
	is a part of Taurus
	is an instance of shell star
pleistocene epoch	has duration 1 million years
	has start time 1 million years ago
	is a kind of quaternary period
pliocene epoch	has duration 13 million years
	has start time 13 million years ago
	is a kind of tertiary period
Pluto	has albedo < 0.25
	has definition The most distant known planet from the Sun. Its orbit has the highest eccentricity and highest inclination to the ecliptic of any planet and some astronomers suggest that it may be an escaped satellite of Neptune. In the mid-1970s Pluto crosses Neptune's orbit on its way in, and for the rest of this century Pluto will be closer to the Sun than Neptune (Pluto and Neptune, however, are never less than 2.6 AU apart). Its mass and radius have not been determined with any great certainty.
	has discoverer Clyde Tombaugh
	has discovery date 1930
	has distance from Sun 39.44 AU
	has eccentricity 0.249
	has inclination to the ecliptic 17°.17
	has mass 6 × 1026 g, 0.1 to 0.2 MEarth
	has orbital period 248.43 years
	has orbital velocity Vorb 4.7 km s-1
	has perhelion date 1989
	has radius < 2900 km
	has rotation period 6d9h17m49s
	has synodic period 366.7 days
	has temperature 50-60 K
	is a part of our solar system
	is an instance of superior planet
	is an instance of terrestrial planet
plutonium	has image
	is a kind of transuranium element
Pockels cell	has definition An electro-optic crystal used as a reversible waveplate by applying alternately high positive and negative voltage.
	has wavefront modification polarization
	is a kind of wavefront modifier
point	has definition A geometrical object with zero dimensions, a location and possibly a time
	has dimensions 0
	is a kind of geometrical object
Poisson distribution	has definition An approximation to the binomial distribution used when the probability of success in a single trial is very small and the number of trials is very large.
	is an instance of statistical distribution
polar motion	has definition The irregularly varying motion of the Earth's pole of rotation with respect to the Earth's crust. (See celestial ephemeris pole.)
	is a kind of axis motion
Polar-disk equatorial telescope	has mounting equatorial with polar-disk
	is a kind of equatorial telescope
Polaris	has B-V magnitude 0.60
	has declination +89 15 51
	has definition A supergiant F8 Ib, F3 V visual binary, with an orbital period of thousands of years. The primary (a Cepheid with a pulsation period of 3.97 days) is itself a single-lined spectroscopic double with a period of 29.6 years. There are at least two more faint (12th mag) components of the system.
	has distance 330 light-years
	has period 3.97 days
	has primary F8 Ib
	has right ascension 2 31 50.5
	has spectral type F7:Ib-IIv
	has synonym alpha UMi
	has synonym HR 424
	has synonym North Star
	has synonym Pole star
	has use The star that lies near the direction in the sky toward which the North Pole of the Earth points.
	has V magnitude 2.02
	is a part of Ursa Minor
	is an instance of binary star
	is an instance of F star
	is an instance of naked eye star
	is an instance of Population I Cepheid
	is an instance of supergiant
pole	has definition One of two points at which the Earth's axis of rotation intersects the geoid.
	has height 0
	has terrestrial longitude 0
	is a kind of geocentric coordinate
Pollux	has B-V magnitude 1.00
	has declination +28 1 34
	has definition A K0 III star.
	has distance 11 pc
	has right ascension 7 45 18.9
	has spectral type K0IIIb
	has synonym beta Gem
	has synonym HR 2990
	has V magnitude 1.14
	is a part of Gemini
	is an instance of bright giant
	is an instance of K star
	is an instance of naked eye star
polonium	has image
	is a kind of group VI element
	is a kind of metallic metalloid
Population I Cepheid	has absolute magnitude <Mv> = -0.5 to -6
	has definition A Cepheid which is about 4 times more luminous than Population II Cepheids, probably because of their higher metal content (although mass may also be a factor).
	has period 5-10 days
	has synonym classical Cepheid
	has synonym young disk Cepheid
	is a kind of Cepheid
	is a kind of Population I star
Population I star	has author Baade (1944)
	has definition Young star typical of those found in galaxy spiral arms.
	has definition Young star with relatively high abundances of metals, and are usually found in the disk of a galaxy, especially the spiral arms, in dense regions of interstellar gas.
	has definition Younger stars, generally formed towards the edge of a galaxy, of the dusty material in the spiral arms, including the heavy elements. The brightest of this Population are hot, white stars.
	has definition Youngest observed stars, like our sun, formed from hydrogen, helium, and a large range of heavier elements (like carbon and oxygen) believed to have been created in the interiors of earlier Population II stars and Population III stars and then blown out into space.
	has synonym arm population
	has synonym disk star
	is a kind of star
	is a part of disk
Population II Cepheid	has absolute magnitude <Mv> = 0 to -3
	has definition A bright yellow star that pulsates like a Cepheid but is older and fainter.
	has period 10 to 30 days
	has synonym W Virginis star
	is a kind of Cepheid
	is a kind of Population II star
	occur in globular clusters
Population II star	has author Baade (1944)
	has definition Old stars typical of those found in the halo of the Galaxy.
	has definition Older observed stars formed mostly from hydrogen and helium.
	has definition Older star with relatively low abundances of metals, usually found in the nucleus of a galaxy or in globular clusters. The Sun is a rather old Population I star.
	has definition Older stars, generally formed towards the centre of a galaxy, containing few heavier elements. The brightest of this Population are red giants.
	has definition Star that has high spatial velocity and low metallicity. This is not an observational definition.
	has synonym halo star
	is a kind of star
	is a part of halo
Population III star	has author Baade (1944)
	has definition Star older than Population II stars.
	is a kind of star
Poseidon	has definition Unofficial name for J VIII, the next outermost satellite of Jupiter.
	has discoverer Melotte
	has discovery date 1908
	has eccentricity e = 0.4
	has inclination i = 147°
	has orbital period P = 737d
	has synonym Jupiter VIII
	is a part of Jupiter
	is an instance of natural satellite
positive muon g factor	applies to particle positive muon
	has symbol gμ
	has uncertainty 0.0000000013
	has value 2.0023318320
	is an instance of particle g factor
positive muon magnetic moment	applies to particle positive muon
	has symbol μμ
	has uncertainty 0.00000022 × 10-26 J T-1
	has value 4.49044813 × 10-26 J T-1
	is an instance of particle magnetic moment
positron	has antiparticle electron
	has charge 1
	has definition The antiparticle of the electron, discovered by Anderson in 1934. It has the same mass and spin as the electron, but opposite charge and magnetic moment.
	has discoverer Anderson
	has discovery date 1934
	has synonym antielectron
	is an instance of antiparticle
	is an instance of charged particle
	is an instance of lepton
	is an instance of radioactive particle
positronium	has definition A positron and electron bound together electrostaticaly.
	is an instance of boson
	is an instance of neutral particle
	is an instance of radioactive particle
post-asymptotic branch star	has definition F-type supergiant with strong sulfur lines.
	is a kind of F star
	is a kind of supergiant
potassium	has abundance 1.45 × 105 in Sun relative to H = 1 × 1012
	has abundance 21000 p.p.m. in Earth's crust
	has abundance 5 × 106 p.p.m. in seawater
	has atomic emission line 404.414 nm for K I
	has atomic emission line 691.108 nm for K I
	has atomic emission line 693.877 nm for K I
	has atomic emission line 769.896 nm for K I
	has atomic emission line 766.491 nm for K I (strong)
	has atomic number 19
	has atomic radii 227 pm
	has biological role essential to all living things
	has boiling point 1047 K
	has chief source sylvite, carnalite, alunite
	has covalent radii 203 pm
	has crystal cell dimension a = 533.4 pm
	has crystal type b.c.c
	has daily dietary intake 1.4 - 7.4 g
	has definition soft white metal which is silvery when first cut but oxidizes rapidly in air
	has density 828 kg m-3 for liquid at 336.80 K melting point
	has density 862 kg m-3 for solid at 293 K
	has discoverer Sir Humphry Davy
	has discovery date 1807 (isolated)
	has discovery location London, England
	has electrical resistivity 6.15 × 10-8 Ω m at 273 K
	has electron affinity 48.4 kJ mol-1 from K to K-
	has electron configuration [Ar]4s1 in ground state
	has electronegativity 0.82 Pauling
	has hazard excess ingestion of KCl (dietary supplement) can be fatal
	has heat capacity 20.786 J K-1 mol-1 for gas at constant pressure 0.1 MPa at 298.15 K
	has heat capacity 29.58 J K-1 mol-1 for solid at constant pressure 0.1 MPa at 298.15 K
	has heat of fusion 2.40 kJ mol-1
	has heat of vaporization 77.53 kJ mol-1
	has image
	has ionic radii 133 pm for K+
	has isotope mass range 35 to 51
	has lethal intake 2600 mg kg-1
	has level in humans 16000 p.p.m. in liver
	has level in humans 16000 p.p.m. in muscle
	has level in humans 1620 mg dm-3 in blood
	has level in humans 2100 p.p.m. in bone
	has linear expansion coefficient 83 × 10-6 K-1
	has longest lived isotope potassium 39
	has main mining area Germany, Spain, Canada, USA, Italy
	has mass absorption coefficient 143 cm2 g-1 for CuKalpha X-ray diffraction
	has mass absorption coefficient 15.8 cm2 g-1 for MoKalpha X-ray diffraction
	has mass magnetic susceptibility +6.7 × 10-9 kg-1 m3 for solid
	has mass of element in person 140 g for a 70 kg average person
	has melting point 336.80 K
	has mineral alunite, carnalite, orthoclase (mined for porcelain, ceramics and glass), sylvite
	has molar volume 45.36 cm3
	has name origin potash from English
	has neutron scattering length 0.367 × 10-12 cm
	has number of isotopes 18 including nuclear isomers
	has number of protons 19
	has ocean oxidation state I
	has ocean residence time 21000 years
	has poisson's ratio 0.35 GPa at 83 K
	has pronunciation poh-tass-ium
	has registry number 7440-09-7 for Chemical Abstracts System database
	has relative atomic mass 39.0983 in units of 12C = 12.000
	has reserves > 1 × 1010 tonnes
	has rigidity modulus 1.30 GPa
	has space group Im3m
	has specimen metal chunks (in mineral oil) or ingots. Warning!
has symbol K
has symbol name origin kalium from Greek
has synthesis mechanism combine sodium metal with potassium chloride
has term symbol 2S1/2 in ground state
has thermal conductivity 102.4 W m-1 K-1 at 300 K
has thermal neutron capture cross section 2.1 barns
has toxic intake 4 g of KCl
has uses compounds are used in fertilizers, chemicals and glass
has van der Waals radii 231 pm
has world production 200 tonnes year-1 for metal form
has world production 51 × 106 tonnes year-1 for salts
has young's modulus 3.53 GPa at 83 K
is a kind of accumulating oceanic element
is a kind of alkali metal
is a kind of lithophile element
reacts with water violently
potassium 38	has atomic mass 37.969080
	has decay mode β+ (5.913 Mev)
	has decay mode γ
	has half life 7.63 minutes
	has nuclear magnetic moment μ = -1.37 nuclear magnetons with diamagnetic correction
	has nuclear spin I = 3+ h/2π
	has number of neutrons 19
	has number of nucleons 38
	has symbol 38K
	is an instance of potassium
potassium 39	has atomic mass 38.963707
	has magnetogyric ratio 1.2483 × 107 rad T-1 s-1
	has natural abundance 93.2581%
	has NMR frequency 4.667 MHz where 1H = 100 MHz; 2.3488 T
	has NMR receptivity 2.69 where 13C = 1.00
	has nuclear magnetic moment μ = +0.391465 nuclear magnetons with diamagnetic correction
	has nuclear quadrupole moment 0.0601 × 10-28 m2
	has nuclear spin I = 3/2+ h/2π
	has number of neutrons 20
	has number of nucleons 39
	has relative NMR sensitivity 5.08 × 10-4 where 1H = 1.00
	has symbol 39K
	has uses isotopically enriched samples available for experimental purposes
	has uses isotopically enriched samples available for experimental purposes
	has uses Nuclear Magnetic Resonance
	is an instance of potassium
potassium 40	has atomic mass 39.963999
	has decay mode β- (1.32 Mev)
	has decay mode γ
	has decay mode EC
	has half life 1.25 × 109 years
	has natural abundance 0.0117 %
	has nuclear magnetic moment μ = -1.298009 nuclear magnetons with diamagnetic correction
	has nuclear quadrupole moment -0.0749 × 10-28 m2
	has nuclear spin I = 4- h/2π
	has number of neutrons 21
	has number of nucleons 40
	has symbol 40K
	is an instance of potassium
potassium 41	has atomic mass 40.961825
	has magnetogyric ratio 0.6851 × 107 rad T-1 s-1
	has natural abundance 6.7302%
	has NMR frequency 2.561 MHz where 1H = 100 MHz; 2.3488 T
	has NMR receptivity 0.0328 where 13C = 1.00
	has nuclear magnetic moment μ = +0.214869 nuclear magnetons with diamagnetic correction
	has nuclear quadrupole moment 0.0733 × 10-28 m2
	has nuclear spin I = 3/2+ h/2π
	has number of neutrons 22
	has number of nucleons 41
	has relative NMR sensitivity 8.40 × 10-4 where 1H = 1.00
	has symbol 41K
	has uses isotopically enriched samples available for experimental purposes
	has uses Nuclear Magnetic Resonance
	is an instance of potassium
potassium 42	has atomic mass 41.962402
	has decay mode β- (3.523 Mev)
	has decay mode γ
	has half life 12.36 hours
	has nuclear magnetic moment μ = -1.1425 nuclear magnetons with diamagnetic correction
	has nuclear spin I = 2- h/2π
	has number of neutrons 23
	has number of nucleons 42
	has symbol 42K
	has uses medical diagnosis
	has uses research
	is an instance of potassium
potassium 43	has atomic mass 42.960717
	has decay mode β- (1.82 Mev)
	has decay mode γ
	has half life 22.3 hours
	has nuclear magnetic moment μ = +0.163 nuclear magnetons with diamagnetic correction
	has nuclear spin I = 3/2+ h/2π
	has number of neutrons 24
	has number of nucleons 43
	has symbol 43K
	has uses medical diagnosis
	is an instance of potassium
potassium 44	has atomic mass 43.96156
	has decay mode β- (5.66 Mev)
	has decay mode γ
	has half life 22.1 minutes
	has nuclear magnetic moment μ = -0.856 nuclear magnetons with diamagnetic correction
	has nuclear spin I = 2- h/2π
	has number of neutrons 25
	has number of nucleons 44
	has symbol 44K
	is an instance of potassium
potassium 45	has atomic mass 44.960696
	has decay mode β- (4.20 Mev)
	has decay mode γ
	has half life 17.8 minutes
	has nuclear magnetic moment μ = +0.1734 nuclear magnetons with diamagnetic correction
	has nuclear spin I = 3/2+ h/2π
	has number of neutrons 26
	has number of nucleons 45
	has symbol 45K
	is an instance of potassium
potential energy	is a kind of energy
Potsdam Refractor	has altitude 107 m
	has aperture 0.80 m
	has creation date 1899
	has focal ratio f/15.0
	has latitude 52° 23' N
	has lens maker C.A. Steinheil
	has location Telegrafenberg, Potsdam, Germany
	has longitude 13°04'E
	has mounting manufacturer Repsold
	has owner Zentralinstitut fur Astrophysik
	is an instance of German equatorial telescope
	is an instance of refractor
power	is a kind of quantity
power unit	is a kind of unit
	is a unit of power
PP I	has definition A thermonuclear reaction in which hydrogen nuclei are transformed into helium nuclei producing 4 × 10-5 ergs of energy. Although the neutrinos from this reaction are detectable, they have not been observed.
	has reaction 1H(p, β+v)2D(p, γ)3He(3He, 2p)4He
	is an instance of proton-proton chain
PP II	has definition A thermonuclear reaction in which hydrogen nuclei are transformed into helium nuclei. Although the neutrinos from this reaction are detectable, they have not been observed.
	has reaction 1H(p, β+v)2D(p, γ)3He(4He, γ)7Be(β+v)7Li(p,α)4He
	is an instance of proton-proton chain
PP III	has definition A thermonuclear reaction in which hydrogen nuclei are transformed into helium nuclei. Occurs once in 1000 times.
	has reaction 1H(p, β+v)2D(p, γ)3He(4He,γ)7Be(p,γ)8B (β+v)8Be → 2 4He
	has reaction probability once in 1000 times
	is an instance of proton-proton chain
Praesepe	has distance 160 pc
	has Messier number 44
	has synonym Beehive Cluster
	has synonym M 44
	has synonym NGC 2632
	is a part of Cancer
	is an instance of Messier object
	is an instance of naked eye object
	is an instance of open cluster
praseodymium	has image
	has ocean oxidation state III
	is a kind of rare Earth
	is a kind of unclassified oceanic element
precambrian era	has duration 2500 million years
	has start time 3100 million years ago
	is a kind of geological era
precession	has definition A slow, periodic conical motion of the rotation axis of a spinning body. In the case of Earth's precession it is due to the fact that Earth's axis of rotation is not perpendicular to the ecliptic but is inclined about 23°.5 and is thus affected by gravitational perturbations from other bodies in the solar system. The Moon and Sun pull harder on that part of the Earth's equatorial bulge nearest them than on that farthest away; this causes a torque which precesses the Earth's rotational axis.
	has definition The slow (once per twenty-six thousand years) gyration of the Earth's axis.
	has definition The uniformly progressing motion of the pole of rotation of a freely rotating body undergoing torque from external gravitational forces. In the case of the Earth, the component of precession caused by the Sun and Moon acting on the Earth's equatorial bulge is called lunisolar precession; the component caused by the action of the planets is called planetary precession. The sum of lunisolar and planetary precession is called general precession. (See nutation.)
	has rate
	is a kind of axis motion
prediction	is a kind of theory related concept
pressure	has definition The force exerted over a surface divided by its area.
	has unit pressure unit
	is a kind of quantity
pressure broadening	has cause pressure which in turn is caused by the surface gravity of the star
	has definition Line broadening caused by pressure.
	has example white dwarfs have very broad absorption lines
	is a kind of line broadening
pressure ionization	has definition A state found in white dwarfs and other degenerate matter in which the atoms are packed so tightly that the electron orbits encroach on each other to the point where an electron can no longer be regarded as belonging to any particular nucleus and must be considered free.
	is a kind of ionization
pressure unit	is a kind of unit
	is a unit of pressure
primary mirror	has definition The first mirror encountered by incident light in a telescope system.
	is a kind of mirror
prime focus	has definition The focal point of the large primary reflecting mirror in astronomical telescopes when the light source is extremely distant. This focus actually falls at a point just within the upper structure of the telescope itself and is therefore accessible to CCD cameras and other instruments; it provides a large field of view.
	is a kind of focus
primordial black hole	has definition Small black hole hypothesized to have formed during the first 10-43 seconds of the universe, when quantum effects were very large.
	is a kind of black hole
primordial nucleosynthesis	has definition Production of atomic nuclei occurring during the first three minutes after the big bang.
	has definition The creation of elements that occurred just minutes after the Big Bang. According to standard theory, primordial nucleosynthesis gave the universe only five nuclei, all lightweight: hydrogen-1, hydrogen-2 (or deuterium), helium-3, helium-4, and lithium-7.
	has definition The process, which took place between one second and 3-4 minutes after the beginning, in which the protons and neutrons of the primordial soup condensed to form the lightest atomic nuclei: Deuterium, Helium-3, Helium-4, and Lithium-7. See isotope and Lithium.
	has definition The production of heavy nuclei from the fusion of lighter ones during the Big Bang. The infant universe consisted of only hydrogen, the lightest of all atomic nuclei, because any heavier nuclei would have come apart in the intense heat. All other elements would have to be formed later, in nucleosynthesis processes. It is believed that most of the helium, the next lightest element after hydrogen, was formed when the universe was a few minutes old
	has synonym big-bang nucleosynthesis
	is a kind of nucleosynthesis
primordial quark	has definition All baryons and mesons are believed to be composed of quarks, which are elementary particles of fractional charge. In the high-density, hot-temperature phase of the very early universe, prolific numbers of quarks would have been present in equilibrium with the other elementary particles. As the universe expanded and cooled, some of these quarks may have been frozen out. To what extent independent free quarks could survive is an unresolved issue of elementary particle physics.
	is a kind of quark
probable error	has acronym p.e.
	has definition The error which will not be exceeded by 50 percent of the cases. The probable error is equal to 0.6745 times the standard error.
	is a kind of statistical quantity
problem	is a kind of theory related concept
process	is a kind of physical phenomena
Procyon	has B-V magnitude 0.42
	has declination +05 13 30
	has definition One of the nearest stars, it is the eighth brightest star. An F star. It is a visual binary; its companion is a DF8 white dwarf.
	has distance 3.5 pc
	has orbital period 40 years
	has parallax 0.283
	has right ascension 07 39 18.1
	has spectral type F5 IV-V
	has synonym alpha CMi
	has synonym HR 2943
	has V magnitude 0.38
	is a part of Canis Minor
	is an instance of F star
	is an instance of naked eye star
	is an instance of subgiant
	is an instance of visual binary
prograde motion	has definition Motion in the same direction as the prevailing direction of motion.
	is a kind of motion
	is a kind of motion
program star	has definition The star being observed or measured, as contrasted with the comparison stars.
	is a kind of star
promethium	has image
	is a kind of rare Earth
	is a kind of synthetic element
prominence	has definition A region of cool, high-density gas embedded in the hot (106 K), low-density solar corona. Prominences are the flamelike tongues of gas that appear above the limb of the Sun.
	has definition Mass of hot, hydrogen rising from the Sun's chromosphere, best observed indirectly during a total eclipse. There are two kinds of prominence : erruptive prominence and quiescent prominence.
	has temperature 104 K
	is a part of corona
proper motion	has definition Apparent angular rate of motion of a star across the line of sight on the celestial sphere.
	has definition The apparent movement of a star, year after year, caused by the star's velocity across the line of sight. If the star's distance is known, this velocity-called the tangential velocity, can be computed. The star with the largest proper motion is Barnard's Star, whose proper motion is 10.3 arc-seconds per year.
	has definition The projection onto the celestial sphere of the space motion of a star relative to the solar system; thus the transverse component of the space motion of a star with respect to the solar system. Proper motion is usually tabulated in star catalogs as changes in right ascension and declination per year or century.
	is a kind of motion
protactinium	has image
	has ocean oxidation state V
	is a kind of actinide
	is a kind of unclassified oceanic element
proto-planet	has definition Early stage in the formation of planets according to the theory by which planetary systems evolve through the condensation of gas clouds surrounding a young star. The theory is not, however, generally accepted.
	is a kind of planet
protogalaxy	has definition A galaxy during the early phase, before it has developed its present shape and mix of stars.
	has definition A galaxy in the process of formation. None are observed nearby, indicating that all or most galaxies formed long ago.
	is a kind of galaxy
proton	has charge 1
	has composition two up quarks and one down quark
	has definition A baryon made of two up quarks and a down quark. It possesses a positive electromagnetic charge and can only be found in atomic nuclei. A single proton is a hydrogen nucleus.
	has definition A massive particle with positive electrical charge found in the nuclei of atoms. Composed of two up quarks and one down quark. The proton's mass is 938.3 MeV, slightly less than that of the neutron.
	has definition A positively charged elementary particle; the nucleus of a hydrogen atom. Mass of proton 1.00728 amu = 1.6726 × 10-24 g = 1836.12 me.
	has definition A subatomic particle with positive electric charge. Every atom has at least one proton in its nucleus; the number of protons determines the element. For example, all atoms with one proton are hydrogen, all atoms with two protons are helium, and so on.
	has definition One of the constituents of the atomic nucleus. It is a spin-1/2 particle carrying positive electric charge. The proton is the lightest baryon and, as a result, is the particle into which all other baryons eventually decay. It is believed to be absolutely stable, but certain theories (GUTs) predict it will decay very, very slowly.
	has symbol p
	is a part of nucleus
	is an instance of charged particle
	is an instance of elementary particle
	is an instance of nucleon
	is an instance of nucleus
proton Compton wavelength	applies to particle proton
	has equation
	has symbol λC,p
	has uncertainty 0.000000010 × 10-15 m
	has value 1.321409847 × 10-15 m
	is an instance of particle Compton wavelength
proton decay	has definition Spontaneous disintegration of the proton, predicted by grand unified theory but never observed experimentally.
	is an instance of prediction
proton g factor	applies to particle proton
	has equation
	has symbol
	has uncertainty 0.000000057
	has value 5.585694675
	is an instance of particle g factor
proton gyromagnetic ratio	applies to particle proton
	has equation
	has symbol γp
	has uncertainty 0.00000011 × 108 s-1 T-1
	has value 2.67522212 × 108 s-1 T-1
	is an instance of particle g factor
proton magnetic moment	applies to particle proton
	has symbol μp
	has uncertainty 0.000000058 × 10-26 J T-1
	has value 1.410606633 × 10-26 J T-1
	is an instance of particle magnetic moment
proton mass	applies to particle proton
	has symbol mp
	has uncertainty 0.00000013 × 10-27 kg
	has value 1.67262158 × 10-27 kg
	is an instance of particle mass
proton-proton chain	has definition A series of thermonuclear reactions in which hydrogen nuclei are transformed into helium nuclei. The temperature and density required are about 107 K and 100 g cm-3. It is the main source of energy in the Sun, where 1038 of these reactions occur every second. All parts of this reaction have been observed in the laboratory, except for the first step 1H(p, β+v)2D, which occurs only a few times in 1012 collisions of protons. But the first two reactions provide about one-third of the Sun's total energy release. The p-p chain divides into three main branches: PP I, PP II and PP III.
	has definition An important nuclear fusion reaction that occurs in stars. It begins with the fusion of two hydrogen nuclei, each of which consists of a single proton.
	has definition Process of nuclear fusion by which relatively cooler stars produce and radiate energy; hotter stars commonly achieve the same result by means of the carbon-nitrogen cycle.
	has definition The nuclear sequence by which the Sun and all other main-sequence stars with less than 1.5 solar masses fuse hydrogen into helium.
	has definition thermonuclear reaction in which two protons collide a very high velocities and combine to form deuteurium, the deuteurium can capture a proton to form tritium and tritium can capture a proton to form helium
	has location the center of the Sun an other stars
	has minimum temperature 107 K
	has next higher temperature reaction carbon cycle
	has optimum temperature 5 million Kelvin
	has part intermediate product deuterium
	has part intermediate product tritium
	has part product helium
	has part reactant proton
	has required reactant concentration high
	has synonym p-p chain
	has temperature dependence E ∝ T4
	is a kind of hydrogen burning
	is a kind of pycnonuclear reaction
Proxima Centauri	has bolometric magnitude M bol = 11.66
	has definition An flare star probably associated with the alpha Centauri system. It is the closest star to the Sun.
	has distance 4.25 light-years
	has mass M = 0.1 Msun
	has parallax 0'.765
	has radius R = 1.3 × 1010 cm
	has spectral type dM4e
	is a part of Centaurus
	is an instance of flare star
PSR B 1257+12	has definition A pulsar in the constellation Virgo and the site of the first solar system to be discovered outside our own. The planets were detected in 1991.
	is an instance of pulsar
pulsar	has acronym PSR
	has definition A fast-spinning neutron star that emits radiation toward Earth every-time it rotates.
	has definition An object discovered at Cambridge University in 1967 which has the mass of a star and a radius no larger than that of Earth and which emits radio pulses with a very high degree of regularity (periods range from 0.03 s for the youngest to more than 3 s for the oldest). All pulsars are characterized by the general properties of dispersion, periodicity, and short duty cycle. Pulsars are believed to be rotating, magnetic (surface magnetic fields of 1010 to 1014 gauss are estimated) neutron stars which are the end products of supernovae. Type S pulsars have a simple pulse shape: Type C, complex: Type D have drifting subpulses.
	has definition Neutron stars that spin rapidly and have strong magnetic fields, which produce electromagnetic radiation. (See neutron star.)
	is a kind of neutron star
	is a kind of radio star
pulsating nova	has definition A variable star, probably not a true nova, in which the change between more and less luminous stages is extreme. (also called recurrent novae)
	is a kind of cataclysmic variable
Puppis	has acronym Pup
	has genitive Puppis
	has synonym Ship's Stern
	is a part of celestial sphere
	is an instance of constellation formerly part of Argo Navis
Puppis A	has age 104 to 105 years
	has definition A supernova remnant. It is an extended nonthermal radio source, and also a source of soft X-rays.
	has distance 1 to 2 kpc
	has synonym 2U 0821-42
	is a part of Puppis
	is an instance of supernova remnant
	is an instance of X-ray source
pycnonuclear reaction	has definition Nuclear process that take place at relatively low temperatures and that are not strongly temperature-dependent.
	has minimum temperature relatively low
	has temperature dependence weak
	is a kind of nuclear process
Pyrex	is a kind of mirror
Pyxis	has acronym Pyx
	has genitive Pyxidis
	has synonym Ship's Compass
	is a part of celestial sphere
	is an instance of constellation formerly part of Argo Navis
	is an instance of constellation named by de Lacaille
Q-branch	has definition A set of lines in the spectra of molecules corresponding to changes in vibrational energy with none in rotational energy.
	is a kind of set of molecular lines
Quadrantid	has duration 0.4 days
	has radiant ???
	has rate 80 per hour
	has start time 3 January
	is an instance of meteor shower
quadrature	has definition Elongation of a planet when it makes a 90° angle with the Sun as seen from Earth. (b) A configuration in which two celestial bodies have apparent longitudes (see longitude, celestial) that differ by 90° as viewed from a third body. Quadratures are usually tabulated with respect to the Sun as viewed from the center of the Earth.
	has elongation 90°
	is a kind of planetary elongation event
quantity	has definition A number with specific units
	has unit
	is a kind of number
quantum chromodynamics	has acronym QCD
	has definition Gauge theory describing the interactions of quarks through the strong color field.
	has exchange quantum gluon
	has gauge symmetry SU(3)C (non-Abelian group)
	is an instance of gauge theory
	see also Yang-Mills theories
quantum electrodynamics	has acronym QED
	has definition Gauge theory describing the interactions between electrically charged particles through the electro-magnetic field.
	has exchange quantum photon
	has gauge symmetry U(1) (Abelian group)
	is an instance of gauge theory
quantum field theory	has applicability elementary particles
	has definition Relativistically invariant version of quantum mechanics used to describe the physics of elementary particles. The action of forces is a result of the exchange of sub-atomic particles.
