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galaxy
subject			fact

galaxy	has number of stars 106 to 1012
	has catalog galaxy catalog
	has definition Vast system of celestial objects, typically consisting of between 106 and 1012 stars, plus interstellar gas and dust. There are three basic types: spiral (further subdivided into normal spirals and spirals with a "bar" at the centre, and yet further subdivided according to the "openness" of the spiral arms), elliptical (subdivided according to ellipticity) and irregular (subdivided according to whether they are made up of Population I or Population II stars). Another not uncommon type of galaxy is a lenticular form mid-way between the spiral and the elliptical.
	has definition A huge collection of millions, billions, or trillions of stars. When referring to the Milky Way, "galaxy" is capitalized, otherwise not; thus: "Andromeda is the nearest giant galaxy to the Galaxy".
	has definition A collection of matter which usually manifests itself by the production of stars.
	has definition A large aggregation of stars, bound together gravitationally. There are three major classifications of galaxies - spiral, elliptical, and irregular - and several subclassifications. The sun belongs to a spiral galaxy, the Milky Way galaxy.
	has definition A large (108-1013 Msun), gravitationally bound aggregate of stars and interstellar matter. Galaxy formation is currently believed to have occurred around z ≈ 3-4.
	has definition 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.
	is a kind of collection of stars
celestial body	has spectra
physical object	has location or center of gravity
	has angular momentum
	has mass
	has velocity
	has momentum
	has temperature
	has volume
	has extent
	has material

Kinds of galaxy :

• active galaxy (5 kinds, 18 facts) - Any galaxy which is emitting large quantities of non-thermal radiation., Active galactic nuclei are very luminous. Their energy output is in two forms: nonthermal continuum and thermal emission line.
• blue compact galaxy (2 facts) (BCG)
• compact galaxy (2 facts) - 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.
• dwarf galaxy (21 kinds, 138 facts) - A galaxy with low luminosity., A small, faint galaxy, exemplified by those that orbit the Milky Way: Ursa Minor, Draco, Sculptor, Sextans, Carina, Fornax, Leo II, and Leo I.
• elliptical galaxy (8 kinds, 41 facts) - 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.
• emission line galaxy (8 kinds, 29 facts)
• field galaxy (2 facts) - 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.
• galaxy containing Cepheids (10 kinds, 41 facts)
• gravitational lens (2 facts) - 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.
• infrared galaxy (3 kinds, 8 facts)
• irregular galaxy (9 kinds, 71 facts) - A galaxy with amorphous structure and with relatively low mass (10⁸-10¹⁰ M_sun). Fewer than 10% of all galaxies are classified as irregular.
• low surface brightness galaxy (3 facts) (LSBG) - A galaxy which is very faint because it contains a very limited number of stars.
• lyman break galaxy (2 facts) (LBG)
• N galaxy (2 facts) - 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.
• protogalaxy (3 facts) - A galaxy in the process of formation. None are observed nearby, indicating that all or most galaxies formed long ago., A galaxy during the early phase, before it has developed its present shape and mix of stars.
• radio galaxy (19 kinds, 72 facts) - 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.
• ring galaxy (2 facts) - 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.
• S0 galaxy (1 kind, 7 facts) - Galaxy with nuclei surrounded by disklike structure without arms.
• spiral galaxy (17 kinds, 110 facts) - A galaxy with a prominent nuclear bulge and luminous spiral arms of gas, dust, and young stars that wind out from the nucleus.
• starburst galaxy (2 facts) - Any galaxy in which an anomalously large rate of star formation is taking place.

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