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Glossary

16th century scientist
a scientist born between 1500 and 1600 AD
17 Leporis
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.
17th century scientist
a scientist born between 1600 and 1700 AD
18th century scientist
a scientist born between 1700 and 1800 AD
19th century scientist
a scientist born between 1800 and 1900 AD
20th century scientist
a scientist born between 1900 and 2000 AD
3 alpha process
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)
3-kpc arm
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.
30 Doradus Nebula (NGC 2070, Great Looped Nebula, Tarantula Nebula)
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.
4-kpc arm
A component of the Sagittarius arm with noncircular gas motions.
40 Eridani
A nearby triple system, 5 pc distant. Component A is K0 V; component B is a DA white dwarf; component C is M5e V.
47 Tucanae
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.
4N nucleus (even-even nucleus)
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.
53 Arietis
Runaway star which diverges from a comparatively small area in Orion.
61 Cygni
The first star other than the Sun to have its parallax, and hence distance, measured. The star is a double orange dwarf.
[WR] star
WR star that is the central object of a planetary nebula.
A band
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)
A star
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.
ablation
Removal of material from a solid by heating, vaporization or collisions
absolute magnitude
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.
absolute magnitude
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.
absolute temperature
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.
absolute zero
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).
absorption
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.
absorption edge
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.
absorption line
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.
absorption line
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.
abstraction
a general concept formed by extracting common features from specific examples
acceleration unit (m·s-2)
meter per second squared
accretion
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).
accretion
Collection of material together, generally to form a single body.
accumulation theory
The theory by which planetesimals are assumed to collide with one another and coalesce, eventually sweeping up enough material to form the planets.
Achilles
Trojan asteroid (60°) ahead of Jupiter. It was the first Trojan to be discovered.
achromatic lens
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.
achromatic objective
A lens of two or more components with different refraction indices (e.g., crown glass and flint glass), used to correct for chromatic aberration.
actinide
Element with atomic number between 89 (Actinium) and 103
actinium (Ac, Ac, 2D3/2 in ground state)
soft, silvery-white, radioactive metal which glows in the dark
active galaxy
Active galactic nuclei are very luminous. Their energy output is in two forms: nonthermal continuum and thermal emission line.
active galaxy
Any galaxy which is emitting large quantities of non-thermal radiation.
active optics
Controlling the shape of a telescope mirror at a relatively slow rate.
active Sun
The Sun during its 11-year cycle of activity when spots, flares, prominences, and variations in radiofrequency radiation are at a maximum.
adaptive optics
Compensating for atmospheric distortions in a wavefront by high-speed changes in the shape of a small, thin mirror.
advance of the perihelion
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).
AE Aurigae
Runaway star which diverges from a comparatively small area in Orion.
Ae star
A-type star with emission in one or several Balmer lines.
aeon (eon)
In astronomical terms, 1000 million years.
aether
In Aristotelian physics, the fifth element, of which the stars and planets are made.
aether
In Classical physics, an invisible medium that was thought to suffuse all space.
AI Velorum star
A class of dwarf Cepheids. They are all RR Lyrae stars with periods shorter than 0.25 days.
airglow (nightglow)
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.
alchemy
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.
Aldebaran (alpha Tau, HR 1457)
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.
Algol (beta Per, Demon star)
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.
Algol (beta Per, Demon star)
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.
alkali earth metal
Element in the second column of the periodic table (from the left)
alkali metal (group I element)
Metal in the first column of the periodic table
alpha Centauri (Rigil Kent, HR 5459)
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.
alpha decay
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.
alpha decay
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.
alpha Persei cluster
A young open cluster with a high mean rotational velocity.
alpha-particle
Nucleus formed by the α-process (q.v.) (see even-even nuclei).
alpha-particle
Particle first discovered in radioactive α decay, and later identified as helium nuclei (two protons and two neutrons bound together).
alpha-particle
The nucleus of a 4He atom, consisting of two protons and two neutrons. Mass of α-particle 4.00260 amu.
alpha-process
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.
alpha2 CVn
A spectrum variable with spectrum showing strong, profuse lines of rare earths, iron-peak elements, and Si.
Altair (alpha Aql, HR 7557)
A bright A7 V star.
altazimuth telescope
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)
aluminium (Al, 2P1/2 in ground state)
soft and malleable metal
Am star
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.
Am star
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.
Amalthea (Jupiter V, Barnard's satellite)
The innermost satellite of Jupiter.
americium (Am, 8S7/2 in ground state)
radioactive silvery metal which does not occur naturally
amount-of-substance concentration unit (mol·m-3)
mole per cubic meter
ampere (A)
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
ampere (A)
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 1946, Resolution 2, approved by the 9th CGPM 1948). A current of 1 A is equivalent to the passage along the filament of a light bulb of about 6 × 1018 electronic charges per second.
Andromeda galaxy (M 31)
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.
Andromeda galaxy (M 31)
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.
Andromeda galaxy (M 31)
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.
Andromeda galaxy (M 31)
The largest member of the local group. It is a giant spiral galaxy that lies 2.4 million light-years away.
Andromeda I
Dwarf spheroidal galaxy in the Andromeda subgroup of the Local Group. It is among the intrinsically faintest members of the Local Group.
Andromeda II
Dwarf spheroidal galaxy in the Andromeda subgroup of the Local Group. It is among the intrinsically faintest members of the Local Group.
Andromeda III
Dwarf spheroidal galaxy in the Andromeda subgroup of the Local Group. It is among the intrinsically faintest members of the Local Group.
angstrom (Å)
A unit of length used when expressing wavelengths.
angular momentum (L)
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.
angular momentum (L)
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.
angular size
The angle subtended by an object on the sky. For example, the angular size of the moon is 30 arcminutes.
anisotropy
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.
anisotropy
The condition in which the universe appears different in different directions.
annular eclipse
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.
annular eclipse
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.
anomalistic month
The interval between two successive perigee passages of the Moon.
anomalistic year
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.
anomalistic year
The interval between two successive perihelion passages of Earth.
anomalous Zeeman effect
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.
antapex
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.
Antares (alpha Sco, HR 6134)
A red M1 Ib supergiant. It has a B3 V companion, which is a radio source.
anthropic principle
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.
anthropic principle
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.)
anthropocentrism
The belief that humans are central to the universe.
anthropomorphism
The projection of human attributes onto nonhuman entities such as animals, the planets, or the universe as a whole.
anti-reflection coating
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.
anticenter
The direction of the sky opposite to that of the galactic center.
anticenter
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.
antimony (Sb, stibium from latin, 4S3/2 in ground state)
metalloid element with three forms. The metallic form is the more stable and is bright, silvery, hard and brittle
antineutrino
The antiparticle of a neutrino.
antineutron
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.
antiparticle
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.
antiparticle
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.
antiparticle
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).
antiparticle
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.
antiparticle
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.
antiproton
The antiparticle of a proton, identical in mass and spin but of opposite (negative) charge.
antiquark
The antiparticle of the quark.
Ap star
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.
Ap star
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.
Ap star
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.
apastron
The point in the orbit of one component of a binary system where it is farthest from the other.
aphelion
The point in a planetary orbit that is at the greatest distance from the Sun.
aplanatic system
A system of three lenses which, taken together, correct for spherical aberration, chromatic aberration, and coma.
apocenter
The point in the orbit of one component of a binary system which is farthest from the center of mass of the system.
apogalacticon
The point in a star's orbit farthest from the Galactic center.
apogee
The point at which a body in orbit around the Earth reaches its farthest distance from the Earth.
Apollo
Prototype of a small group of asteroids whose orbits intersect that of Earth.
Apollo asteroid
One of a small group of asteroids whose orbits intersect that of Earth. They are named for the prototype, Apollo.
Apollo space program
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.
apparent magnitude
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.
apparent magnitude
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.
apsidal motion
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.
arcminute
A unit of angle equal to 1/60 of a degree.
arcsecond
One sixtieth of an arcminute, or 1/3600 of a degree.
Arcturus (Alpha Boötis, HR 5340)
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.
Arcturus (Alpha Boötis, HR 5340)
An old subgiant disk star (K2 IIIp, mv = 0.06) about 11 pc distant.
area unit (m2)
square meter
argon (Ar, 1S0 in ground state)
colourless, odourless gas comprising 1% of the atmosphere
argument of the perihelion (ω, longitude of the perihelion)
An orbital element representing the longitude of the ascending node plus the angle along the orbit from the ascending node to the perihelion point.
argument of the perihelion (ω, longitude of the perihelion)
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.
Ariel
Second satellite of Uranus.
Aristotelian physics
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.
array telescope
a telescope composed of an array of separete individual elements acting in concert
arsenic (As, 4S3/2 in ground state)
a metalloid element with two main forms, grey α arsenic and β arsenic
artifact
an object made by humans
artificial satellite
a satellite made by humans which is gravitationaly bound and in orbit of a larger physical object
AS Eri
An eclipsing binary whose secondary is close to its Roche limit.
ascending node
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.
aspheric
An optical surface with departures in shape from a perfect sphere in order to cancel optical imperfections or aberrations.
association
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.
asterism
a group of bright stars which form a conspicuous pattern on the celestial sphere
asteroid (minor planet, planetoid)
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
asteroid (minor planet, planetoid)
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.
asteroid (minor planet, planetoid)
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).
asteroid belt
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.
astrology
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.
astrology
The belief that human affairs and people's personalities and characters are influenced by (or encoded in) the positions of the planets.
astrometry
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.
astronomer
a scientist specializing in astronomy
astronomical horizon event
The time and azimuth at which a celestial body crosses the astronomical horizon of an oberver.
Astronomical Image Processing System (AIPS)
Developed by the National Radio Astronomy Observatory.
astronomical refraction
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.
astronomical unit (AU)
Mean distance between the Earth and the Sun: 1.495985 × 1011 m.
astronomical unit (AU)
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.
astronomical unit (AU)
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.
astronomical unit (AU)
The mean distance from the earth to the sun, equal to 92.81 million miles or 499.012 light-seconds.
astronomical unit (AU)
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.
astronomy
The science that studies the natural world beyond the earth.
Astrophysical Data Facility (ADF)
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).
astrophysics
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.
asymptotic branch star
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.
atmophile element
volatile element that tends to be found in the atmosphere of a planet or asteroid
atmosphere
A unit of pressure.
atmosphere
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.
atmospheric dispersion corrector
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.
atmospheric extinction
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.
atomic mass constant (mu)
One-twelfth the mean mass of an atom of carbon 12 (including the orbital electrons).
atomic mass number
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.
atomic mass unit (u, dalton)
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.
atomic number (charge number)
The number of protons in an atom's nucleus. This determines the type of element.
attenuation
The falling off of the energy density of radiation with distance from the source, or with passage through an absorbing or scattering medium.
Auger transition
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.
aurora
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.
aurora
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.
aurora
the light produced by excited atoms and ions in the upper atmosphere of a planet
aurora australis
an aurora ocurring in the southern hemisphere of the Earth
aurora borealis
an aurora ocurring in the northern hemisphere of the Earth
autoionization
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)
autumnal equinox
See equinox
Avogadro number (NA)
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).
axion
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".
axis
Theoretical straight line through a celestial body, around which it rotates.
axis motion
the motion of the axis of a system
axisymmetric collapse
Collapse of mass in such a way that the mass maintains the symmetry of a cylinder.
azimuth
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.
azimuth
Directional bearing around the horizon, measured in degrees from north (0°).
azimuthal quantum number (k)
A measure of the minor axis of an elliptic orbital of an electron according to the Bohr-Sommerfeld theory.
B band
Telluric lines due to O2 absorption in Earth's atmosphere, but originally thought to originate in the Sun by Fraunhofer.
B star
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.
b-lines
A triplet of spectral lines of Mg I.
B[e] star
Be stars exhibiting forbidden lines in emission. Jaschek, C., Jaschek, M. 1995 The Behavior of Chemical Elements in Stars, Cambridge University Press
Baade's Window
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.
backscatter
Scattering of radiation (or particles) through angles greater than 90° with respect to the original direction of motion.
Bahcall-Soneira Model
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.
Bailey type
A classification of RR Lyrae stars according to the shape and amplitude of their light variation
Baily's beads (diamond ring effect)
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.
Balmer jump (Balmer discontinuity)
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.
Balmer line
The series lies in the visible portion of the spectrum. (Deuterium Hα is 1.785 Å short-ward of hydrogen Hα)
Bamberga
Asteroid with the darkest known surfaces in the solar system, the only minor planet known to have such a low albedo.
band head
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.
bar
An absolute unit of pressure equal to 106 dyn cm-2.
barium star (Ba star, Ba II star)
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.
barium star (Ba star, Ba II star)
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.
Barnard's star
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.
barotropic gas
A gas in which the pressure is a function of the density only.
barred spiral
(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.
baryon
Heavy subatomic particle composed of 3 quarks.
base SI unit
a unit assumed to be mutually independent from which all other units are derived
Be star
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.
Be star
Non-supergiant B-type stars, which have shown emission in at least one of the Balmer lines at some time.
beat Cepheid
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.
becquerel (Bq)
Unit of radioactivity; 1 becquerel represents one disintegration, or other nuclear transformation, per second.
Bellatrix (gamma Orionis)
A B2 III star.
beryllium (Be, 1S0 in ground state)
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.
Besselian element
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.
Besselian year
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).
beta Cephei star (beta Canis Majoris star)
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.
beta Lyrae star
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.
beta Persei star
A class of eclipsing binary (see Algol) with periods of from 2 to 5 days, the depth of whose secondary minimum is almost negligible.
beta-decay
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 (np + 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).
beta-decay
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.
beta-decay
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.
beta-decay
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.)
beta-particle
An electron or a positron emitted from an excited nucleus when it returns to its ground state via β-decay.
beta-particle
Particles first discovered in radioactive β decay - later identified as electrons.
Betelgeuse (alpha Ori)
A red semiregular variable supergiant. Betelgeuse is also a strong infrared emitter - at 2 µ the brightest in the sky.
Betelgeuse (alpha Ori)
A red supergiant star in the constellation Orion and the brightest red supergiant in Earth's sky.
biased galaxy formation
The theory that bright galaxies form preferentially from anomalously overdense perturbations in the early universe.
big bang (Friedmann-Lemaitre universe)
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.
big bang (Friedmann-Lemaitre universe)
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.
big bang (Friedmann-Lemaitre universe)
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.
big bang (Friedmann-Lemaitre universe)
According to standard cosmology, the explosion that started the universe expanding 10 to 15 billion years ago.
big bang (Friedmann-Lemaitre universe)
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.)
big bang (Friedmann-Lemaitre universe)
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.
big bang (Friedmann-Lemaitre universe)
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.
big bang (Friedmann-Lemaitre universe)
The initial point of creation.
big bang (Friedmann-Lemaitre universe)
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.
big bang (Friedmann-Lemaitre universe)
The state of extremely high (classically, infinite) density and temperature from which the universe began expanding.
big bang (Friedmann-Lemaitre universe)
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).
Big bang crossover effect
The epoch during the radiation era when the universe switched from being radiation-dominated to being matter-dominated.
Big Bang era
A theoretical era postulated to have occurred after the Big Bang. They are in order :
  1. Planck era
  2. hadron era
  3. inflation era
  4. lepton era
  5. nucleosynthetic era
  6. radiation era
  7. matter era
big crunch
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.
big crunch
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.
big crunch
The state of extremely high density and temperature into which a closed universe will recollapse in the distant future.
binary galaxy (double galaxy)
Two galaxies orbiting each other owing to their mutual gravitational attraction.
binary star
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.)
binding energy
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.
biology
The scientific study of life and living matter.
BL Lacertae (BL Lac)
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.)
BL Lacertae (BL Lac)
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.
black dwarf
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.
black hole
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.
black hole
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.
black hole
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.
black hole
An object that is maximally gravitationally collapsed, and from which not even light can escape.
black hole
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.)
blazar
A highly variable active galaxy which, in general, displays no emission lines in its spectrum.
blazar
A term collectively used to refer to Optically Violent Variables (OVVs) and BL Lac objects.
blaze angle
The tilt of the facets or grooves of a diffraction grating.
blazed grating
Diffraction grating so ruled that the reflected light is concentrated into only a few orders, or even a single order, of the spectrum.
BLS1
Broad Line Seyfert 1
blue dwarf
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.
blue giant
A giant star with spectral type O or B. Croswell, K. 1995 The Alchemy of the Heavens, Anchor Books
blue giant
High-temperature star (as opposed to red stars). Blue giants are generally on or near the main sequence of the Hertzsprung-Russell diagram.
blue halo star
Hot star in the horizontal-branch, post-horizontal-branch, and post-asymptotic branch phases of evolution.
blue haze
A condition in the Martian atmosphere which sometimes makes it opaque to radiation in the blue-violet end of the visible spectrum.
blue horizontal-branch star (HZ star)
Blue horizontal-branch star, the first catalog of which was compiled by Humason and Zwicky.
blue horizontal-branch star (HZ star)
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.
blue supergiant
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.
blueshift
The shift of spectral lines toward shorter wavelengths in the spectrum of an approaching source of radiation.
BM Orionis
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.
Bohr atom
The model of an atom whose electrons are pictured as describing "Keplerian" orbits about the central nucleus.
Bohr magnetonB)
Magnetic moment of an electron in the first Bohr orbit.
Bohr radius (a0, a0)
A unit of length based on the radius of the first Bohr orbit of hydrogen 1.
Bok globule
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.
bolometer
A device for measuring the total amount of radiant energy received from a celestial object.
bolometric absolute magnitude (Mbol)
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.
Boltzmann constant (k)
The constant of proportionality relating the mean kinetic energy of an atom to its absolute temperature.
boron (B, 2P1/2 in ground state)
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.
Bose-Einstein nucleus
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.
boson
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.
boson
A particle which does not obey Pauli's exclusion principle. It is denoted by an integer (or zero) spin.
boson
A particle, or pattern of string vibration, with a whole number amount of spin; typically a messenger particle.
boson
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.
boson
Any particle with integer spin: 0, h bar, 2h bar etc.
boson
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.
boson
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.
bosonic string theory
First known string theory; contains vibrational patterns that are all bosons.
bottom
A flavor of quark. See flavor.
bottom-up scenario
A galaxy-formation scenario in which small galaxies form first. Larger and larger structures are then formed in due course.
bottom-up scenario
The idea that small structures, perhaps galaxies or even smaller substructures, form first in the universe, followed later by larger structures.
bound electron
Zone in which the electron in atoms reside. Its radius is determined by the quantum principle, its population by the exclusion principle.
bound-bound transition
Transition between energy levels of an electron bound to a nucleus (the electron is bound both before and after the transition).
bound-free transition
Transition in which a bound electron in any energy level is liberated or its reverse process.
Bowen fluorescence mechanism
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).
Boyle's law
The pressure of an ideal gas kept at constant temperature varies inversely as the volume, i.e., directly as the density.
Bp star
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.
Bragg angle
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.
brane
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.
bremsstrahlung (deceleration radiation, free-free transition)
Electromagnetic radiation given out by electrons interacting with the ions in an ionized gas.
bremsstrahlung (deceleration radiation, free-free transition)
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.)
bright blue variable
Early-type high-luminosity star with peculiar spectra and large-amplitude light variations over a long time scale.
bright point
Bright region (in X-ray and XUV) observed on the Sun during Skylab missions.
Brillouin scattering
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.
brown dwarf
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).
brown dwarf
A substellar object that is below the minimum mass required for nuclear fusion reactions to occur in its core.
brown dwarf
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.
Bruce medalist
a scientist who was awarded the Bruce Medal
bulge
The stellar population that lies within several thousand light-years of the Galactic center. The bulge is old, dense, and metal-rich.
Bw star
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.
C star
A class of carbon star, defined by Morgan and Keenan to replace the Harvard R and N spectral classes.
C star
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.
Ca H line
A spectral line of singly ionized calcium.
Ca K line
A spectral line of singly ionized calcium.
calcium (Ca, 1S0 in ground state)
silvery-white, relatively soft metal
calendar year
is an approximation to the tropical year.
Callisto (Jupiter IV, J IV)
A Galilean satellite of Jupiter. It has the lowest density, lowest albedo, and highest temperature of any of the four main satellites of Jupiter.