	has exchange quantum
	has implication quantum fields are the ultimate reality and particles are merely the localized quanta of these fields.
	has symmetry relativistic invariance
	is a kind of quantum mechanics
quantum gravity	has definition A general term used to describe attempts to quantize gravity. The elementary particle of the gravitational field is the graviton.
	has definition A theory of gravity that would properly include quantum mechanics. To date, there is no complete and self-consistent theory of quantum gravity, although successful quantum theories have been found for all the forces of nature except gravity. (See quantum mechanics.)
	has definition A theory that successfully mergers quantum mechanics and general relativity, possibly involving modifications of one or both. String theory is an example of a theory of quantum gravity.
	is a kind of unified theory
quantum law	has domain quantum mechanics
	is a kind of law
quantum mechanics	has applicability microscopic scales of atoms and subnuclear particles but not restricted to this
	has constraint certain quantities (e.g. energy, angular momentum, light) can only exist in certain discrete amounts, called quanta.
	has definition Physics theory that explains wave-particle duality, the uncertainty principle and the exclusion principle.
	has discoverer Planck
	has equation Any physical system, such as an atom, may be viewed as existing as a combination of its possible states, each of which has a certain probability.
	has implication physical systems must be described in terms of probabilities.
	has synonym quantum theory
	is a kind of physics theory
quantum mechanics interpretation	is a kind of quantum mechanics
quantum of circulation	has equation
	has symbol h/2me
	has uncertainty 0.000000027 × 10-4 m2 s-1
	has value 3.636947516 × 10-4 m2 s-1
	is an instance of atomic constant
quantum quantity	is a kind of quantity
quantum solid	has definition A degenerate gas in which the densities are so great that the nuclei are fixed with respect to each other so that they resemble a crystalline lattice.
	is a kind of solid
quark	has antiparticle antiquark
	has definition A fermion with fractional electric charge.
	has definition A particle that is acted upon by the strong force. Quarks exist in six varieties: up, down, charm, strange, top, bottom and three "colors" (red, green, blue).
	has definition A sub-atomic particle which is a fundamental building block of the hadrons.
	has definition Fundamental particle of which protons, neutrons and electrons are now thought perhaps to be made up. There are possibly three or four types of quark. It is even possible that quarks themselves may be made up of still more fundamental particles.
	has definition Fundamental particles from which all hadrons are made. According to the theory of quantum chromodynamics, protons, neutrons, and their higher-energy cousins are composed of trios of quarks, while the mesons are each made of one quark and one antiquark. Held together by the strong nuclear force, quarks are not found in isolation in nature today; see asymptotic freedom.
	has definition One of the fundamental, indestructible particles of nature, out of which many other subatomic particles are made. Five types of quarks have been discovered, and it is believed that a sixth also exists. Quarks interact mainly via the strong nuclear force and the electromagnetic force.
	has definition The hypothetical constituent of the elementary particles that interacts via glue forces. Originally only three quarks were hypothesized; today it appears that six are required. For a variety of theoretical reasons, free quarks can never be seen.
	has spin 1/2
	is a kind of charged particle
	is a kind of fermion
	is a kind of parton
	is a part of nucleon
quark color	has definition An attribute which distinguishes otherwise identical quarks of the same flavor. Three colors red, green and blue - are required to distinguish the three valence quarks of which baryons are composed. It must be stressed that these colors are just labels and have nothing to do with ordinary color. Color is the source of the strong force which binds quarks together inside baryons and mesons, and so the three colors (r, g, b) can be thought of as three different color charges analogous to electric charge.
	has definition Each flavor of quark can exist in three variations, called colors, usually labeled as red, green, and blue. The color of a quark has no relation to its visual appearance, but the word color is used because there are three variations, in analogy with the three primary colors. Measurable properties of the quarks, such as electric charge and mass, depend on the flavor but not the color, but the color is responsible for the interactions that bind the quarks together (see Yang-Mills theories). Individual quarks cannot exist independently, but are forever confined within baryons or mesons, each of which is colorless. Baryons achieve colorlessness by being composed of three quarks, one of each color, while mesons achieve colorlessness by pairing each colored quark with its corresponding antiquark.
	has definition Property of quarks that expresses their behavior under the strong force. Analogous to the concept of charge in electromagnetism, except that, whereas there are two electrical charges (plus and minus), the strong force involves three color charges - red, green, and blue. The term is whimsical, and has nothing to do with color in the conventional sense, any more than quark "flavor", which determines the weak force behavior of quarks, has anything to do with taste.
	is an instance of quantum quantity
quasar	has acronym QSO
	has catalog quasar catalog
	has definition An intensely bright extragalactic object which superficially resembles a star. Most exist at very high redshifts and are therefore thought to be the nuclei of active galaxies.
	has definition An object with a dominant starlike (i.e., diameter less than 1") component, with an emission line spectrum showing a large redshift - up to z = 3.53 (0.91c) for OQ 172. (The largest redshift known for a normal radio galaxy is z = 0.637 for 3C 123.) Many have multiple absorption redshifts; a few have multiple emission redshifts. (Bahcall system: class I, zabs ≈ zem; class II, zabs significantly less than zem.) The light of most if not all quasars is variable over time intervals between a few days and several years, so their diameters must not be much larger than the diameter of the solar system; yet they are the intrinsically brightest objects known (for 3C 273 (z = 0.158), Mv = -27.5 if its redshift is cosmological). The energy output of a typical quasar at "cosmological" distance is of the order of 1047 ergs per second - which would require a mass of 1010 Msun if it derives its energy solely from nuclear fusion. (Energy requirement under the "local" hypothesis is on the order of 1042 ergs per second.) The basic problem of quasars is that they emit too much radiation in too short a time from too small an area.
	has definition Compact-looking objects, often radio sources, with emission lines in their spectrum which are displaced by very large amounts towards the red. These redshifts correspond to velocities which are a large fraction of the speed of light, and hence these objects are believed to lie at great distances.
	has definition Extremely distant and luminous astronomical objects that are much smaller than a galaxy and much more luminous. Quasars may be the central regions of certain very energetic galaxies at an early stage of their evolution. It is believed that the power of a quasar derives from a massive black hole at its center.
	has definition The brightest objects in the universe, quasars can generate over a trillion times as much light as the Sun from a region little larger than the solar system. Most are extremely distant, which means that they existed long ago.
	has luminosity very large
	has redshift large
	is a kind of violent galaxy
quasar catalog	has object type quasar
	is a kind of galaxy catalog
quaternary period	is a kind of cenozoic era
quiescent prominence	has definition A relatively pacific prominence which may last for months.
	is a kind of prominence
R Coronae Borealis	has amplitude 6th magnitude to 14th magnitude
	has definition Star with a large infrared excess and a rather high 7Li abundance.
	has outburst spectra with many emission-lines
	has outburst start time within a few weeks
	has recovery time several months to a year
	has reference Clayton, G.C. : 1996, PASP 108. The R Coronae Borealis Stars
	has reference Clayton, G.C., Whitney, B.A., Stanford, S. A., Drilling, J.S. : 1992, ApJ 397, 652. Observations of R Coronae Borealis stars in decline - Empirical arguments for dust formation near the stellar surface
	has reference Whitney, B.A., Soker, N., Clayton, G.C. : 1991, AJ 102, 284. Model for R Coronae Borealis stars
	has spectral type F star (F8-G0 Ib)
	is a part of Corona Borealis
	is an instance of R Coronae Borealis variable
R Coronae Borealis variable	has amplitude 8 magnitudes
	has definition A very luminous helium-rich, carbon-rich, hydrogen-poor eruptive variable supergiant whose light declines up to 8 magnitudes at irregular intervals.
	has prototype R Corona Borealis
	is a kind of cataclysmic variable
	is a kind of hydrogen-deficient C star
	is an instance of supergiant
R star	has definition Star with spectral characteristics similar to those of K stars except that molecular bands of C2, CN, and CH are present instead of TiO bands.
	is a kind of C star
R zone	has definition Region in the solar corona in which short-lived radiofrequency variations are observed.
	is a part of corona
R-branch	has definition A set of lines in the spectra of molecules corresponding to unit decreases in rotational energy.
	is a kind of set of molecular lines
r-process	has definition The capture of neutrons on a very rapid time scale (i.e., one in which a nucleus can absorb neutrons in rapid succession, so that regions of great nuclear instability are bridged), a theory advanced to account for the existence of all elements heavier than bismuth (up to A ≈ 298) as well as the neutron-rich isotopes heavier than iron. The essential feature of the r-process is the release of great numbers of neutrons in a very short time (less than 100 seconds). The presumed source for such a large flux of neutrons is a supernova, at the boundary between the collapsing neutron star and the ejected material. However, other proposed sources have included such things as supernova shocks and black-hole-neutron-star collisions. The heavier r-process elements are synthesized at a temperature of about 109 K and an assumed neutron density of 1020-1030 per cm3. The r-process is terminated by neutron-induced fission. The existence of 244Pu (half-life 82 million years) in the early solar system shows that at least one r-process event had occurred in the Galaxy just before the formation of the solar system.
	has definition The creation of elements heavier than zinc through the rapid bombardment of other elements by neutrons. The r process occurs in supernovae. Examples of reprocess elements are gold, iodine, and europium.
	is a kind of nucleosynthetic reaction
rad	has definition Unit of radiation, equal to 100 ergs of ionizing energy absorbed per gram of absorber.
	is an instance of radioactivity unit
	is an instance of radioactivity unit
radial velocity	has definition The speed at which an object moves toward or away from us. It can be measured from a star's spectrum: a star moving toward us has a blueshifted spectrum, and a star moving away from us has a redshifted spectrum. The larger the blueshift or redshift, the larger the radial velocity. The present radial-velocity champion is a star in the constellation Lacerta named Giclas 233-27, which moves toward us at 583 kilometers per second.
	has definition Velocity along the line of sight toward (-) or away from (+) the observer.
	is a kind of velocity
radian	has base unit m·m-1 = 1
	has definition A measure of angular distance; 2π radians equals 360 degrees.
	has definition Unit of angular measure equal to the angle subtended at the centre of a circle by an arc the length of which is equal to the radius. There are 2π radians in a circle. The angle π/4 is called an octant and a thousandth of a radian is sometimes called a mil, which is equal to 3 minutes 26.5 seconds of arc.
	has equivalent 57.29578°
	has proposal date 1873
	has proposer James Thomson
	has symbol rad
	is an instance of angle unit
	is an instance of named derived SI unit
radiation direction modification	changes property direction
	is a kind of radiation modification
radiation era	has definition The Big Bang era when the temperature had dropped to 109K and the rate of electron-positron pair annihilation exceeded the rate of their production, leaving radiation the dominant constituent of the universe.
	has duration 1012 s
	has start time 10 s after Big Bang
	has temperature 109K
	is a kind of Big Bang era
	is followed by matter era
	is preceded by nucleosynthetic era
radiation intensity modification	changes property intensity
	is a kind of radiation modification
radiation measurement	has unit radiation unit
	is a kind of quantity
radiation modification	changes property some aspect of the incomming radiation
	is a kind of optical process
radiation pressure	has definition The transfer of momentum by electromagnetic radiation incident on a surface: prad = (4/3)σT4 / c.
	is a kind of pressure
radiation temperature	has definition The temperature that a blackbody of similar dimensions would have that radiated the same intensity at the same frequency.
	is a kind of temperature
radiative recombination	has definition See radiative capture.
	is a kind of emission
	is a kind of recombination
radiative zone	has definition Region in a stellar interior where conduction and radiation dominates the heat flow
	is a part of solar interior
radio	has definition Electromagnetic radiation with the lowest energy and longest wavelength. Unlike visible light, radio waves penetrate dust and can be detected from throughout the Galaxy.
	has frequency 3 GHz or less
	has wavelength 10 cm or more
	is a kind of photon
radio astronomy	has definition Study of the universe at the radio wavelengths of electromagnetic energy.
	is a kind of astronomy
radio galaxy	has definition A galaxy that is extremely luminous at radio wavelengths. A radio galaxy is usually a giant elliptical - the largest galaxy in a cluster - and is a strong emitter of synchrotron radiation. M87 and M82 are examples.
	is a kind of galaxy
	is a kind of radio source
radio interferometer	has definition Type of radio telescope that relies on the use of two or more aerials at a distance from each other to provide a combination of signals from one source which can be analyzed by computer. Such an analysis results in a resolution that is considerably better than that of a parabolic dish aerial by itself because of the greater effective diameter.
	is a kind of array telescope
	is a kind of radio telescope
	requires signals from different antenna to be combined
radio lobes	has definition Extended regions of diffuse radio emission, often dumbbell shaped, that surround a radio galaxy.
	is an instance of radio source
radio loud quasar	has definition A quasar with detectable radio emission.
	is a kind of quasar
	is a kind of radio galaxy
radio recombination line	has definition Radio recombination lines are the result of electron transitions between high-n (n > 50) levels in an atom or ion.
	is a kind of emission line
radio source	has catalog radio source catalog
	has definition A source of extraterrestrial radio radiation. The strongest known is Cassiopeia A, followed by Cyg A and the Crab Nebula (Tau A) (the capital letters following the name of a constellation refer to the radio sources of the constellation, A being the strongest source). Radio sources are divided into two main categories: Class I, those associated with our Galaxy (which is a weak radio source), and Class II, extragalactic sources. Most radio sources are galaxies, supernova remnants, or H II regions.
	has wavelength radio
	is a kind of celestial body
	is a part of celestial sphere
radio source catalog	is a kind of astronomical catalog
radio star	has definition Star with detectable emission at radio wavelengths. They include pulsars, flare stars, some infrared stars, and some X-ray stars.
	is a kind of star
radio telescope	has definition A device for gathering and amplifying radio energy typically by using a metallic dish antenna
	has definition Non-optical telescope (of various types) which, instead of focusing light received from a distant object, focuses radio signals onto a receiver-amplifier.
	has definition Sensitive radio antennae employed to detect the radio energy emitted by nebulae, galaxies, pulsars, etc.
	has reference astroweb
	is a kind of electromagnetic telescope
	is a kind of electromagnetic telescope
radioactive element	has definition an unstable radioactive element which has an excess or deficit of neutron relative to the stable element
	is a kind of element
	is a kind of radioactive particle
radioactive particle	has decay products the products produced immediately after decay
	has definition A particle which can produce harmful radioactivity directly through its decay or by interacting with other particles.
	has desintegration energy the total energy produced when the particle decays
	has lifetime The average time in which a particle decays
	is a kind of particle
radioactivity unit	is a kind of unit
	is a unit of radioactivity
radium	has image
	has ocean oxidation state II
	is a kind of alkali earth metal
	is a kind of unclassified oceanic element
radius	is a kind of length
radon	has image
	has ocean oxidation state 0
	is a kind of inert gas
	is a kind of unclassified oceanic element
ram pressure	has definition Motion of a blunt body at supersonic velocity through an ambient gaseous medium causes a strong drag or ram pressure to be exerted on the body. In the case of a galaxy moving through the intergalactic gas, the ram pressure is capable of stripping the galaxy of much of its interstellar gas.
	is a kind of pressure
Raman effect	has definition In spectroscopy, the change in the wavelength of light scattered by molecules.
	has definition The change of wavelength on scattering. It arises from radiation exciting (or de-exciting) atoms or molecules from their initial states.
	is a kind of scattering
Rankine	has absolute zero 0
	has definition A temperature scale with the same division as the Fahrenheit scale and the zero point at 0° absolute. 0 °R = - 470 °F.
	has definition Unit which is 5/9 of the Kelvin. Rankine temperatures have the same temperature interval as those on the Fahrenheit scale.
	has triple point of water 491.688
	is an instance of temperature unit
	is named after W. J. M. Rankine (1820-1872)
	to convert to Fahrenheit subtract 459.69
rare Earth	has definition Element with atomic number between 57 and 71 in the top row of the inner-transition elements of the periodic table
	has filling orbital 4f
	has synonym lanthanide
	is a kind of inner transition metal
Rayleigh scattering	has definition Selective scattering (i.e., preferential scattering of shorter wavelengths) of light by very small particles suspended in the Earth's atmosphere, or by molecules of the air itself. The scattering is inversely proportional to the fourth power of the wavelength.
	is a kind of scattering
Rayleigh-Jeans law	has definition An approximation of Planck's blackbody formula valid at long wavelengths (hv << kT). It is often used in radio astronomy; it gives the brightness temperature of a radio telescope.
	is an instance of quantum law
reaction	has duration
	has location
	has optimum density
	has optimum pressure
	has optimum temperature
	has part catalyst
	has part intermediate product
	has part product
	has part reactant
	has reaction rate
	has required reactant concentration
	is a kind of physical process
reasoning process	is a kind of abstraction
Reaumur	has absolute zero -218.52
	has boiling point of water 80
	has definition An arbitrary scale in which the freezing and boiling points of water are taken to be 0 and 80°R respectively. The scale is based on the thermal expansion of an alcohol and water mixture. If the 'length' is 1000 units at the ice point the length expands to 1080 units at the boiling point, hence the peculiar figure of 80 in this scale.
	has proposal date 1730
	has triple point of water 0
	is an instance of temperature unit
	is named after R. A. F. Reaumur (1683-1757)
recent quaternary epoch	is a kind of quaternary period
recession	has definition Motion (increasing distance) away.
	is a kind of motion
recombination	has antonym ionization
	has definition The capture of an electron by a positive ion.
	is a kind of atomic process
recombination radiation	has definition See radiative capture.
	is a kind of bound-free transition
	is a kind of emission
reconnaissance spacecraft	is a kind of military spacecraft
recoverable spacecraft	has definition a spacecraft which returns to Earth intact
	is a kind of artificial satellite
recurrent nova	has initial rise of light curve typically 6 magnitudes
	has prototype star
	is a kind of nova
recycled oceanic element	has ocean concentration increasing with depth
	has ocean residence time 103 to 105 years
	is a kind of oceanic element
red	is a kind of optical
red dwarf	has abundance 70 percent of all stars
	has definition A main-sequence star with spectral type M. Red dwarfs are much fainter, cooler, and smaller than the Sun but are the most common type of star in the Galaxy, accounting for 70 percent of all stars.
	has spectral type M
	is a kind of dwarf
red giant	has definition A giant star with spectral type M. Such stars are in a more advanced state of evolution than the Sun, for they do not burn hydrogen into helium at their cores. Instead, they may fuse hydrogen into helium in a layer surrounding their cores, or they may fuse helium into carbon and oxygen, or they may do both. Often, astronomers use "red giant" loosely, to include not only M giants but G and K giants, too.
	has definition A late-type (K or M) high-luminosity (brighter than Mv = 0) star that occupies the upper right portion of the H-R diagram. Red giants are post-main-sequence stars that have exhausted the nuclear fuel in their cores. The red-giant phase corresponds to the establishment of a deep convective envelope. Red giants in a globular cluster are about 3 times more luminous than RR Lyrae stars in the same cluster.
	has definition Large, highly luminous but relatively cool star that has reached a late stage in its "life". It is running out of nuclear "fuel" and has accordingly expanded greatly and become less dense. Many also become variable stars of long periodicity. Its next evolutionary stage is to become a white dwarf, in developing into which the star has to cross the main sequence on the Hertzsprung-Russell diagram.
	is a kind of giant
	is a kind of M star
Red Spot	has definition An elliptical spot on Jupiter. Its color and intensity vary with time. It has been observed for at least a century, and an examination of earlier records shows that Cassini had sketched it in the seventeenth century.
	has dimension 40000 × 15000 km
	has location southern hemisphere of Jupiter
	is a part of Jupiter
red supergiant	has definition A supergiant with spectral type M. Red supergiants are the largest stars in the universe: if put in place of the Sun, some would touch Saturn. The two brightest red supergiants in Earth's sky are Betelgeuse and Antares.
	is a kind of supergiant
redshift	has definition The shift of spectral lines toward longer wavelengths in the spectrum of a receding source of radiation.
	has value z = Δλ/λ where λ is the laboratory wavelength of the spectral line and Δλ is the difference between the laboratory and the observed wavelengths
	is a kind of Doppler shift
reduced proper motion	has definition The observed proper motion of a star (in seconds of arc per year) reduced to absolute proper motion (in kilometers per second).
	is a kind of proper motion
reflection nebula	has definition A cloud of interstellar gas and dust whose spectrum contains absorption lines characteristic of the spectrum of nearby illuminating stars. The emission component of its spectrum is due to gas; the reflection component, to dust (see also diffuse nebula).
	is a kind of nebula
reflector	has aperture or primary mirror diameter
	has definition A device for gathering and amplifying light or other energy by means of a mirror.
	has definition Telescope that uses mirrors to magnify and focus an image onto an eyepiece. (reflector)
	has mirror diameter equal to aperture (except for Schmidt which has aperture smaller than mirror diameter)
	has mirror maker the person, company or institution that created the mirror
	has mirror type material and other engineering details
	has primary mirror shape
	has secondary mirror shape
	is a kind of electromagnetic telescope
reflector with corrector plate	has primary mirror diameter
	is a kind of optical telescope
	is a kind of reflector
refraction	has definition Deflection (or "bending") of light - or any ray as it passes from one medium into another of greater or lesser density, representing a change in overall speed of the ray. Refracting telescopes rely on the refraction of light through lenses. The refractive index of a medium (e.g., glass) is a measure of the medium's "bending" power.
	is a kind of radiation direction modification
	obeys Snell's law
refractor	has definition A device for gathering and amplifying light by means of a lens.
	has definition A telescope in which the light is focused by a lens at the viewing side of the telescope. By contrast, a reflecting telescope is one in which light is focused by a mirror.
	has definition Telescope that uses lenses to magnify and focus an image onto an eyepiece. (refractor)
	has lens maker the person, company or institution that created the lens
	has lens type doublet
	has optical design visual refractor
	is a kind of optical telescope
regolith	has definition The layer of fragmentary debris produced by meteoritic impact on the surface of the Moon or a planet.
	is a part of Moon
regression of the nodes	has definition The slow (19°.35 per year, 360° in 18.6 years), westward motion of the nodes of the Moon's orbit due to perturbations of the Earth and Sun.
	is a kind of motion
Regulus	has definition A visual triple B8 V star.
	has distance 26 pc
	has spectral type B8 V
	has synonym alpha Leo
	is a part of Leo
	is an instance of B star
	is an instance of dwarf
relativity process	is a kind of physical process
relativity theory	is a kind of physics theory
religious belief	is a kind of belief
resistance	has unit resistance unit
	is a kind of electromagnetic quantity
resistance unit	is a kind of electromagnetic unit
	is a unit of resistance
resonance line	has definition The longest-wavelength line arising from the ground state.
	is a kind of spectral line
rest-mass energy	has definition The energy which a particle has even when it is at rest. According to the famous relation E = mc2 of special relativity, this rest energy is equal to the rest mass of the particle-the mass it has when a rest-times the square of the speed of light. If the mass is in grams and the speed of light in centimeters per second (c = 2.998 × 1010 centimeters per second), then the energy is given in ergs.
	is a kind of energy
retardation point	has altitude inversely proportional to the mass of the meteoroid
	has definition the point in the flight of a meteoroid through the atmosphere where it loses its cosmic velocity and falls freely due to gravity
	has duration zero (it is a point in time)
	has start time shortly after the time of initial contact with atmosphere but before impact
	is a kind of meteor event
	does not occur for meteoroids heavier than 10 tons they impact with a significant fraction of their cosmic velocity
Reticulum	has acronym Ret
	has genitive Reticuli
	has historical image
	has synonym Net
	has synonym Reticle (crosshairs in telescope eyepiece)
	is a part of celestial sphere
	is an instance of constellation named by de Lacaille
retrograde motion	has definition Apparent motion of a planet in a direction opposite to its normal progress across the sky produced by the orbital motion of the earth.
	has definition In a backwards direction; in astronomy this means in a direction corresponding to east-to-west.
	is a kind of motion
Rhea	has albedo 0.57
	has definition Sixth satellite of Saturn
	has diameter about 1500 km
	has discoverer Cassini
	has discovery date 1672
	has rotation period 4d12h25m
	is a part of Saturn
	is an instance of natural satellite
rhenium	has image
	has ocean oxidation state III
	is a kind of transition metal
	is a kind of unclassified oceanic element
rhodium	has image
	is a kind of transition metal
rich cluster	has definition Galaxy cluster with 100 or more galaxies within a volume comparable to that of a loose group. Scale of cluster, about 1 Mpc.
	has diameter 1 Mpc
	has number of galaxies > 100
	is a kind of galaxy cluster
Rigel	has B-V magnitude -0.03
	has declination -08 12 06
	has definition A blue supergiant. It is a multiple star. The seventh brightest star in the night sky.
	has distance 900 light-years
	has right ascension 05 14 32.2
	has spectral type B8 Iae
	has synonym beta Ori
	has synonym HR 1713
	has V magnitude 0.12
	is a part of Orion
	is an instance of B star
	is an instance of blue supergiant
	is an instance of naked eye star
right ascension	has definition Angular distance on the celestial sphere measured eastward along the celestial equator from the equinox to the hour circle passing through the celestial object. Right ascension is usually given in combination with declination.
	has symbol α
	is an instance of angle
	is an instance of equatorial coordinate component
ring	has definition A system of four concentric rings, only about 2-4 km thick. The outermost ring is ring A, then comes Cassini's division, then ring B (also called the bright ring), then Lyot's division, then ring C (the crepe ring), then ring D (discovered in 1969). The rings are a swarm of solid particles, probably jagged rocks about 1 meter to 1 km across (1973), not ice as previously had been assumed, inside the Roche limit. Bobrov (1969) estimates the total mass of the rings to be about 0.01 the lunar mass.
	has width
	is a kind of celestial body
	is a part of Saturn ring system
ring galaxy	has definition A galaxy with a ring-like appearance. The ring contains luminous blue stars, but relatively little luminous matter is present in the central regions. It is believed that such a system was an ordinary galaxy that recently suffered a head-on collision with another galaxy.
	is a kind of galaxy
ring gap	has definition A gap between the rings of Saturn.
	has width
	is a kind of celestial body
	is a part of Saturn ring system
Ring Nebula	has definition A famous planetary nebula (M57, NGC 6720) in the constellation Lyra.
	has distance 2000 light years
	has illuminating star
	has image
	has Messier number 57
	has progenitor star
	has synonym M 57
	is a part of Lyra
	is an instance of Messier object
	is an instance of Messier object
	is an instance of planetary nebula
rise time	has definition The time required for the vehicle to achieve its optimum height (in rocket or balloon astronomy).
	is an instance of time
Ritchey-Chrétien	has definition A class of reflecting telescope with a hyperbolic primary and secondary.
	has definition A system of two mirrors, aspherized to give an image at the secondary (Cassegrain) focus free from spherical aberration and coma.
	has optical design Ritchey-Chrétien
	has primary mirror shape concave hyperboloid
	has secondary mirror shape convex hyperboloid
	is a kind of reflector
Rocard scattering	has definition Linearly anisotropic scattering.
	is a kind of scattering
root mean square	has acronym rms
	has definition The square root of the mean square value of a set of numbers (see also random walk).
	is a kind of statistical quantity
ROSAT	has definition ROentgen SATellite, was an X-ray observatory developed through a cooperative program between the Germany, the United States, and the United Kingdom. The satellite was designed and operated by Germany, and was launched by the United States on June 1, 1990. It was turned off on February 12, 1999.
	is an instance of grazing-incidence telescope
ROSAT PRC97-32	has distance greater than 400 light years
	has image
	is a part of Corona Australis
	is a part of Milky Way
	is an instance of neutron star
rotating coordinate	has frame of reference rotating reference frame
	is a kind of coordinate
rotating reference frame	has definition A frame of reference which rotates with the same period as the system.
	is a kind of frame of reference
rotation	has definition Of a single body in space: spinning on an axis. Of a planetary system, rotation is generally planar in relation to the parent star.
	is a kind of motion
Rotten Egg Nebula	has image
	has synonym OH231.8+4.2
	is an instance of planetary nebula
Royal Greenwich Observatory	has definition Primary national Observatory in Great Britain, first sited at Greenwich in 1675, but in 1958 moved to Herstmonceux, Sussex. From the first, Directorship of the Observatory has entailed appointment as Astronomer Royal. In the 1980s the Observatory will lose its primary national status with the completion of the Northern Hemisphere Observatory in Las Palmas, the Canary Islands.
	is an instance of astronomical institution
Royal Society	has definition English organization founded in the seventeenth century and dedicated to the advancement of science.
	is an instance of science institution
RR Lyr	has definition An old metal-poor white or yellow-white giant star that pulsates like a Cepheid and therefore varies in brightness. Most RR Lyrae stars have periods of under one day, which is shorter than periods for Cepheids. RR Lyrae stars are also fainter than Cepheids, with absolute magnitudes around +0.7, corresponding to a luminosity about 45 times the Sun's. RR Lyrae stars are excellent distance indicators because they all have nearly the same intrinsic brightness. They take their name from the star RR Lyrae, in the constellation Lyra.
	is an instance of RR Lyrae star
RR Lyrae star	has definition A large class of pulsating (amplitude variation about 1 mag) blue giants of anomalous spectral type (A2-F6) with periods of less than 1 day. Their average absolute magnitude is about +0.8. which makes them almost 50 times more luminous than the Sun. They are Population II objects often (but not always) present in globular clusters. RR Lyrae stars are valid distance indicators out to more than 200 kpc.
	has definition Periodic variable with period less than one day, and of spectral types A to early F.
	has definition Type of short-period variable stars. Spectrally classified as A to F giants. They were once called cluster-cepheids.
	has synonym cluster variable
	has synonym cluster variable
	is a kind of horizontal branch star
	is a kind of periodic variable
RRab star	has amplitude larger than RRc star
	has definition A sub-class of Bailey type RR Lyrae variables, having asymmetric light curves of large amplitude.
	has definition RR Lyr star characterised by a sharp rise to maximum then a slow fall to minimum. Usually combined with RRb star.
	is a kind of Bailey type
RRc star	has amplitude smaller than RRa star
	has definition RR Lyr star characterised by equally long rise and fall time. This type has the smallest amplitude
	is a kind of Bailey type
RS CVn star	has definition Close binary, which show the HK lines in emission.
	is a kind of binary star
RS Ophiuchi	is an instance of recurrent nova
rubidium	has image
	has ocean oxidation state I
	has ocean residence time 800000 years
	is a kind of accumulating oceanic element
	is a kind of alkali metal
runaway star	has definition Early type star (O and early B) outside the Galactic plane, which reached large distances (from the Galactic plane) because of their high velocities.
	has definition Star of spectral type O or early B with unusually high space velocities. Runaway stars are thought to be produced when there is a supernova explosion in a close binary system.
