Galaxy
This article is about the astronomical structure. For other uses, see Galaxy (disambiguation).
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NGC 4414, a typical spiral galaxy in theconstellation Coma Berenices, is about 55,000 light-years in diameter and approximately 60 million light-years away from Earth.
A galaxy is a massive, gravitationally bound system consisting of stars,stellar remnants, an interstellar mediumof gas and dust, and dark matter, an important but poorly understood component.[1][2] The word galaxy is derived from the Greek galaxias(γαλαξίας), literally "milky", a reference to the Milky Way. Examples of galaxies range from dwarfs with as few as ten million (107) stars[3] to giants with one hundred trillion (1014) stars,[4] each orbiting their galaxy's own center of mass.
Galaxies contain varying numbers ofplanets, star systems, star clusters and types of interstellar clouds. In between these objects is a sparse interstellar medium of gas, dust, and cosmic rays. Many galaxies are believed to haveSupermassive black holes (SMBH) at their center. These are thought to be the primary driver of active galactic nucleifound at the core of such galaxies. The Milky Way's presumed SMBH, known asSagittarius A*, has a mass four million times that of our sun.[5]
Galaxies have been historically categorized according to their apparent shape, usually referred to as their visual morphology. A common form is theelliptical galaxy,[6] which has an ellipse-shaped light profile. Spiral galaxies are disk-shaped with dusty, curving arms. Those with irregular or unusual shapes are known as irregular galaxies[7] and typically originate from disruption by the gravitational pull of neighboring galaxies. Such interactions between nearby galaxies, which may ultimately result in a merger, sometimes induce significantly increased incidents of star formation leading to starburst galaxies.
There are probably more than 170 billion galaxies in the observable universe.[8] Most are 1,000 to 100,000parsecs in diameter and usually separated by distances on the order of millions of parsecs (or megaparsecs).Intergalactic space (the space between galaxies) is filled with a tenuous gas of an average density less than one atomper cubic meter. The majority of galaxies are organized into a neither fully random nor fully deterministic set of associations known as galaxy groupsand clusters, which, in turn usually form larger superclusters. At the largest scale, these associations are generally arranged into sheets and filaments, which are surrounded by immensevoids.[9]
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