Callisto (Jupiter IV, J IV)
Fifth (known) moon out from jupiter, and its second largest.
Cambridge catalog
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.
candela (cd)
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
candela (cd)
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."
Canopus (alpha Car, HR 2326)
A supergiant, the second brightest star in the southern sky.
Capella (HR 1708, Alpha Aurigae)
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.
carbon (C, 3P0 in ground state)
pure forms occur as graphite, diamond and buckminsterfullerene C60
carbon burning
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.
carbon cycle (Bethe-Weizsäcker cycle, CNO cycle)
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.)
carbon cycle (Bethe-Weizsäcker cycle, CNO cycle)
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.
carbon cycle (Bethe-Weizsäcker cycle, CNO cycle)
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.
carbon cycle (Bethe-Weizsäcker cycle, CNO cycle)
Use of carbon and nitrogen as intermediates in the nuclear fusion process of the Sun. Cooler stars undergo the proton-proton cycle.
carbon detonation supernova model
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.
carbon monoxide (CO)
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.
carbon star
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.)
Carina galaxy
A dwarf galaxy that orbits the Milky Way.
Carina OB 2
A rich association of OB stars near η Carinae.
carrier boson (force carrier)
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.
carrier boson (force carrier)
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.
Cassegrain
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.
Cassegrain
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.
Cassegrain
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.
Cassini's division
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.
Cassiopeia A (3C 461, 3U 2321+58)
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.
cataclysmic variable (eruptive variable, explosive variable, CV)
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.
cataclysmic variable (eruptive variable, explosive variable, CV)
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.
catastrophism
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.
causality (causation)
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.
CDA
Centre de Donnees Astronomiques (Strasbourg, France).
celestial body
natural object visible in the sky
celestial ephemeris pole
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.
celestial equator
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.
celestial event
Event involving one or more celestial objects
celestial latitude
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.
celestial longitude
Angular distance along the ecliptic from the vernal equinox eastward.
celestial longitude
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.
celestial mechanics
Study of the movements and physical interactions of objects in space; astrophysical mathematics.
celestial pole
One of two points at which the Earth's axis of rotation, if extended, would intersect the celestial sphere.
celestial sphere
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.
Celsius (°C, Centigrade)
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.
Cen X-2
sporadic X-ray source
Cen X-4
sporadic X-ray source
Centaurus A (3U 1322-42, NGC 5128)
A strong radio source. Optically, it is an elliptical galaxy with a dark obscuring lane. It is the nearest known violent galaxy.
Centaurus cluster (3U 1247-41)
A cluster of galaxies. Its radio counterpart is compact and located inside NGC 4696.
Centaurus X-3 (3U 1118-60, Krzeminski's star)
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.
Cepheid
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.
Cepheid
A type of variable star whose period of variation is tightly related to its intrinsic luminosity.
Cepheid
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.
Cepheid
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.
Cepheid
Strictly periodic variables with periods 1-50 days, of spectral types F, G and K.
Ceres
Largest asteroid, and the first to be discovered.
CERN
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.
CH star
G-type giant (G5 to K5) in which the molecular bands of CH are very strong.
chalcophile element
element that tends to concentrate in sulfides
Chandra (AXAF)
NASA's premier x-ray observatory
Chandrasekhar limit
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.
Chandrasekhar limit
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.
Chandrasekhar-Schonberg limit
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.
Chandrasekhar-Schonberg limit
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.
charge
The fundamental property of a particle that causes it to be affected by the electromagnetic force.
charge multiplet
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.
charge-coupled device (CCD)
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.
charge-coupled device (CCD)
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.
charged particle
A particle with non-zero charge
Charles's law
The pressure of an ideal gas at constant volume varies directly as the absolute temperature.
charm
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.
charm
The fourth flavor of quarks. Predicted by theory, charmed quarks were discovered in 1974.
charm
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.)
chemistry
The scientific study of chemicals and chemical reactions.
Cherenkov detector
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.
Chi squared test2)
A least-squares statistical test that measures the probability of randomness in a distribution.
chirality (chiral)
An expression of the basic handedness of nature. Fundamental theories of the elementary particles and of superstrings must possess chirality.
chirality (chiral)
Feature of fundamental particle physics that distinguishes left- from right-handed, showing that the universe is not fully left-right symmetric.
chlorine (Cl, 2P3/2 in ground state)
yellow-green, dense, sharp-smelling gas (Cl2) which is a key industrial chemical
chromium (Cr, 7S3 in ground state)
hard, blue-white metal which resists oxidation in air, can be polished to a high shine
chromosphere
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.
chromosphere
The part of the Sun's atmosphere immediately above the surface (the photosphere) and beneath the corona.
chromospheric network
A large-scale cellular pattern along the boundaries of which lie bright and dark mottles seen in Hα and other regions.
Circinus X-1 (3U 1516-56)
A highly variable X-ray source. Many of its properties are similar to those of Cygnus X-1.
circle
An ellipse possessing but one focus.
circumpolar region
a region defined by an equatorial latitude above which stars are visible all year (depends on observers location on Earth)
circumpolar star
Star which never sets, from the viewpoint of an observer on Earth.
classical physics
Physics prior to the introduction of the quantum principle. Classical physics incorporates Newtonian mechanics, views energy as a continuum, and is strictly causal.
closed string
A type of string that is in the shape of a loop.
closed universe
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.
closed universe
A standard universe with a spherical three-dimensional spatial geometry. Such a universe is finite in both space and time, and recollapses.
closed universe
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.)
closed universe
Any model of the Universe in which the gravity of the matter content can reverse the expansion and cause a collapse.
closed universe
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.
closed universe
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.
CN-strong star
Late type giant with strong CN bands. Metallic lines are also stronger than in normal giants.
CN-weak star
High-velocity star with both weak metallic lines and weak CN bands.
CNO bi-cycle
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.
CNO tri-cycle
Similar to the CNO bi-cycle, with the addition of the cycle 17O(p, γ) 18F(β+ν) 18O(p, α) 15N.
co-moving coordinate
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.
co-moving coordinate
Coordinates fixed with respect to the overall Hubble flow of the universe, so that they do not change as the universe expands.
Coalsack
A prominent dark nebula readily visible to the naked eye, located on the galactic plane.
cobalt (Co, 4F9/2 in ground state)
lustrous, silvery-blue, hard metal which is also ferromagnetic. Cobalt 60 is an important radioisotope
coherent scattering
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.
cold dark matter (CDM)
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.
cold dark matter (CDM)
Any dark matter candidate which was non-relativistic at the point of decoupling.
cold dark matter (CDM)
Hypothetical subatomic particles that move slowly compared with the speed of light.
collapse
Sudden contraction of a celestial body
collection of stars
Two or more stars forming a gravitationally bound system
collider
A particle accelerator in which beams of particles with equal but opposite momentum are made to collide head on.
color temperature
A stellar temperature determined by comparison of the spectral distribution of the star's radiation with that of a blackbody.
coma (comet head)
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.
Coma cluster (Coma X-1, Abell 1656)
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.
Coma open cluster
Open cluster similar to the Hyades in overall binary frequency.
Coma X-1 (3U 1257+28)
An extended X-ray source in the Coma cluster of galaxies.
comet
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.
comet nucleus
The stellar-appearing frozen core, containing almost the entire cometary mass, in the head of a comet.
comet tail
The long streamer behind the comet head which does not usually appear until the comet is inside the orbit of Mars.
cometary nebula
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.
commensurate orbit
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.
compact galaxy
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.
compact group
Galaxy group with a few galaxies separated by a few galaxy diameters.
compact H II region
A dense (ne ≥ 103 cm-3) H II region of small linear dimensions (≤ 1 pc).
composite spectrum star
Object with a spectrum due to superposition of the spectra of two different stars.
Compton scattering
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.
Compton scattering
Scattering of a photon due to the Compton effect (see also noncoherent scattering).
Compton scattering
The scattering of photons by free electrons in an ionized medium.
condensation
from gas to liquid or from gas to solid
conjunction
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.
conservation law
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.
conservation law
Law that identifies a quantity, such as energy, that remains unchanged throughout a transformation. All conservation laws are thought to involve symmetries.
conservation of angular momentum
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.
conservation of angular momentum
The total angular momentum of an isolated dynamical system does not change during the course of its evolution.
conservation of energy
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.
conservation of energy
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.
conservation of mass and energy
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.
conservation of matter
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.
conservation of momentum
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.
conservative scattering
Scattering that occurs in the absence of absorption.
consistent
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.
constellation
Precisely defined area of the celestial sphere, associated with a grouping of stars, that the International Astronomical Union has designated as a constellation.
constellation formerly part of Argo Navis
a constellation created by the international astronomical union by breaking up the constellation Argo Navis
constellation named by Bayer
a constellation named by Johann Bayer the author of the Uranometria star atlas 1607
constellation named by de Lacaille
a constellation named by Abbé Nicolas Louis de Lacaille
contraction
A mass motion towards the center of gravity of a celestial body
convection
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.
convective zone
Region in a stellar interior where convection dominates the heat flow
coordinate
Quantities that provide references for locations in space and time.
coordinate component
A component of a coordinate system, components are usually an angle or a length
coordinate transformation
Method of relating observations from one frame of reference to another.
Copenhagen interpretation
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.
core-halo galaxy
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.
corona
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.
coronal green line
An emission line of Fe XIV, the strongest line in the solar corona.
coronal hole
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.
coronene
The first ultraviolet phosphor to be tried on the surface of a CCD.
corrector plate
Thin lens-like optical piece which removes certain optical aberrations.
Cosmic Background Explore (COBE)
satellite that studies the microwave background.
cosmic matter density
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.
cosmic rays (corpuscular radiation)
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.
cosmic string
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.
cosmic string
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.
cosmic string
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.
cosmic year
Time the Sun takes to "orbit" in galactic rotation: about 225 million years.
cosmogony
The study of the origin of celestial systems, especially the solar system.
cosmological constant
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.
cosmological constant
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.)
cosmological constant
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.
cosmological constant
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).
cosmological constant
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).
cosmological constant
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.
cosmological constant
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.
cosmological constant
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.
cosmological constant problem
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.
cosmological redshift (z)
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.
cosmology
The study of the origin, structure, and evolution of the Universe on the largest possible scale. In present usage, it frequently includes cosmogony.
Coster-Kronig transition
An Auger transition in which the vacancy is filled by an electron from a higher subshell of the same shell.
coudé focus
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)
coudé focus
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.
coulomb (C)
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.)
coulomb (C)
Unit of charge.
Coulomb collision
The collision between two charged particles.
Coulomb's law
The force between two charged particles varies directly as the size of the charges and inversely as the square of the distance between them.
CP violation
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.
CPT invariance
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.
CPT invariance
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.
Crab Nebula (M 1, Taurus A, 2U 0531+22)
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.
Crab Nebula (M 1, Taurus A, 2U 0531+22)
A supernova remnant.
Crab pulsar (NP 0532)
A pulsar associated with the Crab Nebula. It has the shortest period of any known pulsar.
creationism
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.
Crepe Ring (C ring)
Rather transparent inner ring of the saturn ring system.
Crepe Ring (C ring)
The second innermost ring of Saturn, it has fewer particles and is less dense than the outer rings; therefore, it is harder to observe.
critical density
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).
critical density
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.
critical density
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.
critical density
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.)
critical equatorial velocity
In rotating early-type stars, that velocity at which the ratio of centrifugal force to gravity at the equator is unity.
crossing time
The time it takes a particle to travel from one point in its orbit to another point 180° away.
crucial experiment
An experiment that has the power to decide between two competing theories.
CS star
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.
CS star
Star exhibiting combined characteristics of C- and S-type stars - i.e. the presence of both C2 and ZrO bands.
culmination (meridian passage)
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.
culmination (meridian passage)
The instant at which a celestial object crosses the meridian.
curie
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.
curie
Unit of radioactivity.
Curie temperature scale (magnetic temperature)
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.
current density
Amount of charge passing through a unit area per unit time.
current density unit (A·m-2)
ampere per square meter
CV Serpentis
A sometimes-eclipsing binary composed of a Wolf-Rayet star and a B0 star.
cyanogen band
Molecular absorption band found in the spectra of stars of type G0 and later.
Cygnus A (3C 405, 2U 1957+40)
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).
Cygnus Loop (Veil Nebula, NG6 6992, Cygnus X-5)
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.
Cygnus X-1 (3U 1956+35)
A black hole candidate in the constellation Cygnus and a source of X-rays.
Cygnus X-1 (3U 1956+35)
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.
Cygnus X-2 (3U 2142+38)
An X-ray source optically identified with an irregular variable star.
Cygnus X-3 (2U 2030+40)
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.
D galaxy
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).
D layer
The lowest part of Earth's ionosphere.
D line
Spectral line of neutral sodium, also the strongest interstellar line.
D ring
The innermost ring of Saturn.
d-electron
An orbital electron whose l quantum number is 2.
D1 line
Spectral line of neutral sodium.
D2 line
Spectral line of neutral sodium.
DA white dwarf
only strong hydrogen lines present
dark flight
phase in the flight of a meteor after the ratardation point when the meteoroid is no longer incandescent
dark halo
Massive, nonluminous matter of unknown kind that surrounds and envelope of a galaxy
dark halo
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.
dark halo
The unseen mass that is believed to surround each galaxy and whose gravitational effects are believed to hold the galaxy together.
dark matter (hidden mass)
Any form of matter which exists in the Universe in a non-luminous form.
dark matter (hidden mass)
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.
dark matter (hidden mass)
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.
dark matter (hidden mass)
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.
dark matter (hidden mass)
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.
dark matter (hidden mass)
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.
dark matter (hidden mass)
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.
dark nebula
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.
day (d)
An interval of 86400.51 seconds, unless otherwise indicated.
db galaxy
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.
DB white dwarf
only strong helium lines present
DC white dwarf
continuous spectrum, no lines clearly visible
de Sitter model
A geometrical model (1917) of an empty universe, based on Einstein's field equations.
de Sitter model
A model of the universe that contains no matter but only a positive cosmological constant. It expands exponentially forever.
de Sitter model
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.
dead person
a person that has died
deceleration parameter (q0)
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.
deceleration parameter (q0)
A parameter (that denotes the rate of change with time of the Hubble constant.
deceleration parameter (q0)
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.)
deceleration parameter (q0)
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.
declination (δ)
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.
declination (δ)
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.
declination (δ)
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.
declination (δ)
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°.
declination (δ)
Location on the sky in a north-south direction. Lines of declination are the celestial equivalent of latitude on Earth. Compare right ascension.
deduction
Process of reasoning in which a conclusion is derived from a given premise or premises, without a need for additional information. Compare induction.
deformable mirror
A very thin mirror whose shape can be changed by the force applied by many small pistons behind the mirror.
degeneracy pressure
Pressure in a degenerate electron or neutron gas.
degeneracy pressure
Pressure in a degenerate electron or neutron gas.
degenerate gas (Fermi gas)
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.
degenerate gas (Fermi gas)
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.
degenerate gas (Fermi gas)
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.
degree
An angle subtended in the sky: From the zenith to the horizon is 90 degrees.
Deimos
The outer satellite of Mars. Mariner 9 has shown that both Phobos and Deimos are locked in synchronous rotation with Mars.
delay time
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.
delta
Short-lived baryon.
Delta Cephei
A pulsating star in the constellation Cepheus. It was the second Cepheid discovered and lent its name to the entire class of stars.
Delta Del star
A type of late A-type and early F-type star with very weak Ca II HK lines.
delta-ray
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.
Demeter
Unofficial name for Jupiter X.
Deneb (alpha Cyg, HR 7924)
An A2 Ia supergiant at the head of the Northern Cross. Most distant first-magnitude star.
density
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.
density
The amount per unit volume
density
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.
density wave theory
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.
descending node
The point in the orbit of a solar-system body where the body crosses the ecliptic from north to south.
DESI
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.
detached binary
Binary star which is not in contact and in which no significant mass exchange is occurring.
determinism
The doctrine that all events are the predictable effects of prior causes.
deuterium (D, 2H, hydrogen 2)
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.
deuteron (d)
The nucleus of a deuterium atom. md = 2.01355 amu.
deuteron (d)
The nucleus of deuterium, an isotope of hydrogen. It consists of one proton and one neutron bound together.
device
an instrumentality invented for a particular purpose
diatomic molecule
a molecule containing only two atoms which can be of different elements
differential rotation
As an object rotates, different parts of it may move at different rates. The Galaxy rotates differentially.
differential rotation
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.
diffraction
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.
diffraction
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.
diffraction
The bending of light in passing a sharp edge or tiny aperture.
diffraction grating
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.
diffraction grating
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.
diffraction grating
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.
Dione
Fifth satellite of Saturn
direct motion
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.
direct motion
Motion of a solar-system body from west to east across the sky.
disk
The central plane of a spiral galaxy, as distinguished from the halo or the nucleus.
disk
The plate-shaped component of a spiral galaxy, in which the spiral arms are found.
dispersion
Resolution of white light into its component wavelengths, either by refraction or by diffraction.
dispersion
The separation of a beam of light into the individual wavelengths of which it is composed by means of refraction or diffraction.
dissociative recombination
Capture of an electron by a positive molecular ion, wherein part of the recombination energy dissociates the molecule into two neutral atoms.
distance
The distance to a celestial object measured from the center of the Earth or Sun, in units of parsec or lightyear
diurnal motion
The apparent daily motion of celestial bodies across the sky from east to west, caused by the Earth's rotation.
diurnal motion
The apparent westward motion of celestial bodies, as seen from Earth, due to the Earth's axial rotation.
dKe star
K dwarf with hydrogen emission lines.
dMe star
M dwarf with hydrogen emission lines.
DO white dwarf
both He and H lines present
Doppler broadening
Line broadening caused by the thermal, turbulent, or mass motions of atoms along the line of sight.
Doppler shift
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.
Doppler shift
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.
Doppler shift
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.
Doppler shift
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).
Doppler shift
The alteration in frequency of electromagnetic radiation due to relative motion between the source and observer.
Doppler shift
The alteration in frequency of electromagnetic radiation due to relative motion between the source and observer.
Doppler shift
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.
Doppler shift
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.
Doppler shift
The change in frequency of a wave (light, sound, etc.) due to the relative motion of source and receiver.
Doppler shift
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.
double line spectroscopic binary
a spectroscopic binary in which periodic Doppler shift is detected in both components of the binary star each out of phase by 180 degrees
double radio source
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.
double star
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).
doublet
In a spectrum, a pair of associated lines arising from transitions having a common lower energy level.
down
A flavor of quark.
DQ Herculis (Nova Herculis 1934)
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.
DQ white dwarf
carbon lines (atomic or molecular) present.
Draco galaxy
A dwarf elliptical galaxy, the intrinsically faintest (Mv = - 8.5) member of the Local Group (next to And I-III).
Dumbbell Nebula (M 27, NGC 6853)
A planetary nebula of large apparent diameter and low surface brightness.
dust
The dust component of a galaxy, includes dark cloud
dust tail (type II comet tail)
Curved part of comet tail composed of dust driven by radiation pressure
dwarf (d, main sequence star)
Star that is on the main sequence - that is, a star fusing hydrogen into helium at its core.
dwarf (d, main sequence star)
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.
dwarf (d, main sequence star)
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.
dwarf Cepheid (delta Scuti star, ultrashort-period Cepheid)
A group of pulsating variable stars with small variations in amplitude. They lie in the lower part of the Cepheid instability strip.
dwarf Cepheid (delta Scuti star, ultrashort-period Cepheid)
Type I Cepheids ( <Mv> +4 to +2).
dwarf galaxy
A galaxy with low luminosity.
dwarf galaxy
A small, faint galaxy, exemplified by those that orbit the Milky Way: Ursa Minor, Draco, Sculptor, Sextans, Carina, Fornax, Leo II, and Leo I.
dwarf nova
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.
dwarf nova
Cataclysmic variable in which the brightness increases suddenly at intervals ranging from several days to years.
dynamical equinox
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.
dynamical parallax
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.
dynamics
Study, in physics, of the motion and equilibrium of systems under the influence of force.
dynamics
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.
dyne
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.
dyne
The force necessary to cause a mass of one gram to accelerate at one centimeter per second per second.