	has spectral type O star, B star
	is a kind of early star
ruthenium	has image
	is a kind of transition metal
rutherfordium	is a kind of transactinide
RV Tau variable	has definition Periodic variable with periods 60-100 days, and of spectral types G and K.
	has period 60 - 100 days
	has spectral type G star, K star
	is a kind of periodic variable
RV Tauri star	has definition A class of about 100 semiregular variable yellow supergiants of late spectral type (G-K), similar to W Virginis stars but with longer periods. Their spectra often contain emission lines, and their light curves have alternating deep and shallow minima. They have a large infrared flux. RVa stars maintain an approximately constant mean brightness; RVb stars have long-term (on the order of 1000 days) periodicity.
	has period 1000 days (order of magnitude)
	is a kind of semiregular variable
RW Aurigae	has definition A T Tauri star with a strong ultraviolet excess.
	has spectral type G5e V
	is an instance of dwarf
	is an instance of G star
	is an instance of T Tauri star
rydberg	has definition Unit of energy (R = h bar3c / me4) equal to 13.5978 eV (the ionization potential of hydrogen). Rydberg correction has definition
	is an instance of energy unit
Rydberg constant	has equation
	has equation
	has symbol R∞
	has uncertainty 0.000083 m-1
	has value 10973731.568549 m-1
	is an instance of atomic constant
S Andromedae	has definition A supernova seen in 1885 in the Andromeda galaxy.
	is an instance of supernova
S Doradus	has definition A supergiant eclipsing binary (an Eta Carinae-type object) in the Large Magellanic Cloud.
	has orbital period 40 years
	is a part of Dorado
	is a part of Large Magellanic Cloud
	is an instance of eclipsing binary
S star	has definition Late type giant (K5 to M) showing distinct bands of ZrO.
	has definition Late type giants (K5 to M) showing distinct bands of ZrO. [JJ95]
	has definition Red-giant star of spectral type S are similar to M stars except that the dominant oxides are those of the metals of the fifth period (Zr, Y, etc.) instead of the third (Ti, Sc, V). They also have strong CN bands and contain spectral lines of lithium and technetium. Pure S stars are those in which ZrO bands are very strong and TiO bands are either absent or only barely detectable. Almost all S stars are LPVs. (S1,0. The number following the comma is an abundance parameter.)
	has definition Red-giant stars of spectral type S are similar to M stars except that the dominant oxides are those of the metals of the fifth period (Zr, Y, etc.) instead of the third (Ti, Sc, V). They also have strong CN bands and contain spectral lines of lithium and technetium. Pure S stars are those in which ZrO bands are very strong and TiO bands are either absent or only barely detectable. Almost all S stars are LPVs. (S1,0. The number following the comma is an abundance parameter.) [H76]
	is a kind of heavy-metal star
	is a kind of late star
s-electron	has definition An orbital electron whose l quantum number is zero.
	has orbital quantum number 0
	is a kind of bound electron
s-process	has definition The process by which elements heavier than copper are formed through a slow flux of neutrons. The s-process operates in red giant stars; prominent s-process elements include barium, zirconium, yttrium, and lanthanum.
	is a kind of nucleosynthetic reaction
S0 galaxy	has definition Galaxy with nuclei surrounded by disklike structure without arms.
	is a kind of galaxy
Sa spiral	has definition In Hubble's classification, a spiral with a large nuclear bulge and closely coiled arms.
	has definition Spiral galaxy with arms tightly wound around the nucleus.
	has synonym early-type spiral
	is a kind of spiral galaxy
Sackur-Tetrode constant	has pressure 101.325 kPa
	has symbol S0/R
	has temperature 1 K
	has uncertainty 0.0000044
	has value -1.1648678
	is an instance of physico chemical constant
Sagitta	has acronym Sge
	has boundary
	has genitive Sagittae
	has synonym Arrow
	is a part of celestial sphere
	is an instance of constellation -has URL: http://www.seds.org/Maps/Stars_en/Fig/sagitta.html
Sagittarius	has acronym Sgr
	has boundary
	has genitive Sagittarii
	has synonym Archer
	is a part of Zodiac
	is an instance of zodiacal constellation
Sagittarius A	has definition A radio source (the galactic center). (Sgr A West is a thermal source; Sgr A East is a nonthermal source.)
	has definition The very center of the Milky Way, Sagittarius A* is a strong source of radio waves and probably a massive black hole.
	has diameter 12 pc
	is a part of Sagittarius
	is an instance of radio source
Sagittarius arm	has definition One of the spiral arms of the Milky Way, lying between us and the center of the Galaxy in the direction of Sagittarius.
	has density of interstellar gas 1.2 atoms cm-3
	has distance from galaxy center 8.7 kpc
	has distance from Sun 1.5 kpc
	is a part of disk
Sagittarius B2	has definition A massive (3 × 106 Msun), dense (up to 108 particles per cm3) H II region and molecular cloud complex - the richest molecular source in the Galaxy. It is in the galactic plane, near the galactic center.
	has distance 10 kpc distant
	is a part of Sagittarius
	is an instance of radio source
samarium	has image
	has ocean oxidation state III
	has ocean residence time 200 years
	is a kind of rare Earth
	is a kind of recycled oceanic element
SAMOS	is a kind of reconnaissance spacecraft
satellite	has approximately elliptical orbit
	has definition a physical object gravitationaly bound and in orbit around a larger physical object
	has orbital inclination
	has orbital period
	is a kind of physical object
	orbits a more massive physical object
satellite elongation	has definition The geocentric angle between a satellite and its primary, measured in the plane of the satellite, planet and Earth. Satellite elongations are measured from 0° east or west of the planet.
	is a kind of elongation
satellite galaxy	has definition A galaxy that orbits a larger one. The Milky Way has at least ten satellite galaxies.
	is a kind of dwarf galaxy
	orbits
Saturn	has albedo 0.50
	has atmosphere composition hydrogen and helium
	has definition Sixth major planet out from the Sun. The most spectacular of the Solar System, it is circled by a series of concentric rings. All the satellites of Saturn are locked in synchronous rotation.
	has eccentricity e = 0.056
	has equatorial diameter 116340 km
	has escape velocity Vesc = 33.1 km s-1
	has inclination i = 2°29'33"
	has mass 5.7 × 1029 g = 95.2 MEarth
	has mean density 0.7 g cm-3
	has mean distance from Sun 9.540 AU
	has number of satellites 10
	has oblateness 0.1
	has obliquity 26°44'
	has orbital velocity Vorb = 9.65 km s-1
	has rotation period at equator 10h14m
	has rotation period at poles 10h38m
	has sidereal period 29.458 years
	has surface gravity 11 m s-2
	has synodic period 378 days
	has temperature Teff ≈ 160 K
	is a part of our solar system
	is an instance of gas giant
	is an instance of naked eye planet
	is an instance of superior planet
Saturn ring system	is a part of Saturn
Sb spiral	has definition Spiral galaxy with arms spread out from the nucleus.
	is a kind of spiral galaxy
SBa barred spiral	has definition Barred spiral galaxy with arms tightly wound around the nucleus
	is a kind of barred spiral
SBb barred spiral	has definition Barred spiral galaxy with arms wound around the nucleus
	is a kind of barred spiral
SBc barred spiral	has definition Barred spiral galaxy with arms widely spread out from the nucleus
	is a kind of barred spiral
Sc spiral	has definition Spiral galaxy with arms widely spread out from the nucleus.
	is a kind of spiral galaxy
SC star	has definition Star which appears to be intermediate in type between S stars and carbon stars (C/O ratio near unity).
	is a kind of S star
scandium	has image
	has ocean oxidation state III]
	has ocean residence time 5000 years
	is a kind of recycled oceanic element
	is a kind of transition metal
scattering	has definition Collision between a photon and a particle
	has definition The process whereby light is absorbed and reemitted in all directions, with essentially no change in frequency. Scattering by free electrons was the dominant source of opacity in the early universe.
	is a kind of collision
	is a kind of radiation direction modification
scavenged oceanic element	has ocean concentration decreasing with depth
	has ocean residence time 103 years or less
	is a kind of oceanic element
Schmidt	has definition A telescope with a spherical primary mirror and a thin refractive corrector plate with a complex, non-spherical shape. Very wide-field performance for surveys.
	has definition A type of reflecting telescope (more accurately, a large camera) in which the coma produced by a spherical concave mirror is compensated for by a thin correcting lens placed at the opening of the telescope tube. The Schmidt has a usable field of 0°.6.
	has definition Telescopic camera incorporating an internal corrective lens or plate that compensates for optical defects and chromatic faults in the main mirror. The system was invented by Bernhard Schmidt.
	has optical design Schmidt
	has primary mirror shape concave spheroid
	has secondary mirror shape none
	is a kind of equatorial telescope
	is a kind of reflector
Schmidt plate	has definition Photographic plate obtained with a Schmidt telescope, which is a type of telescope with a particularly large field of view.
	is a kind of integrating detector
Schmidt Telescope	has altitude 75 m
	has aperture 0.80 m
	has creation date 1968
	has focal ratio f/3.0
	has latitude 56° 47' N
	has location Riga, Latvia
	has longitude 24° 24' E
	has mirror diameter 1.2 m
	has mirror maker Zeiss (Jena)
	has mounting manufacturer Zeiss (Jena)
	has operator Latvian Academy of Sciences
	has owner Radioastrophysical Observatory
	is an instance of Schmidt
scholastics	has definition Adherents to the philosophy and cosmology of Aristotle. Their dominance in the universities, which had been founded largely to study Aristotle, constituted an obstacle to acceptance of the Copernican system advocated by Kepler and Galileo.
	is a kind of outdated belief
Schwarzschild black hole	has definition A nonrotating, spherically symmetric black hole derived from Karl Schwarzchild's 1916 exact solution to Einstein's vacuum field equations.
	is a kind of black hole
Schwarzschild singularity	has definition The center of a black hole. According to Einstein's theory of general relativity, the entire mass of a black hole is concentrated at a point at its center, the "singularity". It is believed that quantum mechanical effects, not included in the theory, would cause the mass to spread out over a tiny but nonzero region, thus preventing an infinite density of matter and doing away with the singularity.
	is a kind of singularity
science	has definition Systematic study of Nature, based upon the presumption that the universe is based upon rationally intelligible principles and that its behavior can therefore be predicted by subjecting observational data to logical analysis.
	is a kind of abstraction
science institution	is a kind of institution
scientist	has career
	has greatest achievement
	is a kind of person
scintillation counter	has definition A device used with a photomultiplier tube to detect or count charged particles or gamma rays.
	is a kind of high energy detector
scintillator	has definition A detector for high-energy photons such as gamma-rays. The impact of a gamma-ray causes a burst of light which can be observed with a PMT.
	is a kind of high energy detector
Sco-Cen association	has definition An association of very young stars. The most luminous member is a B star of Mv = - 4.9.
	has distance 200 pc
	is an instance of association
	is part a of Gould Belt
Scorpius	has acronym Sco
	has boundary
	has genitive Scorpii
	has historical origin beast that killed the Great Hunter, Orion
	has synonym Scorpion
	is a part of Zodiac
	is an instance of zodiacal constellation
Scorpius OB1	has definition An extremely young association of OB stars.
	has distance 2 kpc
	is a part of Scorpius
	is an instance of OB association
Scorpius X-1	has apparent magnitude brightest X-ray source in the sky (besides the Sun)
	has definition A compact eclipsing X-ray source. It has day-to-day variations (period about 0.78 days?) of as much as 1 mag; it also has optical and radio counterparts but no correlation has been found among the flares observed at the three different wavelengths. It is a thermal X-ray source, probably associated with a rotating collapsed star surrounded by an extensive envelope. Tentative optical identification with the 13th mag blue variable V818 Sco. The spectrum of Sco X-1 is similar to that of an old nova. (3U 1617-15)
	has discovery date 1962
	has distance 250 to 500 pc
	is a part of Scorpius
	is an instance of eclipsing binary
	is an instance of X-ray source
Sculptor	has acronym Scl
	has genitive Sculptoris
	has synonym Sculptor
	is a part of celestial sphere
	is an instance of constellation named by de Lacaille
Sculptor Dwarf	has definition A dwarf elliptical galaxy that orbits the Milky Way (Mv = - 11.28 mag), in the Local Group.
	has discovery date 1938
	has distance 85 kpc
	has distance from galaxy center 255000 light-years
	has mass 3 × 106 Msun
	is a part of Local Group
	is a part of Sculptor
	is an instance of dwarf elliptical satellite galaxy
	orbits Milky Way
Sculptor Group	has brightest member NGC 253
	has definition The nearest group of galaxies to the Local Group.
	has distance 4 to 10 million light-years
	is a part of Local Supercluster
	is a part of Sculptor
	is an instance of galaxy cluster
Scutum	has acronym Sct
	has boundary
	has genitive Scuti
	has historical image
	has synonym Shield
	is a part of celestial sphere
	is an instance of constellation
Scutum arm	is a part of Sagittarius arm
sdB	has definition Subdwarf B star with very broad and shallow Balmer lines; fewer lines of the Balmer series are visible than for normal dwarfs.
	is a kind of B star
	is a kind of subdwarf
sdO	has definition Subdwarf O star showing few very broad and shallow Balmer lines and a very strong He II 4686 line.
	is a kind of O star
	is a kind of subdwarf
seaborgium	is a kind of transactinide
second	has definition A unit of time defined as the duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom. In 1967 the General Conference of Weights and Measures (CGPM) adopted this as the tentative definition of the second in SI units, replacing the ephemeris second, which remains in the IAU system of astronomical constants.
	has definition duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom
	has historical origin was defined originally as the fraction 1/86400 of the mean solar day
	has symbol s
	is an instance of base SI unit
	is an instance of second unit
second (angle)	has symbol "
	has synonym arc second
	has value in SI unit (1/60)' = (π/648000) rad
	is an instance of non SI unit
second law of thermodynamics	has definition A physical law formulated in the nineteenth century and stating that any isolated system becomes more disordered in time. (See entropy.)
	has definition Law stating that total entropy always increases.
	is an instance of thermodynamics law
second order phase transition	A phase transition where one phase evolves into the other as the temperature changes, so the two phases never coexist. -has source: [G97]
	is a kind of phase transition
second radiation constant	has equation
	has symbol c2
	has uncertainty 0.0000025 × 10-2 m K
	has value 1.4387752 × 10-2 m K
	is an instance of physico chemical constant
second unit	is a kind of time unit
secondary cosmic ray	has definition Atomic fragment - mainly muons - produced by collisions between primary cosmic rays and the molecules in Earth's atmosphere.
	is a kind of particle
secondary mirror	has definition The second reflecting surface encountered by the light in a telescope. The secondary is usually suspended in the beam and therefore obstructs part of the primary.
	is a kind of mirror
secular parallax	has definition A parallax based on solar motion; i.e., the baseline is the distance the Sun moves in a given interval of time with respect to the local standard of rest.
	has value 4.09 AU per year
	is a kind of parallax
segmented mirror	has definition A large mirror construction technique in which many smaller elements are built and then actively controlled to conform to the shape of the required large mirror.
	is a kind of mirror
segmented mirror telescope	has definition a reflecting telescope whose mirror is composed of multiple segments
	has number of segments
	is a kind of reflector
selective absorption	has definition The process by which light from an astronomical object grows red as the light travels through interstellar dust. Dust scatters blue light more than red, thus leaving predominantly red light transmitted.
	has definition The reddening of starlight in passing through fine particles of interstellar dust.
	has synonym reddening
	is a kind of absorption
selenium	has image
	has ocean oxidation state VI some IV
	has ocean residence time 3000 years
	is a kind of chalcophile element
	is a kind of group VI element
	is a kind of nonmetallic element
	is a kind of recycled oceanic element
selenocentric coordinate	has coordinate origin Moon
	has definition With reference to the center of the Moon.
	is a kind of coordinate
semi-forbidden line	has definition Spectral lines from "semiforbidden" transitions, i.e., those whose transition probabilities are perhaps 1 in 106 instead of about 1 in 109 for forbidden transitions. One bracket - e.g., [C III] - is used to indicate semiforbidden lines.
	is a kind of emission line
semimajor axis	has definition An orbital element representing half the length of the major axis of an ellipse.
	has symbol a
	is an instance of length
	is an instance of orbital element
semiminor axis	has definition Half the length of the minor axis of an ellipse; a standard element used to describe an elliptical orbit.
	is an instance of length
semiregular variable	has amplitude variable
	has definition A class of giant and supergiant pulsating stars of spectral class M, K, N, R, or S with a periodic (or semiperiodic) light curve of varying amplitude.
	is a kind of late star
	is a kind of variable
Serpens	has acronym Ser
	has genitive Serpentis
	has synonym Serpent
	is a part of celestial sphere
	is an instance of constellation
set of lines	has definition A set of absorption or emission lines in the spectra of a celestial object
	is a kind of spectral feature
set of molecular lines	has species molecule
	is a kind of set of lines
SETI	has definition The Search for Extraterrestrial Intelligence, by using radiotelescopes to listen for signals transmitted by intelligent alien beings.
	is an instance of astronomical institution
Sextans	has acronym Sex
	has genitive Sextantis
	has synonym Sextant
	is a part of celestial sphere
	is an instance of constellation
Sextans Dwarf	has definition A dwarf companion to the Milky Way. Discovered by computer.
	has discovery date 1990
	has distance from galaxy center 300 kpc
	is a part of Local Group
	is a part of Sextans
	is an instance of satellite galaxy
	orbits Milky Way
Seyfert galaxy	has definition A type of spiral galaxy first discovered by Karl Seyfert in the 1940s. The central region of a Seyfert galaxy is distinguished by powerful radiation, much of it focused into narrow frequencies.
	has definition One of a small class of galaxies (many of which are spirals) of very high luminosity and very blue continuum radiation with small, intensely bright nuclei whose spectra show strong, broad, high-excitation emission lines probably caused by discrete clouds moving at velocities that are higher than the escape velocity. Seyferts possess many of the properties of QSOs, such as the ultraviolet excess of the continuum, the wide emission lines, and the strong infrared luminosity. The energy sources in their nuclei are unexplained; presumably the energy input can be associated with some process that liberates gravitational binding energy to accelerate relativistic particles. Seyferts comprise about 1 percent of the bright galaxies. The brightest Seyfert known is NGC 1068. Weedman-Khachikian classification: class 1 Seyferts have broad Balmer line wings; class 2 have no obvious Balmer line wings.
	is a kind of spiral galaxy
Seyfert's Sextet	has brightest member NGC 6027
	has definition A compact group of galaxies surrounding NGC 6027. It has both spiral and irregular members.
	has number of galaxies 6
	is a part of Serpens
	is an instance of compact group
shadow matter	has definition Theoretical classes of particles, their existence intimated by supersymmetry, theory, that participate in few if any of the four known fundamental forces. Planets, stars, and galaxies made of shadow matter could conceivably exist in the same space and time we occupy without our sensing their presence.
	is a kind of hypothetical particle
Shajn 2.6-m Reflector	has aperture 2.64 m
	has creation date 1960
	has focal ratio f/3.8, 15.7, 16.4, 40
	has latitude 44° 44' N
	has location Nauchny, Ukraine
	has longitude 34° 00' E
	has owner Crimean Astrophysical Obs.
	has synonym Crimean 102 inch
	is an instance of Fork equatorial telescope
	is an instance of optical telescope
	is an instance of reflector
Shane-Wirtanen catalog	has definition A catalog of all galaxies brighter than seventeenth magnitude (a measure of brightness). There are about a million galaxies in the Shane-Wirtanen catalogue.
	is a kind of galaxy catalog
Shapley	has associate Hale as director of Mt. Wilson
	has associate Russel as his mentor
	has associate van Maanen as a collegue at Mt. Wilson
	has birth date 2 November 1885
	has career 1914-1920 : Mt. Wilson 1921-1952 : Director Harvard College Observatory 1952-1972 : Emeritus
	has death date 20 October 1972
	has degree 1911 : B.Sc. astronomy, University of Missouri 1913 : PhD. Princeton University ("Eclipsing binary stars")
	has greatest achievement structure and scale of our galaxy determined from accurate globular cluster distances
	has greatest gaffe ignored Humason's observations of Cepheids in Andromeda galaxy
	has image
	has name Harlow Shapley
	has orbituary Kopal, Z. : 1972, Nature 240, 429.
	is an instance of 19th century scientist
	is an instance of astronomer
Shapley-Ames catalog	has definition A catalog of galaxies brighter than thirteenth magnitude, completed in 1932. There are about 1200 galaxies in this catalogue.
	is a kind of galaxy catalog
shell star	has definition A hot main-sequence star, usually of spectral class B-F, whose spectrum shows bright emission lines presumed to be due to a gaseous ring or shell surrounding the star.
	has definition A star in which two different types of line profiles co-exist.
	has spectral type B star, A star, F star
	is a kind of emission line star
SHF band	has frequency 3 to 30.0 GHz
	has wavelength 1 to 10 cm
	is a kind of microwave
SHF-G band	has frequency 3.95 to 5.85 GHz
	has wavelength 5.1 to 7.6 cm
	is a kind of SHF band
SHF-H band	has frequency 7.05 to 10.0 GHz
	has wavelength 3.0 to 4.25 cm
	is a kind of SHF band
SHF-J band	has frequency 5.3 to 8.2 GHz
	has wavelength 3.7 to 5.7 cm
	is a kind of SHF band
SHF-K band	has frequency 18.0 to 26.5 GHz
	has wavelength 1.1 to 1.67 cm
	is a kind of SHF band
SHF-M band	has frequency 10.0 to 15.0 GHz
	has wavelength 2.0 to 3.0 cm
	is a kind of SHF band
SHF-P band	has frequency 12.4 to 18.0 GHz
	has wavelength 1.67 to 2.4 cm
	is a kind of SHF band
SHF-R band	has frequency 26.5 to 40.0 GHz
	has wavelength 0.75 to 1.1 cm
	is a kind of SHF band
SHF-S band	has frequency 2.6 to 3.95 GHz
	has wavelength 7.6 to 11.5 cm
	is a kind of SHF band
SHF-X band	has frequency 8.2 to 12.4 GHz
	has wavelength 2.4 to 3.7 cm
	is a kind of SHF band
Shoemaker-Levy 9 collision	has duration 7 days
	has start time July 16, 1994
	is an instance of comet impact
shuttle	is a kind of recoverable spacecraft
SI unit	has definition International System of Units. A practical system of units of measurement adopted in 1969 by the 11th International General Conference of Weights and Measures (CGPM). The seven base units are the meter, the kilogram, the second, the ampere, the kelvin, the mole, and the candela.
	is a kind of unit
	is an acronym for Syteme Internationale
sidereal day	has definition The length of time between two successive meridian transits of the vernal equinox (cf. mean solar day). Because of precession the sidereal day is about 0.0084 second shorter than the period of rotation of Earth relative to a fixed direction (23h56m4s.099).
	has value 23h56m4s.091
	is an instance of day
sidereal year	has definition A period of time based on the revolution of the Earth around the Sun, where a year is defined as the mean period of revolution with respect to the background stars.
	is an instance of year
siderophile element	has definition element that tend to concentrate in metallic iron
	has occurrence in planet metallic core and associated with iron
	is a kind of planetary element
siemens	has base unit m-2·kg-1·s3·A2
	has definition The SI unit of electrical conduction.
	has symbol S
	has synonym reciprocal ohm
	has unit A·V-1
	is an instance of conductance unit
	is an instance of named derived SI unit
	represents electric conductance
sievert	has base unit m2·s-2
	has symbol Sv
	has unit J·kg-1
	is an instance of named derived SI unit
	is an instance of radioactivity unit
	represents dose equavalent
sigma	has definition Short-lived baryon.
	has definition Short-lived baryon.
	is an instance of hyperon
	is an instance of hyperon
silicon	has abundance 0.03 p.p.m. in Atlantic surface seawater
	has abundance 0.03 p.p.m. in Pacific surface seawater
	has abundance 0.82 p.p.m. in deep Atlantic seawater
	has abundance 277100 p.p.m. in Earth's crust
	has abundance 4.09 p.p.m. in deep Pacific seawater
	has abundance 4.47 × 107 in Sun relative to H = 1 × 1012
	has atomic emission line 251.611 nm for Si I
	has atomic emission line 288.156 nm for Si I
	has atomic emission line 504.103 nm for Si II
	has atomic emission line 505.598 nm for Si I
	has atomic emission line 566.956 nm for Si I
	has atomic emission line 634.710 nm for Si II
	has atomic emission line 637.136 nm for Si II
	has atomic number 14
	has atomic radii 117 pm
	has biological role essential to some species and possibly to humans
	has boiling point 2628 K
	has chief source quartz, talc, mica
	has covalent radii 117 pm
	has crystal cell dimension a = pm
	has crystal type
	has daily dietary intake 18 - 1200 mg
	has definition ultrapure crystals of silicon have a blue-grey metallic sheen
	has density 2329 kg m-3 for solid at 293 K
	has density 2525 kg m-3 for liquid at 1683 K melting point
	has discoverer J.J. Berzelius
	has discovery date 1824
	has discovery location Stockholm, Sweden
	has electrical resistivity 0.001 Ω m at 273 K
	has electron affinity 133.3 kJ mol-1 from Si to Si-
	has electron configuration [Ne]3s23p2 in ground state
	has electronegativity 1.90 Pauling
	has hazard some silicate fibres are carcinogenic such as asbestos
	has heat capacity 20.00 J K-1 mol-1 for solid at constant pressure 0.1 MPa at 298.15 K
	has heat capacity 22.251 J K-1 mol-1 for gas at constant pressure 0.1 MPa at 298.15 K
	has heat of fusion 39.6 kJ mol-1
	has heat of vaporization 383.3 kJ mol-1
	has image
	has ionic radii 26 pm for Si4+
	has ionic radii 271 pm for Si4-
	has isotope mass range 24 to 34
	has lethal intake non-toxic
	has level in humans 100 - 200 p.p.m. in muscle
	has level in humans 12 - 120 p.p.m. in liver
	has level in humans 17 p.p.m. in bone
	has level in humans 3.9 mg dm-3 in blood
	has linear expansion coefficient 4.2 × 10-6 K-1
	has longest lived isotope silicon 28
	has main mining area talc in Austria, Italy, India, South Africa, Australia, mica in Canada, USA, India, Brazil
	has mass absorption coefficient 6.44 cm2 g-1 for MoKα X-ray diffraction
	has mass absorption coefficient 60.6 cm2 g-1 for CuKα X-ray diffraction
	has mass magnetic susceptibility -1.8 × 10-9 kg-1 m3 for solid
	has mass of element in person 1 g for a 70 kg average person
	has melting point 1683 K
	has mineral silicates
	has molar volume 12.06 cm3
	has name origin silicis = flint from Latin
	has neutron scattering length 0.41543 × 10-12 cm
	has number of isotopes 11 including nuclear isomers
	has number of protons 14
	has ocean oxidation state IV
	has ocean residence time 30000 years
	has poisson's ratio 0.42 GPa
	has pronunciation sil-i-kon
has registry number 7440-21-3 for Chemical Abstracts System database
has relative atomic mass 28.0555 in units of 12C = 12.000
has reserves unlimited × 10 tonnes
has rigidity modulus 39.7 GPa
has space group
has specimen available as powder, pieces or lumps. Safe.
has symbol Si
has synthesis mechanism reduction of sand (SiO2) with carbon produces black amorphous silicon
has term symbol 3P0 in ground state
has thermal conductivity 148 W m-1 K-1 at 300 K
has thermal neutron capture cross section 0.171 barns
has toxic intake non-toxic
has uses semiconductors, alloys and polymers
has van der Waals radii 200 pm
has world production 3.4 × 106 tonnes year-1 for metallurgical grade
has world production 480000 tonnes year-1 for metallurgical grade
has world production 5000 tonnes year-1 for electronic grade (very pure)
has young's modulus 113 GPa
is a kind of group IV element
is a kind of lithophile element
is a kind of nonmetallic metalloid
is a kind of recycled oceanic element
reacts HF acid or hot alkali solutions by dissolving
silicon 28	has atomic mass 27.9769271
	has natural abundance 92.23%
	has nuclear magnetic moment μ = 0
	has nuclear spin I = 0+
	has number of neutrons 14
	has number of nucleons 28
	has symbol 28Si
	has uses experimental
	is an instance of silicon
silicon 29	has atomic mass 28.9764949
	has magnetogyric ratio -5.3146 × 107 rad T-1 s-1
	has natural abundance 4.67%
	has NMR frequency 19.865 MHz where 1H = 100 MHz; 2.3488 T
	has NMR receptivity 2.09 where 13C = 1.00
	has nuclear magnetic moment μ = -0.55529
	has nuclear spin I = 1/2+
	has number of neutrons 15
	has number of nucleons 29
	has relative NMR sensitivity 7.84 × 10-3 where 1H = 1.00
	has symbol 29Si
	has uses experimental, Nuclear Magnetic Resonance
	is an instance of silicon
silicon 30	has atomic mass 29.9737707
	has natural abundance 3.10%
	has nuclear magnetic moment μ = 0
	has nuclear spin I = 0+
	has number of neutrons 16
	has number of nucleons 30
	has symbol 30Si
	has uses experimental
	is an instance of silicon
silicon 31	has atomic mass 30.975362
	has decay mode β- (1.49 Mev)
	has decay mode γ
	has half life 2.62 hours
	has nuclear spin I = 3/2+
	has number of neutrons 17
	has number of nucleons 31
	has symbol 31Si
	is an instance of silicon
silicon 32	has atomic mass 31.974148
	has decay mode β- (0.227 Mev)
	has decay mode no γ
	has half life 160 years
	has nuclear spin I = 0+
	has number of neutrons 18
	has number of nucleons 32
	has symbol 32Si
	has uses research
	is an instance of silicon
silicon burning	has definition The end of the line for a high-mass star, silicon burning creates iron and other elements of similar mass and presages a supernova.
