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A galaxy is a system of stars, stellar remnants, interstellar gas, dust, and dark matter bound together by gravity. [1] [2] The word is derived from the Greek ...
The giant elliptical galaxy ESO 325-4. An elliptical galaxy is a type of galaxy with an approximately ellipsoidal shape and a smooth, nearly featureless image. They are one of the three main classes of galaxy described by Edwin Hubble in his Hubble sequence and 1936 work The Realm of the Nebulae, [1] along with spiral and lenticular galaxies.
The Milky Way [c] is the galaxy that includes the Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars in other arms of the galaxy, which are so far away that they cannot be individually distinguished by the naked eye.
The Galactic Center is the barycenter of the Milky Way and a corresponding point on the rotational axis of the galaxy. [1] [2] Its central massive object is a supermassive black hole of about 4 million solar masses, which is called Sagittarius A*, [3] [4] [5] a compact radio source which is almost exactly at the galactic rotational center.
Composite image of five galaxies clustered together just 600 million years after the Universe's birth [1]. A galaxy cluster, or a cluster of galaxies, is a structure that consists of anywhere from hundreds to thousands of galaxies that are bound together by gravity, [1] with typical masses ranging from 10 14 to 10 15 solar masses.
Four of the seven members of galaxy group HCG 16 [1]. A galaxy group [2] or group of galaxies [3] (GrG [4]) is an aggregation of galaxies comprising about 50 or fewer gravitationally bound members, each at least as luminous as the Milky Way (about 10 10 times the luminosity of the Sun); collections of galaxies larger than groups that are first-order clustering are called galaxy clusters. [5]
An irregular galaxy is a galaxy that does not have a distinct regular shape, unlike a spiral or an elliptical galaxy. [1] Irregular galaxies do not fall into any of the regular classes of the Hubble sequence , and they are often chaotic in appearance, with neither a nuclear bulge nor any trace of spiral arm structure. [ 2 ]
This is in contrast to extragalactic astronomy, which is the study of everything outside our galaxy, including all other galaxies. Galactic astronomy should not be confused with galaxy formation and evolution, which is the general study of galaxies, their formation, structure, components, dynamics, interactions, and the range of forms they take.