DZ white dwarf
no He nor H lines, but metallic lines present
E layer (Kennelly-Heaviside layer)
Former name for the D and E layers (q.v.).
E layer (Kennelly-Heaviside layer)
The part of Earth's ionosphere where the temperature gradient reverses and starts to rise.
E line
Spectral line consisting of a blend of Fe I and Ca I.
e-process
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.
early star
Hot star of spectral types O, B, A, and early F.
Earth
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).
Earth based telescope
A telescope located on the surface of the Earth.
east point
The point on the celestial horizon 90° clockwise from the north point. At the equinoxes the Sun rises in the east point.
EBS
Electron Bombarded Silicon.
eccentricity (e)
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.
eccentricity (e)
A parameter that specifies the shape of a conic section; one of the standard elements used to describe an elliptic orbit. (See elements, orbital.)
eccentricity (e)
An orbital element representing the eccentricity of the elliptical orbit
eccentricity (e)
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.
eccentricity (e)
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.
eccentrics
In Ptolemaic cosmology, displacement of the center of a rotating celestial sphere from the center of the universe.
echelle
A type of diffraction grating with groove angles of 90°. With the grating at an angle of 45° the grooves resemble a staircase.
eclipse
Obscuration of one astronomical object (such as the sun) by another such object (such as the moon).
eclipse
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.
eclipse
The obscuration of a celestial body caused by its passage through the shadow cast by another body.
eclipse
The total or partial obscuration of the light from a celestial body caused by its passage into the shadow of another body (cf. occultation).
eclipse year
The interval of time between two successive passages of the Sun through the same node of the Moon's orbit.
eclipsing binary
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.
eclipsing binary
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.
eclipsing binary
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.
ecliptic
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.
ecliptic
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.)
ecliptic
The mean plane of the Earth's orbit around the Sun.
ecliptic coordinate
A system of coordinates based on the plane defined by Earth's orbit around the Sun, which is inclined to the celestial equator.
Eddington approximation
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.
Eddington limit
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.
Eddington's standard model
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.
Eddington-Lemaitre universe
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.
effective temperature (Teff)
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.
elastic collision
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.
elastic collision
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.
electrodynamics
Study of the behavior of electromagnetic force in motion.
electromagnetic gauge symmetry
Gauge symmetry underlying quantum electrodynamics.
electromagnetic telescope
telescope designed to collect and record electromagnetic radiation
electromagnetism
Force field of the electromagnetic force, consisting of electric and magnetic lines of force at each point in space.
electromagnetism
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).
electromagnetism
One of the four fundamental forces of nature, governing the electric and magnetic interaction between particles.
electromagnetism
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.
electromagnetism
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.
electromagnetism
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.
electron
A lepton with an electric charge of -1. An electron is also a fermion because it has a spin of one half.
electron
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.
electron
A small, negatively charged particle that appears in every neutral atom, surrounding the positively charged nucleus like bees around honey.
electron
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.
electron
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.
electron
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.
electron bombarded charge-coupled device (EBCCD)
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.
electron temperature
The temperature that appears in the Maxwell distribution of velocities for electrons.
electron volt (eV, eV)
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.
electron volt (eV, eV)
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)
electrostatic unit (esu)
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.
electroweak (weak force)
A unification of electromagnetism and the weak nuclear force.
electroweak (weak force)
Responsible for certain kinds of radioactivity; for example, the disintegration of a neutron into a proton, electron, and antineutrino.
electroweak (weak force)
The combination of the electromagnetic force and the weak nuclear force which takes place at high energy.
electroweak (weak force)
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.
electroweak (weak force)
The unified description of the weak interactions and electromagnetism, developed between 1967 and 1970 by Sheldon Glashow, Steven Weinberg, and Abdus Salam.
electroweak (weak force)
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.
element (atom)
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.
element (atom)
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.
element (atom)
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.
element (atom)
The smallest component of matter that retains its chemical properties. An atom consists of a nucleus and at least one electron.
elementary particle
A particle considered to be fundamental, i.e. not composite, a particle which cannot be separated into components
elementary particle symmetry
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.
elevation (altitude)
Angular distance above the horizon.
elevation (altitude)
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).
elevation (altitude)
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.
eleven-dimensional supergravity
Promising higher-dimensional supergravity theory developed in the 1970s, subsequently ignored, and more recently shown to be an important part of string theory.
ellipse
A plane curve in which the sum of the distances of each point along its periphery from two points - its "foci" - are equal.
elliptical galaxy
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.
emersion
The reappearance of a celestial body after eclipse or occultation.
emission
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.
emission line
Bright line superposed on a continuous spectrum. Can be produced in the laboratory by a glowing gas under low pressure.
emission line
Bright lines produced in a spectrum by a luminous source, such as a star or a bright nebula.
empiricism
An emphasis on sense data as a source of knowledge, in opposition to the rationalist belief that reasoning is superior to experience.
Enceladus
Third satellite of Saturn
Encke's comet
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.)
Encke's division
A region of decreased brightness in the outermost ring of Saturn.
Encke's division
Gap within Saturn's A ring.
endothermic fusion process
a fusion process which requires energy
energy level
Allowed energy state for electrons orbiting the nucleus of an atom.
energy level
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.
energy unit
(1) The capacity to do work. (2) Manifestation of a particular variety of force.
enthalphy (H)
The heat content of a body. H = U + pV, where U is the internal energy, p is the pressure, and V is the volume.
ephemeris second
The length of a tropical second (1/31556925.97474 of the tropical year) on 1900 January 0.5 ephemeris time.
epicycle
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.
epicycle
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.
epicycle
In Ptolemaic cosmology a circular orbit around a point that itself orbits another point.
epoch (T)
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.
epoch (T)
A point of time selected as a fixed reference.
epoch (T)
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).
epoch (T)
An orbital element representing the time of perihelion passage.
epsilon Aurigae
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.
Epsilon Eri
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.
Epsilon Eridani
A young orange dwarf star that is visible to the naked eye.
Epsilon Indi
An old orange dwarf star in the southern constellation Indus.
equator
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.)
equatorial sky area
A fixed region on the sky defined using equatorial coordinates
equatorial telescope
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.
equatorial velocity
The velocity at the surface of a rotating body, on its equator
equinox
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.)
equinox
Either of the two points on the celestial sphere where the celestial equator intersects the ecliptic.
equinox
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.
equivalence principle
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.
equivalence principle
In a freely falling and nonrotating laboratory the laws of physics, including their numerical content, are the same everywhere including gravity-free space.
equivalence principle
States that inertial mass is indistinguishable from gravitational mass.
equivalence principle
The principle that it is impossible to distinguish between gravitational and inertial forces; gravitational mass is precisely equal to inertial mass.
equivalence principle
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.
era
A system of chronological notation reckoned from a given date.
erg (dyne cm)
Unit of energy; the work done by a force of 1 dyne acting over a distance of 1 cm.
ergodic motion
Motion by one or more particles which fills phase space with uniform density after a sufficiently long time.
Eros
A small Asteroid.
erruptive prominence
A violent prominence which may reach heights of 2 million km
escape velocity
Speed a satellite must attain in order to free itself from returning to the parent body under the effects of gravity.
escape velocity
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.
escape velocity
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).
Eta Aquilae
A pulsating star in the constellation Aquila. It was the first Cepheid discovered, in 1784.
etalon
Essentially an optical filter that operates by multiple-beam interference of light reflected and transmitted by a pair of parallel flat reflecting plates.
EUMETSAT satellite
meteorological satellite launched and maintained by Europe's Meteorological Satellite Organization
Europa (Jupiter II)
One of the Galilean satellites of Jupiter
even-odd nucleus
Nucleus that contain even numbers of protons but odd numbers of neutrons.
event (world point)
A point in four-dimensional spacetime referenced by three spatial coordinates and a complementary temporal ordinate.
Evershed effect
The radial motion outward (from the central umbra) of the gases in the penumbral regions of sunspots.
excitation potential
Amount of energy required to bring an electron from its ground state to a given excited state (measured in electron volts).
exclusion principle (Pauli exclusion principle)
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).
exothermic fusion process
A fusion reaction which linerates energy.
expanding arm
A spiral arm of neutral hydrogen receding from the galactic nucleus at about 135 km s-1.
explosive galaxy formation
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.
explosive nucleosynthesis
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.
explosive nucleosynthesis
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.
exponential expansion
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.
exponential expansion
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.
extinction level event
impact event which can cause the extinction of many species on a planet
extragalactic astronomy
The field that deals with objects beyond the Milky Way, especially galaxies and quasars.
extremal black hole
Black hole endowed with the maximal amount of force charge possible for a given total mass.
F corona (F component)
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)
F layer (Appleton layer)
One of two layers in the Earth's ionosphere immediately above the E layer.
F region
Region of the ionosphere above the F layers.
F star
Star of spectral type F in which lines of hydrogen and Ca II are of about equal strength. Metal lines also become noticeable.
f-electron
An orbital electron whose l quantum number is 3.
Fabry-Perot interferometer (FP)
A device that measures distances and changes of distance very accurately, using the pattern of overlap of waves of light.
Fabry-Perot interferometer (FP)
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.
faculae
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.
faculae
Bright regions of the photosphere seen in white light, visible only near the limb of the Sun.
Fahrenheit
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.
faint blue galaxy
A distant, irregularly shaped galaxy in which a large amount of star formation is taking place.
Fechner's law
The intensity of a sensation increases as the logarithm of the stimulus. (See Pogson's ratio.)
fermi
A unit of distance used for describing nuclear distances just as the ångström is used for atomic distances.
fermi
A unit of length.
Fermi level
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.
Fermi National Accelerator Laboratory (Fermilab)
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).
Fermi-Dirac nuclei
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.).
Fermi-Dirac-Sommerfeld law (FDS)
A law which gives the algebraic number of a quantized system of particles which have velocities within a small range.
fermion
A particle, or pattern of string vibration, with half a whole odd number amount of spin; typically a matter particle.
fermion
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).
fermion
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.
fermion
Elementary particle with fractional spin. The proton, electron, neutron, and other elementary particles are all fermions.
fermion
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.
fibrille
Striation or streak which is observed to form whirls in the solar chromosphere.
field galaxy
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.
field horizontal branch star
High-velocity metal-weak star of either B or A spectral type.
field lens
A lens placed in or near the focal plane of a telescope to create an image of the primary mirror inside the instrument.
field star
A star that is not part of any star cluster. Most stars, including the Sun, are field stars.
field star
Star distributed at random in space and not belonging to any particular star cluster.
filament
A prominence seen in projection on the solar disk.
fine structure
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.
fine-structure constant (α, α)
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.
fine-structure constant (α, α)
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.
fine-structure constant (α, α)
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.
fine-tuning
A phrase meaning a highly constrained and implausible adjustment of the parameters of a theory.
FIRST
Far-InfraRed Space Telescope.
first law of thermodynamics
The law of conservation of energy
first order phase transition
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.
fission
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.
fission
Interaction in which nucleons previously united in an atomic nuclei are disjoined, releasing energy. Fission powers atomic bombs.
five-minute oscillations
Vertical oscillations of the solar atmosphere with a well-defined period of 5 minutes.
flare star (UV Ceti star)
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.)
flare star (UV Ceti star)
A star that emits flares, which can outshine the entire star. Many red dwarfs are flare stars.
flare star (UV Ceti star)
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.
flare star (UV Ceti star)
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.
flat universe (Einstein-de Sitter universe)
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.
flat universe (Einstein-de Sitter universe)
A Friedmann model of the Universe in which the spacetime continuum is not curved.
flat universe (Einstein-de Sitter universe)
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.
flat universe (Einstein-de Sitter universe)
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.
flat universe (Einstein-de Sitter universe)
A particular solution to Einstein's cosmological equations in which the universe is flat. (See flat universe.)
flat universe (Einstein-de Sitter universe)
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.
flat universe (Einstein-de Sitter universe)
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.)
flat universe (Einstein-de Sitter universe)
A world model of a static universe with a positive cosmological constant, whose radius of curvature is constant and independent of time.
flat universe (Einstein-de Sitter universe)
Big Bang model that was formulated by Friedmann and Lemaitre which has a zero curvature, or flat (Euclidian). This space is unbounded.
flat universe (Einstein-de Sitter universe)
The flat (k = 0), pressureless standard model of the universe.
flat universe (Einstein-de Sitter universe)
The Friedmann-Lemaitre model in which space is Euclidean was advocated by Einstein and de Sitter in 1932.
flatness problem
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.
flatness problem
Poses the question: why, out of an infinite number of possibilities, is our Universe so close to the one special case: the "flat" Universe?
flatness problem
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.)
flatness problem
The riddle of why the universe is neither dramatically open nor closed, but appears to be almost perfectly balanced between these states.
flavor
Designation of quark types - up down, strange, charmed, top, and bottom. Flavor determines how the weak nuclear force influences quarks.
flavor
In particle physics, another word for "type". For example, there are 6 flavors of quarks, meaning that there are 6 different types of quarks.
flavor
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.
flavor
The term used to describe different quark types. There are six quark flavors: up, down, strange, charm, bottom and top.
Flexible image transport system (FITS)
A method for saving image data which has become standard in astronomy.
fluorescence
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.
fluorescence
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.
fluorine (F, 2P3/2 in ground state)
pale yellow gas (F2) which is the most reactive of all the elements, and is the strongest oxidizing agent
FOC
Faint Object Camera (Hubble).
focal reducer
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.
Fomalhaut (alpha PsA, HR 8728)
An A star. The brightest star in the constellation Piscis Austrinus.
forbidden line (nebular line)
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].
Forbush decrease
A decrease in cosmic-ray intensity during active Sun.
force quantity
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.
formaldehyde (H2CO)
An organic molecule, the first polyatomic molecule to be discovered in interstellar space. In 1973 It was discovered in two external galaxies.
formamide (HCONH2)
An interstellar molecule discovered in space at a wavelength of 6.5 cm.
formic acid (H2CO2)
The first organic acid to be detected in interstellar space at 18.3 cm. Formic acid is the "sting" of an insect.
Fornax galaxy
A dwarf spheroidal galaxy, in the Local Group, that orbits the Milky Way. (Mv ≈ -12).
Four-shooter
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.
fractal
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.
fractal
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.
frame of reference
A set of axes to which positions and motions in a system can be referred.
frame transfer
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.
Fraunhofer line
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.
free electron
Electron not bound to a nucleus.
free parameter
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.
free-fall
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.
free-free transition
The acceleration of an unbound (or free) electron by a proton or atomic nucleus results in the emission of electromagnetic radiation.
frequency (ν)
Number of oscillations per second of an electromagnetic wave. The amplitude of a wave depends on the intensity; the wavelength, on the frequency.
frequency (ν)
The number of complete wave cycles a wave completes each second.
frequency (ν)
The number of cycles or complete alternations per unit time of a carrier wave, band, or oscillation.
frequency (ν)
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.
frequency (ν)
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.
frequency distribution
A statistical arrangement of numerical data according to size or magnitude (see also distribution function).
FU Ori star (fuors)
A kind of T Tau stars with considerable changes in brightness. The post-eruption spectrum is that of a late supergiant.
fundamental force
One of four fundamental forces which mediate particle interaction.
fundamental frequency
The lowest characteristic frequency of oscillation of a dynamical system.
fundamental star
Star for which coordinates have been determined to a very high degree of accuracy.
fusion
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.
fusion
Interaction in which nucleons are forged together, creating new atomic nuclei and releasing energy. Fusion powers hydrogen bombs.
fusion
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.
G band
A band of CH. It is conspicuous in the spectra of G-K stars.
G star
Spectral type for yellow stars.
G star
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.
galactic astronomy
The study of the Milky Way.
galactic center
Now thought to comprise black holes - which would explain why the centre of our Galaxy appears strangely obscure, and emits only infrared radiation.
galactic center
The direction of the sky toward the center of the Milky Way.
galactic coordinate
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: lIIlI + 32°.31. Galactic coordinates are independent of precession.
galactic equator
The primary circle defined by the central plane of the Galaxy.
galactic latitude
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.
galactic longitude
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.
galactic nucleus
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.
galactic nucleus
The central region of a galaxy.
galactic plane
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.
galactic pole
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.
galactic pole
The poles of the galactic plane. The new system puts the galactic north pole in Coma at α(1950) = 12h49m, δ(1950) = 27°.4
galactic rotation
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.
galactic velocity component
One of three components of a star's motion with respect to the center of a galaxy.
galactic wind
A hypothetical outflow of tenuous material from a galaxy, analogous to the solar wind.
Galactocentric distance
A star's distance from the Galactic center. The Sun's Galactocentric distance is about 27000 light-years.
galaxy
A collection of matter which usually manifests itself by the production of stars.
galaxy
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".
galaxy
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.
galaxy
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.
galaxy
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.
galaxy
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.
galaxy cluster
A conglomeration of hundreds or thousands of galaxies. The nearest large galaxy cluster is the Virgo cluster.
galaxy cluster
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.
galaxy group
A small gathering of galaxies, smaller than a cluster. The Milky Way is part of the Local Group, which contains about thirty galaxies.
galaxy merger
The formation of a galaxy from the collision of two or more separate galaxies.
galaxy peculiar velocity
A deviation in the velocity of a galaxy from that expected on the basis of a uniform expansion of the universe. (See Hubble law.)
Galilean satellite
The largest satellites of Jupiter - Io, Europa, Ganymede, and Callisto. All are locked in synchronous rotation with Jupiter.
Galilean transformation
The non-relativistic method of relating observations from one frame of reference to another.
gamma ray
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.
gamma ray
Electromagnetic radiation similar to X-radiation, although of shorter wavelength, emitted spontaneously by some radioactive substances from atomic nuclei during radioactive decay.
gamma ray
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.
gamma ray
Extremely short-wavelength electromagnetic energy.
gamma ray
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.
gamma ray
Photons of very high frequency and very short wavelength; the most energetic and penetrating form of electromagnetic radiation.
gamma ray
Rays first discovered in radioactive material, and later identified as very high energy photons.
Gamma Velorum (HR 3207)
A triple system (WC8, B1 IV, O9 I) embedded in the Gum Nebula. It is the brightest Wolf-Rayet star in the sky.
gamma-ray burst
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.
Ganymede (Jupiter III)
The largest satellite of Jupiter.
gas
The gaseous component of a galaxy, includes nebula
gas
A phase of matter consisting of a low density collection of particles which have no short range order
gas giant (Jovian planet)
Giant planet that has a gaseous surface.
gaseous fragmentation
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.
gaseous nebula (emission nebula)
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.
gauge symmetry
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.)
gauge symmetry
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.
Gauge theory (Yang-Mills theories)
A theory that treats force in a geometrical way in terms of global and local symmetries.
Gauge theory (Yang-Mills theories)
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.
Gauge theory (Yang-Mills theories)
Account of forces that views them as arising from broken symmetries.
Gauge theory (Yang-Mills theories)
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.
gauss
Unit of magnetic flux density.
Gaussian distribution
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.
Gaussian distribution
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.
Gaussian gravitational constant (k)
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.
Gaussian year
The period associated with Kepler's third law with a = 1.
gegenschein (counterglow)
A very faint glow (about 10° across) which can occasionally be seen in a part of the sky opposite the Sun.
gegenschein (counterglow)
Faint nebulous light about 20° across near the ecliptic and opposite the Sun, best seen in September and October. Also called counterglow.
gegenschein (counterglow)
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".
general precession
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.
general relativity (Einstein's theory of gravity)
Gravitation is a curvature in four-dimensional space-time rather than as a force existing between two masses.
geocentric coordinate
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.
geocentric coordinate
The longitude and latitude of a point on the Earth relative to the geoid. These coordinates are influenced by local gravity anomalies.
geocentric cosmology
A model of the universe in which the earth is centrally located, and the Sun, planets, and stars revolve around the Earth.
geocentric cosmology
School of ancient theories that depicted the earth as standing, immobile, at the center of the universe.
geocorona
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.
geodesy
Measurement and study of the Earth's size and shape.
geodetic coordinate
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.)
geodetic precession
A small, relativistic, direct motion of the equinox along the ecliptic.
geology
Scientific study of the dynamics and history of the earth, as evidenced in its rocks, chemicals, and fossils.
geosynchronous satellite
an artificial satellite which orbits Earth every 24 hours
giant
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.
giant
High-luminosity star that lies above the main sequence on the Hertzsprung-Russell diagram.
giant elliptical galaxy
Massive elliptial galaxy which is too far for individual stars to be resolved.