	has duration 1 day
	has optimum density 3 × 107 g cm-3
	has optimum temperature 2.7 × 109 K
	has part product iron 56
	has reactant silicon
	has reactant silicon involved in many hundreds of nuclear reactions
	is a kind of exothermic fusion process
silicon	does not react oxygen, water or acids
silicon monosulfide	has symbol SiS
	is an instance of diatomic molecule
	is an instance of interstellar molecule
silicon monoxide	has symbol SiO
	is an instance of diatomic molecule
	is an instance of interstellar molecule
Silkworm nebula	has image
	has synonym IRAS 17441-2411
	is an instance of planetary nebula
silurian period	has duration 20 million years
	has start time 425 million years ago
	is a kind of paleozoic era
silver	has image
	has ocean oxidation state I
	has ocean residence time 5000 years
	is a kind of chalcophile element
	is a kind of recycled oceanic element
	is a kind of transition metal
SIMBAD	is an instance of database
single line spectroscopic binary	has definition a spectroscopic binary in which periodic Doppler shift is detected in only one component of the binary star
	is a kind of spectroscopic binary
singularity	has definition A place, either in space or in time at which some quantity, such as density, becomes infinite. The laws of physics cannot describe infinite quantities and, in fact, physicists believe that infinities do not exist in nature. All singularities, such as the Schwarzschild singularity, are therefore probably the artifacts of inadequate theories rather than real properties of nature. According to Einstein's theory of general relativity, the universe began in a singularity of infinite density, the big bang. Physicists believe that an improved and yet-to-be discovered modification of general relativity, incorporating quantum mechanics, will show that the universe did not begin as a singularity. (See Schwarzschild singularity.)
	has definition A point in space-time at which the space-time curvature and other physical quantities become infinite and the laws of physics break down.
	has definition A point of infinite curvature of space where the equations of general relativity break down. A black hole represents a singularity; so, perhaps, did the universe at the first moment of time.
	has definition Anomaly in space-time at which a state not in accord with the classical laws of physics obtains. An example is a black hole; another is the moment of the big bang.
	has definition If the standard big bang theory is extrapolated all the way back to time zero, one reaches an instant of infinite density, infinite pressure, and infinite temperature - an instant that is frequently called the initial singularity. This singularity is sometimes said to mark the beginning of time, but it is more realistic to recognize that an extrapolation to infinite density cannot be trusted.
	has definition Location where the fabric of space or spacetime suffers a devastating rupture.
	is a kind of theoretical celestial body
Sirius	has B-V magnitude 0.00
	has declination -16 42 58
	has definition The brightest star in the sky. Its companion (Sirius B) is a white dwarf of about 0.96 Msun but only about 0.03 Rsun, the nearest white dwarf to Earth.
	has distance 8.6 light-years
	has orbital period 49.9 years
	has right ascension 6 45 8.9
	has spectral type A1Vm
	has synonym alpha CMa
	has synonym Dog Star
	has synonym HR 2491
	has V magnitude -1.46
	is a part of Canis Major
	is an instance of A star
	is an instance of binary star
	is an instance of dwarf
	is an instance of naked eye star
Sirius B	has definition Companion of Sirius, a white dwarf.
	has mass 1 solar mass
	has radius 0.03 solar radii
	has surface temperature Teff = 32000 K
	is a part of Canis Major
	is an instance of white dwarf
sky survey	is a kind of database
Slipher	has birth date 11 November 1875
	has career 1901-1916 : Lowell Observatory 1913 : blue-shift observed in Andromeda galaxy (Bull. Lowell Obs., 2, 56) 1914 : first galaxy redshifts observed (Bull. Lowell Obs., 2, 6) 1916-1954 : Director Lowell Observatory 1917 : most galaxies have redshifts (The Observatory, 40, 304) 1925 : red-shifts of galaxies suggest a distance/velocity relationship 1929 : Hubble combines Slipher's redshift measurements with Hubble's distance estimates and postulates Hubble's Law
	has death date 8 November 1969
	has degree Indiana Univ.
	has image
	has name Vesto Melvin Slipher
	has publication 1918, PASP 30, 346.. Discussion about the spectra of 'spiral nebula' NGC 4449 and NGC 4214, but no spectra actually published (claims 200 km/s and 300 km/s recession velocities respectively)
	has refererence Hall, J.S. : 1989, PASP 101, 887. : He discovered the high velocities and rotation of nebulous objects later identified as galaxies. He measured the velocities of 41 of these objects. In 1929 Hubble derived his important velocity-distance relationship using, as he later wrote Slipher, "your velocities and my distances"
	is an instance of 19th century scientist
	is an instance of astronomer
Sloan Digital Sky Survey Telescope	has acronym SDSS
	has altitude 2800 m
	has aperture 2.5 m
	has comment To take CCD imagery of a quarter of sky in 5 colors (u, g, r, i, z) and measure redshifts of 1 million galaxies
	has creation date (1998)
	has latitude 32° 47' N
	has location Apache Point, New Mexico, US
	has longitude 105° 49' W
	has mounting manufacturer L & F Industries and University of Washington
	has owner Astrophys. Res. Consortium Obs.
	has synonym Sloan 2.5 m
	is an instance of altazimuth telescope
	is an instance of optical telescope
	is an instance of Ritchey-Chrétien
slow nova	has definition A nova whose light curve shows a much more gradual development - i.e., rise time of several days, maximum of several weeks, slower decline, amplitude only about 10 mag.
	is a kind of nova
Small Magellanic Cloud	has acronym SMC
	has declination -72d49m43s
	has definition The second largest, and the second nearest, of the galaxies that orbit the Milky Way. It lies in the southern sky, 190000 light-years away.
	has diameter 320 × 185 arcminutes
	has distance from galaxy center 60 kpc
	has magnitude 2.7
	has mass 1 billion solar masses
	has NED data
	has right ascension 00h52m44.8s
	is a part of Local Group
	is a part of Tucana
	is an instance of Magellanic Cloud
	orbits Milky Way
SMC X-1	has definition An X-ray source in the Small Magellanic Cloud. It is a binary system. Identified with Sanduleak No. 160, a B0 I supergiant (mv = + 13.6). Because no radial-velocity variations are apparent in Sk 160, the mass of the X-ray emitter must be small relative to Sk 160 (about 2 Msun if Sk 160 is 20 Msun), unlike the compact member of CygX-1.
	has orbital period 3.89-day
	has synonym 2U 0115-73
	is a part of Small Magellanic Cloud
	is a part of Tucana
	is an instance of binary star
Smithsonian Astronomical Observatory	has acronym SAO
	is an instance of astronomical institution
smoke trail	has composition tiny roughly spherical micrometeorites
	has definition condensation of vaporized rock removed from the surface of a meteoroid by ablation
	has destination surface of celestial body
	has duration hours
	has time of occurence when meteoroid collides with atmosphere
	is a kind of meteor event
	can produce produce light visible only at night
SN 1987A	has distance 167000 light years
	has image
	is a part of Large Magellanic Cloud
	is an instance of type II supernova
Snell's law	has definition For a refracted light beam, the ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant. (also called the Law of Refraction)
	is an instance of optical law
sodium	has abundance 1.91 × 106 in Sun relative to H = 1 × 1012
	has abundance 10500 p.p.m. in seawater
	has abundance 23000 p.p.m. in Earth's crust
	has atomic emission line 313.548 nm for Na II
	has atomic emission line 589.595 nm for Na I
	has atomic emission line 818.326 nm for Na I
	has atomic emission line 819.482 nm for Na I
	has atomic emission line 588.995 nm for Na I (strong, used in atom absorption spectrometry)
	has atomic number 11
	has atomic radii 154 pm
	has biological role essential to most species including humans
	has boiling point 1156.1 K
	has chief source halite, trona
	has crystal cell dimension a = 376.7, c = 615.4 pm for α-Na
	has crystal type hexagonal for α-Na
	has daily dietary intake 2 - 15 g
	has definition soft, silvery-white metal which oxidizes rapidly when cut
	has density 928 kg m-3 for liquid at 370.96 K melting point
	has density 971 kg m-3 for solid at 293 K
	has discoverer Sir Humphry Davy
	has discovery date 1807 (isolated)
	has discovery location Royal Institution, London, England
	has electrical resistivity 4.2 × 10-8 Ω m at 273 K
	has electron affinity 52.9 kJ mol-1 from to -
	has electron configuration [Ne]3s in ground state
	has electronegativity 0.93 Pauling
	has hazard compounds are not hazardous, but excess sodium chloride can be toxic by ingestion
	has heat capacity 20.786 J K-1 mol-1 for gas at constant pressure 0.1 MPa at 298.15 K
	has heat capacity 28.24 J K-1 mol-1 for solid at constant pressure 0.1 MPa at 298.15 K
	has heat of fusion 2.64 kJ mol-1
	has heat of vaporization 89.04 kJ mol-1
	has image
	has ionic radii 98 pm for Na+
	has isotope mass range 19 to 31
	has lethal intake 3000 mg kg-1 of chloride in rat
	has level in humans 10000 p.p.m. in bone
	has level in humans 1970 mg dm-3 in blood
	has level in humans 2000 - 4000 p.p.m. in liver
	has level in humans 2600 - 7800 p.p.m. in muscle
	has linear expansion coefficient 70.6 × 10-6 K-1
	has longest lived isotope sodium 23
	has main mining area halite in Germany, Poland, USA, UK; trona in Kenya, USA
	has mass absorption coefficient 3.21 cm2 g-1 for MoKalpha X-ray diffraction
	has mass absorption coefficient 30.1 cm2 g-1 for CuKalpha X-ray diffraction
	has mass magnetic susceptibility +8.8 × 10-9 kg-1 m3 for solid
	has mass of element in person 100 g for a 70 kg average person
	has melting point 370.96 K
	has mineral halite, trona, occurs in many others but these are not mined as a source of sodium
	has molar volume 23.68 cm3
	has name origin soda from English
	has neutron scattering length 0.358 × 10-12 cm
	has number of isotopes 14 including nuclear isomers
	has number of protons 11
	has ocean oxidation state I
	has ocean residence time 1 × 108 years
	has poisson's ratio 0.34 GPa
	has pronunciation so-dee-uhm
	has registry number 7440-23-5 for Chemical Abstracts System database
	has relative atomic mass 22.989768 in units of 12C = 12.000
	has reserves almost unlimited
	has rigidity modulus 2.53 GPa
	has space group P63/mmc for α-Na
	has specimen ingots or lumps, in sealed ampoules under nitrogen, or spheres and sticks stored under mineral oil. Warning!
	has symbol Na
has symbol name origin natrium from Latin
has synthesis mechanism electrolysis of molted sodium chloride
has term symbol 2S1/2 in ground state
has thermal conductivity 141 W m-1 K-1 at 300 K
has thermal neutron capture cross section 0.530 barns
has toxic intake 12 g kg-1 of chloride in humans
has uses manufacture of chemicals and metals
has uses nuclear reactor heat exchanger
has van der Waals radii 231 pm
has world production 168 × 106 tonnes year-1 as salt
has world production 200000 tonnes year-1 as sodium metal
has world production 29 × 106 tonnes year-1 as sodium carbonate
has young's modulus 6.80 GPa
is a kind of accumulating oceanic element
is a kind of alkali metal
is a kind of lithophile element
reacts with water by producing hydrogen gas
sodium 22	has atomic mass 21.994434
	has decay mode β+ (2.842 Mev) 90%
	has decay mode γ
	has decay mode EC 10%
	has half life 2.605 years
	has nuclear magnetic moment μ = +1.746 nuclear magnetons with diamagnetic correction
	has nuclear spin I = 3+ h/2π
	has number of neutrons 11
	has number of nucleons 22
	has symbol 22Na
	has uses research, medical diagnostic
	is an instance of sodium
sodium 23	has atomic mass 22.989767
	has magnetogyric ratio 7.0761 × 107 rad T-1 s-1
	has natural abundance 100%
	has NMR frequency 26.451 MHz where 1H = 100 MHz; 2.3488 T
	has NMR receptivity 525 where 13C = 1.00
	has nuclear magnetic moment μ = +2.217520 nuclear magnetons with diamagnetic correction
	has nuclear quadrupole moment 0.1089 × 10-28 m2
	has nuclear spin I = 3/2+ h/2π
	has number of neutrons 12
	has number of nucleons 23
	has relative NMR sensitivity 0.0925 where 1H = 1.00 using NaCl (aq) reference
	has symbol 23Na
	has uses Nuclear Magnetic Resonance
	is an instance of sodium
sodium 24	has atomic mass 23.990961
	has decay mode β- (5.514 Mev)
	has decay mode γ
	has half life 14.96 years
	has nuclear magnetic moment μ = +1.690 nuclear magnetons with diamagnetic correction
	has nuclear spin I = 4+ h/2π
	has number of neutrons 13
	has number of nucleons 24
	has symbol 24Na
	has uses research, medical diagnostic
	is an instance of sodium
soft ultraviolet	is a kind of ultraviolet
soft X-ray	is a kind of X-ray
solar apex	has definition A point on the celestial sphere toward which the Sun and the solar system are moving with respect to the local standard of rest at a rate of about 19.4 km per second (about 4.09 AU per year).
	is a part of Hercules
	is an instance of equatorial coordinate
	is opposite of antapex
solar atmosphere	has definition The gaseous outer layers of the Sun, including, from the deeper layers outward, the photosphere, the chromosphere, and the corona. The atmosphere constitutes those layers of the Sun that can be observed directly.
	is a part of Sun
solar burst	has definition Suddenly enhanced nonthermal radio emission from the high solar corona immediately following a solar flare, probably due to energetic electrons trapped in the coronal magnetic field. Bursts are divided into several types, depending on their time frequency characteristics (type III is the most common). They are classified on a scale of importance ranging from -1 (least important) to +3. Bursts are generally attributed to a sudden acceleration of some 1035-36 electrons to energies greater than 100 keV in less than 1 second.
	is a kind of solar event
	is preceeded by solar flare
solar cycle	has definition The 11-year period between maxima (or minima) of solar activity. Every 11 years the magnetic field of the Sun reverses polarity; hence the more basic period may be 22 years.
	has period 11 years
	is an instance of periodic celestial event
	is an instance of solar event
solar eclipse	has definition An eclipse in which the Earth passes through the shadow cast by the Moon. It may be total (observer in the Moon's umbra), partial (observer in the Moon's penumbra), or annular. (See eclipse, annular.)
	is a kind of eclipse
solar event	is a kind of celestial event
	occurs on Sun
solar flare	has cause sudden release of large amounts of energy in a relatively small volume above the solar surface.
	has classification scale of importance ranging from 3+ (largest area) to 1- (smallest area).
	has definition Sudden and short-lived brightening of a region of the solar chromosphere.
	has duration as short as 300 s
	has effect on Earth ionization in Earth's atmosphere may increase by several orders of magnitude producing aurorae, magnetic storms and radio interference. (electron density 1011 compared with 108 in solar quiet times).
	has ejected mass up to 1016 g
	has energy up to 1032 ergs
	has location in the vicinity of a sunspot
	has mass ejection velocity up to 1500 km s-1
	is a kind of solar event
	is followed by solar burst
solar interior	is a part of Sun
solar mass	has definition Unit of mass equal to the amount of mass in the Sun, and the unit in which stellar and galactic masses are expressed.
	is an instance of mass unit
solar motion	has definition The velocity of the Sun through space, relative to the local standard of rest. The solar motion is U = -9 kilometers per second, V = +12 kilometers per second, and W = +7 kilometers per second.
	is a kind of mass motion
	is a kind of peculiar velocity
solar neutral region	has definition A region where the magnetic field strength approaches zero. Generally, neutral regions occur between regions of opposite polarity.
	is a part of photosphere
solar parallax	has definition Angle subtended by the equatorial radius of the Earth at a distance of 1 AU.
	has value 8.79 arcsecond
	is a kind of parallax
solar radius	has definition A unit of length based on radius of Sun
	is an instance of length unit
	is an instance of radius
solar rotation	has definition Differential rotation, the equatorial rotation taking less time than the polar by up to 9.4 Earth-days.
	is a kind of differential rotation
	is a kind of solar motion
solar system	has definition A star with planets
	is a kind of celestial body
solar velocity	has definition Velocity of the Sun (19.4 km sin the direction lII = 51°, bII = 23°) with respect to the local standard of rest.
	is an instance of space velocity
solar wind	has definition A radial outflow of hot plasma from the solar corona which carries both mass and angular momentum away from the sun. It is the effects of the solar wind that produce aurorae in the Earth's upper atmosphere, that cause the tails of comets to stream back from the Sun, and that distort the symmetry of planetary magnetospheres.
	has electron temperature 20000 K
	has ion temperature 10000 K
	has magnetic field 5 × 10-5 gauss
	has mass loss rate 10-13 Msun per year
	has number density 5 per cm3 (1971)
	has velocity 600 km s-1
	has velocity at Earth 400 km s-1
	is a part of solar atmosphere
solid	has definition a high density collection of particle which do not move relative to one another (except for thermal oscillations)
	is a kind of phase of matter
solid angle	has definition A measure of the angular size of an extended object, equal to the area it subtends on the surface of a sphere of unit radius.
	is a kind of quantity
solid angle unit	is a kind of unit
	is a unit of solid angle
solidification	has final phase solid
	has initial phase liquid
	has inverse process melting
	is a kind of first order phase transition
solstice	has definition One of the two points on the ecliptic at which the Sun appears to be farthest away from the celestial equator (representing therefore mid-summer or mid-winter).
	has period 1 year
	is a kind of periodic celestial event
Sombrero galaxy	has definition A spiral galaxy in the constellation Virgo. It was the first galaxy whose rotation was detected.
	has Messier number 104
	has synonym M 104
	has synonym NGC 4594
	is a part of Virgo
	is an instance of Messier object
	is an instance of spiral galaxy
sound wave in gas	has definition A longitudinal wave propagated through a gas as a pattern of alternating high and low pressure.
	is a kind of longitudinal wave
	is a kind of mass motion
South Atlantic Anomaly	discovered by Orbiting Atronomical Observatory
	has acronym SAA
	has definition A disturbance in the geomagnetic field (a region of intense charged-particle fluxes) over the south part of the Atlantic Ocean. It was discovered in early OAO (Orbiting Atronomical Observatory) flights that when the detector passed over that area, the data it collected were not valid.
	has location south part of the Atlantic Ocean
	is a part of magnetosphere
south celestial pole	has acronym SCP
	has declination -90 degrees
	is a part of Octans
	is an instance of celestial pole
	is opposite of north celestial pole
south galactic pole	has acronym SGP
	has definition A point in the constellation Sculptor toward which our line of sight is perpendicular to and below the Galactic disk.
	has galactic latitude -90 degrees
	is a part of Sculptor
	is an instance of galactic pole
	is opposite of north galactic pole
south pole	has definition The astronomical coordinate which coincides with the southern intersection of Earth's axis with the geoid.
	has terrestrial latitude -90 degrees
	is a part of Earth
	is an instance of pole
	is opposite of north pole
Southern African Large Telescope	has acronym SALT
	has altitude 1798 m
	has aperture 9.1 m
	has comment A twin of the Hobby-Eberly Telescope
	has creation date (2004)
	has focal ratio (f/1.4) f/4.7
	has latitude 32° 23' S
	has location Sutherland, South Africa
	has longitude 20° 49'E
	has mounting altazimuth fixed in altitude but rotates in azimuth
	has optical design Spherical figure
	has owner South African Astronomical Obs.
	is an instance of altazimuth telescope
	is an instance of optical telescope
	is an instance of reflector
space motion	has definition Velocity of a star with respect to the Sun; hypotenuse of the right triangle formed by its radial and tangential velocities (cf. peculiar velocity). Space motion vectors are U (in the direction of the galactic anticenter), V (in the direction of galactic rotation), and W (in the direction of the galactic north pole).
	is a kind of velocity
Space Physics Analysis Network	has acronym SPAN
	is a kind of astronomical software
space science institution	is a kind of institution
space station	is a kind of manned spacecraft
space station Freedom	is an instance of space station
space telescope	is a kind of artificial satellite
	is a kind of electromagnetic telescope
space velocity	has definition A star's total velocity with respect to the local standard of rest. This is the combination of the star's U, V, and W velocities: space velocity = sqrt (U2 + V2 + W2). For example, the Sun (U = -9, V = +12, W = +7) has a space velocity of 17 kilometers per second.
	is a kind of galactic velocity component
spallation	has definition The process in which an incoming beam of particles or energy collides with a substance, reacts with it, and knocks off pieces of it.
	is a kind of ablation
spark chamber	has definition A means of detecting high energy particles by the trail of ionizations left as they pass through a chamber containing many charge plates.
	is a kind of high energy detector
sparticle	has definition Hypothetical particle which is predicted by some Grand Unified Theories.
	is a kind of hypothetical particle
special relativity	has author Einstein (1905)
	has definition Relativity theory that explains Lorentz contraction and time dilation for observers in relative motion.
	has equation E = mc2 (where m is mass, E is energy and c is the speed of light)
	has phenomena Lorentz contraction and time dilation
	has proposition A spaceship (or other enclosed vessel) traveling at uniform speed through space contains its own space-time continuum. A ray of light passes an observer at the speed of light no matter how (uniformly) fast nor in what direction the observer is travelling.
	is an instance of relativity theory
specific volume unit	has definition cubic meter per kilogram
	has symbol m3·kg-1
	is a kind of derived SI unit
spectral feature	has definition A feature in the spectra of a celestial object which occurs at a specific wavelength or range of wavelengths
	has species
	has wavelength
	has width
	is a kind of radiation measurement
	occurs in
spectral line	has definition A line corresponding to an image of the entrance slit of the spectrometer, seen when light is either emitted by or interrupted by a hot rarefied gas such as hydrogen. The pattern is characteristic of the gas and the wavelength at which the features are observed to occur is indicative of the velocity of the object.
	has definition Dark lines visible in an absorption spectrum, or bright lines that make up an emission spectrum. They are caused by the transference of an electron in an atom from one energy level to another; strong lines are produced at levels at which such transference occurs easily, weak where it occurs with difficulty. Ionization of certain elements can affect such transferences and cause problems in spectral analysis.
	has definition Discrete emissions (or absorptions) in frequency, usually formed by atomic transitions. The essential difference between optical line spectra and X-ray spectra is that the former correspond to energy changes in the outer electrons in an atom, and the latter to energy changes in the inner electron orbitals. Gamma rays usually correspond to energy changes in the nucleus. Infrared radiation is produced by high-n transitions of atoms or by the vibration or rotation of molecules. Thermal radio emission is usually produced by still higher-n transitions (the notation 109α corresponds to a transition in a hydrogenic atom between the principal quantum number n = 109 and n' = Δn = n + 1 = 110; similarly, a β-transition indicates Δn = 2. etc.).
	has definition Emission or absorption at a discrete wavelength or frequency, caused by atomic or molecular transitions. In the case of atoms, the transitions involve the jump of an electron from one orbit to another; a quantum of light is emitted if the electron jumps toward the nucleus and absorbed if it jumps outward.
	has equivalent width
	has transition
	is a kind of spectral feature
spectral series line	has definition A spectral line belonging to a series of lines of a given atom arising from transitions with a common lower energy level.
	has lower energy level
	has series limit
	is a kind of spectral line
spectrograph	has definition A device, usually based on a finely etched grate that performs the function of a prism, for breaking up light into its constituent parts and making a photographic or electronic record of the resulting spectrum. When lacking a means for recording the spectrum, the device is called a spectroscope.
	has definition A spectroscope fitted with a device such as a photoelectric cell for measuring the spectra observed with it.
	has definition An instrument that records the amount of light in each range of wavelength, that is, in each range of color. In general, each type of astronomical object, such as a star or a galaxy, will emit a characteristic spectrum of light. (See spectrum.)
	has synonym spectrometer
	is a kind of detector
spectrograph dispersing element	has definition The part of a spectrograph which disperses the light into component colors
	has dispersion
	has wavefront modification wavelength
	is a kind of wavefront modifier
spectroheliograph	has definition Device with which spectra of the various regions of the Sun are obtained and photographed.
	is a kind of spectrograph
spectroscopic binary	has definition Star whose binary nature can be detected from the periodic Doppler shifts of their spectra, owing to their varying velocities in the line of sight. Double-lined spectroscopic binaries have two sets of spectral features, oscillating with opposite phases. Single-lined spectroscopic binaries have only one set of oscillating spectral lines, owing to the dimness of the secondary component. Spectroscopic binaries are typically of spectral type B, with almost circular orbits (whereas long-period M-type binaries have highly eccentric orbits).
	is a kind of binary star
spectrum variable	has definition Main-sequence Am or Ap stars whose spectra show anomalously strong lines of metals and rare earths which vary in intensity by about 0.1 mag over periods of about 1-25 days. They are characterized by large magnetic fields (103-104 gauss) at the surface, small variations in light and color, and small projected rotational velocities. These peculiarities are sometimes interpreted in terms of an oblique rotator.
	is a kind of star
speed of light in vacuum	has symbol c
	has uncertainty 0
	has value 299792458 m s-1
	is an instance of universal constant
sphere	has definition The outer surface of a ball. The surface of a familiar three-dimensional ball has two dimensions (which can be labeled by two numbers such as "latitude" and "longitude," as on the surface of the earth). The concept of a sphere, though, applies more generally to balls and hence their surfaces, in any number of dimensions. A one-dimensional sphere is a fancy name for a circle; a zero-dimensional sphere is two points (as explained in the text). A three-dimensional sphere is harder to picture; it is the surface of a four-dimensional ball.
	has dimensions 3
	is a kind of geometrical object
spheres	has definition Concept probably older than the ancient Greeks, in which the Sun, Moon, planets and the stars were thought to orbit the Earth travelling on their own crystalline but - except for that of the stars - transparent spheres.
	is a kind of outdated belief
spherical collapse	has definition Initial stage in the collapse of a star, followed by gravitational collapse and finally singularity.
	has symmetry spherical
	is a kind of collapse
	is followed by gravitational collapse
Spica	has B-V magnitude -0.23
	has declination -11 9 41
	has definition A double-lined spectroscopic binary (B1.5 V, late B). Component A (10.9 Msun) is a beta Cephei star which seems to be near core hydrogen exhaustion.
	has distance 80 pc
	has eccentricity e = 0.146
	has inclination i = 65°
	has orbital period 4.01452 days
	has right ascension 13 25 11.5
	has spectral type B1III-IV+B2V
	has synonym alpha Vir
	has synonym HR 5056
	has V magnitude 0.98
	is a part of Virgo
	is an instance of B star
	is an instance of giant
	is an instance of naked eye star
	is an instance of spectroscopic binary
spicule	has definition A short-lived, narrow jet of gas spouting out of the solar chromosphere. Spicules tend to cluster at the edges of supergranulation cells.
	has lifetime 5 minutes
	has location edge of supergranulation cell
	has synonym mottle
	is a part of chromosphere
spin	has definition A quantum property of all particles which denotes the intrinsic angular momentum of the particle.
	has definition A quantum-mechanical version of the familiar notion of the same name; particles have an intrinsic amount of spin that is either a whole number or half a whole number (in multiples of Plancks constant), and which never changes.
	has definition The intrinsic angular momentum of an elementary particle, as by the particle's spinning on its axis. Spin is quantized in units of Planck's constant of action, h, so that, e.g., "spin 1," means spin = 1h. Particles with integral spin (0, 1) are called bosons; those with half spin are fermions.
	has definition The intrinsic angular momentum possessed by many particles. It can be thought of as resulting from the particles spinning about an axis through their centers. In contrast to orbital angular momentum, spin is quantized in integer and half-integer units of h bar. Fundamentally, spin describes how quantum fields transform under the transformations of special relativity.
	is an instance of quantum quantity
spin-flip collision	has definition Collision between particles in which the direction of the spin angular momentum changes. Since the total angular momentum is conserved, the orbital angular momentum must be changed in magnitude or direction or both.
	is a kind of collision
	see also 21-cm radiation
spiral density wave	has definition A wave, due to a local increase in the gravitational field, that produces a series of alternate compressions and rarefactions as it propagates with fixed angular velocity in a rotating galaxy. The compression also acts on interstellar gas in the galaxy, which is triggered to form stars on the leading edges of the spiral arms. The large-scale structure of spiral galaxies can be understood in this way.
	is a kind of galaxy theory
spiral galaxy	has classification criterion Subdivided according to the openness of the spiral arms as Sa spiral, Sb spiral or Sc spiral.
	has definition A galaxy with a prominent nuclear bulge and luminous spiral arms of gas, dust, and young stars that wind out from the nucleus.
	has mass 1010 to 1012 Msun
	is a kind of galaxy
spiritualism	has antonym materialism
	has definition Belief that material interactions alone cannot account for all phenomena, and that some - e.g., thought - are due to the fundamentally insensible actions of intangibles.
	is a kind of religious belief
spontaneous emission	has definition Radiation emitted by an isolated body.
	is an instance of bound-bound transition
	is an instance of emission
spontaneous fission	is a kind of energy source
	is a kind of fission
spontaneous symmetry breaking	has definition Any situation in physics in which the ground state (i.e. the state of minimum energy) of a system has less symmetry than the system itself. For example, the state of minimum energy for an iron magnet is that in which the atomic spins are all aligned in the same direction, giving rise to a net macroscopic magnetic field. By selecting a particular direction in space. the magnetic field has broken the rotational symmetry of the system. However, if the energy of the system is raised, the symmetry may be restored (e.g. the application of heat to an iron magnet destroys the magnetic field and restores rotational symmetry).
	has definition The breaking of an exact symmetry of the underlying laws of physics by the random formation of some object. For example, the rotational in variance of the laws of physics can be broken by the randomly chosen orientation of an orthorhombic crystal that condenses as the material is cooled. In the standard model of particle physics, the symmetry between electrons and neutrinos is spontaneously broken by the values that are randomly chosen by the Higgs fields. In grand unified theories, the symmetry between electrons, neutrinos, and quarks is spontaneously broken by the values chosen randomly by the Higgs fields.
	is a kind of phase transition
Sporer's law of zones	has definition The equatorward drift of average sunspot latitudes.
	is a kind of solar motion
Sputnik 1	has definition First artificial Earth satellite, launched by the Soviet Union on 4 October 1957. (lit.: companion)
	is an instance of artificial satellite
SS Cygni star	has definition A subclass of dwarf nova. SS Cyg is a double-lined, noneclipsing spectroscopic binary (sdBe, dG5) with an orbital period of 6h38m. Mean time between eruptions, 54 days. It may be a sporadic source of soft X-rays.
	is a kind of dwarf nova
SSPM	has definition Solid State Photomultiplier.
	is a kind of detector
stable Lagrangian point	has equilibrium stable
	is a kind of Lagrangian point
	see also Trojan asteroid
standard deviation	has definition The root mean square deviation from the arithmetic mean.
	has symbol σ
	is a kind of statistical quantity
standard error	has acronym s.e.
	has definition The standard deviation of a distribution of means or any other statistical measure computed from samples. It is equal to 1.4826 times the probable error.
	is a kind of statistical quantity
standing wave	has definition A pattern of oscillations in space in which the regions of maximum displacement and of zero displacement (the nodes) remain fixed in position. This pattern is formed when two waves of the same amplitude and frequency move simultaneously through a medium in opposite directions.
