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Seen behind a veil of foreground stars which lie within our own galaxy, this face-on Spiral Galaxy ESO 269-57 is about 150 million light-years away and 200,000 light-years across.
Stars are almost always found in collections called galaxies, together with gas, dust, and " dark matter"; ~10-20% of a galaxy is composed of stars, gas, and dust. Galaxies are held together by gravitational attraction and the galactic components orbit a common centre. There is some evidence that black holes may exist at the centre of some, or most, galaxies. Galaxies " evolve" from protogalaxies.The word galaxy was derived from the name of our own galaxy, the Milky Way, using the Greek word gala (gen. galaktos) meaning milk.
Galaxies come in three main types: ellipticals, spirals, and irregulars. A slightly more extensive description of galaxy types is given by the Hubble sequenceThe Hubble sequence is a classification of galaxy types developed by Edwin Hubble in 1936. It is also called the tuning-fork diagram as a result of the shape of its graphical representation. Galaxy types are divided as follows: ) Elliptical galaxies (E0-7. Our own galaxy, the Milky WayThis article is about the galaxy called the Milky Way. For the candy bar of the same name, see Milky Way candy bar. The Milky Way (a translation of the Latin Via Lactea in turn derived from the Greek Galaxia gala, galactos means "milk")) is a hazy band of, sometimes simply called the Galaxy (with uppercase), is a large barred spiral galaxyNGC 253, a barred spiral starburst gallery (2MASS). A barred spiral galaxy is a spiral galaxy with a band of bright stars emerging from the center and running across the middle of the galaxy. Spiral arms appear to emerge from the ends of the "bar" in thes about 30 kiloparsecsTo help compare distances at different orders of magnitude this page lists lengths from 11,000 light years and 110,000 light years (1020 m and 1021 m). See also lengths of other orders of magnitude. Distances shorter than 1020 m 16,000 light years 1. or 100,000 light yearA light year abbreviated ly is the distance light travels in one year: roughly 9. 46 × 1012 kilometres (9. 46 petametres, or about 5. 88 × 1012 miles). More specifically, a light year is defined as the distance that a photon would travel, in free space ans in diameter, contains about 300 billion stars and has a total mass of about a trillion times the mass of the sun.
In spiral galaxies, the spiral arms have the shape of approximate logarithmic spiralLogarithmic spiral (pitch 10° Cutaway of a nautilus shell ''showing the chambers arranged in an ''approximately logarithmic spiral A logarithmic spiral equiangular spiral or growth spiral is a special kind of spiral curve which often appears in nature.s, a pattern that can be theoretically shown to result from a disturbance in a uniformly rotating mass of stars.
Like the stars, the spiral arms also rotate around the center, but they do so with constant angular velocityAngular velocity is the vector physical quantity that represents the rotation of a spinning body. It is usually represented by the symbol Omega or omega . The magnitude of the angular velocity is the angular speed (or angular frequency and is denoted by &. That means that stars pass in and out of spiral arms. The spiral arms are thought to be areas of high density or density waves. As stars move into an arm, they slow down, thus creating a higher density; this is akin to a "wave" of slowdowns moving along a highway full of moving cars. The arms are visible because the high density facilitates star formation and they therefore harbor many bright and young stars.
The space between galaxies is relatively empty, except for intergalactic gas clouds.
Only few galaxies exist by themselves; these are known as field galaxies. Most galaxies are gravitationally bound to a number of other galaxies. Structures containing up to about 50 galaxies are called groups of galaxies, and larger structures containing many thousands of galaxies packed into an area a few megaparsecs across are called clusters. Superclusters are giant collections containing tens of thousands of galaxies, found in clusters, groups and sometimes individually; as far as we can tell the universe is uniform at scales above this.
Our galaxy is a member of the Local Group, and together with the Andromeda Galaxy dominates it; overall the Local Group contains about 30 galaxies in a space about one megaparsec across. The Local Group is part of the Local Supercluster, also known as the Virgo Supercluster.
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