GIS
Gas Imaging Spectrometer (ASCA X-ray satellite).
global inertial frame
A coordinate system or frame of reference anchored with respect to the overall distribution of matter in the universe.
globular cluster
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.
glueball
Theoretical particles made exclusively of gluons. Tentative evidence of the existence of glueballs had been found in accelerator experiments by the mid-1980s.
gluon
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.
gluon
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".
gluon
Smallest bundle of the strong force field; messenger particle of the strong force.
gluon
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.
gluon
The hypothetical particle that carries the force between quarks.
GMS satellite
Geostationary Meteorological Satellite
Golay cell
A gas bulb used to detect infrared radiation.
Goldstone boson
A massless spin-0 particle which arises whenever a (continuous) global symmetry is spontaneously broken.
Gould Belt
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.
grand unified theory (grand unification, GUT)
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.
grand unified theory (grand unification, GUT)
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.
grand unified theory (grand unification, GUT)
Class of theories that merge all three nongravitational forces into a single theoretical framework.
grand unified theory (grand unification, GUT)
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.
grand unified theory (grand unification, GUT)
Theory in physics that attempt to explain the forces of nature as manifestations of a single underlying force.
granule
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.
granule
The mottled appearance of the solar photosphere, caused by gases rising from the interior of the Sun.
gravitational collapse
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.
gravitational lens
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.
gravitational lensing (deflection of light)
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.
gravitational lensing (deflection of light)
Gravitational effect that bends a ray of light. Such an effect was predicted within the general theory of relativity, although previously considered impossible.
gravitational lensing (deflection of light)
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.
gravitational lensing (deflection of light)
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.
gravitational lensing (deflection of light)
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.
gravitational lensing (deflection of light)
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.
gravitational mass
That property of matter which makes it create a gravitational field and attract other particles (cf. inertial mass; equivalence principle).
gravitational potential energy
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.
gravitational potential energy
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.
gravitational radius
The radius which an object should have in order that light emitted from its surface just ceases to escape from its surface.
gravitational redshift (Einstein effect)
Displacement of spectral lines toward longer wavelengths due to the effects of gravity.
gravitational redshift (Einstein effect)
Light is emitted at a lower frequency and longer (or redder) wavelength in a gravitational field than in the absence of a gravitational field.
gravitational redshift (Einstein effect)
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.
gravitino
Hypothetical force-carrying particles predicted by supersymmetry theories. The gravitino's spin would be 1/2. Its mass is unknown.
graviton
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.
graviton
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.
graviton
A quantum of gravitational radiation.
graviton
Smallest bundle of the gravitational force field; messenger particle for the gravitational force.
graviton
The hypothetical quantum particle of the gravitational field. It could also be thought of as a quantized element of space-time curvature.
graviton
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.
gravity
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.
gravity
Fundamental force of nature, generated by all particles that possess mass. Interpreted by means of Newtonian mechanics or by the general theory of relativity.
gravity
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.
  • gravity
    One of the four fundamental forces of nature, and the one most different from the other three.
    gravity
    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.
    gravity
    The universal ability of all material objects to attract each other; F = Gm1m2 / r2.
    gravity
    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.)
    gravity
    The weakest of the four fundamental forces of nature. Described by Newton's universal theory of gravity, and subsequently by Einstein's general relativity.
    grazing-incidence telescope (XRT)
    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.
    great circle
    A circle on the surface sphere whose diameter equal to the diameter of the sphere.
    Great Rift
    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.
    Great Wall
    A sheet of galaxies which stretches more than 500 million light-years across the sky.
    greatest elongation
    The instants when the geocentric angular distances of Mercury or Venus are at a maximum from the Sun.
    Gregorian
    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.
    Gregorian
    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.
    grism
    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.
    Groombridge 1830
    A famous halo star whose proper motion, discovered in 1841, was then the largest known, displacing that of 61 Cygni.
    ground state
    The state in which all electrons are in the lowest possible energy states.
    gyrofrequency
    The frequency with which an electron or other charged particle executes spiral gyrations in moving across a magnetic field.
    h and Chi Persei (Perseus OB1, double cluster in Perseus)
    A double stellar association. It contains many young O and B stars and also many M supergiants.
    H and K emission line star
    Late objects (F4 to M), which exhibit emission features in their Ca II HK lines.
    H II condensation
    A high-density H II region.
    H II region
    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.
    H II region
    Hydrogen gets ionized by hot O and B stars in H II regions. The most famous H II region is the Orion Nebula.
    H II region
    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).
    H magnitude
    The magnitude derived from observations at 1.6 microns.
    h-line
    Singly ionized magnesium resonance line.
    Hades (Jupiter IX)
    An unofficial name for Jupiter IX, the outermost satellite of Jupiter.
    hadron
    High mass elementary particles that are influenced by the strong nuclear force.
    hadron era (hadron barrier)
    The Big Bang era during which quantum and general-relativistic effects are expected to modify each other in an unknown way.
    hadron era (hadron barrier)
    The Big Bang era when the Universe was matter-dominated, containing many hadrons in equilibrium with the radiation field and when kTmπ. 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).
    half-life1/2)
    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.
    half-power beamwidth (HPBW)
    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.
    Halley's comet
    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.
    halo
    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.
    halo
    the galactic halo, however, describes the spherical collection of stars forming a surrounding "shell" for our otherwise compact, discoid Galaxy.
    halo
    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.
    harmonic motion (periodic motion)
    A motion that repeats itself in equal intervals of time. An oscillating particle in harmonic motion is a harmonic oscillator.
    harmonic overtone
    Any integral multiple of the fundamental frequency
    Haro galaxy
    Blue object whose spectra show sharp emission lines.
    head-tail galaxy
    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".
    HEAO
    High-Energy Astronomical Observatory.
    heavy-metal star
    A class of peculiar giant that includes the Ba II stars and the S stars.
    height
    Elevation above ground or distance upwards from a given level (especially sea level) to a fixed point. (See altitude.)
    Hektor
    Largest and brightest Trojan asteroid.
    heliocentric cosmology
    School of models in which the sun was portrayed as standing at the center of the universe.
    heliograph
    Device for recording the positions of sunspots.
    helium (He, 1S0 in ground state)
    helium (He, 1S0 in ground state)
    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.
    helium burning
    The stage when a star fuses helium into carbon and oxygen. All stars born with more than half a solar mass eventually burn helium.
    helium flash
    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.
    helium problem
    Poses the question: what physical process caused the current abundance of helium in the Universe?
    helium shell flash
    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.
    helium temperature scale
    The vapour pressure of helium 4 is used as an indication of temperature in the region 1 to 5.2 K.
    helium variable star
    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.
    helium-strong star
    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.
    helium-weak star
    B-type star in which the helium lines are weaker than in normal stars.
    Helix Nebula (NGC 7293)
    A planetary nebula with the largest known angular diameter.
    Hera (Jupiter VII)
    Unofficial name for Jupiter VII.
    Hercules cluster (3U 1551 + 15)
    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.
    Hercules X-1 (3U 1653+35)
    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.
    hertz (Hz)
    A unit of frequency equal to one cycle (or wave) per second.
    Hestia (Jupiter VI)
    Unofficial name for Jupiter VI.
    heterotic e-string theory (E8 × E8)
    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.
    heterotic o-string theory (O(32))
    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.
    heterotic string theory
    Gross's version of string theory in which space-times of different dimensions are associated with the same closed loop.
    Hidalgo
    Asteroid with the largest known orbit with the second highest inclination to the ecliptic and second highest eccentricity of any asteroid.
    hierarchy problem
    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.
    Higgs boson
    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).
    Higgs boson
    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.
    high energy detector
    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.
    high-velocity star
    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.
    high-velocity star
    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.
    higher-dimensional supergravity
    Class of supergravity theories in more than four spacetime dimensions.
    Hind's nebula (NGC 1554, NGC 1555)
    A reflection nebula illuminated by the star T Tauri. It is remarkable for its changes in brightness.
    Hirayama family
    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.
    HK lines
    Two spectral lines of singly ionized calcium. The second strongest interstellar line.
    hold-time
    The time taken to use up all the liquid cryogens, like LN2, in a cooled CCD cryostat.
    Holmberg radius
    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.
    hologram
    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.
    honeycomb mirror
    A construction method for a large mirror in which the back is hollowed-out to leave a ribbed structure that resembles a honeycomb.
    horizon
    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.
    horizon
    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.
    horizon problem (homogeneity problem)
    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.
    horizon problem (homogeneity problem)
    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.
    horizon problem (homogeneity problem)
    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.
    horizon problem (homogeneity problem)
    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.)
    horizontal branch star
    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.
    horizontal parallax
    The difference between the topocentric and geocentric positions of an object, when the object is on the astronomical horizon.
    Horsehead Nebula (NGC 2024)
    An absorption nebula in the middle of Orion.
    hot dark matter
    Any form of dark matter which was relativistic at its point of decoupling.
    hour circle
    A great circle passing through the celestial poles - i.e., perpendicular to the celestial equator.
    Hourglass Nebula (MyCn18)
    A compact H II region in the center of Lagoon Nebula.
    Hoyle-Narlikar theory
    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.
    HRS
    High-Resolution Spectrograph (Hubble).
    Hubble constant (H0)
    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.
    Hubble constant (H0)
    Hubble's constant in units of 100 km s-1 Mpc-1.
    Hubble constant (H0)
    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.
    Hubble constant (H0)
    The present expansion rate of the universe, in units of kilometers per second per megaparsec. The larger the Hubble constant, the younger the universe.
    Hubble constant (H0)
    The rate at which the universe expands, equal to approximately fifty kilometers of velocity per megaparsec of distance.
    Hubble constant (H0)
    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.
    Hubble flow
    The movement of the galaxies away from us caused by the expansion of the Universe.
    Hubble law
    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.
    Hubble nebula (NGC 2261)
    A cometary nebula whose apex star is R Mon.
    Hubble radius (c/H)
    The radius of the observable universe ().
    Hubble Space Telescope (HST, HST)
    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.
    Hubble time (H0-1)
    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.
    Hubble time (H0-1)
    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.
    Hulse-Taylor pulsar (PSR 1913+16)
    A binary pulsar probably consisting of a neutron star and an even more compact object in an eccentric orbit.
    hybrid array
    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).
    hydrodynamic soliton
    A finite-amplitude disturbance which is propagated through a fluid without any change of shape. MHD solitons are also known.
    hydrodynamics
    The study of how gases and fluids flow under applied forces.
    hydrogen (H, 2S1/2 in ground state)
    hydrogen (H, 2S1/2 in ground state)
    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.
    hydrogen burning
    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.
    hydrogen line
    Set of spectral features associate with neutral hydrogen (H I)
    hydrogen-deficient C star
    A subgroup of high-luminosity C stars with weak or absent hydrogen lines, mostly of types F and G.
    hydrogen-deficient early type star
    Early type star of type O, B or A in which the hydrogen lines are very weak or absent.
    hydroxyl radical (OH)
    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).
    hypergalaxy
    A system consisting of a dominant spiral galaxy surrounded by a cloud of dwarf satellite galaxies, often ellipticals.
    Hyperion
    Eighth satellite of Saturn
    hyperon
    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.
    hypothesis
    A scientific proposition that purports to explain a given set of phenomena; less comprehensive and less well established than a theory.
    Iapetus
    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.
    Icarus
    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.
    ideal gas (perfect gas)
    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.
    ideal gas (perfect gas)
    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.
    image dissector scanner
    A specialized television camera used as a light detector (instead of a photographic plate) in the 1970's.
    image intensifier
    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.
    image photon counting system (IPCS)
    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.
    Image Reduction and Analysis Facility (IRAF)
    An extensive suite of programs developed for astronomy applications and supported by the US National Optical Astronomy Observatories.
    image tube (image intensifier)
    An electronic camera in which electrons, emitted from a photocathode surface exposed to light, are focused electronically onto a phosphor or photographic plate.
    imaging spectrometer
    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.
    immersion
    The disappearance of a celestial body due to eclipse or occultation.
    impact event
    The collision of two celestial bodies
    incandescent flight
    phase in the flight of a meteor before the ratardation point when the meteoroid is producing its own incandescent light
    inclination (i)
    An orbital element representing the inclination of the object's orbital plane to the ecliptic
    inclination (i)
    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.
    inclination (i)
    The angle between two planes or their poles; usually the angle between an orbital plane and a reference plane.
    induction
    System of reasoning in which the conclusion, though implied by the premises and consistent with them, does not necessarily follow from them.
    inelastic collision
    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.
    inflation era
    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.)
    inflation era
    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.
    inflation era
    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.
    inflation era
    Big Bang era in which universe undergoes a brief burst of enormous expansion.
    inflation era
    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.
    infrared (IR)
    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.)
    infrared (IR)
    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.
    infrared magnitude
    The magnitude derived from the observations at an infrared wavelength
    inner bremsstrahlung
    The continuous electromagnetic radiation that accompanies the β-decay of nuclei.
    inner Lagrangian point (L1)
    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.
    instrument
    a device that requires skill for proper use
    integrating detector
    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.
    integration time
    The interval of time used to collect photons of light on a detector and build-up a strong signal.
    intercombination line
    Spectral line emitted in transitions between two levels with different values of S.
    interference filter
    A filter used to shut out all light except the desired wavelengths.
    interference filter
    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.
    interline transfer
    A CCD construction consisting of vertical strips which are alternately opaque and light sensitive. The opaque strips conceal charge transfer registers.
    Intermediate Frequency (IF)
    The beat frequency between the signal and the local oscillator in a radio detection system.
    internal symmetry
    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.
    International temperature scale (ITS90)
    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 : [JN92] -has source: J. A. Hall, 1967
    interstellar gas
    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.
    interstellar line
    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.
    interstellar molecule
    Molecule present in interstellar space. As of late 1974, at least 33 molecular species had been identified with reasonable certainty.
    intrinsic luminosity
    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.
    intrinsic luminosity
    The energy per second emitted by an astronomical object, analogous to the wattage of a light bulb.
    invariable plane
    The plane through the center of mass of the solar system perpendicular to the angular momentum vector of the solar system.
    invariant plane
    The plane defined by the total angular momentum of the solar system. It is within about 1°.5 of the ecliptic.
    inverse beta-decay
    The relatively rare process p + vbar → n + e+. Free-electron capture (e + pn + v) is sometimes called inverse β-decay in astrophysics.
    inverse bremsstrahlung
    Absorption of a photon by an electron in the field of a nucleus.
    inverse Compton effect
    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.
    inverse P Cygni line
    An emission line in which the emission is on the violet side of the absorption. It is usually interpreted to mean infall of matter.
    Io (Jupiter I, Jupiter I)
    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.
    ionic tail (type I comet tail)
    Straight part of comet tail composed of ions driven by the solar wind
    ionization
    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.
    ionization potential
    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.
    ionized helium line
    Set of spectral features associate with ionized helium (He II)
    ionosphere
    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.
    IRAS galaxy
    Any galaxy which was discovered by the Infra-Red Astronomical Satellite (IRAS) to possess an excessive amount of infrared emission.
    iron (Fe, 5D4 in ground state, ferrum = iron from Latin)
    lustrous, silvery and soft or workable metal when absolutely pure
    irregular galaxy
    A galaxy with amorphous structure and with relatively low mass (108-1010 Msun). Fewer than 10% of all galaxies are classified as irregular.
    isomer
    Nucleus with the same A and Z numbers but in different energy states.
    isomer shift
    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.
    isoplanatic patch
    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.
    isotone
    Nucleus with the same number of neutrons but with different A and Z numbers.
    isotope
    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.)
    isotope
    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.
    isotropy (rotational invariance)
    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.
    isotropy (rotational invariance)
    Having equal and uniform properties at all points and in all directions. In astronomy the term describes microwave background radiation.
    isotropy (rotational invariance)
    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.)
    isotropy (rotational invariance)
    Quality of being the same in all directions.
    isotropy (rotational invariance)
    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.
    isotropy (rotational invariance)
    The same in all directions.
    J magnitude
    The magnitude derived from observations at 1.3 microns.
    Janus
    The innermost satellite of Saturn, just outside Saturn's rings. Named Janus for the first and the last.
    Jeans mass
    The critical mass a volume of space must contain before it will collapse under the force of its own gravity.
    Jeans mass
    The mass enclosed within a sphere of diameter equal to the Jeans length.
    Jet Propulsion Laboratory (JPL)
    Funded by NASA and operated by the California Institute of Technology (Caltech).
    Jodrell Bank
    A telescope in England designed to detect radio waves emitted by astronomical objects.
    joule (J)
    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.
    joule (J)
    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.
    Jovian aurora
    an aurora ocurring on the planet Jupiter
    Julian year
    A period served as the basis for the Julian calendar.
    Juno
    An asteroid with a relatively large albedo.
    Jupiter
    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.)
    K alpha
    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.
    K corona (K component)
    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)
    K edge
    The absorption edge of the K shell (see absorption edges).
    K electron
    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.
    k line
    Singly ionized magnesium resonance line.
    K magnitude
    The magnitude derived from observations at 2.2 microns.
    K star
    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).
    Kaluza-Klein theory
    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.
    Kaluza-Klein theory
    Class of theories incorporating extra curled-up dimensions, together with quantum mechanics.
    kaon (K meson)
    Meson with non-zero strangeness quantum number.
    Kapteyn Universe
    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.
    Kapteyn's star (HD 33793, CD -45 1841)
    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.
    kbTop
    Astronomy
    Kelvin (K, absolute)
    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.
    Kelvin (K, absolute)
    Unit of thermodynamic temperature, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water
    Kelvin contraction (Kelvin-Helmholtz contraction)
    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.
    Kelvin timescale
    The time it takes a star to contract gravitationally from infinite radius down to its present radius by radiating its thermal energy.
    Kepler's first Law
    Each planetary orbit is an ellipse with the Sun at one focus.
    Kepler's Law
    One of three orbital laws formulated by Kepler
    Kepler's second Law (law of areas)
    Equal areas are swept out in equal times.
    Kepler's Supernova
    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)
    Kepler's third Law (harmonic law)
    The square of the period is proportional to the cube of the distance.
    Keplerian orbit (osculating orbit)
    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.
    Keplerian orbit (osculating orbit)
    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.
    Kerr black hole
    A rotating, axisymmetric black hole based on Kerr's 1963 solution to Einstein's field equations.
    kilogram (kg)
    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.
    kilogram (kg)
    Unit of mass equal to the international prototype of the kilogram
    kinematic relativity
    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.
    kinematics
    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.
    kinetic energy
    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.
    kinetic temperature
    A measure of the average random motion of the particles in a system.
    Kirchhoff's first law
    To each chemical species there corresponds a characteristic spectrum.
    Kirchhoff's law
    (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.
    Kirchhoff's second law
    Every element is capable of absorbing the radiation which it is able to emit; this is the phenomenon of the reversal of the lines.
    Kirkwood gap
    Region in the asteroid belt which has been swept clear of asteroids by the perturbing effects of Jupiter.
    Klein-Alfvén cosmology
    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.
    Klein-Alfvén cosmology
    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).
    Kolmogorov-Smirnov test
    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.
    KREEP
    Lunar basaltic material rich in radioactive elements. (from K for potassium, REE for rare-earth elements, P for phosphorus)
    Kruger 60 AB
    A faint dM binary in the Solar neighborhood. It may be a subluminous star.
    kurtosis
    The peakedness or flatness in the graphical representation of a statistical distribution.
    L corona
    The part of the solar corona whose spectrum consists of emission lines.
    L magnitude
    The magnitude derived from observations at 3.5 microns.
    L2
    An unstable Lagrangian point beyond the most massive body on the line passing through the centers of mass of the two bodies.
    L3
    An unstable Lagrangian point beyond the less massive body on the line passing through the centers of mass of the two bodies.
    L4
    A stable Lagrangian point in the orbit of the less massive component which is equidistant from the two main components in the orbit direction
    L5
    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
    Lagoon Nebula (M 8, NGC 6523)
    An emission nebula in Sagittarius 2 kpc distant.