	has synonym stationary wave
	is a kind of wave
Stanford Linear Accelerator Center	has acronym SLAC
	has definition It is distinguished by having a 2-mile-long linear accelerator in which electrons and positrons can be accelerated for subsequent injection into storage rings such as PEP, an e+e- collider which was commissioned in 1980. It was in the SPEAR rings at SLAC that the J / ψ meson and the τ lepton were first observed in the mid-1970s. However, the most fascinating of SLAC's facilities is the novel SLC (Stanford Linear Collider), consisting of the old linear accelerator together with two new collider arcs.
	has location Stanford University, California, USA.
	is an instance of high energy physics institution
star	has absolute magnitude
	has age
	has apparent magnitude
	has B magnitude
	has catalog star catalog
	has definition A celestial object that generates energy by means of nuclear fusion at its core. To do this it must have more than about 0.08 the sun's mass. If, for instance, the planet Jupiter were some fifty to one hundred times more massive than it is, fusion reactions would transpire in its core and it would be a star. See planet.
	has definition a spinning ball of hot gases held together by gravity
	has energy production which takes place primarily within the core
	has energy source gravitational contraction and or fusion
	has mass greater than 0.08 the sun's mass
	has material hydrogen, helium
	has parallax from the point of view of Earth's orbit
	has position on celestial sphere from the point of view of Earth
	has proper motion
	has radiation at surface which is diffused out from the hotter core
	has spectral type
	has star surface temperature
	has surface density which depends on luminosity class
	has surface temperature greater than 1000 Kelvin
	has U magnitude
	has V magnitude
	has velocity determined from proper motion and radial velocity
	is a kind of celestial body
star catalog	has object type star
	is a kind of astronomical catalog
star cluster	has definition Gravitationally bound aggregations of stars, smaller and less massive than galaxies.
	is a kind of collection of stars
star gravitational contraction	is a kind of gravitational contraction
star merger	has definition The formation of a new star from the gradual merging of two stars which spiral into each other within a common envelope of gas.
	is a kind of merger
star system	has abundance half the stars in the solar neighborhood are members of star systems
	has definition A few stars that orbit each other.
	is a kind of collection of stars
starburst galaxy	has definition Any galaxy in which an anomalously large rate of star formation is taking place.
	is a kind of galaxy
Stark effect	has cause the influence of an electric field on a radiating atom or ion
	has definition Line broadening due to the influence of electric fields.
	has effect splitting of a spectral line
	has use indicator of atmospheric pressure in a stellar atmosphere and hence of the star's luminosity
	is a kind of line broadening
	is proportional to the ion and electron density in a plasma
STARLINK	has acronym STARLINK
	has definition A software environment and suite of programs for astronomical data analysis developed in the UK and supported by the Rutherford-Appleton Labs.
	is a kind of astronomical software
stat-coulomb	has definition Unit of charge in the cgs electrostatic system.
	has equivalent 3.3 × 10-10 coulombs.
	is an instance of CGS unit
	is an instance of charge unit
static universe	has definition A universe whose radius of curvature is constant and independent of time, as in the Einstein universe.
	is a kind of cosmology theory
statistical concept	is a kind of mathematical concept
statistical distribution	has definition The range of variation of some quantity in a population, obtained by sampling many members of the population. For example, the statistical distribution of the height of American males could be obtained by sampling 10,000 randomly chosen males and counting the number of them within each range of heights. In cosmology, the distance between pairs of galaxies, averaged over a large number of galaxies, would constitute a statistical distribution.
	is a kind of statistical concept
statistical error	has definition The uncertainty resulting from a measurement of purely random events. Such an uncertainty is defined as bracketing a range of values within which the correct value has a 66% chance of lying. For example, a value of (100 ± 10) obtained from a given measurement means that the true value has a 66% chance of lying between 90 and 110, and a 34% chance of being either above or below this range.
	is a kind of statistical quantity
statistical quantity	is a kind of quantity
	is a kind of statistical concept
statistical test	is a kind of statistical concept
Steady State Theory	has definition A cosmological theory propounded by Bondi, Gold, and Hoyle in which the Universe has no beginning and no end and maintains the same mean density, in the face of its observed expansion, by the continuous creation of matter at the current rate of 2.8 × 10-46 g cm-3 s-1 (or roughly one nucleon per cubic kilometer per year). Discovery of the microwave background has persuaded most astronomers to reject the steady-state theory.
	has definition Theory that the expanding universe was never in a state of appreciably higher density - i.e., that there was no "big bang" - and that matter is constantly being created out of empty space in order to maintain the cosmic matter density.
	is a kind of cosmology theory
Stefan's law	has definition The flux of radiation from a blackbody is proportional to the fourth power of its absolute temperature: L = 4πR2σT4.
	is an instance of quantum law
Stefan-Boltzmann constant	has definition A constant relating the energy radiated by a black body to its absolute temperature and incorporated in the Stefan-Boltzmann radiation law which states that the energy radiated per unit time is given by σ(T4 - T40), where σ is the Stefan-Boltzmann constant and T and T0 are the absolute temperatures of the body and its surroundings expressed in kelvins.
	has definition The constant of proportionality relating the luminosity of a star to its absolute temperature: σ = 5.67 × 10-5 ergs cm-2 (deg-K)-4 s-1.
	has equation
	has symbol σ
	has uncertainty 0.000040 × 10-8 W m-2 K-4
	has value 5.670400 × 10-8 W m-2 K-4
	is an instance of physico chemical constant
stellar nucleosynthesis	has definition The production of heavy nuclei from the fusion of lighter ones within a star. The Sun presently converts hydrogen into helium.
	is a kind of nucleosynthesis
stellar parallax	is a kind of parallax
stellar theory	has domain star
	is a kind of astronomy theory
stellarator	has definition A type of plasma machine. It has a twisted-field configuration in the form of a figure 8 to fold the plasma back on itself; therefore, unlike a pinch machine, it has no ends where the plasma can leak out. Stellarators and tokomaks resemble each other in that both are toroidal devices that attain equilibrium and MHD stability through rotational transform and shear; they differ mainly in the way they attain these properties.
	is a kind of fusion device
Stephan's Quartet	has number of galaxies 4
	is an instance of compact group
Stephan's Quintet	has definition A highly disturbed cluster of five peculiar galaxies (NGC 7317, NGC 7318A, NGC 7318B, NGC 7319, NGC 7320) in Pegasus which seem to exhibit gaseous connecting bridges. Four have large redshifts (of the order of 5700-6700 km s-1), but the fifth member (NGC 7320) has a much smaller redshift (800 km s-1). Discovered in 1877 by M. E. Stephan.
	has number of galaxies 5
	is a part of Pegasus
	is an instance of compact group
steradian	has approval date 1900
	has base unit m2·m-2 = 1
	has definition Unit of solid (three-dimensional) angular measure. One steradian is equal to the angle subtended at the centre of a sphere by an area of surface equal to the square of the radius. The surface of a sphere subtends an angle of 4π steradians at its centre.
	has proposal date 1880
	has symbol sr
	has symbol sr, Ω0
	is a unit of solid angle
	is an instance of named derived SI unit
	is an instance of solid angle unit
stimulated emission	has definition Radiation emitted by a body, such as an atom, when it is bombarded by radiation. The stimulated radiation has the same wavelength and direction as the bombarding radiation.
	has synonym induced emission
	is an instance of bound-bound transition
	is an instance of emission
Stingray Nebula	has distance 18000 light years
	has extent 130 solar systems
	has image
	has synonym Hen-1357
	is an instance of planetary nebula
	is part of Ara
strange	has charge -1/3
	has definition A flavor of quark.
	is an instance of light quark
strangeness	has definition A property ascribed to certain hyperons whose lifetimes before decay are abnormally long (about 10-8 to 10-10 seconds) relative to their rates of production (about one every 10-23 seconds). Like parity, strangeness is conserved in strong interactions but not in weak ones.
	has definition A quantum number associated with the strange quark. Strangeness is conserved by the strong nuclear force.
	has definition A quantum number used in quark theory.
	is an instance of quantum quantity
stratosphere	has altitude 15 to 50 km
	has definition The region of Earth's atmosphere immediately above the troposphere. The temperature increases with altitude.
	has temperature 240 to 270 K
	is a part of atmosphere
Stratospheric Observatory for Far-Infrared Astronomy	has acronym SOFIA
	is an instance of airborne telescope
string theory	has definition A theory in which the fundamental constituents of matter are not particles but tiny one-dimensional objects, which we can think of as strings. These strings are so minute (only 10-33 cm long) that, even at current experimental energies, they seem to behave just like particles. So, according to string theory, what we call "elementary particles" are actually tiny strings. each of which is vibrating in a way characteristic of the particular "elementary particle".
	has definition Fundamental one-dimensional object that is the essential ingredient in string theory.
	has definition Nambu's original idea that the elementary particles could be described as extended, one-dimensional objects was called string theory. Since the ends of Nambu's strings whipped around at the speed of light they were also called light strings. Later attempts to include the spin half fermions within a string theory led to the term spinning strings. Strings that possess supersymmetry are called superstrings. Heterotic strings combine spaces of two different dimensionalities. The term string is used in a generic way to describe all these different variations, including superstrings.
	has definition The hypothesized, basic constituents of matter, according to new theories of physics. In earlier theories of physics, the basic constituents of matter were point-like particles, such as electrons, which interacted with other particles at a point. According to the string theory, the basic constituents are 1-dimensional structures called strings. There are completely different strings, called cosmic strings, which can form according to some theories and which may extend for great distances in space. Postulated to have formed as a result of processes in the early universe, cosmic strings are 1-dimensional structures of enormous energy, extending for perhaps thousands or millions of light years in space. There is no good observational evidence that either kind of strings exist. (See superstring theory.)
	has definition Theory that subatomic particles actually have extension along one axis, and that their properties are determined by the arrangement and vibration of the strings.
	has definition Unified theory of the universe postulating that fundamental ingredients of nature are not zero-dimensional point particles but tiny one-dimensional filaments called strings. String theory harmoniously unites quantum mechanics and general relativity the previously known laws of the small and the large, that are otherwise incompatible. Often short for superstring theory.
	is a kind of unified theory
strong force	has carrier boson gluon
	has definition Fundamental force of nature that binds quarks together, and holds nucleons (which are comprised of quarks) together as the nuclei of atoms. Portrayed in quantum chromodynamics as conveyed by quanta called gluons.
	has definition One of the four fundamental forces of nature. It governs the interaction between particles in atomic nuclei.
	has definition Strongest of the four fundamental forces, responsible for keeping quarks locked inside protons and neutrons and for keeping protons and neutrons crammed inside of atomic nuclei.
	has definition The interactions which bind quarks together to form rotons, neutrons, and other particles. The residual effects of these forces are responsible for the forces between protons and neutrons. See Yang-Mills theories.
	has definition The short-range nuclear force which is assumed to be responsible for binding the nucleus together. Strong interactions are so called because they occur in the extremely short time of about 10-23 seconds. Strong interactions can occur only when the particles involved are less than 3 fermis apart.
	has definition The short-range nuclear interactions responsible for holding nuclei together. The characteristic range of the strong interaction is 10-13 cm, and the time scale over which it operates is 10-33 second.
	has range
	has strength (relative to electromagnetism)
	has synonym nuclear force
	is a kind of fundamental force
strong force symmetry	has definition Gauge symmetry underlying the strong force, associated with invariance of a physical system under shifts in the color charges of quarks.
	is a kind of gauge symmetry
strontium	has image
	has ocean oxidation state II
	has ocean residence time 4 × 106 years
	is a kind of alkali earth metal
	is a kind of recycled oceanic element
SU Ursa Major	is a part of Milky Way
	is an instance of dwarf nova
SU(2)	has definition The symmetry of the weak nuclear interaction.
	is a kind of elementary particle symmetry
SU(2) x U(1)	has definition The symmetry of the unified electroweak interaction.
	is a kind of elementary particle symmetry
SU(3)	has definition Symmetrical Unitary of Order 3: A symmetry found in sub-nuclear spectra. It is a concept in group theory, by which Gell-Mann and others, using eight quantum numbers, have been able to combine particles into family groups or supermultiplets, as the lowest-lying eightfold group of the nucleon doublet, the Λ singlet, the Σ triplet, and the Ξ doublet. The SU(3) theory applies only to the strongly interacting particles.
	has definition The symmetry corresponding to quark theory and the strong nuclear interaction.
	is a kind of elementary particle symmetry
SU(5)	has definition One of the suggested symmetries of the grand unified theory in which the gluon and electroweak forces are united. It includes the group SU(3) × SU(2) × U(1).
	has definition Symmetrical Unitary of Order 5: The simplest type of grand unified theory, proposed in the 1970s. (See grand unified theories.)
	is a kind of elementary particle symmetry
Subaru Telescope	has altitude 4215 m
	has aperture 8.3 m
	has comment Thin primary (Coming ULE) under active control
	has creation date (1999)
	has focal ratio f/1.8, 12.5, 35
	has latitude 19° 49' N
	has location Mauna Kea, Hawaii, US
	has longitude 155° 28' W
	has mounting manufacturer Mitsubishi
	has owner National Astronomy Obs. (Japan)
	has synonym Subaru
	is an instance of altazimuth telescope
	is an instance of optical telescope
	is an instance of Ritchey-Chrétien
subdwarf	has definition A metal-poor main-sequence star. On the H-R diagram, subdwarfs lie slightly below the metal-rich Main Sequence, because they are fainter than metal-rich main-sequence stars of the same color.
	has definition Late-type object whose observed color and absolute magnitude place it below the Main Sequence.
	has definition Star whose luminosity is 1.5 to 2 magnitudes lower than that of main-sequence stars of the same spectral type. Subdwarfs are primarily Population II and lie just below the Main Sequence on the H-R diagram.
	has luminosity class VI
	has symbol sd
	is a kind of star
subgiant	has definition A star whose position on the H-R diagram is intermediate between that of main-sequence stars and normal giants of the same spectral type.
	has luminosity class IV
	is a kind of star
subgiant CH star	has definition Hot Ba star
	has spectral type < G5
	is a kind of barium star
sublimation	has final phase gas
	has initial phase solid
	has inverse process condensation
	is a kind of first order phase transition
submillimeter	has frequency 300 GHz to 3 THz
	has wavelength 100 μm to 1 mm
	is a kind of microwave
Sudbury Neutrino Observatory	has acronym SNO
	has component deuterium detector
	is an instance of neutrino telescope
sulfur	has abundance 1.6 × 107 in Sun relative to H = 1 × 1012
	has abundance 260 p.p.m. in Earth's crust
	has abundance 870 p.p.m. in seawater
	has atomic emission line 545.38 nm for S II
	has atomic emission line 547.36 nm for S II
	has atomic emission line 550.97 nm for S II
	has atomic emission line 560.61 nm for S II
	has atomic emission line 565.99 nm for S II
	has atomic emission line 792.40 nm for S I
	has atomic emission line 964.99 nm for S I
	has atomic number
	has atomic radii 104 pm
	has biological role essential to all living things; part of the amino acids methionine and cysteine
	has boiling point 717.824 K
	has chief source native sulfur, pyrite, H2S in natural gas
	has covalent radii 104 pm
	has critical pressure 20700 kPa
	has critical temperature 1314 K
	has crystal cell dimension a = 1046.46, b = 1286.60, c = 2448.60 pm for α form
	has crystal type orthorhombic for α form
	has daily dietary intake 850 - 930 mg
	has definition the α-S8 orthorhombic form of sulfur is yellow
	has density 1819 kg m-3 for liquid at 393 K
	has density 1957 kg m-3 for β solid at 293 K
	has density 2070 kg m-3 for α solid at 293 K
	has discovery date prehistoric
	has electrical resistivity 2 × 1015 Ω m at 293 K
	has electron affinity 200.4 kJ mol-1 from S to S-
	has electron configuration [Ne]3s23p4 in ground state
	has electronegativity 2.58 Pauling
	has hazard elemental form is harmless unless ingested; ignited it emits highly toxic SO2 fumes
	has heat capacity 22.64 J K-1 mol-1 for α solid at constant pressure 0.1 MPa at 298.15 K
	has heat capacity 23.673 J K-1 mol-1 for gas at constant pressure 0.1 MPa at 298.15 K
	has heat of fusion 1.23 kJ mol-1
	has heat of vaporization 9.62 kJ mol-1
	has image
	has ionic radii 184 pm for S2-
	has ionic radii 29 pm for S6+
	has ionic radii 37 pm for S4+
	has isotope mass range 29 to 39
	has lethal intake 175 mg kg-1 for rabbits
	has level in humans 1800 mg dm-3 in blood
	has level in humans 500 - 2400 p.p.m. in bone
	has level in humans 5000 - 11000 p.p.m. in muscle
	has level in humans 7000 - 12000 p.p.m. in liver
	has linear expansion coefficient 74.33 × 10-6 K-1
	has longest lived isotope sulfur 32
	has main mining area USA (native sulfur), Spain
	has mass absorption coefficient 89.1 cm2 g-1 for CuKα X-ray diffraction
	has mass absorption coefficient 9.55 cm2 g-1 for MoKα X-ray diffraction
	has mass magnetic susceptibility -5.83 × 10-9 kg-1 m3 for β solid
	has mass magnetic susceptibility -6.09 × 10-9 kg-1 m3 for α solid
	has mass of element in person 140 g for a 70 kg average person
	has melting point 386.0 K for α form
	has mineral occurs naturally as native sulfur deposits associated with oil-bearing strata
	has molar volume 15.49 cm3
	has name origin sulvere = sulfur from Sanskrit (sulphurium from Latin)
	has neutron scattering length 0.2847 × 10-12 cm
	has number of isotopes 11 including nuclear isomers
	has number of protons 16
	has ocean oxidation state VI
	has ocean residence time 8 × 106 years
	has pronunciation sul-fer
	has registry number 7704-34-9 for Chemical Abstracts System database
has relative atomic mass 32.066 in units of 12C = 12.000
has reserves 2.5 × 109 tonnes
has space group Fddd for α form
has specimen powder and flakes. Safe.
has symbol S
has term symbol 3P2 in ground state
has thermal conductivity 0.269 W m-1 K-1 for α at 300 K
has thermal neutron capture cross section 0.53 barns
has toxic intake elemental form is not very toxic, but simple derivatives are (SO2, H2S, etc.)
has uses key industrial chemical, starting point for sulfuric acid
has van der Waals radii 185 pm
has world production 54 × 106
is a kind of accumulating oceanic element
is a kind of chalcophile element
is a kind of group VI element
is a kind of nonmetallic element
reacts with air when heated
reacts with oxidising acids
sulfur 32	has atomic mass 31.97207070
	has magnetogyric ratio 2.0534 × 107 rad T-1 s-1
	has natural abundance 95.02%
	has NMR frequency 24.664 MHz where 1H = 100 MHz; 2.3488 T
	has NMR receptivity 0.0973 where 13C = 1.00
	has nuclear magnetic moment μ = 0 nuclear magnetons with diamagnetic correction
	has nuclear quadrupole moment -0.678 × 10-28 m2
	has nuclear spin I = 0+ h/2π
	has number of neutrons 16
	has number of nucleons 32
	has relative NMR sensitivity 2.26 × 10-3 where 1H = 1.00
	has symbol 32S
	is an instance of sulfur
sulfur 33	has atomic mass 32.97145843
	has natural abundance 0.75%
	has nuclear magnetic moment μ = +0.643821 nuclear magnetons with diamagnetic correction
	has nuclear spin I = 3/2+ h/2π
	has number of neutrons 17
	has number of nucleons 33
	has symbol 33S
	has uses Nuclear Magnetic Resonance
	is an instance of sulfur
sulfur 34	has atomic mass 33.96786665
	has natural abundance 4.21%
	has nuclear magnetic moment μ = 0 nuclear magnetons with diamagnetic correction
	has nuclear spin I = 0+ h/2π
	has number of neutrons 18
	has number of nucleons 34
	has symbol 34S
	is an instance of sulfur
sulfur 35	has atomic mass 34.969031
	has decay mode β- (0.1674 Mev)
	has decay mode no γ
	has half life 87.2 days
	has nuclear magnetic moment μ = +1.00 nuclear magnetons with diamagnetic correction
	has nuclear quadrupole moment 0.0471 × 10-28 m2
	has nuclear spin I = 3/2+ h/2π
	has number of neutrons 19
	has number of nucleons 35
	has symbol 35S
	has uses research
	is an instance of sulfur
sulfur 36	has atomic mass 35.96708062
	has natural abundance 0.02%
	has nuclear magnetic moment μ = 0 nuclear magnetons with diamagnetic correction
	has nuclear spin I = 0+ h/2π
	has number of neutrons 20
	has number of nucleons 36
	has symbol 36S
	is an instance of sulfur
sulfur 37	has atomic mass 36.971125
	has decay mode β- (4.865 Mev)
	has decay mode γ
	has half life 5.05 minutes
	has number of neutrons 21
	has number of nucleons 37
	has symbol S
	is an instance of sulfur
sulfur 38	has atomic mass 37.971162
	has decay mode β- (2.94 Mev)
	has decay mode γ
	has half life 2.84 hours
	has nuclear magnetic moment μ = +1.00 nuclear magnetons with diamagnetic correction
	has nuclear spin I = 0+ h/2π
	has number of neutrons 22
	has number of nucleons 38
	has symbol 38S
	is an instance of sulfur
sulfur monoxide	has symbol SO
	is an instance of diatomic molecule
	is an instance of interstellar molecule
summer solstice	is an instance of solstice
Sun	has absolute bolometric magnitude Mbol = +4.67
	has absolute visual magnitude Mv = +4.85
	has age 4.6 billion years
	has apparent magnitude -26.74
	has central density 155 g cm-3 (Bahcall 1973)
	has central temperature 14-15 × 106 K
	has definition Star that Earth orbits. Central body of solar system. It takes about 1-10 million years for photons to diffuse from the Sun's interior to its surface. About 3% of the energy radiated is in the form of neutrinos. Every second about 655 million tons of H are being converted into 650 million tons of He. A grazing light ray is deflected 1".7 by the Sun. If the total angular momentum of the solar system were concentrated in the Sun, its equatorial rotation speed would be about 100 km s-1.
	has density at surface 3 × 10-7 g cm-3
	has distance from galactic center 27000 light-years
	has distance from galactic plane 35 light-years
	has energy generating mass 0.35 Msun
	has energy source proton-proton reaction
	has escape velocity Vesc = 618 km s-1
	has galactic orbital period 220 million years (e ≈ 0)
	has galactic orbital velocity Vorb = 250-300 km s-1
	has inclination of rotational axis to pole of ecliptic 7°15'
	has luminosity 3.83 × 1033 ergs s-1
	has luminosity class V (main-sequence)
	has magnetic field 1-2 gauss as high as 10-1000 gauss in active regions
	has mass 1.989 × 1033 g
	has mean density 1.409 g cm-3
	has mean rotation speed 1.9 km s-1
	has radius 695990 km
	has rotational period at equator 24d6h
	has rotational period at poles 35 days
	has spectral type G2
	has surface gravity 27398 cm s-2
	has surface temperature 5785 K
	has velocity relative to nearby stars 20 km s-1 toward R.A. 18h4m, declination +30° (towards in Hercules)
	is a part of our solar system
	is a part of our solar system
	is a part of our solar system
	is a part of Zodiac
	is an instance of dwarf
	is an instance of G star
	is an instance of naked eye object
	is an instance of Population I star
Sun Kelvin timescale	has definition The time it takes the Sun to contract gravitationally from infinite radius down to its present radius by radiating its thermal energy.
	has value 2 to 3 × 107 years
	is an instance of Kelvin timescale
Sun nuclear time scale	has definition Time required for the Sun to evolve a significant distance off the main sequence; the time it takes the Sun to convert all its available hydrogen into helium.
	has value 1010 years.
	is an instance of nuclear time scale
Sun orbital event	has participants planet, Sun
	is a kind of orbital event
sunrise	has definition The times at which the apparent upper limb of the Sun is on the astronomical horizon; i.e., when the true zenith distance, referred to the center of the Earth, of the central point of the disk is 90°50', based on adopted values of 34' for horizontal refraction and 16' for the Sun's semidiameter.
	has period 1 day
	is an instance of astronomical horizon event
sunset	has definition The times at which the apparent upper limb of the Sun is on the astronomical horizon; i.e., when the true zenith distance, referred to the center of the Earth, of the central point of the disk is 90°50', based on adopted values of 34' for horizontal refraction and 16' for the Sun's semidiameter.
	has period 1 day
	is an instance of astronomical horizon event
sunspot	has definition A temporary disturbed area in the solar photosphere that appears dark because it is cooler than the surrounding areas. Sunspots usually occur in pairs of opposite polarity about 30° N and S of the equator, and move in unison across the face of the Sun as it rotates. The leading (or preceding) spot is called the p-spot; the following, the f-spot. Some sunspots have magnetic fields as high as 1000 gauss (highest observed was 5000 gauss
	has definition Comparatively dark spot on the Sun's photosphere, commonly one of a (not always obvious) group of two. The center of a vast electrostatic field and a magnetic field of a single polarity (up to 4000 gauss), a sunspot represents a comparatively cool depression (at a temperature of approximately 4500 °C). Sunspots occur in cycles of about 11 Earth-years in period although their individual duration - a matter of Earth-days only - is affected by the differential rotation of the Sun; they tend to form at high latitudes and drift towards the solar equator. They are also sources of strong ultra-shortwave radio emissions.
	is a part of photosphere
sunspot radiation	has definition Intense, variable, circularly polarized radio waves in a noise storm.
	is a part of sunspot
Sunyaev-Zel'dovich process	has definition Compton scattering between the photons of the cosmic microwave background radiation and electrons in galaxy clusters.
	is a kind of Compton scattering
super-metal-rich star	has definition Used in reference to stars, or stellar populations, which are richer in metals than the Hyades.
	is a kind of metal-rich star
supercluster	has definition A cluster of clusters of galaxies. Superclusters are typically about one hundred million (108) light-years in diameter and contain tens of thousands of galaxies.
	has number of clusters
	is a kind of collection of galaxies
superconducting super collider	has definition A proposed accelerator of great size and high energy.
	is an instance of collider
Supergalactic Plane	has definition An apparent plane of symmetry, passing through the Virgo cluster of galaxies, about which many of the brightest galaxies in the sky are concentrated. These galaxies form the Local Supercluster.
	is a kind of plane
supergiant	has definition An extremely luminous star of large diameter and low density. No supergiants are near enough to establish a trigonometric parallax.
	has luminosity class I
	is a kind of star
supergranulation cell	has definition Convective cell in the solar photosphere, distributed fairly uniformly over the solar disk, that last as long as a day. New sunspots develop in the intersections of adjacent supergranulation cells. Most of the magnetic flux through the photosphere is concentrated in the supergranule boundaries.
	has diameter 15000 to 30000 km
	has distribution uniform
	has mean lifetime 1 day
	is a part of photosphere
supergravity	has definition A supersymmetric theory of gravity in which the graviton is accompanied by a spin-3/2 particle called the "gravitino". In supergravity theories, supersymmetry has been promoted to the status of a local gauge symmetry.
	has definition Class of point-particle theories combining general relativity and supersymmetry
	is a kind of supersymmetry
superior planet	has definition Planets farther from the Sun than the Earth is (i.e., Mars to Pluto).
	is a kind of planet
supermultiplet	has definition A multiplet of multiplets.
	is a kind of multiplet
supernova	has definition A gigantic stellar explosion in which the star's luminosity suddenly increases by as much as a billion times. Most of the star's substance is blown off, leaving behind, at least in some cases, an extremely dense core which may be a neutron star.
	has energy release 1049 to 1051 ergs
	has peak brightness 108 solar luminosity units
	has use supernova peak brightness distance determination
	is a kind of cataclysmic variable
	is a kind of emission line star
supernova explosion	is a kind of gravitational collapse
supernova produced element	has definition element heavier than iron produced within the core of an exploding supernova
	has origin exploding core of a supernova
	is a kind of element
supernova produced radioactive element	has definition a radioactive element produced within the core of an exploding supernova
	is a kind of radioactive element
	is a kind of supernova produced element
supernova remnant	has acronym SNR
	has definition A gaseous nebula, the expanding shell ejected by a supernova, and deriving its energy (at least in some cases) from the conversion by the remanent neutron star of its rotational energy into a stream of high-energy particles being continually accelerated in the SNR. About 100 SNRs are known in our Galaxy. Supernova remnants are usually powerful radio sources.
	has definition The expanding shell of gas ejected at high speed by a supernova explosion, observed as an expanding diffuse gaseous nebula, often with a shell-like structure.
	has ejection velocity 10000 km s-1
	is a kind of expanding emission nebula
	is a kind of radio source
	produces synchrotron radiation
superpartner	has definition Particle whose spins differ by 1/2 unit and that are paired by supersymmetry.
	is a kind of hypothetical particle
superstring theory	has definition A new type of theory in physics that unifies all the forces of nature, including the gravitational force, and that may be capable of explaining all of the fundamental laws and particles of nature. In superstring theories, the basic constituent of matter is a 1-dimensional structure, called a string, rather than a point-particle structure. According to superstring theory, space has more than 3 dimensions.
	has definition A proposal for the ultimate laws of nature, a "theory of everything," stemming primarily from discoveries in the mid 1980's. The fundamental entity in this theory is an ultramicroscopic string-like object, with a length of typically 10-33 centimeters and effectively zero thickness. At present our understanding of string theory is very limited. The simplest predictions of superstring theory concern processes at the Planck energy, and so far very little is known about the consequences of string theory at lower energies.
	has definition String theory that incorporates supersymmetry.
	is a kind of string theory
supersymmetric quantum field theory	has definition Quantum field theory incorporating supersymmetry.
	is a kind of supersymmetry
supersymmetric standard model	has definition Generalization of the standard model of particle physics to incorporate supersymmetry. Entails a doubling of the known elementary particle species.
	is a kind of supersymmetry
supersymmetry	has definition A mathematical property of some theories of physics proposing that every particle of integer spin (intrinsic angular momentum) has a partner of half integer spin. For example, the photon, which is the particle of light, has a spin of 1 unit. Its hypothesized super symmetric partner is called the photino, which would have a spin of 1/2 units.
	has definition A symmetry principle that relates the properties of particles with a whole number amount of spin (bosons) to those with half a whole (odd) number amount of spin (fermions).
	has definition A symmetry relating fermions and bosons. If supersymmetry is a true symmetry of nature, then every "ordinary" particle has a corresponding "superpartner" which differs in spin by half a unit.
	has definition A symmetry that relates the fermions (fractional spin particles) to the bosons (elementary particles with integral spin).
	has definition Class of theories that seek to identify symmetrical relationships linking fermions and bosons - i.e., particles of half integer spin, like electrons, protoins, and neutrinos, with those of integral spin, like photons and gluons. If attainable, a fully realized supersymmetry theory would provide a unified account of all four fundamental forces, and might well shed light on the very early evolution of the universe as well.
	is a kind of unified theory
superunified theory	has definition Hypothetical theory that presumably would show how all four fundamental forces of nature functioned as a single force in the extremely early universe. The best current candidates for such a potential achievement are thought to be supersymmetry and string theory.
	has definition While grand unified theories attempt to describe three of the four known interactions of nature - the weak, strong, and electromagnetic interactions - in a unified way, the fourth interaction, gravity, is omitted. Theories which attempt to include gravity as well, such as superstrings, are called superunified.
	is a kind of unified theory
supra-thermal proton bremsstrahlung	has definition Ordinary electron-proton bremsstrahlung viewed from the rest frame of the electron rather than the proton; in other words, the electron is at rest and the heavy particle (proton) is moving.
	is a kind of bremsstrahlung
Swan band	has definition A C2 band which passes through a minimum between spectral types R4 and R6 and increase again toward N6.
	has discoverer W. Swan
	has discovery date 1856
	is a kind of C2 band
	occurs in carbon star
SX Phoenicis	has definition A dwarf Cepheid (spectral type A) with the shortest known period (1h19m).
	is a part of Phoenix
	is an instance of dwarf Cepheid
symbiotic star	has definition A term originally used by P. Merrill to describe stars of two essentially dissimilar kinds which seem to occur together and which seem to "need" each other. In practice, it has come to signify a peculiar group of objects (usually spectral type Me) that display a combination of low-temperature absorption spectra and high-temperature emission lines. These objects undergo semiperiodic nova-like outbursts and display the spectral changes of a slow nova superposed on the features of a late-type star. Their spectra are midway between those of planetary nebulae and true stellar objects. A symbiotic star is now usually taken to be a small, hot, blue star surrounded by an extensive variable envelope. As of 1973 about 30 were known.
	has definition Object exhibiting a spectrum corresponding to a low-temperature star (generally a giant) plus emission lines corresponding to a hot plasma.
	has synonym combination variable
	is a kind of composite spectrum star
	is a kind of variable
symmetry	has antonym symmetry breaking
	has definition A property of a physical system that does not change when the system is transformed in some manner. For instance, a sphere is rotationally symmetrical since its appearance does not change if it is rotated.
	has definition State of a system such that it has a significant quantity that remains invariant after a transformation. More generally, an apt or pleasing proportion based upon such a state.
	has definition The property of being unchanged after some transformation. A square, for example, has a 4-sided rotational symmetry. It appears the same after it is rotated by 90 degrees.
	is a kind of mathematical concept
symmetry breaking	has antonym symmetry
	has definition A reduction in the amount of symmetry a system appears to have, usually associated with a phase transition.
	has definition A set of mathematical transformations that represent a symmetry.
	has definition A violation of symmetry.
	has definition In cosmology and particle physics, a state in which traces of an earlier symmetry may be discerned. A broken symmetry condition differs from chaos in that its parts can in theory be united in a symmetrical whole, like the pieces of a jigsaw puzzle.
	has definition The process by which an intrinsic symmetry of a system is disrupted. For example, a compass, in the absence of any outside magnetic field, has rotational symmetry and is equally likely to point in any direction. The magnetic field of the earth breaks the symmetry and causes the compass to point in a particular direction, toward the earth's north magnetic pole. In some cosmological models, the infant universe was much more symmetric than it is today. As the universe aged and cooled, some of these symmetries were permanently broken.
	has synonym asymmetry
	is a kind of mathematical concept
synchronous rotation	has definition Rotation whose period is equal to the orbital period.
	has period equal to orbital period
	is a kind of rotation
synodic month	has definition The period of time between two successive identical phases of the Moon e.g., new Moon to new Moon or full Moon to full Moon (cf. lunation).
	has value 29.53 days
	is an instance of month
synthetic element	has definition element which cannot be found as a mineral deposit
	has lifetime
	has synthesis mechanism
	is a kind of element
T association	has definition Association containing many T Tauri stars. About 20 are known.
	is a kind of association
T Pyxidis	has acronym T Pyx
	has definition a recurrent nova
	has distance from Earth 6000 light-years
	has erupted in 1966, 1944, 1920, 1902, and 1890.
	has image
	has reference Shara, M.M., Zurek, D.R., Williams, R.E., Prialnik, D., Roberto.G., Moffat, A.F.J. : 1997, AJ 114, 258. HST Imagery of the Non-Expanding, Clumped Shell of the Recurrent Nova T Pyxidis
	is an instance of recurrent nova
T Tauri star	has definition Also called T Tauri variable, a type of variable star of spectral classification F, G or K (giants above the main sequence on the Hertzsprung-Russell diagram) that loses an appreciable proportion of its mass in its (irregular) more luminous periods, and is thus surrounded by volumes of gas and dust.
	has definition Eruptive variable subgiant star associated with interstellar matter and believed to be still in the process of gravitational contraction on their way to the main sequence. They are found only in nebulae or very young clusters. They have low-temperature (G-M) spectra with strong emission lines and broad absorption lines. Their absolute magnitudes are brighter than those of main-sequence stars of similar spectral types. They have a high lithium abundance. T Tau itself is dG5e. (sometimes called RW Aurigae stars)
	has definition Late type irregular variables associated with bright or dark nebulosity. The spectrum exhibits emission in both CaII and H lines.
	has definition Luminous variable stars with low effective temperatures and strong emission lines, associated with interstellar gas clouds and found in very young clusters. They are believed to be still in the process of gravitational contraction from their protostellar phase and have not yet arrived at the main sequence and begun to burn hydrogen.
	has synonym nebular variable
	is a kind of emission line star
	is a kind of irregular variable
	is a kind of late star
tachyon	has definition Particle whose mass (squared) is negative; its presence in a theory generally yields inconsistencies.
	is a kind of hypothetical particle
tangential velocity	has definition A star's velocity across an observer's line of sight. To calculate a star's tangential velocity, one must know the star's distance and proper motion.
	has synonym transverse velocity
	is a kind of velocity
tantalum	has image
	has ocean oxidation state V
	is a kind of transition metal
	is a kind of unclassified oceanic element
Tarantula Nebula	has image
	is a part of Large Magellanic Cloud
	is an instance of gaseous nebula
tau	has charge -1
	has symbol τ
	is an instance of charged particle
	is an instance of lepton
	is an instance of radioactive particle
tau Compton wavelength	applies to particle tau
	has equation
	has symbol λC,τ
	has uncertainty 0.00011 × 10-15 m
	has value 0.69770 × 10-15 m
	is an instance of particle Compton wavelength
tau mass	applies to particle tau
	has symbol mτ
	has uncertainty 0.00052 × 10-27 kg
	has value 3.16788 × 10-27 kg
	is an instance of particle mass
Taurid	is an instance of meteor shower
Taurus	has acronym Tau
	has genitive Tauri
	has historical image
	has historical origin beast hunted by Orion, also Zeus in a disguise used to elope with Europa
	has synonym Bull
	is a part of Zodiac
	is an instance of zodiacal constellation
TDRSS	has definition Transmission Data and Relay Satellite System.
	is an instance of geosynchronous satellite
technetium	has image
	is a kind of synthetic element
	is a kind of transition metal
telescope	has angular resolving power
	has aperture
	has definition A device for gathering and amplifying light or X rays, gamma rays, or other forms of energy.
	has effective diameter
	has focal ratio
	has location
	has operator
	has purpose observe celestial objects
	has wavelength sensitivity
	is a kind of instrument
Telescopio Nazionale Galileo	has altitude 2370 m
	has aperture 3.58 m
	has creation date (1998)
	has focal ratio f/2.5, 6, 11
	has latitude 28° 45' N
	has location La Palma, Canary Islands
	has longitude 17° 54'W
	has mirror maker Zeiss
	has mirror type Zerodur
	has owner Obs. del Roque de los Muchachos
	has synonym Galileo
	is an instance of altazimuth telescope
	is an instance of optical telescope
	is an instance of Ritchey-Chrétien
Telescopium	has acronym Tel
	has genitive Telescopii
	has synonym Telescope
	is a part of celestial sphere
	is an instance of constellation named by de Lacaille
tellerium	has ocean oxidation state VI some IV
	is a kind of scavenged oceanic element
telluric lines	has definition Spectral lines or bands that originate from absorption by gases such as O2, H2O, or CO2 in the Earth's atmosphere.
	has species O2, H2O, or CO2
	is a kind of molecular band
tellurium	has image
	is a kind of group VI element
	is a kind of nonmetallic metalloid
temperature	has definition A measure of the average kinetic energy of the particles of a system.
	has unit temperature unit
	is a kind of quantity
temperature unit	has absolute zero -has source: [JM92]
	has definition A temperature unit is a measure of the average kinetic energy per degree of freedom of the constituent molecules.
	has triple point of water -has source: [JM92]
	is a kind of unit
	is a unit of temperature
tera	has symbol T
	has value 1012
	is an instance of unit prefix
terbium	has image
	has ocean oxidation state III
	is a kind of rare Earth
	is a kind of recycled oceanic element
terrestrial latitude	has definition Angular distance on the Earth measured north or south of the equator along the meridian of a geographic location.
	has definition On Earth, distance north or south on the equator along a line connecting the poles.
	is an instance of angle
	is an instance of geocentric coordinate component
terrestrial longitude	has definition Angular distance measured along the Earth's equator from the Greenwich meridian to the meridian of a geographic location.
	has definition On Earth, distance east or west of Greenwich, England, measured along lines drawn parallel to the equator.
	is an instance of angle
	is an instance of geocentric coordinate component
terrestrial planet	has composition mostly made of heavier non-volatile elements
	has definition A planet having a compact rocky surface like Earth
	has material silicates, oxides, water
	is a kind of planet
tertiary mirror	has definition The third mirror to be encountered by the light in a telescope system. A tertiary mirror is required on alt-az telescopes to direct light to the stationary Nasmyth foci.
	is a kind of mirror
tertiary period	is a kind of cenozoic era
tesla	has base unit kg·s-2·A-1
	has definition The derived SI unit of magnetic flux density.
	has equivalent 1 maxwell per square centimeter
	has equivalent 1 Wb m-2
	has equivalent 104gauss
	has symbol T
	has unit Wb·m-2
	is an instance of magnetic flux density unit
	is an instance of named derived SI unit
tethered satellite	is a kind of artificial satellite
Tethys	has definition Fourth satellite of Saturn
	has diameter 1000 km
	has discoverer Cassini
	has discovery date 1684
	has orbital period P = 1.87 days
	is a part of Saturn
	is an instance of natural satellite
tevatron	has definition A particle accelerator capable of attaining an energy of 1 TeV.
	is a kind of particle accelerator
thallium	has image
	has ocean oxidation state I
	has ocean residence time 10000 years
	is a kind of accumulating oceanic element
	is a kind of group III element
	is a kind of metallic element
The Thaw Refractor	has altitude 380 m
	has aperture 0.76 m
	has comment Originally had a Brashear visual objective (1914); present one is corrected for red light
	has creation date 1985
	has focal ratio f/18.6
	has latitude 40° 29' N
	has lens maker R.E. Sumner
	has location Pittsburgh, PA, US
	has longitude 80° 01' W
	has mounting manufacturer Warner and Swasey
	has optical design Red-light refractor
	has owner Allegheny Observatory
	has synonym 30 inch
	is an instance of German equatorial telescope
	is an instance of refractor
Themis	has definition A satellite of Saturn discovered by Pickering in 1900, but since lost.
	is a part of Saturn
	is an instance of natural satellite
theoretical celestial body	has definition natural object postulated by theory but not yet observed
	is a kind of natural object
theory	has author or reasearch group
	has date or a range of dates for which the theory was active
	has definition A rationally coherent account of a wider range of phenomena than is customarily accounted for by a hypothesis.
	has domain a field of research
	has validity correct or incorrect with caveats
	is a kind of hypothesis
theory of everything	has acronym TOE
	has definition Theory of Everything. A quantum-mechanical theory that encompasses all forces and all matter.
	is a kind of unified theory
theory related concept	is a kind of abstraction
thermal bremsstrahlung	has definition A mode of X-ray production by very energetic electrons accelerated in the field of a positive ion.
	is a kind of bremsstrahlung
thermal energy	has definition Energy associated with the motions of the molecules, atoms, or ions in a substance.
	is a kind of energy
thermodynamics	has definition Laws developed in the nineteenth century to describe aspects of heat, work, energy, entropy, and their mutual evolution in a physical system.
	has definition The area of physics that analyzes the behavior of a system with very many members, such as a gas with many individual molecules. In such a situation, the behavior of the whole system is obtained by averaging over the behavior of individual members.
	has definition The study of the behavior of heat (and, by implication, other forms of energy) in changing systems.
	has synonym statistical mechanics
	is a kind of physics
thermodynamics law	has domain thermodynamics
	is a kind of law
thermohaline convection	has definition A type of hydrodynamic instability.
	is a kind of convection
theta pinch	has definition A fusion device in which the magnetic field runs parallel to the plasma column. It is a long cylindrical tube enclosed in a one-turn magnet coil.
	is a kind of pinch machine
Theta2 Orionis	has definition A spectroscopic binary tentatively identified with 2U 0525-06.
	has orbital period 21.03 days
	has spectral type O9.5 Vp
	has synonym 2U 0525-06
	is a part of Orion
	is an instance of dwarf
	is an instance of O star
	is an instance of spectroscopic binary
thick disk	has definition The stellar population that contains Arcturus and about 4 percent of the other stars near the Sun. It has a scale height of about 3500 light-years and consists of old stars.
	has height 3500 light-years
	is a part of disk
thin disk	has definition The stellar population that contains the Sun and most other nearby stars. Most of its stars have a scale height of 1000 light-years and orbit the Galaxy on fairly circular orbits. The stars of the thin disk range in age from 0 to about 10 billion years. The thin disk breaks into two subpopulations, the young thin disk and the old thin disk. The young thin disk has a smaller scale height than the old thin disk, and the former's stars have more circular orbits.
	has height 1000 light-years
	has star age 0 to about 10 billion years
	is a part of disk
thioformaldehyde	has symbol H2CS
	is an instance of interstellar molecule
third law of thermodynamics	has definition All substances have zero entropy at 0 K.
	has synonym Nernst theorem
	is an instance of thermodynamics law
Thompson Refractor	has altitude 34 m
	has aperture 0.66 m
	has creation date 1897
	has focal ratio f/10.4
	has latitude 50° 52' N
	has lens maker Grubb
	has location Herstmonceux, Sussex, England
	has longitude 0° 20' E
	has mounting manufacturer Grubb
	has owner Royal Greenwich Observatory
	is an instance of German equatorial telescope
	is an instance of refractor
Thomson cross section	has equation
	has symbol σe
	has uncertainty 0.000000015 × 10-28 m2
	has value 0.665245854 × 10-28 m2
	is an instance of area
	is an instance of atomic constant
Thomson scattering	has definition The limit of Compton scattering at low energies.
	is a kind of Compton scattering
thorium	has image
	has ocean oxidation state IV
	has ocean residence time 50 years
	is a kind of actinide
	is a kind of scavenged oceanic element
thought experiment	has definition An experiment that cannot be or is not carried out in practice, but can, given sufficient imagination and rigor, be reasoned through by thought and intuition alone.
	is a kind of theory related concept
three body recombination	has definition Capture of an electron by an ion with the binding energy absorbed by a third free electron
	is a kind of recombination
three-phase	has definition A CCD construction in which three overlapping metal electrodes are used to define a pixel and effect the transfer of charge, in either direction along a column, by the charge-coupling method. If only two electrodes are used then the device is two-phase.
	has number of electrodes 3
	is a kind of charge-coupled device
threshold energy	has definition Difference between the energy at the first excited level and that of the ground state.
	is a kind of energy
thulium	has image
	has ocean oxidation state III
	is a kind of rare Earth
	is a kind of recycled oceanic element
tidal dwarf galaxy	has acronym TDG
	has definition A self-gravitating entity of dwarf-galaxy mass, built from tidal material expelled during interactions (e.g. AM0547-244, AM1054-325 and AM1353-272)
	is a kind of dwarf galaxy
time	has definition A dimension distinguishing past, present, and future. In relativity, time is portrayed as a geometrical dimension, analogous to the dimensions of space.
	has unit time unit
	is a kind of quantity
time dilation	has definition The flow of time slows down for an observer in motion.
	is a consequence of special relativity
	is an instance of relativity process
time unit	is a kind of unit
	is a unit of time
tin	has image
	has ocean oxidation state IV
	is a kind of group IV element
	is a kind of metallic element
	is a kind of scavenged oceanic element
tired light	has definition The hypothesis that light may be degraded in energy, thereby increasing in wavelength and becoming redshifted, during its passage through intergalactic space. This would provide an alternative to the Big Bang model in accounting for the redshifts of distant galaxies. However, there is no evidence for any such tired-light effect.
	is a kind of cosmology theory
TIROS satellite	is a kind of artificial satellite
Titan	has albedo 0.21
	has atmosphere composition H2 and CH4
	has definition Seventh (known) moon out from Saturn, its largest and brightest. It is possibly also the largest satellite in the Solar System.
	has discoverer Huyghens
	has discovery date 1655
	has orbital period 15d22h41m
	has radius R ≈ 2900 km
	has rotation period 15d22h41m
	has synonym S VI
	is a part of Saturn
	is an instance of natural satellite
Titania	has definition Fourth satellite of Uranus.
	has discoverer Herschel
	has discovery date 1787
	has orbital period P = 8d17h
	has radius R ≈ 850 km
	is a part of Uranus
	is an instance of natural satellite
titanium	has abundance 1.12 × 105 in Sun relative to H = 1 × 1012
	has abundance 4.8 × 10-4 p.p.m. in seawater
	has abundance 5600 p.p.m. in Earth's crust
	has atomic emission line 323.452 nm for Ti II
	has atomic emission line 336.121 nm for Ti II
	has atomic emission line 364.268 nm for Ti I
	has atomic emission line 365.350 nm for Ti I (used in atom absorption spectrometry)
	has atomic emission line 399.864 nm for Ti I
	has atomic emission line 334.941 nm for Ti II (strong)
	has atomic number 22
	has atomic radii 145 pm
	has biological role none
	has boiling point 3560 K
	has bulk modulus 108.4 GPa
	has chief source ilmenite; sometimes anatase
	has covalent radii 132 pm
	has crystal cell dimension a = 295.11, c = 468.43 pm for α-Ti
	has crystal type h.c.p. for α-Ti
	has daily dietary intake 0.8 mg
	has definition hard, lustrous, silvery metal which resists corrosion due to an oxide layer on its surface
	has density 4110 kg m-3 for liquid at 1933 K melting point
	has density 4540 kg m-3 for solid at 293 K
	has discoverer Rev. W. Gregor
	has discovery date 1791
	has discovery location Creed, Cornwall, England
	has electrical resistivity 42.0 × 10-8 Ω m at 293 K
	has electron affinity 7.6 kJ mol-1 from Ti to Ti-
	has electron configuration [Ar]3d24s2 in ground state
	has electronegativity 1.54 Pauling
	has hazard some compounds are dangerous to handle, such as TiCl3, which is corrosive. Suspected carcinogen.
	has heat capacity 24.430 J K-1 mol-1 for gas at constant pressure 0.1 MPa at 298.15 K
	has heat capacity 25.02 J K-1 mol-1 for solid at constant pressure 0.1 MPa at 298.15 K
	has heat of fusion 20.9 kJ mol-1
	has heat of vaporization 428.9 kJ mol-1
	has image
	has ionic radii 69 pm for Ti4+
	has ionic radii 80 pm for Ti2+
	has isotope mass range 41 to 53
	has lethal intake non-lethal
	has level in humans 0.054 mg dm-3 in blood
	has level in humans 0.9 - 2.2 p.p.m. in muscle
	has level in humans 1.2 - 4.7 p.p.m. in liver
	has linear expansion coefficient 8.35 × 10-6 K-1
	has longest lived isotope titanium 48
	has main mining area Norway, India, Brazil, Canada, USA, Russia
	has mass absorption coefficient 208 cm2 g-1 for CuKalpha X-ray diffraction
	has mass absorption coefficient 24.2 cm2 g-1 for MoKalpha X-ray diffraction
	has mass magnetic susceptibility +4.01 × 10-8 kg-1 m3 for solid
	has mass of element in person 20 mg for a 70 kg average person
	has melting point 1933 K
	has mineral anatase, brookite, ilmenite, perovskite, rutile, titanite
	has molar volume 10.55 cm3
	has name origin Titans = the sons of the Earth goddess from Greek
	has neutron scattering length -0.3438 × 10-12 cm
	has number of isotopes 13 including nuclear isomers
	has number of protons 22
	has ocean oxidation state IV
	has ocean residence time 50
	has poisson's ratio 0.361 GPa
	has pronunciation tit-ayn-iuhm
	has registry number 7440-32-6 for Chemical Abstracts System database
	has relative atomic mass 47.867 in units of 12C = 12.000
	has reserves 440 × 106 tonnes
	has rigidity modulus 45.6 GPa
has space group P63/mmc for α-Ti
has specimen crystals, foil, granules, powder, rod or wire. Safe.
has symbol Ti
has term symbol 3F2 in ground state
has thermal conductivity 21.9 W m-1 K-1 at 300 K
has thermal neutron capture cross section 6.09 barns
has toxic intake low as metal, oxide (TiO2) and inorganic titanium (IV) salts
has uses chemical plants, lightweight alloys, hip replacement joints
has uses paint, because TiO2 has good covering power
has world production 3 × 106 tonnes year-1 TiO2
has world production 99000 tonnes year-1 titanium metal
has young's modulus 120.2 GPa
is a kind of transition metal
is a kind of unclassified oceanic element
reacts with air if powdered metal is ignited
titanium 44	has atomic mass 43.959689
	has decay mode γ
	has decay mode EC (0.265 Mev)
	has half life 67 years
	has nuclear spin I = 0+ h/2π
	has number of neutrons 22
	has number of nucleons 44
	has symbol 44Ti
	has uses research
	is an instance of titanium
titanium 45	has atomic mass 44.958124
	has decay mode β+ (2.063 Mev) 86%
	has decay mode EC 14%
	has decay mode no γ
	has half life 3.078 hours
	has nuclear magnetic moment μ = 0.095 nuclear magnetons with diamagnetic correction
	has nuclear spin I = 7/2- h/2π
	has number of neutrons 23
	has number of nucleons 45
	has symbol 45Ti
	is an instance of titanium
titanium 46	has atomic mass 45.952629
	has natural abundance 8.0%
	has nuclear spin I = 0+ h/2π
	has number of neutrons 23
	has number of nucleons 46
	has symbol 46Ti
	has uses isotopically enriched samples available for experimental purposes
	is an instance of titanium
titanium 47	has atomic mass 46.951764
	has magnetogyric ratio 1.5084 × 107 rad T-1 s-1
	has natural abundance 7.3%
	has NMR frequency 5.637 MHz where 1H = 100 MHz; 2.3488 T
	has NMR receptivity 0.864 where 13C = 1.00
	has nuclear magnetic moment μ = -0.78848 nuclear magnetons with diamagnetic correction
	has nuclear quadrupole moment +0.290 × 10-28 m2
	has nuclear spin I = 5/2- h/2π
	has number of neutrons 25
	has number of nucleons 47
	has relative NMR sensitivity 2.09 × 10-3 where 1H = 1.00
	has symbol 47Ti
	has uses isotopically enriched samples available for experimental purposes
	has uses Nuclear Magnetic Resonance
	is an instance of titanium
titanium 48	has atomic mass 47.947947
	has natural abundance 73.8%
	has nuclear spin I = 0+ h/2π
	has number of neutrons 26
	has number of nucleons 48
	has symbol 48Ti
	has uses isotopically enriched samples available for experimental purposes
	is an instance of titanium
titanium 49	has atomic mass 48.947871
	has magnetogyric ratio 1.5080 × 107 rad T-1 s-1
	has natural abundance 5.5%
	has NMR frequency 5.638 MHz where 1H = 100 MHz; 2.3488 T
	has NMR receptivity 1.18 where 13C = 1.00
	has nuclear magnetic moment μ = -1.10417 nuclear magnetons with diamagnetic correction
	has nuclear quadrupole moment +0.240 × 10-28 m2
	has nuclear spin I = 7/2- h/2π
	has number of neutrons 27
	has number of nucleons 49
	has relative NMR sensitivity 3.76 × 10-3 where 1H = 1.00
	has symbol 49Ti
	has uses isotopically enriched samples available for experimental purposes
	has uses Nuclear Magnetic Resonance
	is an instance of titanium
titanium 50	has atomic mass 49.944792
	has natural abundance 5.4%
	has nuclear spin I = 0+ h/2π
	has number of neutrons 28
	has number of nucleons 50
	has symbol 50Ti
	is an instance of titanium
Tokamak	has definition A type of "magnetic bottle" used in experiments on controlled nuclear fusion. (the name is a Russian acronym)
	is a kind of fusion device
top	has charge 2/3
	has definition The sixth flavor of quark.
	is an instance of heavy quark
top-down scenario	has definition A scenario of galaxy formation in which large structures form first and then fragment to become galaxies.
	is a kind of galaxy theory
topocentric coordinate	has definition With reference to, or pertaining to, a point on the surface of the Earth, usually with reference to a coordinate system.
	is a kind of coordinate
Torino scale 10 impact	has definition impact capable of causing a global climatic catastrophe
	has has average interval time 100000 years or longer
	is a kind of impact event
Torino scale 8 impact	has definition collision capable of causing localized destruction
	has has average interval time 50 to 1000 years
	is a kind of impact event
Torino scale 9 impact	has definition collision capable of causing regional devastation
	has has average interval time 1000 to 100000 years
	is a kind of impact event
Toro	has albedo ≤ 0.15
	has aphelion distance 1.96 AU
	has asteroid number 1685
	has closest distance to Earth 0.13 AU
	has discoverer Wirtanen
	has discovery date 1948
	has eccentricity e = 0.44
	has orbital period 584.2 days
	has perihelion distance 0.77 AU
	has radius 5 × 3 km
	has rotation period 10h11m
	has semi-major axis a = 1.37 AU
	has surface composition rocky, thinly covered with dust
	is a part of Earth-crossing asteroid
	is an instance of Earth-crossing asteroid
torr	has definition A unit of pressure.
	has equivalent 1 mm Hg
	has equivalent 1/760 atmosphere
	is an instance of pressure unit
torus	has definition The topological name for the shape of a donut. While a donut is a two-dimensional surface in a three-dimensional space, the torus can be generalized to higher numbers of dimensions.
	has definition The two-dimensional surface of a doughnut.
	has dimensions 3
	is a kind of geometrical object
total lunar eclipse	has definition An eclipse in which the Moon passes through the shadow cast by the Earth, the Moon passing completely through the Earth's umbra.
	is an instance of lunar eclipse
total solar eclipse	has definition A solar eclipse in which the solar disk is completely covered at maximum eclipse.
	is an instance of solar eclipse
toy theory	has definition A theory which is known to be too simple to describe reality, but which is nonetheless useful for theorists to study because it incorporates some important features of reality. For example, most of what is known about magnetic monopoles in grand unified theories was discovered first in a toy theory that includes only three Higgs fields, while the simplest realistic grand unified theory includes twenty-four of them.
	is a kind of theory
transactinide	is a kind of transuranium element
transformation	is a kind of mathematical concept
transient X-ray source	has definition As of early 1974, four had been detected: Cen X-2, Cen X-1, 2U 1543-47, and Cep X-4. They resemble slow novae.
	is a kind of X-ray source
transit	has definition The passage of a smaller, nearer astronomical object across the face of a larger object in the background, as in a transit of Venus across the Sun.
	is a kind of occultation phase
	is followed by emersion
	is preceeded by immersion
transit telescope	has definition A stationary support structure for a telescope. Motion is allowed along the meridian from the zenith to the horizon, but stars cannot be tracked east/west. Measurements are only possible when the objects "transit" the meridian due to the Earth's rotation.
	is a kind of Earth based telescope
transition metal	is a kind of column grouped element
	is a kind of metallic element
transuranium element	has atomic number greater than 92
	has definition A synthetic element with atomic number greater than 92 (uranium)
	is a kind of actinide
	is a kind of synthetic element
transverse wave	has definition Wave vibrating at right angles to the direction of propagation.
	is a kind of wave
Triangulum	has acronym Tri
	has boundary
	has genitive Trianguli
	has synonym Triangle
	is a part of celestial sphere
	is an instance of constellation
Triangulum Australe	has acronym TrA
	has boundary
	has genitive Trianguli Australis
	has synonym Southern Triangle
	is a part of celestial sphere
	is an instance of constellation named by Bayer
triassic period	has duration 49 million years
	has start time 230 million years ago
	is a kind of mesozoic era
Trifid Nebula	has definition An emission nebula in Sagittarius.
	has distance 1 kpc
	has distance 9000 light years
	has image
	has Messier number 20
	has synonym M 20
	has synonym NGC 6514
	is a part of Sagittarius
	is an instance of gaseous nebula
	is an instance of Messier object
trigonometric pi	has value 3.1415926535
	is an instance of mathematical constant
triple star	has definition A star system having three stars that revolve around one another.
	has number of stars 3
	is a kind of star
	is a kind of star system
tritium	has atomic mass 3.016
	has decay mode β- (0.01861 Mev) 100%
	has decay mode no γ
	has definition A short-lived isotope of hydrogen. Tritium does not exist naturally on Earth.
	has half life 12.26 years
	has magnetogyric ratio 28.5335 × 107 rad T-1 s-1
	has NMR frequency 106.663 MHz where 1H = 100 MHz; 2.3488 T
	has nuclear magnetic moment μ = 2.97896
	has nuclear spin I = 1/2
	has number of neutrons 2
	has number of nucleons 3
	has relative NMR sensitivity 1.21 where 1H = 1.00
	has symbol 3H
	has synonym hydrogen 3
	has uses research, diagnostic
	is an instance of hydrogen
triton	has charge 1
Triton	has definition The inner satellite of Neptune, discovered by Lassell in 1846. It is larger than the Moon (R ≈ 2900 km), with an almost circular retrograde orbit of 5 days 21 hours.
triton	has definition The nucleus of the tritium atom.
	has symbol T
Triton	is a part of Neptune
triton	is an instance of charged particle
Triton	is an instance of natural satellite
triton	is an instance of nucleus
	is an instance of radioactive particle
Trojan asteroid	has definition Asteroid located at the points of Jupiter's orbit around the Sun that are equidistant from the Sun and Jupiter. About 15 are now known.