    Lagrangian point
    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.
    Lallemand camera
    A type of image tube (q.v.).
    Lamb shift
    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.)
    Lamb shift
    The difference in energy levels of the H atom between 2S1/2 and 2P4.
    lambda
    Heavy short-lived baryon.
    lambda Bootis star
    A type of young (usually early A), weak-lined, metal-poor stars with low radial velocities.
    lambda Bootis star
    A-type stars with weak metallic lines, low rotational velocity and low radial velocity.
    lambda doublet
    Two lines in the microwave region of the spectrum of the OH molecule caused by splitting of electronic levels.
    Laplacian determinism
    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.
    Laplacian plane
    For a system of satellites, the fixed plane relative to which the vector sum of the disturbing forces has no orthogonal component.
    Large Magellanic Cloud (LMC)
    The nearest and largest of the many galaxies that orbit the Milky Way. It is 160000 light-years away.
    large-scale motions
    Bulk motions of distant galaxies deviating from the Hubble flow.
    Larmor frequency
    The frequency of precession of a charged particle orbiting in a uniform magnetic field. It is equal to eH /me, where e is the electron charge, me is the electron mass, and H is the magnetic field strength.
    Larmor radius
    The radius of the circular orbit that a charged particle describes transverse to a magnetic field.
    late star
    Star with spectral type later than the sun (G2).
    law
    A theory of such wide and invariable application that its violation is thought to be impossible.
    leap second
    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.
    Lense-Thirring effect
    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.
    Lenz's law
    The current induced by an electromotive force will appear in such a direction that it opposes the charge that produced it.
    Leo dwarf
    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.
    Leo I
    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.
    Leo II
    The second most distant galaxy that orbits the Milky Way. Like Leo I, Leo II is a dwarf galaxy that was discovered in 1950.
    Leonid
    Shower of meteors emanating from an apparent point in Leo every 33 years; the next is due in about 1999.
    lepton
    A class of non-strongly-interacting particles that includes the electron, muon, tau, and their associated neutrinos. See Table 7.1 on page 120.
    lepton
    A fermion which is not made of quarks.
    lepton
    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.
    lepton
    Any fermion that does not participate in the strong interactions. Leptons include the electron family and the muon family.
    lepton
    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.
    lepton
    Elementary particle that does not participate in the strong interactions, including electrons, muons, and neutrinos.
    lepton
    Elementary particle that have no measurable size and are not influenced by the strong nuclear force. Electrons, muons, and neutrinos are leptons.
    lepton
    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.
    lepton era
    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).
    LHe
    The symbol for liquid helium. The temperature of liquid helium is normally 4 K, that is, four degrees above absolute zero.
    libration
    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.
    libration
    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.
    libration
    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.
    libration orbit
    An elliptical orbit associated with a stable Lagrangian point.
    life zone
    The region around a star where a planet can have liquid water and so may support life.
    lifetime (τ, mean life)
    The time it takes for a sample of identical particles to decay to 1/e of its initial population.
    light element
    In astronomy these elements are hydrogen, helium, and lithium; sometimes also beryllium and boron.
    light-year (lt-yr, ly)
    Distance traveled at the speed of light for one Earth-year: 9.46 million million km.
    light-year (lt-yr, ly)
    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.)
    light-year (lt-yr, ly)
    The distance light travels in one year, equal to 5.8 × 1012 (about six trillion) miles.
    light-year (lt-yr, ly)
    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.
    light-year (lt-yr, ly)
    The distance that light traverses in a vacuum during one year.
    light-year (lt-yr, ly)
    The distance traveled in a vacuum by light in one year, equal to 9.46 × 1017cm.
    LIGO
    Large Interferometric Gravitational Wave Observatory.
    line
    A geometrical object with one dimension
    line broadening
    Increase in the range of wavelengths in which some characteristic emission or absorption occurs.
    line of apsides
    The major axis of an elliptical orbit.
    line of nodes
    The intersection between the orbital plane of the Moon or a planet and the plane of the ecliptic.
    liquid
    a high density cohesive collection of particle which move relative to one another and which has short range order
    liquid element
    element which is in liquid form at standard temperature and pressure
    lithium (Li, 2S1/2 in ground state)
    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.
    lithium star
    C star with a very strong Li I 6078 line.
    lithium star
    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.
    lithophile element
    element that tends to concentrate in silicate materials such as igneous rocks
    Littrow
    The configuration of a diffraction grating spectrograph in which the diffracted ray returns along the same direction as the incident ray.
    LN2
    The symbol for liquid nitrogen. The temperature of liquid nitrogen is normally 77 K, that is, 77 degrees above absolute zero.
    local bubble
    The region of the Galaxy near the Sun which has little neutral hydrogen gas.
    Local Group
    A conglomeration of about twenty galaxies including our own Milky Way.
    Local Group
    The association of galaxies to which the Milky Way galaxy belongs.
    Local Group
    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.
    Local Group
    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.
    Local Group
    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.
    Local Group
    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.
    local hypothesis of quasars
    The hypothesis that quasars are not at the distances inferred from their redshifts.
    local inertial frame
    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.
    local standard of rest (LSR)
    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.
    local standard of rest (LSR)
    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.
    Local Supercluster (Virgo supercluster)
    A loosely knit assemblage of some 100 clusters of galaxies, including the Local Group.
    Local Supercluster (Virgo supercluster)
    An aggregation of galaxies - roughly ten thousand of them - to which the Virgo Cluster and our own galaxy belong.
    Local Supercluster (Virgo supercluster)
    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).
    Local Supercluster (Virgo supercluster)
    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.
    location dependent periodic celestial event
    Periodic celestial event which depends on the observers location on the planet
    long-period variable (red variable, Mira variable)
    Cyclic variable with cycles 100-500 days, and of spectral types K, M, S and C.
    long-period variable (red variable, Mira variable)
    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.
    longitude of the ascending node (Ω)
    An orbital element representing the point where the body crosses the ecliptic from south to north
    longitudinal wave
    A wave vibrating along the direction of propagation.
    lookback time
    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.
    lookback time
    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.
    loose group
    Galaxy group with on the order of 10 galaxies separated by 10-100 galaxy diameters.
    Lorentz contraction
    A moving object appears shortened along its direction of motion.
    Lorentz invariance (principle of relativity)
    A law which states that laws of physics are identical for all observers moving at constant velocity relative to each other.
    Lorentz transformation
    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.
    Lorentz transformation
    The transformation which keeps the speed of light invariant between relativistic frames of reference.
    Loschmidt constant (n0)
    Number of molecules of an ideal gas per unit volume (2.687 × 1019 molecules per cm3).
    Loschmidt constant (n0)
    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.
    low surface brightness galaxy (LSBG)
    A galaxy which is very faint because it contains a very limited number of stars.
    low-velocity star
    A star whose U, V, and W velocities are all near zero. Such stars have nearly circular orbits around the Galaxy.
    Lowell's band
    A dark border sometimes found on the Martian polar cap.
    lower chromosphere (reversing layer)
    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.
    lower culmination (culmination below pole)
    Passage of a celestial object across the observer's meridian. The crossing farther from the zenith.
    luminance unit (cd·m-2)
    candela per square meter
    luminosity
    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.
    luminosity
    The intrinsic brightness of a star. Usually defined in terms of absolute magnitude.
    luminosity
    The total amount of energy radiated by a star - that is, its true, or intrinsic, brightness.
    luminosity
    Total radiant energy output per second (absolute brightness, usually expressed in ergs s-1 or in magnitudes). L = 4πR2σT4 (see Stefan's law).
    luminous blue variable (S Dor variable, Hubble-Sandage variable)
    A variable-star designation for the high-luminosity early type objects.
    luminous mass
    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.
    Lumogen
    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.
    lunar eclipse
    An eclipse in which the Moon passes through the shadow cast by the Earth.
    lunar fines
    Small particles of rock or powdered rock on the Moon.
    Lupus Loop
    A radio source, a large broken shell 4°.5 in diameter, identified as a prehistoric supernova remnant.
    Luyten 726-8
    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 ).
    Lyman alpha
    The resonance line of hydrogen. It can only be studied from spacecraft or in the spectra of highly redshifted quasars.
    Lyot division
    A gap between the B ring and the C ring.
    M 32
    An elliptical galaxy that orbits the Andromeda galaxy.
    M 81 (NGC 3031)
    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.
    M 87 (3C 274, Virgo A, 2U 1228+12)
    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.
    M magnitude
    The magnitude derived from observations at 5 microns.
    M star
    Having a spectral type of M, that is, red like Betelgeuse and Antares.
    M star
    Spectral type for red stars, such as Betelgeuse, Antares, and Proxima Centauri.
    M star
    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.
    M-Theory
    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.
    Mach's Principle
    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.
    Mach's Principle
    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.
    Mach's Principle
    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.
    Mach's Principle
    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.
    Magellanic Cloud
    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.
    Magellanic Cloud
    Two galaxies that lie close to the milky way galaxy. They are visible in the southern skies of Earth.
    Magellanic Cloud
    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.
    Magellanic Cloud
    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.
    Magellanic Stream
    A name given to the common envelope of neutral hydrogen in which the LMC and SMC are embedded.
    Magellanic Stream
    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.
    Magellanic Stream
    A strand of gas that the Milky Way has ripped out of the Magellanic Clouds.
    magnesium (Mg, 1S0 in ground state)
    silvery white, lustrous and relatively soft metal
    magnetic field strength unit (A·m-1)
    ampere per meter
    magnetic monopole
    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.
    magnetic monopole
    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.
    magnetic monopole
    a massive particle with but one magnetic pole, the production of which is indicated in some theories of the early universe.
    magnetic monopole
    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.)
    magnetic monopole
    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.
    magnetic monopole problem
    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.
    magnetic pressure
    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.
    Magnetic star
    Star (usually of spectral type A) with strong integrated magnetic fields ranging up to 30000 gauss.
    magnetohydrodynamics (hydromagnetics, MHD)
    The study of the collective motions of charged particles in a magnetic field. (Sometimes called hydromagnetics)
    magnetopause
    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.
    magnetosphere
    The extent of a planet's magnetic field.
    magnetosphere
    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).
    magnitude
    A logarithmic brightness scale for astronomical objects. See Appendix.
    magnitude
    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.
    magnitude
    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).
    magnitude
    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.
    magnitude
    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.
    magnitude
    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.
    main lines
    The transitions that emit radiation at 1665 and 1667 mhz of an OH source.
    Maksutov telescope
    A reflector whose primary mirror is spheroidal instead of parabolic. The light initially passes through a large concave lens to remove the spherical aberration.
    MAMA
    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.
    manganese (Mn, 6S5/2 in ground state)
    hard, brittle, silvery metal
    manganese star
    Star with an anomalously high Mn-Fe ratio, which show deviations from the odd-even effect for phosphorus, gallium, and yttrium.
    many-worlds interpretation
    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.
    mare
    An area on the moon that appears darker and smoother than its surroundings. Lunar maria are scattered basaltic flows.
    Markarian galaxy
    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.
    Mars
    Fourth major planet out from the Sun. Its tiny satellites are locked in synchronous rotation with mars.
    mascons (mass concentrations)
    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.
    mascons (mass concentrations)
    gravity anomalies found on the moon. As of 1971, 12 mascons were known.
    mass
    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.
    mass
    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.
    mass
    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.
    mass density unit (kg m-3)
    kilogram per cubic meter
    mass fraction unit (kg·kg-1 = 1)
    kilogram per kilogram, which may be represented by the number one
    mass motion
    A non-uniform matter flow
    mass of the Galaxy
    The mass of the Milky Way.
    massive black hole
    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.
    massive compact halo object (MACHO)
    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.
    massless black hole
    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.
    massless particle
    A particle with zero mass
    materialism
    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.
    matter era (recombination era, decoupling era)
    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.
    matter era (recombination era, decoupling era)
    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.
    matter era (recombination era, decoupling era)
    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.
    matter era (recombination era, decoupling era)
    The epoch at t ≈ 1013 s after the big bang (T ≈ 3000 K) when matter and radiation decoupled.
    matter era (recombination era, decoupling era)
    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.
    matter era (recombination era, decoupling era)
    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.
    matter era (recombination era, decoupling era)
    The release of photons from constant collisions with massive particles as the universe expanded and its matter density diminished. See decoupling.
    maxwell
    Unit of magnetic flux through 1 cm2 normal to a field of 1 gauss.
    Maxwell distribution
    An expression for the statistical distribution of velocities among the molecules of a gas at a given temperature.
    Maxwell distribution
    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.
    Me star
    Star of spectral type M with emission lines in their spectra.
    mean element
    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.
    mean free path
    Mean length of the path of a particle between collisions.
    mean free path
    The mean distance traversed by a particle before undergoing a significant deflection or collision.
    mean solar day
    The mean length of time between two successive culminations of the Sun (i.e., the mean period from apparent noon to apparent noon).
    mean solar second
    1/86400 of a mean solar day (cf. ephemeris second).
    mechanics
    The study, in physics, of the influence of forces.
    median
    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.
    meniscus mirror
    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.
    Mercury
    Innermost planet of the Solar System. Transits of the Sun occur either 7 or 13 years apart - last transit 1973 November 10.
    meridian
    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.
    meridian
    The great circle on the celestial sphere which passes through the celestial poles and the zenith of the observer.
    meridian
    The great circle passing through the zenith of the observer and the north and south points on his horizon.
    meridian
    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.
    meridional flow
    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.
    meson
    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.
    mesosphere
    The part of Earth's atmosphere immediately above the stratosphere, where the temperature drops from about 270 K to 180 K.
    Messier catalog
    Designation of objects in the Messier Catalog of nebulae, star clusters, and galaxies, published in the Eighteenth Century.
    Messier catalog
    is an instance of nebula catalog
    Messier catalog
    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).
    Messier catalog
    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.
    Messier object
    an object assigned a number by Charles Messier
    metal
    As used in stellar spectroscopy, any element heavier than helium.
    metal
    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.
    metal-rich star (metal-strong star, CN-strong star)
    A small subgroup of A-type stars in which the lines of Mg II are very strong.
    metal-rich star (metal-strong star, CN-strong star)
    Star having metal-to-hydrogen ratios greater than those of the Hyades.
    metallic hydrogen
    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.
    meteor (shooting star)
    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.
    meteor (shooting star)
    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.
    meteor (shooting star)
    when a meteoroid collides with the atmosphere or surface of a celestial body, kinetic energy is converted to heat, light and sound
    meteor event
    The event in a sequence of the events during the passage of a meteor. They are in order :
    1. incandescent flight
    2. fireball
    3. smoke trail
    4. meteoroid break up
    5. retardation point
    6. dark flight
    7. meteorite impact
    meteor shower
    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.
    meteor shower
    temporary increase in the rate at which meteors are observed when the target runs into a swarm of meteoroids that share a common orbit
    meter (m)
    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.
    meter (m)
    length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second
    methanol (CH3OH, methyl alcohol)
    More rotational lines have been observed astronomically for it than for any other molecule.
    methylamine (CH3NH2)
    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.
    microchannel plate
    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.
    micron (μ)
    A unit of length.
    microphotometer
    A device for measuring the variations in density in a photographic emulsion.
    microwave
    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).
    microwave
    An electromagnetic wave with a wavelength between one millimeter and 30 centimeters, or sometimes one meter.
    microwave
    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.
    microwave
    Radio radiation with wavelengths of about 10-4 to 1 meter, equal to 109 to 1013 hertz.
    Mie diffraction
    The diffraction of light by small spherical particles.
    Mie scattering
    Scattering of light (without regard to wavelength) by larger particles, such as those of dust or fog in Earth's atmosphere.
    mile
    The mile employed in this book is the statute mile, equal to 5280 feet.
    Milky Way
    A softly glowing band of light that bisects the skies of Earth, produced by light from stars and nebulae in the galactic disk.
    Milky Way
    Our own galaxy, the second largest in the local group.
    Milky Way
    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.
    Milky Way
    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
    Mimas
    The second innermost satellite of Saturn. The perturbations of Mimas and Janus produce the divisions in Saturn's rings.
    mini black hole
    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.
    Mintaka
    One of the three stars in Orion's belt, and the star along whose line of sight interstellar gas was first spectroscopically detected.
    Mira (omicron Cet)
    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.
    Mira (omicron Cet)
    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.
    Miranda
    The innermost satellite of Uranus.
    missing mass problem
    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.
    missing mass problem
    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.)
    mole (mol)
    amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12
    mole (mol)
    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.
    molecular band
    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.
    molecular hydrogen (H2)
    A molecule consisting of two hydrogen atoms (H2) and the most common molecule in space.
    molecular hydrogen (H2)
    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.
    molecule
    The smallest unit of a chemical compound. A molecule is composed of two or more atoms, linked by interactions of their electrons.
    Monoceros Loop
    A filamentary supernova remnant resembling a loop.
    month
    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.
    Moon
    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.
    moonrise
    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.
    moonset
    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.
    motion
    Process of passing through space or changing position
    MS star
    M-type stars with ZrO bands.
    MS star
    Star sharing the M and S characteristics. They thus exhibit bands of both TiO and SrO.
    Mt. Wilson
    The location, in California, of the 100-inch diameter telescope used by Edwin Hubble and others.
    Mu Columbae
    Runaway star which diverges from a comparatively small area in Orion.
    multiplet
    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.
    Munich Image Data Analysis System (MIDAS)
    A suite of programs and a software environment developed at the European Southern Observatory for astronomy applications.
    muon (μ)
    A second-generation lepton. It is essentially a more massive electron.
    muon (μ)
    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.
    muon (μ)
    Elementary particles produced when cosmic rays enter the upper atmosphere.
    muon (μ)
    Short-lived elementary particle with negative electrical charge. Muons are leptons. They resemble electrons, but are 207 times more massive.
    N galaxy
    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.
    N magnitude
    The magnitude derived from observations at 10 microns.
    N star
    In the old terminology, the cooler C-type stars.
    N star
    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.
    N1 line
    Green forbidden line of doubly ionized oxygen.
    N2 line
    Green forbidden line of doubly ionized oxygen.
    nadir (downward)
    The point on the celestial sphere diametrically opposite to the zenith in the direction of gravity.
    naked eye object
    a physical object visible to the unnaided human eye
    naked eye planet
    a planet in our solar system visible to the unaided eye
    naked eye star
    A star visible without visual aids
    naked singularity
    A singularity that will be visible and communicable to the outside world.
    National Aeronautics and Space Administration (NASA)
    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.
    National Air and Space Development Agency (NASDA)
    National Air and Space Development Agency.
    natural line broadening
    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.
    natural object
    an object occurring naturally, not constructed by humans
    natural satellite
    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).
    natural unit
    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.)
    nautical mile
    A unit of length.
    nebula
    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).
    nebula
    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.
    nebula
    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.)
    nebular hypothesis
    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.
    neon (Ne, 1S0 in ground state)
    colourless, odourless gas
    neon burning
    The stage in which a star burns neon into oxygen and magnesium.
    Neptune
    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.
    Nereid
    The outer satellite of Neptune. It has the most eccentric orbit of any natural satellite.
    neutral particle
    A particle with zero charge
    neutrino
    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.
    neutrino
    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).
    neutrino
    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.
    neutrino
    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.
    neutrino
    Chargeless species of particle, subject only to the weak force.
    neutrino telescope
    telescope designed to detect neutrinos and determine their origin, type and energy
    neutron
    A neutral baryon.
    neutron
    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.
    neutron
    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.
    neutron
    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.
    neutron
    Stable within the nucleus, the neutron if isolated decays, with a ha half-life of fifteen minutes.
    neutron
    typically found in the nucleus of an atom.
    neutron star
    A dead, collapsed star that consists mostly of neutrons and is only about 20 kilometers across. Neutron stars are much denser than white dwarfs.
    neutron star
    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.
    neutron star
    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.
    New General Catalogue (NGC)
    A catalogue of 7840 nebulae, star clusters, and galaxies that was published in 1888 by John Dreyer.
    newton (N)
    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.
    Newton's first law
    Newton's law which states that a body remains in a state of rest or uniform motion when left to itself.
    Newton's law of motion
    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.
    Newton's second law
    Newton's law which states that the net force on a body is equal to the product of its mass and its acceleration.