	is a kind of asteroid
	is a part of asteroid belt
	see also Lagrangian point
tropical year	has definition The interval of time between two successive vernal equinoxes.
	has definition The period of one complete revolution of the mean longitude of the sun with respect to the dynamical equinox. The tropical year is longer than the Besselian year by 0s.148 T, where T is centuries from B1900.0.
	has value 365.242 mean solar days
	is an instance of year
tropopause	has altitude 5 km
	has definition Upper boundary of the troposphere, where the temperature gradient goes to zero.
	is a part of troposphere
troposphere	has altitude 0 to 15 km
	has definition Lowest level of Earth's atmosphere, from zero altitude to about 15 km above the surface. This is the region where most weather occurs. Its temperature decreases from about 290 K to 240 K.
	has temperature 240 to 290 K
	is a part of atmosphere
Tucana	has acronym Tuc
	has boundary
	has genitive Tucanae
	has historical image
	has historical origin Kepler and Riccioli, called it Anser Americanus (the American Goose),
	has synonym Toucan
	is a part of celestial sphere
	is an instance of constellation named by Bayer
Tully-Fisher relation	has definition An observed relation between the intrinsic luminosity of a spiral galaxy and the rotational speed of its stars. More luminous galaxies have stars that are moving faster.
	is a kind of galaxy theory
tungsten	has image
	has ocean oxidation state VI
	is a kind of transition metal
	is a kind of unclassified oceanic element
Tunguska meteorite	is a kind of comet impact
twin-exhaust model	has definition A theoretical model for radio galaxies in which a compact source in the galactic nucleus is assumed to emit twin beams of rapidly moving plasma that traverse hundreds of thousands of light-years, eventually splattering to a halt in the ambient intergalactic gas, where the resulting dissipation energizes the radio lobes.
	is a kind of galaxy theory
Twistor Theory	has definition Model of the Universe proposed by Roger Penrose, based on the application of complex numbers (involving (-1)1/2) used in calculations in the microscopic world of atoms and quantum theory to the macroscopic ordinary world of physical laws and relativity. The result is an eight-dimensional concept of reality that although complicated is possibly a more logical understanding of the constitution of the Universe.
	is a kind of cosmology theory
Two-Micron All-Sky Survey	has acronym 2MASS
	is an instance of sky survey
Tycho's star	has definition Remnant of a Type I supernova (B Cas), 3-5 kpc distant, which Tycho observed and described in 1572. At its peak it was as bright as Venus and was visible in the daytime, reaching a magnitude of about -4. It is an X-ray source (2U 0022+63). (3C 10)
	is an instance of type I supernova
Tycho-2 catalog	is a kind of star catalog
type I string theory	has definition One of the five superstring theories; involves both open and closed strings.
	is a kind of superstring theory
type I supernova	has absolute magnitude Mv = -14 to -17
	has definition A gigantic stellar explosion in which the star's luminosity suddenly increases by as much as a billion times. Most of the star's substance is blown off, leaving behind, at least in some cases, an extremely dense core which may be a neutron star.
	has definition A supernova which may be produced by the thermonuclear detonation of a highly degenerate core produced by mass transfer from a companion star.
	has ejecta velocity 10000 km s-1
	has frequency less common than type II supernova
	has location spiral and elliptical galaxies
	has mass lower than type II supernova
	has spectra nonhydrogen lines
	is a kind of supernova
type II supernova	has absolute magnitude Mv = - 12 to - 13.5
	has definition A gigantic stellar explosion in which the star's luminosity suddenly increases by as much as a billion times. Most of the star's substance is blown off, leaving behind, at least in some cases, an extremely dense core which may be a neutron star.
	has ejecta velocity 5000 km s-1
	has frequency 0.01 to 0.05 per year (in our Galaxy)
	has location young, massive stars near the edge of spiral arms
	has mass higher than type II supernova
	has spectra hydrogen lines
	is a kind of supernova
type IIA string theory	has definition One of the five superstring theories; involves closed strings with left-right symmetric vibrational patterns.
	is a kind of superstring theory
type IIB string theory	has definition One of the five superstring theories; involves closed strings with left-right asymmetric vibrational patterns.
	is a kind of superstring theory
type III supernova	has definition A gigantic stellar explosion in which the star's luminosity suddenly increases by as much as a billion times. Most of the star's substance is blown off, leaving behind, at least in some cases, an extremely dense core which may be a neutron star.
	has definition Supernova similar to Type II
	has mass much higher than type II supernova
	is a kind of supernova
Télescope Combiné de Schmidt	has altitude 105 m
	has aperture 0.84 m
	has creation date 1958
	has focal ratio f/2.5
	has latitude 50° 48' N
	has location Uccle, Bruxelles, Belgium
	has longitude 4° 21'E
	has mirror diameter 1.2 m
	has mirror maker Cox, Hargreaves, Thomson
	has mirror type borosilicate
	has mounting manufacturer Zeiss (Jena)
	has owner Observatoire Royal de Belgique
	is an instance of Schmidt
Télescope de Schmidt	has altitude 1270 m
	has aperture 0.90 m
	has creation date 1981
	has focal ratio f/3.5
	has latitude 43° 45' N
	has location Calern, France
	has longitude 6° 56'W
	has mirror diameter 1.52 m
	has mirror maker Jean Texereau
	has mirror type Cer-Vit
	has mounting manufacturer C.M.G-Paris
	has owner Observatoire de Calem
	has synonym Calem Schmidt
	is an instance of Schmidt
U Geminorum	has image
	is a part of Milky Way
	is an instance of U Geminorum star
U Geminorum star	has definition A type of dwarf nova. All U Geminorum stars are binaries containing a white dwarf and a red dwarf with total masses of roughly 1-2 Msun and with periods of less than 12 hours (period of U Gem, 1.5 × 104 seconds). About 150 are known.
	has prototype U Geminorum
	is a kind of dwarf nova
U line	has species sodium (Na I)
	has wavelength 3302 Å
	is a kind of spectral line
U velocity	has definition The component of a star's motion away from the Galactic center. If a star moves away from the Galactic center, the star's U velocity is positive; if a star moves toward the Galactic center, the U velocity is negative; and if the star moves neither toward nor away from the Galactic center, the U velocity is zero. The Sun has a U velocity of -9 kilometers per second, so the Sun is moving toward the Galactic center at 9 kilometers per second.
	is a kind of galactic velocity component
U(1)	has definition The symmetry of the electromagnetic field.
	is a kind of elementary particle symmetry
UBC-Laval Telescope	has acronym LMT
	has altitude 50 m
	has aperture 2.7 m
	has creation date 1992
	has focal ratio f/1.887
	has latitude 49° 07' N
	has limitation views a 21' field at local zenith (centered on declination +49° 1)
	has location Vancouver, BC, Canada
	has longitude 122° 35' W
	has mirror maker P Hickson
	has mirror type rotating liquid mercury container
	has mounting manufacturer Univ. of British Col.
	has optical design Paraboloid, field corr.
	has owner Univ. of B.C. and Laval Univ.
	is an instance of fixed vertical mount telescope
	is an instance of Newtonian
	is an instance of optical telescope
UHF band	has frequency 300 MHz to 3 GHz
	has wavelength 10 cm to 1 m
	is a kind of radio
ULF band	has frequency 30 Hz or less
	has wavelength 10000 km and higher
	is a kind of radio
ultra-luminous infrared galaxy	has acronym ULIRG
	is a kind of infrared galaxy
ultraviolet	has acronym UV
	has definition Electromagnetic radiation "beyond the violet" with wavelengths in the approximate range 100-4000 Å.
	has definition Electromagnetic radiation of a wavelength slightly shorter than that of visible light.
	has definition Part of the electromagnetic spectrum immediately above visible light (but below gamma-rays and X-rays); it therefore comprises a range of radiation of shorter wavelength and higher frequency than those of visible light.
	has frequency 750 THz to 30 PHz
	has wavelength 10 to 400 nm
	is a kind of photon
ultraviolet Ga star	has definition Bp star which exhibits a strong 1414 line of Ga II in the ultraviolet spectrum .
	is a kind of Bp star
ultraviolet star	has definition Very hot prewhite-dwarf star; usually the hot central star of a planetary nebula which is contracting toward the white-dwarf state.
	is a kind of central star of planetary nebula
ultraviolet-bright star	has definition Star that is brighter than horizontal-branch stars and bluer than giant-branch stars.
	is a kind of central star of planetary nebula
Umbriel	has definition A satellite of Uranus.
	has discoverer Lassell
	has discovery date 1851
	has orbital period 4.1 days
	has radius 200 km
	is a part of Uranus
	is an instance of natural satellite
Umklapp scattering	has definition The contribution to scattering caused when the exchange of momentum crosses the boundary of a Brillouin zone.
	is a kind of scattering
uncertainty principle	has corollary it is impossible to measure an atomic or nuclear process without at the same time disturbing or altering the process.
	has definition The principle that the fundamental uncertainty in a variable times that in its canonical conjugate is of the order of Planck's constant.
	has discoverer Werner Heisenberg
	has equation Relation Conjugates ΔxΔp ≥ h position, momentum ΔEΔt ≥ h energy, time
	has example the uncertainty in the measurement of the position of an electron varies inversely as the uncertainty in the measurement of its momentum.
	has implication the microscopic realm is a roiling frenzy, awash in a violent sea of quantum fluctuations.
	has synonym Heisenberg uncertainty principle
	is an instance of quantum law
unclassified globular cluster	is a kind of globular cluster
unclassified oceanic element	has ocean concentration unknown
	has ocean residence time unknown
	is a kind of oceanic element
unified atomic mass unit	has symbol u
	has value in SI unit 1.66054 × 10-27 kg, approximately
	is an instance of non SI unit
unified theory	has definition Any theory that describes all four forces and all of matter within a single, all-encompassing framework.
	has definition In particle physics, any theory exposing relationships between seemingly disparate classes of particles. More generally, a theory that gathers a wide range of fundamentally different phenomena under a single precept, as in Maxwell's discovery that light and magnetism are aspects of a single, electromagnetic force.
	is a kind of theory
uniformitarianism	has antonym catastrophism
	has definition The hypothesis that the extensive changes in the earth, as evinced in the geological record, have resulted, not from massive catastrophes, but from the slow operation of wind, weather, volcanism, and the like over many millions of years.
	is a kind of doctrine
unit	has historical origin
	is a kind of mathematical concept
	represents
unit prefix	has rule multiple prefixes may not be used any unit prefix can be used with any SI unit
	has value strictly represented by powers of 10 except for information technology units such as bits
	is a kind of constant
unitary transformation	has definition A transformation whose reciprocal is equal to its Hermitian conjugate.
	has symbol U
	is a kind of transformation
United Kingdom Infrared Telescope	has acronym UKIRT
	has altitude 4194 m
	has aperture 3.802 m
	has creation date 1978
	has focal ratio f/2.5, 36 IR
	has latitude 19° 50' N
	has location Mauna Kea, Hawaii, US
	has longitude 155° 28' W
	has mirror maker Grubb-Parsons
	has mirror type Cer-Vit
	has mounting English-yoke equatorial
	has mounting manufacturer Hadfields Ltd. (Sheffield)
	has owner Joint Astronomy Centre
	has purpose infrared work only
	is an instance of Cassegrain
	is an instance of English equatorial telescope
	is an instance of optical telescope
United Kingdom Schmidt Telescope Unit	has acronym UKST
	has altitude 1145 m
	has aperture 1.24 m
	has creation date 1973
	has focal ratio f/2.5
	has latitude 31° 16' S
	has location Siding Spring Mtn., Australia
	has longitude 149° 04' E
	has mirror diameter 1.83 m
	has mirror maker Grubb-Parsons
	has mirror type Cer-Vit
	has mounting manufacturer Grubb-Parsons
	has owner Anglo-Australian Observatory
	has synonym U.K. Schmidt
	is an instance of Schmidt
universal constant	is a kind of fundamental physical constant
Universe	has definition The total celestial cosmos. According to Gott et al. the universe seems to be on a large scale isotropic, homogeneous, matter-dominated, and with negligible pressure. The total proper mass content of about 1023 Msun (Sandage derives 1056 g from his determination of the deceleration parameter q0) and radius of about 2 × 1028 cm are the order of magnitude that most cosmologists would accept if the universe is bounded. Total mass contributed by luminous matter, about 3 × 1053 g (see mass discrepancy). Age about 18 × 109 yr for a Hubble constant H0 = 55 km s-1 Mpc-1.
	is a kind of celestial body
	is a part of kbTop
unmanned spacecraft	has crew none
	has definition an artificial satellite without a crew
	is a kind of artificial satellite
unstable Lagrangian point	has equilibrium unstable
	is a kind of Lagrangian point
up	has charge 2/3
	has definition A flavor of quark
	is an instance of light quark
upper chromosphere	has composition ionized hydrogen
	has definition The upper part of the chromosphere and consists of hot, ionized hydrogen. It has an emission spectrum (see flash spectrum).
	has density ρ ≈ 10-16 g cm-3
	has location 4000 to 12000 km
	has temperature 106 K
	is a part of chromosphere
upper culmination	has definition Passage of a celestial object across the observer's meridian. The crossing closer to the observer's zenith.
	has synonym culmination above pole
	is an instance of culmination
upper precambrian period	has duration million years
	has start time million years ago
	is a kind of precambrian era
uranium	has image
	has ocean oxidation state VI
	has ocean residence time 300000 years
	is a kind of accumulating oceanic element
	is a kind of actinide
uranium 235 decay	is an instance of spontaneous fission
uranium 235 splitting	is an instance of neutron induced fission
Uranus	has albedo 0.66
	has atmosphere composition H2 and CH4
	has definition Seventh planet from the Sun. Has retrograde rotation.
	has discoverer Herschel
	has discovery date March 13, 1781
	has eccentricity e = 0.04
	has escape velocity 22 km s-1
	has inclination i = 0°.8
	has mass 8.78 × 1028 g
	has maximum brightness +5.7 mag
	has mean density 1.21 g cm-3
	has mean distance from Sun 19.18 AU
	has oblateness 0.07
	has obliquity 97°.9
	has orbital period 84.0 years
	has orbital velocity 6.8 km s-1
	has radius 25400 km
	has rotation period 10h49m26s
	has satelites 5 major ones, all of which orbit in its equatorial plane
	has surface gravity 0.96 Earth's
	has surface temperature 110 K
	has synodic period 369.66 days
	is a part of our solar system
	is an instance of gas giant
	is an instance of superior planet
Urca process	has definition A series of nuclear reactions, primarily among the iron group of elements, accompanied by a high rate of neutrino formation and postulated as a cause of stellar collapse. Neutrinos carry away energy quickly and invisibly, so this process was named for the Urca casino in Rio de Janeiro, which carried away money the same way.
	is a kind of nuclear process
Ursa Major	has acronym UMa
	has boundary
	has genitive Ursae Majoris
	has historical origin wagon or cart, a plow, a bull's thigh, and (to the Chinese) the government
	has synonym Big Dipper
	has synonym Great Bear
	is a part of celestial sphere
	is an instance of constellation
Ursa Minor	has acronym UMi
	has boundary
	has genitive Ursae Minoris
	has synonym Little Bear
	has synonym Little Dipper
	is a part of celestial sphere
	is an instance of constellation
Ursa Minor Dwarf	has definition An intrinsically faint (Mv ≈ - 9) dwarf elliptical galaxy, a member of the Local Group.
	has distance 70 pc
	is a part of Local Group
	is a part of Ursa Minor
	is a part of Ursa Minor
	is an instance of dwarf elliptical satellite galaxy
	orbits Milky Way
UV Ceti	has angular separation 1".0
	has apparent magnitude Mv = 15.3 and 15.8
	has definition Late-type dwarf with spectra showing hydrogen emission lines. Faint flare star of very low mass. Like other flare stars, it is a member of a binary system in which both components are of nearly equal brightness. Radio flares have also been observed.
	has distance 2.8 pc
	has eccentricity e = 0.615
	has mass 0.15 Msun
	has orbital period 26.5 years
	has spectral type M6e V
	has synonym Luyten 726-8 B
	is a part of Cetus
	is an instance of binary star
	is an instance of dMe star
	is an instance of flare star
UV Ceti star	has definition Late-type dwarfs with spectra showing hydrogen emission lines.
	has emission line hydrogen
	has prototype UV Ceti
	has spectral type dKe star, dMe star
	is a kind of dwarf
	is a kind of emission line star
UVES	has definition Ultraviolet Echelle Spectrograph.
	is a kind of spectrograph
V velocity	has definition A star's velocity in the direction of Galactic rotation, as measured relative to a nearby star that has a circular orbit. If a star revolves faster than such a star, the V velocity is positive; if it revolves more slowly, the V velocity is negative; and if both revolve at the same rate, the V velocity is zero. The Sun has a V velocity of +12 kilometers per second, so it revolves 12 kilometers per second faster than it would if it had a circular orbit. Since a star on a circular orbit revolves around the Galaxy at 220 kilometers per second, a star with a V velocity of 0 is not stationary; rather, it revolves at 220 kilometers per second. The Sun therefore revolves around the Galaxy at 220 + 12 = 232 kilometers per second.
	is a kind of galactic velocity component
Van Allen belt	has definition One of two doughnut-shaped belts in the Earth's magnetosphere, where many energetic charged particles from the solar wind are trapped in Earth's magnetic field. The energy of the particles is highest in the inner belt.
	has definition One of two toroidal zones of high radiation in Earth's upper atmosphere, above the equator, caused by the trapping of charged particles in the magnetosphere.
	has location above the equator
	has shape toroidal
	is a part of magnetosphere
van der Waals force	has definition The relatively weak attractive force operative between neutral atoms and molecules.
	is a kind of force quantity
van Maanen	has career Mt. Wilson
	has name Adrian van Maanen
	is an instance of 19th century scientist
	is an instance of astronomer
van Maanen's star	has density 4 × 105 g cm-3
	has distance 4 pc
	is an instance of white dwarf
vanadium	has abundance 1.05 × 104 in Sun relative to H = 1 × 1012
	has abundance 1.1 × 10-3 p.p.m. in Atlantic surface seawater
	has abundance 1.6 × 10-3 p.p.m. in Pacific surface seawater
	has abundance 1.8 × 10-3 p.p.m. in deep Pacific seawater
	has abundance 160 p.p.m. in Earth's crust
	has atomic emission line 318.398 nm for V I
	has atomic emission line 318.540 nm for V I (used in atom absorption spectrometry)
	has atomic emission line 399.864 nm for V I
	has atomic emission line 411.178 nm for V I
	has atomic emission line 438.472 nm for V I
	has atomic emission line 437.924 nm for V I (strong)
	has atomic number
	has atomic radii 132 pm
	has biological role essential to some species including humans; stimulates metabolism
	has boiling point 3650 K
	has bulk modulus 158 GPa
	has chief source descloizite, patronite, vanadinite, carnotite
	has crystal cell dimension a = 302.40 pm
	has crystal type b.c.c.
	has daily dietary intake 0.04 mg
	has definition shiny, silvery metal, soft when pure. Resists corrosion due to a protective film of oxide
	has density 5550 kg m-3 for liquid at 2160 K melting point
	has density 6110 kg m-3 for solid at 292 K
	has discoverer A.M. del Rio
	has discovery date 1801
	has discovery location Mexico City, Mexico
	has electrical resistivity 24.8 × 10-8 Ω m at 293 K
	has electron affinity 50.7 kJ mol-1 from V to V-
	has electron configuration [Ar]3d34s2 in ground state
	has electronegativity 1.63 Pauling
	has hazard irritates eyes and lungs; highly toxic fumes
	has hazard mutagenic effect in some experimental compounds
	has heat capacity 24.89 J K-1 mol-1 for solid at constant pressure 0.1 MPa at 298.15 K
	has heat capacity 26.012 J K-1 mol-1 for gas at constant pressure 0.1 MPa at 298.15 K
	has heat of fusion 17.6 kJ mol-1
	has heat of vaporization 458.6 kJ mol-1
	has image
	has ionic radii 59 pm for V5+
	has ionic radii 61 pm for V4+
	has ionic radii 65 pm for V3+
	has ionic radii 72 pm for V2+
	has isotope mass range 44 to 55
	has lethal intake 10 mg kg-1 V2O5 ingested by rat
	has level in humans < 0.0002 mg dm-3 in blood
	has level in humans 0.0035 p.p.m. in bone
	has level in humans 0.006 p.p.m. in liver
	has level in humans 0.02 p.p.m. in muscle
	has linear expansion coefficient 8.3 × 10-6 K-1
	has longest lived isotope vanadium 51
	has main mining area not mined as such, obtained as ore by-product and from Venezuelan oils
	has mass absorption coefficient 233 cm2 g-1 for CuKalpha X-ray diffraction
	has mass absorption coefficient 27.5 cm2 g-1 for MoKalpha X-ray diffraction
	has mass magnetic susceptibility +6.28 × 10-8 kg-1 m3 for solid
	has mass of element in person 0.11 mg for a 70 kg average person
	has melting point 2160 K
	has mineral carnotite, descloizite, patronite, vanadinite
	has molar volume 8.34 cm3
	has name origin Vanadis = Scandinavian goddess from Scandinavian
	has neutron scattering length -0.0382 × 10-12 cm
	has number of isotopes 11 including nuclear isomers
	has number of protons 23
	has ocean oxidation state V
	has ocean residence time 50000 years
	has poisson's ratio 0.365 GPa
has pronunciation van-ay-di-uhm
has registry number 7440-62-2 for Chemical Abstracts System database
has relative atomic mass 50.9415 in units of 12C = 12.000
has rigidity modulus 46.7 GPa
has space group Im3m
has specimen foil, granules, powder, rod, turnings. Care !
has symbol V
has term symbol 4F3/2 in ground state
has thermal conductivity 30.7 W m-1 K-1 at 300 K
has thermal neutron capture cross section 5.08 barns
has toxic intake varies
has uses alloys, especially in steel
has world production 7000 tonnes year-1
has young's modulus 127.6 GPa
is a kind of recycled oceanic element
is a kind of transition metal
reacts with concentrated acids
vanadium 47	has atomic mass 46.954906
	has decay mode β+ (2.927 Mev)
	has decay mode EC
	has decay mode no γ
	has half life 32.6 minutes
	has nuclear spin I = 3/2- h/2π
	has number of neutrons
	has number of nucleons
	has symbol V
	is an instance of vanadium
vanadium 48	has atomic mass 47.952257
	has decay mode β+ (4.015 Mev)
	has decay mode γ
	has half life 15.98 days
	has nuclear magnetic moment μ = 2.01 nuclear magnetons with diamagnetic correction
	has nuclear spin I = 4+ h/2π
	has number of neutrons
	has number of nucleons
	has symbol V
	is an instance of vanadium
vanadium 49	has atomic mass 48.948517
	has decay mode EC (0.601 Mev)
	has decay mode no γ
	has half life 337 days
	has nuclear magnetic moment μ = 4.47 nuclear magnetons with diamagnetic correction
	has nuclear spin I = 7/2- h/2π
	has number of neutrons
	has number of nucleons
	has symbol V
	is an instance of vanadium
vanadium 50	has atomic mass 49.947161
	has magnetogyric ratio 2.6491 × 107 rad T-1 s-1
	has natural abundance 0.25%
	has NMR frequency 9.970 MHz where 1H = 100 MHz; 2.3488 T
	has NMR receptivity 0.755 where 13C = 1.00
	has nuclear magnetic moment μ = +3.34745 nuclear magnetons with diamagnetic correction
	has nuclear quadrupole moment 0.210 × 10-28 m2
	has nuclear spin I = 6+ h/2π
	has number of neutrons 27
	has number of nucleons 50
	has relative NMR sensitivity 0.0555 where 1H = 1.00
	has symbol 50V
	has uses isotopically enriched samples available for experimental purposes
	has uses Nuclear Magnetic Resonance
	is an instance of vanadium
vanadium 51	has atomic mass 50.943962
	has magnetogyric ratio 7.0362 × 107 rad T-1 s-1
	has natural abundance 99.75%
	has NMR frequency 26.289 MHz where 1H = 100 MHz; 2.3488 T
	has NMR receptivity 2150 where 13C = 1.00
	has nuclear magnetic moment μ = +5.1574 nuclear magnetons with diamagnetic correction
	has nuclear quadrupole moment -0.052 × 10-28 m2
	has nuclear spin I = 7/2- h/2π
	has number of neutrons 28
	has number of nucleons 51
	has relative NMR sensitivity 0.38 where 1H = 1.00
	has symbol 51V
	has uses isotopically enriched samples available for experimental purposes
	has uses Nuclear Magnetic Resonance
	is an instance of vanadium
variable	has amplitude
	has definition A star that varies in luminosity. The first variable discovered in a given constellation has the letter R preceding the name of the constellation. Then S, . . . , Z. Then RR, RS, . . . , Rz, SS, . . . , Sz, . . . , ZZ. Then AA, . . . , AZ (the letter J is never used), BB, . . . , BZ, . . . , QQ, . . . QZ. The next variable (the 335th) is given the designation V335.
	has definition A star whose light varies. Some variables vary simply because they consist of two stars, one of which eclipses the other; Algol is the most famous example. Other variables, however, vary because the stars themselves actually change in brightness; the most famous are the Cepheids, RR Lyraes, and Miras, all of which pulsate.
	has definition Star whose luminosity changes over periods of time; there are many reasons and many types. Periods vary widely in length and even regularity. Novae and supernovae are classed as variables. The present brightest variable star is Betelgeuse (Alpha Orionis).
	has definition a star whose apparent magnitude varies by at least 0.1 magnitudes in the visible spectrum (a star whose optical brightness variations can be detected by the human eye)
	has light curve
	has name designated with R, S, T, U, V, W, X, Y, or Z and the genitive of the latin constellation name RR, RS, RT, RU, RV, RW, RX, RY, or RZ and the genitive of the latin constellation name when the single letter designations are exhausted AA...AZ, BB...BZ, etc. (omitting J), which ends with QQ...QZ and the genitive of the latin constellation namewhen the RR...RZ designations are exhausted V 335, V 336, etc., when the double letter designations are exhausted
	has observable variation time scale within a period of decades
	has observational problem some difficulty in distinguishing between various kinds
	has optical brightness variation 0.2 magnitudes or greater
	is a kind of star
variable-mass theory	has definition A theory of Hoyle and Narlikar in which the masses of fundamental particles are assumed to vary with time in a manner that precisely accounts for the Hubble redshift law.
	is a kind of cosmology theory
vector boson	carries the force electroweak
	has acronym IVB
	has definition A hypothetical elementary particle that acts as intermediary for the weak interaction, carrying its effect from one particle to another as the photon does for electromagnetic interactions and as various mesons do for the strong interactions.
	has definition Also called the intermediate vector boson.
	has definition Force-carrying particles of nature. Three vector bosons are responsible for the weak nuclear force. By admitting the photon on an equal footing it is possible to create a unified electroweak theory. As a result of symmetry-breaking processes, however, this photon remains massless while the three other vector bosons pick up mass.
	is a kind of carrier boson
	is a kind of hypothetical particle
	see also Weinberg-Salam theory
vector translation	has definition The small theoretical precession of the axis of an orbiting body due to the gravitational influence of its primary. This effect is predicted by general relativity, but so far it has not been observed.
	is an instance of precession
	is an instance of prediction
Vega	has B-V magnitude 0.00
	has declination +38 47 01
	has definition The fifth brightest star in the night sky.
	has distance 25 light-years
	has right ascension 18 36 56.2
	has spectral type A0Va
	has synonym alpha Lyr
	has synonym HR 7001
	has V magnitude 0.03
	is a part of Lyra
	is an instance of A star
	is an instance of dwarf
	is an instance of naked eye star
Vela	has acronym Vel
	has boundary
	has genitive Velorum
	has synonym Ship's Sail
	is a part of celestial sphere
	is an instance of constellation formerly part of Argo Navis
Vela pulsar	has definition A compact radio source about 400-500 pc distant associated with the Vela supernova remnant (q.v.). It has a nonthermal radio spectrum and is about 20 percent polarized. It is associated with the Gum Nebula, the Vela pulsar, and the X-ray source 2U 0832-45, although the pulsar and the X-ray source are displaced about 0°.7 from the center of the Vela X radio emission. Vela Y and Vela Z are outlying components, also nonthermal, but too weak to exhibit polarization.
	has definition A pulsar about 400-500 pc distant, probably associated with the Vela supernova remnant. Period 0.0892 seconds.
	has synonym PSR 0833-45
	has synonym Vela X
	is a part of Vela
	is an instance of pulsar
Vela satellite	has altitude 120000 km
	has definition A sequence of satellites launched to monitor possible violations of the nuclear test ban treaties. The system consists of four satellites in a circular orbit around the Earth with a radius of 120000 km. The Vela satellites have detected cosmic gamma-ray bursts (q.v.).
	is a kind of military spacecraft
Vela supernova remnant	has definition A gaseous nebula in the middle of the Gum Nebula, the remnant of a Type II supernova whose light reached Earth about 10000 to 30000 years ago. It consists of bright filaments that form a D-shaped ring in Hα and a rough circle in the ultraviolet. It includes the Vela X, Y, and Z radio complexes and is a strong X-ray source.
	is a part of Vela
	is an instance of supernova remnant
Vela X-1	has apparent magnitude 7
	has mass of unseen companion 1.7 to 15 Msun, with a probable value of about 2.6 Msun.
	has orbital period 8.96 days
	has spectral type B0.5 Ib
	has synonym 3U 0900-40
	has synonym HD 77581
	is an instance of B star
	is an instance of eclipsing binary
	is an instance of single line spectroscopic binary
	is an instance of supergiant
	is an instance of X-ray source
velocity	has base unit length per unit time
	has definition The speed and the direction of an object's motion.
	is a kind of quantity
velocity unit	has definition meter per second
	has symbol m·s-1
	is a kind of derived SI unit
velocity-of-light radius	has definition The radius of a rotating neutron star at which the rotational velocity of the plasma approaches the velocity of light. (also called velocity-of-light cylinder)
	is an instance of radius
Venezuela 1-meter Schmidt	has altitude 3610 m
	has aperture 1.00 m
	has comment has a 1-m objective prism
	has creation date 1978
	has focal ratio f/3.0
	has latitude 8° 47' N
	has location Llano del Hato, Merida, Venezuela
	has longitude 70° 52' W
	has mirror diameter 1.52 m
	has mirror maker Askania
	has mounting manufacturer Askania
	has optical design Concentric Schmidt
	has owner Centro "EJ. Duarte"
	is an instance of Bent-yoke equatorial telescope
	is an instance of Schmidt
Venus	has albedo 0.76
	has atmosphere composition 90-95% CO2, remainder primarily N2, traces of water vapor, oxygen, HF, HC1 (by volume)
	has atmospheric pressure 92-95 atm
	has cloud altitude 44 km
	has definition Second planet from the Sun. Has retrograde rotation. Mariner 10 has established that the cloud tops rotate every 4 hours retrograde. Radar experiments have established that the surface is somewhat smoother than the Moon, but there are mountains and there is extensive cratering. Last transit of Sun was in 1882; next one will be 2004. Venus's rotation period is in synchronism with Earth - that is, at inferior conjunction the same side is always toward the Earth.