    Newton's theory of gravity (inverse square law)
    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.
    Newton's theory of gravity (inverse square law)
    Gravitational force decreases as one over distance squared (1/r2), where r is the distance from the source.
    Newton's theory of gravity (inverse square law)
    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.
    Newton's third law
    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.
    Newtonian
    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.
    Newtonian constant of gravitation (G)
    The universal constant of proportionality in the attraction between two unit masses a unit distance apart.
    NGC 1275 (3U 0316+41, Perseus X-1, Perseus A)
    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.
    NGC 205
    An elliptical galaxy that orbits the Andromeda galaxy.
    NGC object
    an object assigned a number in the New General Catalog of non-stellar objects
    nickel (Ni, 3F4 in ground state)
    corrosion resistant, silvery-white, lustrous, malleable and ductile metal
    NICMOS
    Near Infrared Camera for Multi-Object Spectrography.
    Nicol prism
    A device made from a split crystal of Iceland spar with which plane-polarized light can be detected.
    nitrogen (N, 4S3/2 in ground state)
    colourless, odourless gas (N2)
    NML Cygnus (IRC+40448)
    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.
    nodical month (Draconic month)
    The interval of time between two successive transits of the Moon through its ascending node.
    non SI unit
    not an SI unit, but an important and widely used unit
    non-coherent scattering
    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.
    North America Nebula (NGC 7000)
    An emission nebula in Cygnus.
    north galactic pole (NGP)
    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.
    north point
    The point at which the meridian intersects the horizon below the north celestial pole.
    North Polar Spur
    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.
    north pole
    The astronomical coordinate which coincides with the northern intersection of Earth's axis with the geoid.
    nova
    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.
    nova
    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.
    nova
    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.
    nova remnant
    an expanding emission nebula produced by a nova
    nuclear density
    The density of an atomic nucleus (about 1014 g cm-3).
    nuclear disk
    A rotating disk of about 106 Msun of neutral hydrogen in the inner 800 pc of our Galaxy.
    nuclear physics
    That branch of physics that attempts to understand the physics of the nucleus.
    nuclear time scale
    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.
    nucleon
    The generic name for the proton and the neutron.
    nucleosynthesis
    The production of heavy nuclei from the fusion of lighter ones.
    nucleosynthetic era
    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.
    nucleus
    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.
    nucleus
    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.
    nuclide
    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.
    nutation
    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.
    nutation
    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".
    O magnitude
    The magnitude derived from observations at 11 microns.
    O star
    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.
    O((f))
    O-type stars in which N III is present in emission and He is strong in absorption.
    O(f)
    O-type stars in which N III is present in emission and He II is weakly present in absorption or emission.
    OB association
    A loose gathering of O and B stars that typically stretches over hundreds of light-years and contains a few dozen OB stars.
    Oberon
    Outermost satellite of Uranus.
    objective
    The primary mirror of a reflecting telescope (or the primary lens of a refractor).
    objective grating
    A coarse grating placed in front of the telescope objective.
    objective prism
    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.
    observatory
    a place on earth which has one or more telescopes
    Occam's Razor
    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.
    occultation
    The cutoff of the light from a celestial body caused by its passage behind another object.
    occultation phase
    A phase during the occultation of a celestial body
    oceanic element
    an element contained in the ocean
    Oe
    O star with emissions in the Balmer lines.
    Oef
    Early O stars that show double emission lines in He II λ4686.
    oersted (Oe)
    Unit of magnetic field strength. 1 Oe corresponds to 1000/4π amperes per meter.
    Of star
    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.
    Olber's paradox
    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.
    Olber's paradox
    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.
    Olber's paradox
    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.
    old star
    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.
    old thin disk
    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.
    omega
    Heavy short-lived baryon.
    omega
    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.)
    Omega Centauri
    A bright globular cluster.
    Omega Nebula (Swan Nebula, M 17, NGC 6618)
    A bright H II region. It is a double radio source.
    Omicron2 Eridani
    A triple star that was the first white dwarf ever discovered.
    Oort cloud
    H I region extending to more than 100000 AU from the Sun, barely gravitationally bound, postulated as the birthplace of comets.
    Oosterhoff Group globular cluster
    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.
    open cluster
    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.)
    open cluster
    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.
    open string
    A type of string with two free ends.
    open universe
    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.
    open universe
    Any model of the Universe which does not contain enough matter to halt its expansion.
    open universe
    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.
    open universe
    Cosmological model in which the universe continues to expand forever; its space-time geometry is hyperbolic, or "open".
    opposition
    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.
    opposition
    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.
    optical (light)
    Electromagnetic radiation with wavelengths of or close to those detectable by the eye.
    optical fibre
    A long, thin strand of glass capable of excellent transmission of light over large distances.
    optical fibre
    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.
    optical matching
    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.
    optics
    The science of light.
    orbit
    The path in space followed by a celestial body.
    orbital element
    One of seven quantities that must be established from observations in order to define the size, shape, and orientation of an orbit in space.
    orbital event
    Periodic celestial event in the orbit of a celestial body
    orbital quantity
    A quantity used for calculating orbits
    orbital velocity
    Velocity required by a body to achieve a circular orbit around its primary: Vorb = (GM / r)1/2.
    orbiting collision
    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.
    Orion A
    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.
    Orion arm (local arm)
    The spiral arm of the Milky Way in which the Sun is located. It lies between the Sagittarius arm and the Perseus arm.
    Orion B (NGC 2024)
    A radio continuum source.
    Orion Nebula (M 42)
    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.
    Orion Nebula (M 42)
    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).
    Orion Nebula (M 42)
    The Orion Nebula, a star-forming region in the constellation Orion.
    Orion spur
    That part of the local spiral arm in which the Sun is embedded. The Sun is on an inner edge of the Orion spur.
    ortho-hydrogen
    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.
    ortho-spectrum
    Spectrum of triplet (l = 1).
    Oscillating Universe
    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.
    osculating element
    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.
    our solar system
    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.
    Owl Nebula (M 97, NGC 3587)
    A planetary nebula.
    oxygen (O, 3P2 in ground state)
    colourless, odourless gas (O2) which is very reactive
    oxygen burning
    The stage when a star fuses oxygen into silicon and sulfur. It occurs only in stars born with over eight solar masses.
    oxygen-rich giant
    A collective designation for a giant showing metal oxide molecules - thus M, MS and S stars.
    ozone layer
    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.
    P Cygni star
    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.
    P Cygni star
    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).
    P-branch
    A set of lines in the spectra of molecules corresponding to unit increases in rotational energy.
    p-electron
    An orbital electron whose l quantum number is 1.
    p-process
    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.
    P-strong star
    A small subgroup of B-type stars in which P lines are very strong.
    PAH
    Polycyclic Aromatic Hydrocarbon
    pairing energy (δ)
    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.
    Pallas
    The second asteroid to be discovered.
    Palomar
    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.
    Pan (Jupiter XI)
    Unofficial name for Jupiter XI.
    pancake model
    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.)
    PAPA
    Precision Analogue Photon Address.
    para-hydrogen
    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.
    paradox
    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.
    parallax (distance)
    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.
    parallax (distance)
    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.
    parallax (distance)
    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.)
    parallax (distance)
    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.
    parity
    The operation which reverses the signs of the coordinate axes used to describe a system, i.e. (x, y, z) -> (-x, -y, -z).
    parity
    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.
    parsec (pc)
    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.
    parsec (pc)
    Astronomical unit of distance, equal to 3.26 light-years.
    parsec (pc)
    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.
    parsec (pc)
    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)
    partial lunar eclipse
    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.
    particle
    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.
    particle accelerator
    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.
    particle accelerator
    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.
    particle Compton wavelength
    The wavelength of a photon containing the rest energy of a particular particle.
    particle constant
    a fundamental physical constant quantifying a particle property
    particle physics (high-energy physics)
    That branch of physics that attempts to understand the fundamental particles and forces of nature.
    particle physics (high-energy physics)
    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.
    parton
    A generic term used to describe any particle which may be present inside nucleons. It includes quarks, antiquarks and gluons.
    parton
    A hypothetical pointlike constituent of a nucleon, which contains all the charge of the nucleon.
    pascal (Pa)
    The derived SI unit of pressure.
    Paschen-Back effect
    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.
    Patroclus
    Trojan asteroid (60°) behind Jupiter.
    PDA
    Photodiode array.
    peculiar star
    Star with spectra that cannot be conveniently fitted into any of the standard spectral classifications. Denoted by a p after spectral type.
    peculiar velocity
    Velocity with respect to the Local Standard of Rest.
    penumbral lunar eclipse (appulse)
    An eclipse in which the Moon passes through the shadow cast by the Earth, the Moon passing only through the Earth's penumbra.
    pep reaction
    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).
    periapsis
    The point in the orbit of a satellite where it is closest to its primary.
    periastron
    The point in the orbit of one component of a binary system where it is nearest the other component.
    pericenter
    The point in the orbit of one component of a binary system which is closest to the center of mass of the system.
    pericynthion
    The point in the orbit of a satellite around the Moon closest to the Moon.
    perigalacticon
    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.
    perigee
    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.
    perihelion
    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.
    period (P)
    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.
    periodic celestial event
    Celestial event which occurs with a very regular period
    Perseus arm
    The spiral arm that lies next out from the arm containing the Sun.
    Perseus cluster
    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.
    Perseus OB2
    A young association of OB stars.
    Perseus-Pisces region
    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.
    person
    a human living or dead
    PG 1159 star (pre-degenerate)
    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.
    phase angle
    The angle measured at the center of an illuminated body between the light source and the observer.
    phase of matter
    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.
    phase transition
    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.
    Phobos
    The potato-shaped inner satellite of Mars. Phobos lies just outside the Martian Roche limit.
    Phoebe
    The outermost satellite of Saturn. Has retrograde orbit.
    phosphorus (P, 4S3/2 in ground state)
    soft and flammable white solid, the red form is usually non-flammable
    photocathode
    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.
    photodiode
    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.
    photoelectric device
    Any detector which uses the photoelectric effect to convert photons to electrons.
    photoelectric magnitude
    The magnitude of an object as measured with a photoelectric photometer. (mpe) (antiquated term)
    photographic magnitude
    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)
    photoionization
    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.
    photomultiplier
    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.
    photomultiplier
    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.
    photon (electromagnetic radiation)
    A transverse wave of electric and magnetic fields which can propagate through empty space.
    photon (electromagnetic radiation)
    Smallest packet of the electromagnetic force field; messenger particle of the electromagnetic force; smallest bundle of light.
    photon (electromagnetic radiation)
    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.
    photonics
    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.
    photosphere
    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).
    photosphere
    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
    photosynthesis
    a biochemical process operating in green plants in which carbohydrates are formed under the influence of light with chlorophyl serving as a catalyst
    photovisual magnitude
    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)
    physical object
    Anything tangible having existence (living or nonliving)
    physicist
    a scientist specializing in physics
    physics
    The scientific study of the interactions of matter and energy.
    pinch machine
    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.
    Pinwheel galaxy (M 33, NGC 598, Triangulum galaxy)
    A Sc II-III spiral galaxy, a satellite of the Andromeda galaxy, about 700 kpc distant. Mv = - 18.9.
    Pinwheel galaxy (M 33, NGC 598, Triangulum galaxy)
    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.
    Pinwheel galaxy (M 33, NGC 598, Triangulum galaxy)
    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.
    pion (π-meson)
    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.
    pitch angle
    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.
    plage (flocculus)
    The bright rim of a sunspot, observed in emission in monochromatic light of some spectral line (Halpha or Ca II). It is a chromospheric phenomenon associated with and often confused with a facula. (sometimes called flocculus)
    Planck constant (h)
    A universal constant of nature that measures the magnitude of quantum mechanical effects.
    Planck constant (h)
    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.
    Planck constant (h)
    The universal constant of proportionality relating the frequency of a photon to its quantum of energy.
    Planck energy
    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.
    Planck energy
    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.
    Planck era
    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.
    Planck length (lP)
    The dimension at which space is predicted to become "foamlike" and at which Einstein's theory is supposed to break down.
    Planck length (lP)
    The scale below which quantum fluctuations in the fabric of spacetime would become enormous. The size of a typical string in string theory.
    Planck length (lP)
    The size limit at which normal notions of space-time are supposed to break down.
    Planck mass (mP)
    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.
    planet
    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.
    planetary element
    an element which occurs in certain parts of planets with varying concentration
    planetary elongation
    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.
    planetary elongation
    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.
    planetary elongation event
    A point in the orbit of a celestial body with a specific planetary elongation such as 0°, 90° or 180°
    planetary nebula
    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.
    planetary nebula
    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.
    planetary precession
    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.
    planetocentric coordinate
    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.
    planetographic coordinate
    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.
    Plaskett's star (HD 47129)
    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.
    plasma
    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.
    plasmapause
    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.
    Pleione (28 Tau)
    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.
    Pluto
    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.
    Pockels cell
    An electro-optic crystal used as a reversible waveplate by applying alternately high positive and negative voltage.
    point
    A geometrical object with zero dimensions, a location and possibly a time
    Poisson distribution
    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.
    polar motion
    The irregularly varying motion of the Earth's pole of rotation with respect to the Earth's crust. (See celestial ephemeris pole.)
    Polaris (HR 424, Pole star, North Star)
    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.
    pole
    One of two points at which the Earth's axis of rotation intersects the geoid.
    Pollux (beta Gem, HR 2990)
    A K0 III star.
    Population I Cepheid (classical Cepheid, young disk Cepheid)
    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).
    Population I star (arm population, disk star)
    Young star typical of those found in galaxy spiral arms.
    Population I star (arm population, disk star)
    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.
    Population I star (arm population, disk star)
    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.
    Population I star (arm population, disk star)
    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.
    Population II Cepheid (W Virginis star)
    A bright yellow star that pulsates like a Cepheid but is older and fainter.
    Population II star (halo star)
    Old stars typical of those found in the halo of the Galaxy.
    Population II star (halo star)
    Older observed stars formed mostly from hydrogen and helium.
    Population II star (halo star)
    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.
    Population II star (halo star)
    Older stars, generally formed towards the centre of a galaxy, containing few heavier elements. The brightest of this Population are red giants.
    Population II star (halo star)
    Star that has high spatial velocity and low metallicity. This is not an observational definition.
    Population III star
    Star older than Population II stars.
    Poseidon (Jupiter VIII)
    Unofficial name for J VIII, the next outermost satellite of Jupiter.
    positron (antielectron)
    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.
    positronium
    A positron and electron bound together electrostaticaly.
    post-asymptotic branch star
    F-type supergiant with strong sulfur lines.
    potassium (K, 2S1/2 in ground state, kalium from Greek)
    soft white metal which is silvery when first cut but oxidizes rapidly in air
    PP I
    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.
    PP II
    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.
    PP III
    A thermonuclear reaction in which hydrogen nuclei are transformed into helium nuclei. Occurs once in 1000 times.
    precession
    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.
    precession
    The slow (once per twenty-six thousand years) gyration of the Earth's axis.
    precession
    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.)
    pressure
    The force exerted over a surface divided by its area.
    pressure broadening
    Line broadening caused by pressure.
    pressure ionization
    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.
    primary mirror
    The first mirror encountered by incident light in a telescope system.
    prime focus
    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.
    primordial black hole
    Small black hole hypothesized to have formed during the first 10-43 seconds of the universe, when quantum effects were very large.
    primordial nucleosynthesis (big-bang nucleosynthesis)
    Production of atomic nuclei occurring during the first three minutes after the big bang.
    primordial nucleosynthesis (big-bang nucleosynthesis)
    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.
    primordial nucleosynthesis (big-bang nucleosynthesis)
    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.
    primordial nucleosynthesis (big-bang nucleosynthesis)
    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
    primordial quark
    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.
    probable error (p.e.)
    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.
    Procyon (alpha CMi, HR 2943)
    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.
    prograde motion
    Motion in the same direction as the prevailing direction of motion.
    program star
    The star being observed or measured, as contrasted with the comparison stars.
    prominence
    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.
    prominence
    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.
    proper motion
    Apparent angular rate of motion of a star across the line of sight on the celestial sphere.
    proper motion
    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.
    proper motion
    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.
    proto-planet
    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.
    protogalaxy
    A galaxy during the early phase, before it has developed its present shape and mix of stars.
    protogalaxy
    A galaxy in the process of formation. None are observed nearby, indicating that all or most galaxies formed long ago.
    proton (p)
    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.
    proton (p)
    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.
    proton (p)
    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.
    proton (p)
    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.
    proton (p)
    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.
    proton decay
    Spontaneous disintegration of the proton, predicted by grand unified theory but never observed experimentally.
    proton-proton chain (p-p chain)
    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.
    proton-proton chain (p-p chain)
    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.
    proton-proton chain (p-p chain)
    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.
    proton-proton chain (p-p chain)
    The nuclear sequence by which the Sun and all other main-sequence stars with less than 1.5 solar masses fuse hydrogen into helium.
    proton-proton chain (p-p chain)
    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
    Proxima Centauri
    An flare star probably associated with the alpha Centauri system. It is the closest star to the Sun.
    PSR B 1257+12
    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.
    pulsar (PSR)
    A fast-spinning neutron star that emits radiation toward Earth every-time it rotates.
    pulsar (PSR)
    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.
    pulsar (PSR)
    Neutron stars that spin rapidly and have strong magnetic fields, which produce electromagnetic radiation. (See neutron star.)
    pulsating nova
    A variable star, probably not a true nova, in which the change between more and less luminous stages is extreme. (also called recurrent novae)
    Puppis A (2U 0821-42)
    A supernova remnant. It is an extended nonthermal radio source, and also a source of soft X-rays.
    pycnonuclear reaction
    Nuclear process that take place at relatively low temperatures and that are not strongly temperature-dependent.
    Q-branch
    A set of lines in the spectra of molecules corresponding to changes in vibrational energy with none in rotational energy.
    quadrature
    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.
    quantity
    A number with specific units
    quantum chromodynamics (QCD)
    Gauge theory describing the interactions of quarks through the strong color field.
    quantum electrodynamics (QED)
    Gauge theory describing the interactions between electrically charged particles through the electro-magnetic field.
    quantum field theory
    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.
    quantum gravity
    A general term used to describe attempts to quantize gravity. The elementary particle of the gravitational field is the graviton.
    quantum gravity
    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.)
    quantum gravity
    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.
    quantum mechanics (quantum theory)
    Physics theory that explains wave-particle duality, the uncertainty principle and the exclusion principle.
    quantum solid
    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.
    quark
    A fermion with fractional electric charge.
    quark
    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).
    quark
    A sub-atomic particle which is a fundamental building block of the hadrons.
    quark
    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.
    quark
    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.
    quark
    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.
    quark
    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.
    quark color
    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.
    quark color
    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.
    quark color
    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.
    quasar (QSO)
    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.
    quasar (QSO)
    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, zabszem; 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.
    quasar (QSO)
    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.
    quasar (QSO)
    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.
    quasar (QSO)
    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.
    quiescent prominence
    A relatively pacific prominence which may last for months.
    R Coronae Borealis
    Star with a large infrared excess and a rather high 7Li abundance.
    R Coronae Borealis variable
    A very luminous helium-rich, carbon-rich, hydrogen-poor eruptive variable supergiant whose light declines up to 8 magnitudes at irregular intervals.
    R star
    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.
    R zone
    Region in the solar corona in which short-lived radiofrequency variations are observed.
    R-branch
    A set of lines in the spectra of molecules corresponding to unit decreases in rotational energy.
    r-process
    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.
    r-process
    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.
    rad
    Unit of radiation, equal to 100 ergs of ionizing energy absorbed per gram of absorber.
    radial velocity
    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.
    radial velocity
    Velocity along the line of sight toward (-) or away from (+) the observer.
    radian (rad)
    A measure of angular distance; 2π radians equals 360 degrees.
    radian (rad)
    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.
    radiation era
    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.
    radiation pressure
    The transfer of momentum by electromagnetic radiation incident on a surface: prad = (4/3)σT4 / c.
    radiation temperature
    The temperature that a blackbody of similar dimensions would have that radiated the same intensity at the same frequency.
    radiative recombination
    See radiative capture.
    radiative zone
    Region in a stellar interior where conduction and radiation dominates the heat flow
    radio
    Electromagnetic radiation with the lowest energy and longest wavelength. Unlike visible light, radio waves penetrate dust and can be detected from throughout the Galaxy.
    radio astronomy
    Study of the universe at the radio wavelengths of electromagnetic energy.
    radio galaxy
    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.
    radio interferometer
    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.
    radio lobes
    Extended regions of diffuse radio emission, often dumbbell shaped, that surround a radio galaxy.
    radio loud quasar
    A quasar with detectable radio emission.
    radio recombination line
    Radio recombination lines are the result of electron transitions between high-n (n > 50) levels in an atom or ion.
    radio source
    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.
    radio star
    Star with detectable emission at radio wavelengths. They include pulsars, flare stars, some infrared stars, and some X-ray stars.
    radio telescope
    A device for gathering and amplifying radio energy typically by using a metallic dish antenna
    radio telescope
    Non-optical telescope (of various types) which, instead of focusing light received from a distant object, focuses radio signals onto a receiver-amplifier.
    radio telescope
    Sensitive radio antennae employed to detect the radio energy emitted by nebulae, galaxies, pulsars, etc.
    radioactive element
    an unstable radioactive element which has an excess or deficit of neutron relative to the stable element
    radioactive particle
    A particle which can produce harmful radioactivity directly through its decay or by interacting with other particles.
    ram pressure
    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.
    Raman effect
    In spectroscopy, the change in the wavelength of light scattered by molecules.
    Raman effect
    The change of wavelength on scattering. It arises from radiation exciting (or de-exciting) atoms or molecules from their initial states.
    Rankine
    A temperature scale with the same division as the Fahrenheit scale and the zero point at 0° absolute. 0 °R = - 470 °F.
    Rankine
    Unit which is 5/9 of the Kelvin. Rankine temperatures have the same temperature interval as those on the Fahrenheit scale.
    rare Earth (lanthanide)
    Element with atomic number between 57 and 71 in the top row of the inner-transition elements of the periodic table
    Rayleigh scattering
    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.
    Rayleigh-Jeans law
    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.
    Reaumur
    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.
    recession
    Motion (increasing distance) away.
    recombination
    The capture of an electron by a positive ion.
    recombination radiation
    See radiative capture.
    recoverable spacecraft
    a spacecraft which returns to Earth intact
    red dwarf
    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.
    red giant
    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.
    red giant
    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.
    red giant
    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.
    Red Spot
    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.
    red supergiant
    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.
    redshift
    The shift of spectral lines toward longer wavelengths in the spectrum of a receding source of radiation.
    reduced proper motion
    The observed proper motion of a star (in seconds of arc per year) reduced to absolute proper motion (in kilometers per second).
    reflection nebula
    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).
    reflector
    A device for gathering and amplifying light or other energy by means of a mirror.
    reflector
    Telescope that uses mirrors to magnify and focus an image onto an eyepiece. (reflector)
    refraction
    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.
    refractor
    A device for gathering and amplifying light by means of a lens.
    refractor
    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.
    refractor
    Telescope that uses lenses to magnify and focus an image onto an eyepiece. (refractor)
    regolith
    The layer of fragmentary debris produced by meteoritic impact on the surface of the Moon or a planet.
    regression of the nodes
    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.
    Regulus (alpha Leo)
    A visual triple B8 V star.
    resonance line
    The longest-wavelength line arising from the ground state.
    rest-mass energy
    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.
    retardation point
    the point in the flight of a meteoroid through the atmosphere where it loses its cosmic velocity and falls freely due to gravity
    retrograde motion
    Apparent motion of a planet in a direction opposite to its normal progress across the sky produced by the orbital motion of the earth.
    retrograde motion
    In a backwards direction; in astronomy this means in a direction corresponding to east-to-west.
    Rhea
    Sixth satellite of Saturn
    rich cluster
    Galaxy cluster with 100 or more galaxies within a volume comparable to that of a loose group. Scale of cluster, about 1 Mpc.
    Rigel (beta Ori, HR 1713)
    A blue supergiant. It is a multiple star. The seventh brightest star in the night sky.
    right ascension (α)
    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.
    ring
    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.
    ring galaxy
    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.
    ring gap
    A gap between the rings of Saturn.
    Ring Nebula (M 57)
    A famous planetary nebula (M57, NGC 6720) in the constellation Lyra.
    rise time
    The time required for the vehicle to achieve its optimum height (in rocket or balloon astronomy).
    Ritchey-Chrétien
    A class of reflecting telescope with a hyperbolic primary and secondary.
    Ritchey-Chrétien
    A system of two mirrors, aspherized to give an image at the secondary (Cassegrain) focus free from spherical aberration and coma.
    Rocard scattering
    Linearly anisotropic scattering.
    root mean square (rms)
    The square root of the mean square value of a set of numbers (see also random walk).
    ROSAT
    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.
    rotating reference frame
    A frame of reference which rotates with the same period as the system.
    rotation
    Of a single body in space: spinning on an axis. Of a planetary system, rotation is generally planar in relation to the parent star.
    Royal Greenwich Observatory
    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.
    Royal Society
    English organization founded in the seventeenth century and dedicated to the advancement of science.
    RR Lyr
    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.
    RR Lyrae star (cluster variable, cluster variable)
    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.
    RR Lyrae star (cluster variable, cluster variable)
    Periodic variable with period less than one day, and of spectral types A to early F.
    RR Lyrae star (cluster variable, cluster variable)
    Type of short-period variable stars. Spectrally classified as A to F giants. They were once called cluster-cepheids.
    RRab star
    A sub-class of Bailey type RR Lyrae variables, having asymmetric light curves of large amplitude.
    RRab star
    RR Lyr star characterised by a sharp rise to maximum then a slow fall to minimum. Usually combined with RRb star.
    RRc star
    RR Lyr star characterised by equally long rise and fall time. This type has the smallest amplitude
    RS CVn star
    Close binary, which show the HK lines in emission.
    runaway star
    Early type star (O and early B) outside the Galactic plane, which reached large distances (from the Galactic plane) because of their high velocities.
    runaway star
    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.
    RV Tau variable
    Periodic variable with periods 60-100 days, and of spectral types G and K.
    RV Tauri star
    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.
    RW Aurigae
    A T Tauri star with a strong ultraviolet excess.
    rydberg
    Unit of energy (R = h bar3c / me4) equal to 13.5978 eV (the ionization potential of hydrogen). Rydberg correction has definition
    S Andromedae
    A supernova seen in 1885 in the Andromeda galaxy.
    S Doradus
    A supergiant eclipsing binary (an Eta Carinae-type object) in the Large Magellanic Cloud.
    S star
    Late type giant (K5 to M) showing distinct bands of ZrO.
    S star
    Late type giants (K5 to M) showing distinct bands of ZrO. Jaschek, C., Jaschek, M. 1995 The Behavior of Chemical Elements in Stars, Cambridge University Press
    S star
    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.)
    S star
    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.) Hopkins, J. 1976 Glossary of Astronomy and Astrophysics, University of Chicago Press
    s-electron
    An orbital electron whose l quantum number is zero.
    s-process
    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.
    S0 galaxy
    Galaxy with nuclei surrounded by disklike structure without arms.
    Sa spiral (early-type spiral)
    In Hubble's classification, a spiral with a large nuclear bulge and closely coiled arms.
    Sa spiral (early-type spiral)
    Spiral galaxy with arms tightly wound around the nucleus.
    Sagittarius A
    A radio source (the galactic center). (Sgr A West is a thermal source; Sgr A East is a nonthermal source.)
    Sagittarius A
    The very center of the Milky Way, Sagittarius A* is a strong source of radio waves and probably a massive black hole.
    Sagittarius arm
    One of the spiral arms of the Milky Way, lying between us and the center of the Galaxy in the direction of Sagittarius.
    Sagittarius B2
    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.
    satellite
    a physical object gravitationaly bound and in orbit around a larger physical object
    satellite elongation
    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.
    satellite galaxy
    A galaxy that orbits a larger one. The Milky Way has at least ten satellite galaxies.
    Saturn
    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.
    Sb spiral
    Spiral galaxy with arms spread out from the nucleus.
    SBa barred spiral
    Barred spiral galaxy with arms tightly wound around the nucleus
    SBb barred spiral
    Barred spiral galaxy with arms wound around the nucleus
    SBc barred spiral
    Barred spiral galaxy with arms widely spread out from the nucleus
    Sc spiral
    Spiral galaxy with arms widely spread out from the nucleus.
    SC star
    Star which appears to be intermediate in type between S stars and carbon stars (C/O ratio near unity).
    scattering
    Collision between a photon and a particle
    scattering
    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.
    Schmidt
    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.
    Schmidt
    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.
    Schmidt
    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.
    Schmidt plate
    Photographic plate obtained with a Schmidt telescope, which is a type of telescope with a particularly large field of view.
    scholastics
    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.
    Schwarzschild black hole
    A nonrotating, spherically symmetric black hole derived from Karl Schwarzchild's 1916 exact solution to Einstein's vacuum field equations.
    Schwarzschild singularity
    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.
    science
    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.
    scintillation counter
    A device used with a photomultiplier tube to detect or count charged particles or gamma rays.
    scintillator
    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.
    Sco-Cen association
    An association of very young stars. The most luminous member is a B star of Mv = - 4.9.
    Scorpius OB1
    An extremely young association of OB stars.
    Scorpius X-1
    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)
    Sculptor Dwarf
    A dwarf elliptical galaxy that orbits the Milky Way (Mv = - 11.28 mag), in the Local Group.
    Sculptor Group
    The nearest group of galaxies to the Local Group.
    sdB
    Subdwarf B star with very broad and shallow Balmer lines; fewer lines of the Balmer series are visible than for normal dwarfs.
    sdO
    Subdwarf O star showing few very broad and shallow Balmer lines and a very strong He II 4686 line.
    second (s)
    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.
    second (s)
    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
    second law of thermodynamics
    A physical law formulated in the nineteenth century and stating that any isolated system becomes more disordered in time. (See entropy.)
    second law of thermodynamics
    Law stating that total entropy always increases.
    second order phase transition
    A phase transition where one phase evolves into the other as the temperature changes, so the two phases never coexist.
    secondary cosmic ray
    Atomic fragment - mainly muons - produced by collisions between primary cosmic rays and the molecules in Earth's atmosphere.
    secondary mirror
    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.
    secular parallax
    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.
    segmented mirror
    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.
    segmented mirror telescope
    a reflecting telescope whose mirror is composed of multiple segments
    selective absorption (reddening)
    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.
    selective absorption (reddening)
    The reddening of starlight in passing through fine particles of interstellar dust.
    selenocentric coordinate
    With reference to the center of the Moon.
    semi-forbidden line
    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.
    semimajor axis (a)
    An orbital element representing half the length of the major axis of an ellipse.
    semiminor axis
    Half the length of the minor axis of an ellipse; a standard element used to describe an elliptical orbit.
    semiregular variable
    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.
    set of lines
    A set of absorption or emission lines in the spectra of a celestial object
    SETI
    The Search for Extraterrestrial Intelligence, by using radiotelescopes to listen for signals transmitted by intelligent alien beings.
    Sextans Dwarf
    A dwarf companion to the Milky Way. Discovered by computer.
    Seyfert galaxy
    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.
    Seyfert galaxy
    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.
    Seyfert's Sextet
    A compact group of galaxies surrounding NGC 6027. It has both spiral and irregular members.
    shadow matter
    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.
    Shane-Wirtanen catalog
    A catalog of all galaxies brighter than seventeenth magnitude (a measure of brightness). There are about a million galaxies in the Shane-Wirtanen catalogue.
    Shapley-Ames catalog
    A catalog of galaxies brighter than thirteenth magnitude, completed in 1932. There are about 1200 galaxies in this catalogue.
    shell star
    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.
    shell star
    A star in which two different types of line profiles co-exist.
    SI unit (Syteme Internationale)
    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.
    sidereal day
    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).
    sidereal year
    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.
    siderophile element
    element that tend to concentrate in metallic iron
    siemens (S, reciprocal ohm)
    The SI unit of electrical conduction.
    sigma
    Short-lived baryon.
    sigma
    Short-lived baryon.
    silicon (Si, 3P0 in ground state)
    ultrapure crystals of silicon have a blue-grey metallic sheen
    silicon burning
    The end of the line for a high-mass star, silicon burning creates iron and other elements of similar mass and presages a supernova.
    single line spectroscopic binary
    a spectroscopic binary in which periodic Doppler shift is detected in only one component of the binary star
    singularity
    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.)
    singularity
    A point in space-time at which the space-time curvature and other physical quantities become infinite and the laws of physics break down.
    singularity
    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.
    singularity
    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.
    singularity
    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.
    singularity
    Location where the fabric of space or spacetime suffers a devastating rupture.
    Sirius (alpha CMa, Dog Star, HR 2491)
    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.
    Sirius B
    Companion of Sirius, a white dwarf.
    slow nova
    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.
    Small Magellanic Cloud (SMC)
    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.
    SMC X-1 (2U 0115-73)
    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.
    smoke trail
    condensation of vaporized rock removed from the surface of a meteoroid by ablation
    Snell's law
    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)
    sodium (Na, 2S1/2 in ground state, natrium from Latin)
    soft, silvery-white metal which oxidizes rapidly when cut
    solar apex
    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).
    solar atmosphere
    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.
    solar burst
    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.
    solar cycle
    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.
    solar eclipse
    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.)
    solar flare
    Sudden and short-lived brightening of a region of the solar chromosphere.
    solar mass
    Unit of mass equal to the amount of mass in the Sun, and the unit in which stellar and galactic masses are expressed.
    solar motion
    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.
    solar neutral region
    A region where the magnetic field strength approaches zero. Generally, neutral regions occur between regions of opposite polarity.
    solar parallax
    Angle subtended by the equatorial radius of the Earth at a distance of 1 AU.
    solar radius
    A unit of length based on radius of Sun
    solar rotation
    Differential rotation, the equatorial rotation taking less time than the polar by up to 9.4 Earth-days.
    solar system
    A star with planets
    solar velocity
    Velocity of the Sun (19.4 km sin the direction lII = 51°, bII = 23°) with respect to the local standard of rest.
    solar wind
    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.
    solid
    a high density collection of particle which do not move relative to one another (except for thermal oscillations)
    solid angle
    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.
    solstice
    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).
    Sombrero galaxy (M 104, NGC 4594)
    A spiral galaxy in the constellation Virgo. It was the first galaxy whose rotation was detected.
    sound wave in gas
    A longitudinal wave propagated through a gas as a pattern of alternating high and low pressure.
    South Atlantic Anomaly (SAA)
    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.
    south galactic pole (SGP)
    A point in the constellation Sculptor toward which our line of sight is perpendicular to and below the Galactic disk.
    south pole
    The astronomical coordinate which coincides with the southern intersection of Earth's axis with the geoid.
    space motion
    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).
    space velocity
    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.
    spallation
    The process in which an incoming beam of particles or energy collides with a substance, reacts with it, and knocks off pieces of it.
    spark chamber
    A means of detecting high energy particles by the trail of ionizations left as they pass through a chamber containing many charge plates.
    sparticle
    Hypothetical particle which is predicted by some Grand Unified Theories.
    special relativity
    Relativity theory that explains Lorentz contraction and time dilation for observers in relative motion.
    specific volume unit (m3·kg-1)
    cubic meter per kilogram
    spectral feature
    A feature in the spectra of a celestial object which occurs at a specific wavelength or range of wavelengths
    spectral line
    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.
    spectral line
    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.
    spectral line
    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.).
    spectral line
    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.
    spectral series line
    A spectral line belonging to a series of lines of a given atom arising from transitions with a common lower energy level.
    spectrograph (spectrometer)
    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.
    spectrograph (spectrometer)
    A spectroscope fitted with a device such as a photoelectric cell for measuring the spectra observed with it.
    spectrograph (spectrometer)
    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.)
    spectrograph dispersing element
    The part of a spectrograph which disperses the light into component colors
    spectroheliograph
    Device with which spectra of the various regions of the Sun are obtained and photographed.
    spectroscopic binary
    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).
    spectrum variable
    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.
    sphere
    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.
    spheres
    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.
    spherical collapse
    Initial stage in the collapse of a star, followed by gravitational collapse and finally singularity.
    Spica (alpha Vir, HR 5056)
    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.
    spicule (mottle)
    A short-lived, narrow jet of gas spouting out of the solar chromosphere. Spicules tend to cluster at the edges of supergranulation cells.
    spin
    A quantum property of all particles which denotes the intrinsic angular momentum of the particle.
    spin
    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.
    spin
    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.
    spin
    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.
    spin-flip collision
    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.
    spiral density wave
    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.
    spiral galaxy
    A galaxy with a prominent nuclear bulge and luminous spiral arms of gas, dust, and young stars that wind out from the nucleus.
    spiritualism
    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.
    spontaneous emission
    Radiation emitted by an isolated body.
    spontaneous symmetry breaking
    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).
    spontaneous symmetry breaking
    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.
    Sporer's law of zones
    The equatorward drift of average sunspot latitudes.
    Sputnik 1
    First artificial Earth satellite, launched by the Soviet Union on 4 October 1957. (lit.: companion)
    SS Cygni star
    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.
    SSPM
    Solid State Photomultiplier.
    standard deviation (σ)
    The root mean square deviation from the arithmetic mean.
    standard error (s.e.)
    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.
    standing wave (stationary wave)
    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.
    Stanford Linear Accelerator Center (SLAC)
    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.
    star
    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.
    star
    a spinning ball of hot gases held together by gravity
    star cluster
    Gravitationally bound aggregations of stars, smaller and less massive than galaxies.
    star merger
    The formation of a new star from the gradual merging of two stars which spiral into each other within a common envelope of gas.
    star system
    A few stars that orbit each other.
    starburst galaxy
    Any galaxy in which an anomalously large rate of star formation is taking place.
    Stark effect
    Line broadening due to the influence of electric fields.
    STARLINK (STARLINK)
    A software environment and suite of programs for astronomical data analysis developed in the UK and supported by the Rutherford-Appleton Labs.
    stat-coulomb
    Unit of charge in the cgs electrostatic system.
    static universe
    A universe whose radius of curvature is constant and independent of time, as in the Einstein universe.
    statistical distribution
    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.
    statistical error
    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.
    Steady State Theory
    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.
    Steady State Theory
    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.
    Stefan's law
    The flux of radiation from a blackbody is proportional to the fourth power of its absolute temperature: L = 4πR2σT4.
    Stefan-Boltzmann constant (σ)
    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.
    Stefan-Boltzmann constant (σ)
    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.
    stellar nucleosynthesis
    The production of heavy nuclei from the fusion of lighter ones within a star. The Sun presently converts hydrogen into helium.
    stellarator
    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.
    Stephan's Quintet
    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.
    steradian (sr, sr, Ω0)
    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.
    stimulated emission (induced emission)
    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.
    strange
    A flavor of quark.
    strangeness
    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.
    strangeness
    A quantum number associated with the strange quark. Strangeness is conserved by the strong nuclear force.
    strangeness
    A quantum number used in quark theory.
    stratosphere
    The region of Earth's atmosphere immediately above the troposphere. The temperature increases with altitude.
    string theory
    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".
    string theory
    Fundamental one-dimensional object that is the essential ingredient in string theory.
    string theory
    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.
    string theory
    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.)
    string theory
    Theory that subatomic particles actually have extension along one axis, and that their properties are determined by the arrangement and vibration of the strings.
    string theory
    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.
    strong force (nuclear force)
    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.
    strong force (nuclear force)
    One of the four fundamental forces of nature. It governs the interaction between particles in atomic nuclei.
    strong force (nuclear force)
    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.
    strong force (nuclear force)
    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.
    strong force (nuclear force)
    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.
    strong force (nuclear force)
    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.
    strong force symmetry
    Gauge symmetry underlying the strong force, associated with invariance of a physical system under shifts in the color charges of quarks.
    SU(2)
    The symmetry of the weak nuclear interaction.
    SU(2) x U(1)
    The symmetry of the unified electroweak interaction.
    SU(3)
    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.
    SU(3)
    The symmetry corresponding to quark theory and the strong nuclear interaction.
    SU(5)
    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).