	has eccentricity e = 0.0068
	has escape velocity Vesc = 10.3 km s-1
	has inclination i = 3°.39
	has mass 4.872 × 1027 g
	has maximum elongation 48°
	has mean density 5.16 g cm-3
	has mean distance from Sun 0.7233 AU
	has obliquity 3° R
	has orbital period 224.7 days
	has orbital velocity 35 km s-1
	has radius 6056 km
	has rotation period 243.09 ± 0.5 days
	has surface gravity 8 m s2
	has surface temperature 743 ± 8 K (from Venera 8)
	has synodic period 583.9 days
	has temperature of cloud tops 250 K
	is a part of our solar system
	is an instance of naked eye planet
	is an instance of terrestrial planet
vernal equinox	has date on or around 21 March
	has definition The ascending node of the ecliptic on the celestial equator; also the time at which the apparent longitude (see apparent place; longitude, celestial) of the Sun is 0°.
	has definition The point of intersection between the ecliptic and the celestial equator, where the Sun crosses from south to north. It is sometimes called the First Point of Aries because several thousand years ago it was in Aries. Because of precession it has now slid west into Pisces and in 200-300 years it will edge into Aquarius. By definition, the vernal equinox is at α = 0°, δ = 0°.
	has synonym First Point of Aries
	has synonym spring equinox
	is an instance of equinox
Very Large Array	has acronym VLA
	has angular resolving power 1 arcsecond
	has definition A radio telescope distributed along three 13-mile-long arms of a Y-shaped track.
	has element aperture 82 feet
	has image
	has number of elements 27
	has position near Socorro, Atacama desert, New Mexico, USA
	has reference NRAO Very Large Array
	is an instance of radio interferometer
Very Large Telescope	has acronym VLT
	has altitude 2640 m
	has aperture 16 m equivalent
	has comment Four separate telescopes and domes, first unit became operational in 1998
	has creation date 2001
	has diagram
	has element aperture 8.2 m
	has focal ratio f/13.5, 15 (each mirror)
	has image
	has latitude 24° 51' S
	has location Cerro Paranal, Chile
	has longitude 70° 27' W
	has mirror maker REOSC
	has mirror type Schott Zerodur, active optics
	has number of elements 4
	has owner European Southern Observatory
	is an instance of altazimuth telescope
	is an instance of array telescope
	is an instance of optical telescope
	is an instance of Ritchey-Chrétien
Very Large Telescope Interferometer	has acronym VLTI
	is an instance of array telescope
Vesta	has albedo 0.24
	has definition An asteroid in diameter. It is the brightest of all minor planets, at times approaching naked-eye visibility. Its spectrum can also be interpreted to mean a rotation period of 10h40m58s.84.
	has diameter 500 km
	has discoverer Olbers
	has discovery date 1807
	has eccentricity e = 0.09
	has inclination i = 7°.1
	has orbital period P = 1325 days
	has rotation period 5h20m31s.665
	has semi-major axis a = 2.361 AU
	has visual magnitude 5.5 (at mean opposition)
	is a part of asteroid belt
	is an instance of asteroid
VF band	has frequency 30 to 300 Hz
	has uses submarine communication
	has wavelength 1000 to 10000 km
	is a kind of radio
VHF band	has frequency 30 to 300 MHz
	has wavelength 1 to 10 m
	is a kind of radio
vibrational energy	has definition Motion of the pair of nuclei in a diatomic molecule along the direction of the internuclear axis (cf. rotational energy).
	is a kind of energy
Victor M. Blanco Telescope	has altitude 2215 m
	has aperture 4.001 m
	has creation date 1976
	has focal ratio f/2.8, 8.0
	has latitude 30° 10' S
	has location Cerro Tololo, Chile
	has longitude 70° 49' W
	has mirror maker KPNO Optical Shop
	has mirror type Cer-Vit
	has mounting manufacturer Western Gear Corp.
	has owner Cerro Tololo Inter-American Obs.
	has synonym CTIO 4 meter
	is an instance of Horseshoe equatorial telescope
	is an instance of optical telescope
	is an instance of Ritchey-Chrétien
vidicon	has definition General name for the class of vacuum tube imaging devices which employ a scanning electron beam to read out the image.
	is a kind of image tube
violent galaxy	has definition A type of galaxy differentiated only recently. Violent galaxies include QSOs and exploding galaxies like M82. About 1 percent of the galaxies are classified as violent. Violent galaxies release on the average 1058 ergs of energy, compared with a supernova release of 1049 ergs. Nearest violent galaxy is Cen A.
	has synonym eruptive galaxy
	has synonym exploding galaxy
	is a kind of emission line galaxy
violet	is a kind of optical
Virgo	has acronym Vir
	has boundary
	has genitive Virginis
	has historical image
	has synonym Virgin
	is a part of Zodiac
	is an instance of zodiacal constellation
Virgo cluster	has definition An irregular cluster including the giant elliptical M87 (the galaxy of greatest known mass).
	has members M87
	has number of galaxies 2500
	has redshift z = 0.004
	is a part of Local Supercluster
	is a part of Virgo
	is an instance of rich cluster
Virgo infall	has definition The observed gravitational motion of nearby galaxies toward the Virgo cluster of galaxies, about 50 million light years away. The Virgo cluster represents a strong concentration of mass, a strong departure from a uniform distribution of matter, and it therefore causes galaxies in its vicinity to deviate from the Hubble flow.
	is a kind of galaxy motion
virial theorem	has definition For a bound gravitational system the long-term average of the kinetic energy is one-half of the potential energy.
	is an instance of thermodynamics law
virtual pair	has definition Particle and antiparticle that exist for an extremely short time, often as the intermediate stage of a nuclear transition. According to Dirac's theory, the vacuum can be visualized as consisting of a sea of virtual electron-positron pairs that can only be released or separated when sufficient energy is made available.
	is a kind of virtual particle
virtual particle	has definition A particle that exists for an extremely short time in an intermediate stage of a reaction or transition.
	has definition Particles that erupt from the vacuum momentarily; they exist on borrowed energy, consistent with the uncertainty principle, and rapidly annihilate, thereby repaying the energy loan.
	has definition Particles which take part in virtual processes. They are said to be "off mass-shell", meaning that the relation E2 = p2c2 + m02 c4 does not hold.
	has definition Quantum uncertainties in energy make it possible for virtual particles to be constantly created and annihilated during elementary particle interactions. Elementary particles are able to make use of these virtual particles within their interactions.
	is a kind of particle
virtual phase	has definition A type of CCD in which only one electrode is physically outside the silicon and is such as to obscure only half of the pixel. A specially doped layer under the transparent part acts as another or virtual electrode.
	has number of electrodes 1
	is a kind of charge-coupled device
visibility function	has definition The Fourier transformation of a distant radio source, normalized to its value at small antenna spacings.
	is a kind of Fourier transformation
visual binary	has definition A binary star which can be resolved into two components with current telescopes.
	is a kind of binary star
VLBA	has angular resolving power 15 microarcsecond
	has effective diameter 12000 kilometers
	has image
	has reference The Very Long Baseline Array
	is an acronym for Very Long Baseline Array
	is an instance of radio interferometer
VLF band	has frequency 3 to 30 kHz
	has wavelength 10 to 100 km
	is a kind of radio
Volans	has acronym Vol
	has boundary
	has genitive Volantis
	has historical image
	has historical origin sailors in the south seas said they had seen schools of flying fish
	has synonym Flying Fish
	has synonym Piscis Volans
	is a part of celestial sphere
	is an instance of constellation named by Bayer
volt	has base unit m2·kg·s-3·A-1
	has symbol V
	has unit W·A-1
	is an instance of electric potential unit
	is an instance of named derived SI unit
volume unit	has definition cubic meter
	has symbol m3
	is a kind of derived SI unit
von Klitzing constant	has equation
	has symbol RK
	has uncertainty 0.000095 Ω
	has value 25812.807572 Ω
	is an instance of electromagnetic constant
	is an instance of resistance
Vulpecula	has acronym Vul
	has boundary
	has genitive Vulpeculae
	has historical image
	has synonym Little Fox
	is a part of celestial sphere
	is an instance of constellation
VV Cep	is a part of Cepheus
	is an instance of VV Cep star
VV Cep star	has definition A composite spectrum star. One observes a spectrum of a K or M supergiant, showing emissions of hydrogen and [FeII] plus the spectrum of the secondary, which is generally of type B.
	has prototype VV Cep
	is a kind of composite spectrum star
VV Cephei star	has definition Eclipsing binaries with M supergiant primaries and blue (usually B) supergiant or giant secondaries. They have a rich emission spectrum. Sandage (1974) suggests Mv = - 7.3 for the M2p component of VV Cep.
	is a kind of eclipsing binary
VW Hydri	is a part of Milky Way
	is an instance of dwarf nova
W particle	has definition Massive boson thought to have been abundant in the early universe, when the unified electroweak force was manifest.
	has definition Particle that transmits the unified electromagnetic and weak nuclear forces. These particles were predicted by the Weinberg-Salam theory of the 1960s and later discovered in the 1980s.
	has definition See Intermediate Vector Boson.
	is a kind of vector boson
W Ursae Majoris star	has definition A large class of double-lined eclipsing binaries with very short periods (a few hours) whose spectra indicate mass transfer. They are distinguished by the fact that their primary and secondary minima are equal. They are all F or G binaries on or near the main sequence. They may be the progenitors of dwarf novae.
	is a kind of eclipsing binary
W velocity	has definition A star's velocity perpendicular to the Galactic plane. If a star is moving up, its W velocity is positive; if a star is moving down, its W velocity is negative; and if a star does neither, its W velocity is zero. The Sun has a W velocity of +7 kilometers per second, so it is moving up at 7 kilometers per second. In general, the greater a star's W velocity when it crosses the Galactic plane, the farther above and below the plane the star will travel.
	is a kind of galactic velocity component
W3	has definition A dense cloud of gas in the Perseus arm.
	has distance 3 kpc
	is a part of Perseus arm
	is an instance of radio source
W44	has definition A radio source. It is a supernova remnant less than 0°.5 from the galactic plane.
	has distance 3 kpc
	is an instance of supernova remnant
W49	has definition A radio source (a giant H II region). It is the most powerful thermal radio source known in our Galaxy.
	has distance 14 kpc
	is an instance of radio source
W51	has definition A radio source, a supernova remnant. PSR 1919+14 lies within its radio contours.
	is an instance of supernova remnant
water vapor	has symbol H2O
	is an instance of interstellar molecule
	is an instance of neutral particle
watt	has base unit m2·kg·s-3
	has definition Unit of power.
	has equivalent 107 ergs s-1
	has symbol W
	has unit J·s-1
	is an instance of named derived SI unit
	is an instance of power unit
wave	has definition A propagating pattern of disturbance.
	has definition Propagation of energy by means of coherent vibration.
	has propagation direction
	is a kind of harmonic motion
	is a kind of motion
wave number unit	has definition reciprocal meter
	has symbol m-1
	is a kind of derived SI unit
wave-particle duality	has definition Basic feature of quantum mechanics that objects manifest both wavelike and particle-like properties.
	is an instance of quantum law
wavefront modifier	has definition A device which modifies the wavefront shape, polarization or wavelength
	has wavefront modification shape, polarization or wavelength
	is a kind of optical device
wavelength shift	changes property frequency (and wavelength)
	has definition The alteration in frequency of electromagnetic radiation
	is a kind of radiation modification
WC star	has emission line He I, He II, C II, C III or C IV
	is a kind of Wolf-Rayet
weak G-band star	has definition G-type giant (G5 to K5) with a very weak or absent G band of CH and weak CN bands. These stars are C-deficient.
	is a kind of G star
	is a kind of giant
weak gauge symmetry	has definition Gauge symmetry underlying the weak force.
	is a kind of gauge symmetry
weak line star	has definition Late stars in which the lines of all metals are weakened when compared with normal stars of the same temperature. Also called metal-weak stars.
	has metallic line strength small
	is a kind of late star
weak mixing angle	has equation
	has symbol sin2θW
	has uncertainty 0.0019
	has value 0.2224
	is an instance of atomic constant
Weak-lined radio galaxy	has acronym WLRG
	is a kind of radio galaxy
weakly interacting massive particle	has acronym WIMP
	has definition A generic term for a class of hypothetical particle which may form the missing mass. A form of non-baryonic cold dark matter.
	has definition Some astronomers believe that these exotic subatomic particles make up most of the mass of the universe.
	is a kind of cold dark matter
weber	has base unit m2·kg·s-2·A-1
	has definition The derived SI unit of magnetic flux.
	has equivalent 108 maxwells
	has symbol Wb
	has unit V·s
	is an instance of magnetic flux unit
	is an instance of named derived SI unit
week	has definition An arbitrary period of days, usually seven days; approximately equal to the number of days counted between the four phases of the Moon. (See lunar phases.)
	has value 7 days
	is a kind of time unit
Werner line	has definition Spectral line of molecular hydrogen in the ultraviolet, in the same general region as the Lyman lines.
	has species H2
	has wavelength UV
	is a kind of molecular band
Whirlpool galaxy	has discoverer Charles Messier
	has discovery date 1773
	has image
	has Messier number 51
	has synonym M 51
	has synonym NGC 5194
	is a part of Canes Venatici
	is a part of Local Group
	is an instance of Messier object
	is an instance of Sc spiral
whistlers	has definition Radio waves generated by a flash of lightning, which travel along Earth's magnetic field out beyond the ionosphere and back to Earth. They arrive back with a descending pitch because the high-frequency end of the wave train arrives first (see dispersion).
	is a part of ionosphere
white dwarf	has definition A small, faint, dense, dying star that has used up its nuclear fuel and is slowly fading from view. A typical white dwarf has 60 percent of the Sun's mass but is little larger than the Earth. White dwarfs are common, accounting for 10 percent of all stars in the Galaxy; the nearest is Sirius B, just 8.6 light-years away. But no white dwarf is visible to the naked eye.
	has definition Faint star lying five or more magnitudes below the main sequence.
	has definition Faint very-compact stars at the end of their life. Also used as convenient photometric and flux standards.
	has definition Final stage of a star, at which the nuclear energy is exhausted. Cool, and becoming a dead, black body, it is nevertheless extremely dense.
	has definition Star of high surface temperature, low luminosity, and high density (105-108 g cm-3), with roughly the mass of the Sun and the radius of the Earth, that has exhausted most or all of its nuclear fuel, believed to be a star near its final stage of evolution. When the Sun becomes a white dwarf, its radius will be about 0.01 of its present radius. DA white dwarfs are hydrogen-rich; DB white dwarfs are helium-rich; DC are carbon rich; DF are calcium-rich; DP are magnetic stars. White dwarfs have relatively low rotational velocities.
	has symbol D
	has symbol wd
	is a kind of early star
white giant	has definition A giant star of spectral type A. Some RR Lyrae stars are white giants.
	is a kind of A star
	is a kind of giant
white hole	has antonym black hole
	has definition The time-reversal of a black hole. A white hole is a singularity from which matter emerges unpredictably, but into which matter cannot enter. The initial singularity of the standard big bang theory is an example of a white hole. It can be shown that the creation of a new universe from a false vacuum bubble in the context of classical general relativity would require a white hole singularity, which means essentially that it cannot be done, even in principle. However, a false vacuum bubble could conceivably grow to become a new universe through a process of quantum tunneling.
	is a kind of singularity
white supergiant	has definition A supergiant star with a spectral type of A. White supergiants are rare; the nearest is Deneb, which lies 1500 light-years away.
	is a kind of A star
Wien displacement law constant	has equation
	has symbol b
	has uncertainty 0.0000051 × 10-3 m K
	has value 2.8977686 × 10-3 m K
	is an instance of physico chemical constant
Wien's law	has definition The wavelength at which a blackbody emits the greatest amount of radiation is inversely proportional to its absolute temperature.
	is an instance of quantum law
William Herschel Telescope	has acronym WHT
	has altitude 2332 m
	has aperture 4.2 m
	has creation date 1987
	has focal ratio 02.5, 11
	has latitude 28° 46' N
	has location La Palma, Canary Islands
	has longitude 17° 53' W
	has mirror maker Grubb-Parsons
	has mirror type Owens-Illinois Cer-Vit
	has mounting manufacturer Grubb-Parsons
	has operator Royal Greenwich Observatory
	has owner Obs. del Roque de los Muchachos
	is an instance of altazimuth telescope
	is an instance of optical telescope
	is an instance of reflector
winter solstice	is an instance of solstice
Wisconsin-Indiana-Yale-NOAO Telescope	has altitude 2089 m
	has aperture 3.5 m
	has comment mirror by Steward Observatory Mirror Lab (R. Angel)
	has creation date 1994
	has focal ratio (f/1.75) f/6.3
	has latitude 31° 57' N
	has location Kitt Peak, Arizona. US
	has longitude 111° 36' W
	has mirror maker Charles Harmer/NOAO
	has mirror type spin-cast borosilicate honey-comb
	has mounting manufacturer L & F Industries
	has owner WIYN Observatory
	has synonym WIYN 3.5 m
	is an instance of altazimuth telescope
	is an instance of optical telescope
	is an instance of reflector
WN star	has emission line He I, He II, N III, N IV or N V
	is a kind of Wolf-Rayet
WO star	has emission line He I, He II, O IV, O V or O VI
	is a kind of Wolf-Rayet
Wolf 359	has definition A nearby flare star.
	is a part of Leo
	is an instance of flare star
Wolf-Lundmark-Melotte	has declination -15d27m51s
	has definition A dwarf E5 elliptical galaxy, sometimes considered a member of the Local Group.
	has diameter 11.5 × 4.0 arcminute
	has distance 4.2 million light years
	has magnitude 11.03
	has NED data
	has right ascension 00h01m57.9s
	has synonym WLM
	is a part of Local Group
	is an instance of irregular galaxy
Wolf-Rayet	has definition Hot stars characterized by wide emission lines of highly ionized elements, standing out distinctly from the continuous spectrum. There exist three varieties: WN, WC and WO
	has definition One of a class of very luminous, very hot (as high as 50000 K) stars whose spectra have broad emission lines (mainly He I and He II), which are presumed to originate from material ejected from the star at very high (~ 2000 km s-1 ) velocities. Some W-R spectra show emission lines due to carbon (WC stars); others show emission lines due to nitrogen (WN stars). (Hiltner and Schild classification: WN-A, narrow lines; WN-B, broad lines.)
	has ejection velocity 2000 km s-1
	has emission line He I, He II
	has temperature up to 50000 K
	is a kind of early star
	is a kind of emission line star
work function	has definition The amount of energy needed to release an electron from the attraction of positive ions in a metal. It is different for different metals.
	has symbol W
	is a kind of energy
wormhole	has definition A bridge to another universe created by a black hole.
	has definition A tube-like region of space connecting one region of the universe to another.
	has definition An intriguing solution to the equations of general relativity which describes a neck that can connect two completely separate universes. Wormholes arise in the discussion of the creation of a universe in the laboratory, because the new universe disappears through a wormhole and completely detaches itself from the parent universe.
	is a kind of singularity
WR 124	has distance 15000 light-years
	has image emission nebula M1-67
	has reference Grosdidier, Y.; Moffat, A. F. J.; Blais-Ouellette, S.; Joncas, G.; Acker, A. : 1999, Proc. IAU Symp. 193, 356. M1-67, nebula ejected from the 200 km/s runaway WN8 star WR 124
	has spectral type WN 8
	has spectral type WN8
	has velocity 200 km/s
	is a part of Milky Way
	is an instance of WN star
WZ Cassiopeiae	has definition A lithium star, the most super-rich carbon star known.
	has effective temperature 2420 K
	is an instance of lithium star
WZ Sagittae	has definition A recurrent DAe old nova (1913 and 1946) with the shortest known orbital period (about 80 minutes). It is almost certainly a close binary system in which mass is being transferred onto a white-dwarf primary.
	is an instance of recurrent nova
X-process	has definition The unknown nucleosynthetic process that Burbidge, Burbidge, Fowler, and Hoyle said had formed the light nuclei deuterium, lithium, beryllium, and boron.
	is a kind of nucleosynthesis
X-ray	has definition A large band of electromagnetic radiation with wavelengths smaller than extreme ultraviolet light. A typical X-ray photon has over one thousand times as much energy as a photon of visible light.
	has definition Photon of wavelength between about 0.1 Å and 100 Å - more energetic than ultraviolet, but less energetic than a γ-ray.
	has frequency 30 PHz to 3 EHz
	has wavelength 100 pm to 10 nm
	is a kind of photon
X-ray astronomy	has definition Detection of stellar and interstellar X-ray emission. Because X-rays are almost entirely filtered out by the Earth's upper atmosphere, the use of balloon- and rocket-borne equipment is essential.
	is a kind of astronomy
X-ray pulsar	has definition Pulsar (q.v.) that radiates in the X-ray region of the spectrum. Best verified examples are Her X-1 and Cen X-3. They are thought to be rotating, strongly magnetic neutron stars of about 1 Msun in a grazing orbit around a more massive star from which they are accreting matter.
	is a kind of pulsar
X-ray selected BL Lac object	has acronym SBL
	is a kind of BL Lacertae
X-ray source	has definition A class of celestial objects whose dominant mechanism of energy dissipation is through X-ray emission. Galactic X-ray sources appear optically as starlike objects, peculiar in their ultraviolet intensity, variability (on time scales ranging from milliseconds to weeks), and spectral features. All known compact X-ray sources are members of close binary systems; a current popular model is mass accretion onto a compact object from a massive companion. (Four X-ray sources - all variable - are known to be associated with globular clusters.) The 21 known extended X-ray sources associated with clusters of galaxies seem to be clouds of hot gas trapped in the cluster's gravitational field.
	has wavelength X-ray
	is a kind of celestial body
	is a part of celestial sphere
X-ray space telescope	has wavelength sensitivity in gamma ray and X-rays
	is a kind of space telescope
xenon	has image
	has ocean oxidation state 0
	is a kind of inert gas
	is a kind of unclassified oceanic element
XMM	has definition X-ray Multi-mirror Mission
	has owner ESA
	is an instance of X-ray space telescope
year	has definition A period of time based on the revolution of the Earth around the Sun.
	is a kind of time unit
yellow	is a kind of optical
yellow giant	has definition A giant star with a spectral type of G. The nearest and brightest yellow giants are the two composing the double star Capella.
	is a kind of G star
	is a kind of giant
yellow supergiant	has definition A supergiant star with a spectral type of G.
	is a kind of G star
	is a kind of supergiant
Yerkes 40-inch Refractor	has altitude 334 m
	has aperture 1.016 m
	has comment a focal reducer also provides an f/3 focus
	has creation date 1897
	has focal ratio f/19.04
	has latitude 42° 34' N
	has lens maker Alvan Clark & Sons
	has location Williams Bay, Wisconsin, US
	has longitude 88° 33' W
	has mounting manufacturer Warner and Swasey
	has operator University of Chicago
	has owner Yerkes Observatory
	has synonym 40 inch
	is an instance of German equatorial telescope
	is an instance of refractor
ylem	has definition Primordial state of matter - neutrons and their decay products (protons and electrons) - before the Big Bang. The term was taken from Aristotle and used for the α-β-γ theory.
	has definition The word used by Gamow and his collaborators for the primordial material of the Big Bang. In most of his work Gamow assumed that the ylem consisted entirely of neutrons. In inflationary cosmology, the role of the ylem is played by the false vacuum.
	is a kind of phase of matter
yocto	has symbol y
	has value 10-24
	is an instance of unit prefix
yotta	has symbol Y
	has value 1024
	is an instance of unit prefix
young thin disk	has definition A subPopulation In the thin disk whose stars range in age from 0 to 1 billion years old. The stars of the young thin disk have a scale height of 350 light-years and have very circular orbits around the Galaxy.
	has star age 0 to 1 billion years
	is a part of thin disk
ytterbium	has image
	has ocean oxidation state III
	has ocean residence time 400 years
	is a kind of rare Earth
	is a kind of recycled oceanic element
yttrium	has image
	has ocean oxidation state III
	is a kind of transition metal
	is a kind of unclassified oceanic element
YY Orionis	has definition An extremely young star (younger than T Tauri) in the Orion Nebula.
	is a part of Orion
	is an instance of YY Orionis star
YY Orionis star	has definition Very young, late-type, low-mass stars in the gravitationally contracting stage in which the star is still accreting matter from the protostellar cloud.
	has spectra peculiarity inverse P Cyg type emission profiles in the Ca II HK lines
	is a kind of T Tauri star
Z Camelopardalis	has acronym Z Cam
	has definition is a dwarf nova with standstills in the light curve
	has light curve from AAVSO
	has reference Oppenheimer, B.D., Kenyon, S.J., Mattei, J.A.: 1998, AJ 115, 1175. An Analysis of AAVSO Observations of Z Camelopardalis
	is a part of Milky Way
	is an instance of Z Camelopardalis star
Z Camelopardalis star	has definition A class of dwarf nova with standstills in their light curves. Z Cam itself is a semidetached binary (period 7h21m) consisting of a dG1 star and a hot white dwarf or a hot blue subdwarf which is probably degenerate. Mean time between eruptions, 20 days. Peak-to-peak amplitude, about 0.5 mag.
	has prototype Z Camelopardalis
	is a kind of dwarf nova
Z particle	has definition Massive boson thought to have been abundant in the early universe, when the unified electroweak force was manifest.
	has definition Particle that transmits the unified electromagnetic and weak nuclear forces. These particles were predicted by the Weinberg-Salam theory of the 1960s and later discovered in the 1980s.
	is a kind of vector boson
z pinch	has definition A diffuse toroidal pinch in which the magnetic field runs around the plasma column.
	is a kind of pinch machine
Zeeman effect	has cause the influence of an magnetic field on a radiating atom or ion.
	has definition Line broadening due to the influence of magnetic fields.
	has effect splitting of a spectral line into a multiplet of lines with distinct polarization characteristics.
	is a kind of line broadening
	is measured by measuring the difference between right-hand and left-hand polarization across a spectral line.
zenith	has antonym nadir
	has azimuth 0
	has definition The point in the sky directly overhead.
	has definition The point on the celestial sphere directly above the observer's head - i.e., opposite to the direction of gravity (nadir).
	has elevation 90
	is an instance of local coordinate
	is opposite of nadir
zenith distance	has definition Angular distance on the celestial sphere measured along the great circle from the zenith to the celestial object. Zenith distance is 90° minus altitude.
	is an instance of angle
	is an instance of local coordinate component
zepto	has symbol z
	has value 10-21
	is an instance of unit prefix
zero-point energy	has definition The energy of the lowest state of a quantum system. Amount of vibrational energy allowed by quantum mechanics to be associated with atomic particles at 0 K, whereas classical mechanics requires this to be zero. Also, the energy of an electron in its ground state.
	is a kind of energy
zero-point pressure	has definition The pressure contributed by degenerate electrons, which do not come to rest even at absolute zero.
	is a kind of pressure
Zerodur	is a kind of mirror
zeroth law of thermodynamics	has definition The temperature of a body in equilibrium is the same at all points.
	has synonym law of thermal equilibrium
	is an instance of thermodynamics law
zeta Aurigae star	has definition Binary star with a K supergiant primary and a main-sequence secondary.
	is a kind of binary star
Zeta Ophiuchi	has definition A reddened O star (a runaway star from the Sco-Cen association) with a high rotational velocity. It is well known for its strong interstellar absorption lines in the visible part of the spectrum.
	has distance 170 pc
	has rotational velocity 396 km s-1
	has spectral type O9.5 V
	is a part of Ophiuchus
	is an instance of dwarf
	is an instance of O star
Zeta Pup	has definition The brightest Of star known, embedded in the Gum Nebula. It has an envelope which is rapidly accelerating outward.
	has spectral type O4f
	is a part of Puppis
	is an instance of O star
Zeta Puppis	has B-V magnitude -0.26
	has declination -40 0 11
	has right ascension 8 3 35.0
	has spectral type O5Iaf
	has synonym HR 3165
	has V magnitude 2.25
	is a part of Puppis
	is an instance of blue supergiant
	is an instance of naked eye star
	is an instance of O star
zetta	has symbol Z
	has value 1021
	is an instance of unit prefix
zinc	has image
	has ocean oxidation state II
	has ocean residence time 5000 years
	is a kind of chalcophile element
	is a kind of recycled oceanic element
	is a kind of transition metal
zirconium	has image
	has ocean oxidation state IV
	is a kind of transition metal
	is a kind of unclassified oceanic element
Zodiac	has definition A band about 8° wide on the celestial sphere, centered on the ecliptic.
	has historical origin Contains twelve constellations which represent a calendar of the Sun's apparent progress across the celestial sphere. Naked eye planets are found along much the same path (the ecliptic) and caused these constellations to become a focus for astrology.
	has purpose to facilitate planetary observations as most objects in solar orbit are found in this region
	has synonym ecliptic region
	is a part of celestial sphere
	is an instance of equatorial sky area
zodiacal constellation	has definition a constellation within the Zodiac region, each is a sign of the zodiac except for Ophiuchus
	has historical origin a constellation which the Sun passes through
	has historical purpose to approximately represent the month in which the Sun occupies this constellation
	is a kind of constellation
	is a part of Zodiac
Zodiacal Light	has definition A faint glow that extends away from the Sun in the ecliptic plane of the sky, visible to the naked eye in the western sky shortly after sunset or in the eastern sky shortly before sunrise. Its spectrum indicates it to be sunlight scattered by interplanetary dust. (Pioneer 10 has determined that its brightness varies inversely as the square of the distance out to 2.25 AU and then decreases more rapidly.) The zodiacal light contributes about a third of the total light in the sky on a moonless night.
	is a kind of scattering
Zone of Avoidance	has definition An irregular zone near the plane of the Milky Way where the absorption due to interstellar dust is so great that no external galaxies can be seen through it.
	has name origin galaxies appear to "avoid" the Milky Way
	is a part of celestial sphere
	is an instance of equatorial sky area