    SU(5)
    Symmetrical Unitary of Order 5: The simplest type of grand unified theory, proposed in the 1970s. (See grand unified theories.)
    subdwarf (sd)
    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.
    subdwarf (sd)
    Late-type object whose observed color and absolute magnitude place it below the Main Sequence.
    subdwarf (sd)
    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.
    subgiant
    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.
    subgiant CH star
    Hot Ba star
    sulfur (S, 3P2 in ground state)
    the α-S8 orthorhombic form of sulfur is yellow
    Sun
    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.
    Sun Kelvin timescale
    The time it takes the Sun to contract gravitationally from infinite radius down to its present radius by radiating its thermal energy.
    Sun nuclear time scale
    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.
    sunrise
    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.
    sunset
    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.
    sunspot
    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. Typical diameter, 109 cm.
    sunspot
    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.
    sunspot radiation
    Intense, variable, circularly polarized radio waves in a noise storm.
    Sunyaev-Zel'dovich process
    Compton scattering between the photons of the cosmic microwave background radiation and electrons in galaxy clusters.
    super-metal-rich star
    Used in reference to stars, or stellar populations, which are richer in metals than the Hyades.
    supercluster
    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.
    superconducting super collider
    A proposed accelerator of great size and high energy.
    Supergalactic Plane
    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.
    supergiant
    An extremely luminous star of large diameter and low density. No supergiants are near enough to establish a trigonometric parallax.
    supergranulation cell
    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.
    supergravity
    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.
    supergravity
    Class of point-particle theories combining general relativity and supersymmetry
    superior planet
    Planets farther from the Sun than the Earth is (i.e., Mars to Pluto).
    supermultiplet
    A multiplet of multiplets.
    supernova
    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.
    supernova produced element
    element heavier than iron produced within the core of an exploding supernova
    supernova produced radioactive element
    a radioactive element produced within the core of an exploding supernova
    supernova remnant (SNR)
    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.
    supernova remnant (SNR)
    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.
    superpartner
    Particle whose spins differ by 1/2 unit and that are paired by supersymmetry.
    superstring theory
    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.
    superstring theory
    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.
    superstring theory
    String theory that incorporates supersymmetry.
    supersymmetric quantum field theory
    Quantum field theory incorporating supersymmetry.
    supersymmetric standard model
    Generalization of the standard model of particle physics to incorporate supersymmetry. Entails a doubling of the known elementary particle species.
    supersymmetry
    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.
    supersymmetry
    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).
    supersymmetry
    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.
    supersymmetry
    A symmetry that relates the fermions (fractional spin particles) to the bosons (elementary particles with integral spin).
    supersymmetry
    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.
    superunified theory
    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.
    superunified theory
    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.
    supra-thermal proton bremsstrahlung
    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.
    Swan band
    A C2 band which passes through a minimum between spectral types R4 and R6 and increase again toward N6.
    SX Phoenicis
    A dwarf Cepheid (spectral type A) with the shortest known period (1h19m).
    symbiotic star (combination variable)
    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.
    symbiotic star (combination variable)
    Object exhibiting a spectrum corresponding to a low-temperature star (generally a giant) plus emission lines corresponding to a hot plasma.
    symmetry
    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.
    symmetry
    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.
    symmetry
    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.
    symmetry breaking (asymmetry)
    A reduction in the amount of symmetry a system appears to have, usually associated with a phase transition.
    symmetry breaking (asymmetry)
    A set of mathematical transformations that represent a symmetry.
    symmetry breaking (asymmetry)
    A violation of symmetry.
    symmetry breaking (asymmetry)
    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.
    symmetry breaking (asymmetry)
    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.
    synchronous rotation
    Rotation whose period is equal to the orbital period.
    synodic month
    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).
    synthetic element
    element which cannot be found as a mineral deposit
    T association
    Association containing many T Tauri stars. About 20 are known.
    T Pyxidis (T Pyx)
    a recurrent nova
    T Tauri star (nebular variable)
    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.
    T Tauri star (nebular variable)
    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)
    T Tauri star (nebular variable)
    Late type irregular variables associated with bright or dark nebulosity. The spectrum exhibits emission in both CaII and H lines.
    T Tauri star (nebular variable)
    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.
    tachyon
    Particle whose mass (squared) is negative; its presence in a theory generally yields inconsistencies.
    tangential velocity (transverse velocity)
    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.
    TDRSS
    Transmission Data and Relay Satellite System.
    telescope
    A device for gathering and amplifying light or X rays, gamma rays, or other forms of energy.
    telluric lines
    Spectral lines or bands that originate from absorption by gases such as O2, H2O, or CO2 in the Earth's atmosphere.
    temperature
    A measure of the average kinetic energy of the particles of a system.
    temperature unit
    A temperature unit is a measure of the average kinetic energy per degree of freedom of the constituent molecules.
    terrestrial latitude
    Angular distance on the Earth measured north or south of the equator along the meridian of a geographic location.
    terrestrial latitude
    On Earth, distance north or south on the equator along a line connecting the poles.
    terrestrial longitude
    Angular distance measured along the Earth's equator from the Greenwich meridian to the meridian of a geographic location.
    terrestrial longitude
    On Earth, distance east or west of Greenwich, England, measured along lines drawn parallel to the equator.
    terrestrial planet
    A planet having a compact rocky surface like Earth
    tertiary mirror
    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.
    tesla (T)
    The derived SI unit of magnetic flux density.
    Tethys
    Fourth satellite of Saturn
    tevatron
    A particle accelerator capable of attaining an energy of 1 TeV.
    Themis
    A satellite of Saturn discovered by Pickering in 1900, but since lost.
    theoretical celestial body
    natural object postulated by theory but not yet observed
    theory
    A rationally coherent account of a wider range of phenomena than is customarily accounted for by a hypothesis.
    theory of everything (TOE)
    Theory of Everything. A quantum-mechanical theory that encompasses all forces and all matter.
    thermal bremsstrahlung
    A mode of X-ray production by very energetic electrons accelerated in the field of a positive ion.
    thermal energy
    Energy associated with the motions of the molecules, atoms, or ions in a substance.
    thermodynamics (statistical mechanics)
    Laws developed in the nineteenth century to describe aspects of heat, work, energy, entropy, and their mutual evolution in a physical system.
    thermodynamics (statistical mechanics)
    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.
    thermodynamics (statistical mechanics)
    The study of the behavior of heat (and, by implication, other forms of energy) in changing systems.
    thermohaline convection
    A type of hydrodynamic instability.
    theta pinch
    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.
    Theta2 Orionis (2U 0525-06)
    A spectroscopic binary tentatively identified with 2U 0525-06.
    thick disk
    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.
    thin disk
    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.
    third law of thermodynamics (Nernst theorem)
    All substances have zero entropy at 0 K.
    Thomson scattering
    The limit of Compton scattering at low energies.
    thought experiment
    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.
    three body recombination
    Capture of an electron by an ion with the binding energy absorbed by a third free electron
    three-phase
    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.
    threshold energy
    Difference between the energy at the first excited level and that of the ground state.
    tidal dwarf galaxy (TDG)
    A self-gravitating entity of dwarf-galaxy mass, built from tidal material expelled during interactions (e.g. AM0547-244, AM1054-325 and AM1353-272)
    time
    A dimension distinguishing past, present, and future. In relativity, time is portrayed as a geometrical dimension, analogous to the dimensions of space.
    time dilation
    The flow of time slows down for an observer in motion.
    tired light
    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.
    Titan (S VI)
    Seventh (known) moon out from Saturn, its largest and brightest. It is possibly also the largest satellite in the Solar System.
    Titania
    Fourth satellite of Uranus.
    titanium (Ti, 3F2 in ground state)
    hard, lustrous, silvery metal which resists corrosion due to an oxide layer on its surface
    Tokamak
    A type of "magnetic bottle" used in experiments on controlled nuclear fusion. (the name is a Russian acronym)
    top
    The sixth flavor of quark.
    top-down scenario
    A scenario of galaxy formation in which large structures form first and then fragment to become galaxies.
    topocentric coordinate
    With reference to, or pertaining to, a point on the surface of the Earth, usually with reference to a coordinate system.
    Torino scale 10 impact
    impact capable of causing a global climatic catastrophe
    Torino scale 8 impact
    collision capable of causing localized destruction
    Torino scale 9 impact
    collision capable of causing regional devastation
    torr
    A unit of pressure.
    torus
    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.
    torus
    The two-dimensional surface of a doughnut.
    total lunar eclipse
    An eclipse in which the Moon passes through the shadow cast by the Earth, the Moon passing completely through the Earth's umbra.
    total solar eclipse
    A solar eclipse in which the solar disk is completely covered at maximum eclipse.
    toy theory
    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.
    transient X-ray source
    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.
    transit
    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.
    transit telescope
    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.
    transuranium element
    A synthetic element with atomic number greater than 92 (uranium)
    transverse wave
    Wave vibrating at right angles to the direction of propagation.
    Trifid Nebula (M 20, NGC 6514)
    An emission nebula in Sagittarius.
    triple star
    A star system having three stars that revolve around one another.
    tritium (3H, hydrogen 3)
    A short-lived isotope of hydrogen. Tritium does not exist naturally on Earth.
    Triton
    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 (T)
    The nucleus of the tritium atom.
    Trojan asteroid
    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.
    tropical year
    The interval of time between two successive vernal equinoxes.
    tropical year
    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.
    tropopause
    Upper boundary of the troposphere, where the temperature gradient goes to zero.
    troposphere
    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.
    Tully-Fisher relation
    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.
    twin-exhaust model
    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.
    Twistor Theory
    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.
    Tycho's star
    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)
    type I string theory
    One of the five superstring theories; involves both open and closed strings.
    type I supernova
    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.
    type I supernova
    A supernova which may be produced by the thermonuclear detonation of a highly degenerate core produced by mass transfer from a companion star.
    type II supernova
    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.
    type IIA string theory
    One of the five superstring theories; involves closed strings with left-right symmetric vibrational patterns.
    type IIB string theory
    One of the five superstring theories; involves closed strings with left-right asymmetric vibrational patterns.
    type III supernova
    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.
    type III supernova
    Supernova similar to Type II
    U Geminorum star
    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.
    U velocity
    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.
    U(1)
    The symmetry of the electromagnetic field.
    ultraviolet (UV)
    Electromagnetic radiation "beyond the violet" with wavelengths in the approximate range 100-4000 Å.
    ultraviolet (UV)
    Electromagnetic radiation of a wavelength slightly shorter than that of visible light.
    ultraviolet (UV)
    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.
    ultraviolet Ga star
    Bp star which exhibits a strong 1414 line of Ga II in the ultraviolet spectrum .
    ultraviolet star
    Very hot prewhite-dwarf star; usually the hot central star of a planetary nebula which is contracting toward the white-dwarf state.
    ultraviolet-bright star
    Star that is brighter than horizontal-branch stars and bluer than giant-branch stars.
    Umbriel
    A satellite of Uranus.
    Umklapp scattering
    The contribution to scattering caused when the exchange of momentum crosses the boundary of a Brillouin zone.
    uncertainty principle (Heisenberg uncertainty principle)
    The principle that the fundamental uncertainty in a variable times that in its canonical conjugate is of the order of Planck's constant.
    unified theory
    Any theory that describes all four forces and all of matter within a single, all-encompassing framework.
    unified theory
    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.
    uniformitarianism
    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.
    unitary transformation (U)
    A transformation whose reciprocal is equal to its Hermitian conjugate.
    Universe
    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.
    unmanned spacecraft
    an artificial satellite without a crew
    up
    A flavor of quark
    upper chromosphere
    The upper part of the chromosphere and consists of hot, ionized hydrogen. It has an emission spectrum (see flash spectrum).
    upper culmination (culmination above pole)
    Passage of a celestial object across the observer's meridian. The crossing closer to the observer's zenith.
    Uranus
    Seventh planet from the Sun. Has retrograde rotation.
    Urca process
    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.
    Ursa Minor Dwarf
    An intrinsically faint (Mv ≈ - 9) dwarf elliptical galaxy, a member of the Local Group.
    UV Ceti (Luyten 726-8 B)
    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.
    UV Ceti star
    Late-type dwarfs with spectra showing hydrogen emission lines.
    UVES
    Ultraviolet Echelle Spectrograph.
    V velocity
    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.
    Van Allen belt
    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.
    Van Allen belt
    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.
    van der Waals force
    The relatively weak attractive force operative between neutral atoms and molecules.
    vanadium (V, 4F3/2 in ground state)
    shiny, silvery metal, soft when pure. Resists corrosion due to a protective film of oxide
    variable
    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.
    variable
    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.
    variable
    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).
    variable
    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)
    variable-mass theory
    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.
    vector boson (IVB)
    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.
    vector boson (IVB)
    Also called the intermediate vector boson.
    vector boson (IVB)
    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.
    vector translation
    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.
    Vega (alpha Lyr, HR 7001)
    The fifth brightest star in the night sky.
    Vela pulsar (PSR 0833-45, Vela X)
    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.
    Vela pulsar (PSR 0833-45, Vela X)
    A pulsar about 400-500 pc distant, probably associated with the Vela supernova remnant. Period 0.0892 seconds.
    Vela satellite
    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.).
    Vela supernova remnant
    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.
    velocity
    The speed and the direction of an object's motion.
    velocity unit (m·s-1)
    meter per second
    velocity-of-light radius
    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)
    Venus
    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.
    vernal equinox (First Point of Aries, spring equinox)
    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°.
    vernal equinox (First Point of Aries, spring equinox)
    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°.
    Very Large Array (VLA)
    A radio telescope distributed along three 13-mile-long arms of a Y-shaped track.
    Vesta
    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.
    vibrational energy
    Motion of the pair of nuclei in a diatomic molecule along the direction of the internuclear axis (cf. rotational energy).
    vidicon
    General name for the class of vacuum tube imaging devices which employ a scanning electron beam to read out the image.
    violent galaxy (eruptive galaxy, exploding galaxy)
    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.
    Virgo cluster
    An irregular cluster including the giant elliptical M87 (the galaxy of greatest known mass).
    Virgo infall
    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.
    virial theorem
    For a bound gravitational system the long-term average of the kinetic energy is one-half of the potential energy.
    virtual pair
    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.
    virtual particle
    A particle that exists for an extremely short time in an intermediate stage of a reaction or transition.
    virtual particle
    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.
    virtual particle
    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.
    virtual particle
    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.
    virtual phase
    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.
    visibility function
    The Fourier transformation of a distant radio source, normalized to its value at small antenna spacings.
    visual binary
    A binary star which can be resolved into two components with current telescopes.
    volume unit (m3)
    cubic meter
    VV Cep star
    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.
    VV Cephei star
    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.
    W particle
    Massive boson thought to have been abundant in the early universe, when the unified electroweak force was manifest.
    W particle
    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.
    W particle
    See Intermediate Vector Boson.
    W Ursae Majoris star
    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.
    W velocity
    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.
    W3
    A dense cloud of gas in the Perseus arm.
    W44
    A radio source. It is a supernova remnant less than 0°.5 from the galactic plane.
    W49
    A radio source (a giant H II region). It is the most powerful thermal radio source known in our Galaxy.
    W51
    A radio source, a supernova remnant. PSR 1919+14 lies within its radio contours.
    watt (W)
    Unit of power.
    wave
    A propagating pattern of disturbance.
    wave
    Propagation of energy by means of coherent vibration.
    wave number unit (m-1)
    reciprocal meter
    wave-particle duality
    Basic feature of quantum mechanics that objects manifest both wavelike and particle-like properties.
    wavefront modifier
    A device which modifies the wavefront shape, polarization or wavelength
    wavelength shift
    The alteration in frequency of electromagnetic radiation
    weak G-band star
    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.
    weak gauge symmetry
    Gauge symmetry underlying the weak force.
    weak line star
    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.
    weakly interacting massive particle (WIMP)
    A generic term for a class of hypothetical particle which may form the missing mass. A form of non-baryonic cold dark matter.
    weakly interacting massive particle (WIMP)
    Some astronomers believe that these exotic subatomic particles make up most of the mass of the universe.
    weber (Wb)
    The derived SI unit of magnetic flux.
    week
    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.)
    Werner line
    Spectral line of molecular hydrogen in the ultraviolet, in the same general region as the Lyman lines.
    whistlers
    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).
    white dwarf (D, wd)
    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.
    white dwarf (D, wd)
    Faint star lying five or more magnitudes below the main sequence.
    white dwarf (D, wd)
    Faint very-compact stars at the end of their life. Also used as convenient photometric and flux standards.
    white dwarf (D, wd)
    Final stage of a star, at which the nuclear energy is exhausted. Cool, and becoming a dead, black body, it is nevertheless extremely dense.
    white dwarf (D, wd)
    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.
    white giant
    A giant star of spectral type A. Some RR Lyrae stars are white giants.
    white hole
    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.
    white supergiant
    A supergiant star with a spectral type of A. White supergiants are rare; the nearest is Deneb, which lies 1500 light-years away.
    Wien's law
    The wavelength at which a blackbody emits the greatest amount of radiation is inversely proportional to its absolute temperature.
    Wolf 359
    A nearby flare star.
    Wolf-Lundmark-Melotte (WLM)
    A dwarf E5 elliptical galaxy, sometimes considered a member of the Local Group.
    Wolf-Rayet
    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
    Wolf-Rayet
    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.)
    work function (W)
    The amount of energy needed to release an electron from the attraction of positive ions in a metal. It is different for different metals.
    wormhole
    A bridge to another universe created by a black hole.
    wormhole
    A tube-like region of space connecting one region of the universe to another.
    wormhole
    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.
    WZ Cassiopeiae
    A lithium star, the most super-rich carbon star known.
    WZ Sagittae
    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.
    X-process
    The unknown nucleosynthetic process that Burbidge, Burbidge, Fowler, and Hoyle said had formed the light nuclei deuterium, lithium, beryllium, and boron.
    X-ray
    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.
    X-ray
    Photon of wavelength between about 0.1 Å and 100 Å - more energetic than ultraviolet, but less energetic than a γ-ray.
    X-ray astronomy
    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.
    X-ray pulsar
    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.
    X-ray source
    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.
    XMM
    X-ray Multi-mirror Mission
    year
    A period of time based on the revolution of the Earth around the Sun.
    yellow giant
    A giant star with a spectral type of G. The nearest and brightest yellow giants are the two composing the double star Capella.
    yellow supergiant
    A supergiant star with a spectral type of G.
    ylem
    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.
    ylem
    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.
    young thin disk
    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.
    YY Orionis
    An extremely young star (younger than T Tauri) in the Orion Nebula.
    YY Orionis star
    Very young, late-type, low-mass stars in the gravitationally contracting stage in which the star is still accreting matter from the protostellar cloud.
    Z Camelopardalis (Z Cam)
    is a dwarf nova with standstills in the light curve
    Z Camelopardalis star
    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.
    Z particle
    Massive boson thought to have been abundant in the early universe, when the unified electroweak force was manifest.
    Z particle
    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.
    z pinch
    A diffuse toroidal pinch in which the magnetic field runs around the plasma column.
    Zeeman effect
    Line broadening due to the influence of magnetic fields.
    zenith
    The point in the sky directly overhead.
    zenith
    The point on the celestial sphere directly above the observer's head - i.e., opposite to the direction of gravity (nadir).
    zenith distance
    Angular distance on the celestial sphere measured along the great circle from the zenith to the celestial object. Zenith distance is 90° minus altitude.
    zero-point energy
    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.
    zero-point pressure
    The pressure contributed by degenerate electrons, which do not come to rest even at absolute zero.
    zeroth law of thermodynamics (law of thermal equilibrium)
    The temperature of a body in equilibrium is the same at all points.
    zeta Aurigae star
    Binary star with a K supergiant primary and a main-sequence secondary.
    Zeta Ophiuchi
    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.
    Zeta Pup
    The brightest Of star known, embedded in the Gum Nebula. It has an envelope which is rapidly accelerating outward.
    Zodiac (ecliptic region)
    A band about 8° wide on the celestial sphere, centered on the ecliptic.
    zodiacal constellation
    a constellation within the Zodiac region, each is a sign of the zodiac except for Ophiuchus
    Zodiacal Light
    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.
    Zone of Avoidance
    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.