A galaxy may be a gravitationally sure system of stars, stellar remnants, celestial body gas, dust, and matter. The word galaxy springs from the Greek galaxias (γαλαξίας), virtually “milky”, a relevancy the Milky Way System. Galaxies place size from dwarfs with simply a number of hundred million (108) stars to giants with 100 trillion (1014) stars, every orbiting its galaxy’s center of mass.
Galaxies area unit classified per their visual morphology as elliptical, spiral, or irregular. several galaxies area unit thought to possess supermassive black holes at their centers. The milklike Way’s central part, called Sagittarius A*, includes a mass four million times bigger than the Sun. As of March 2016, GN-z11 is that the oldest and most distant discovered galaxy with a comoving distance of thirty two billion light-years from Earth, and discovered because it existed simply four hundred million years when the large Bang.
Research discharged in a pair of016 revised the amount of galaxies within the noticeable universe from a previous estimate of two hundred billion (2×1011) to a recommended 2 trillion (2×1012) or additional, containing additional stars than all the grains of sand on planet Earth. Most of the galaxies area unit one,000 to 100,000 parsecs in diameter (approximately three hundred0 to 300,000 lightweight years) and separated by distances on the order of innumerable parsecs (or megaparsecs). For comparison, the Milky Way System includes a diameter of a minimum of thirty,000 parsecs (100,000 LY) and is separated from the spiral nebula, its nearest giant neighbor, by 780,000 parsecs (2.5 million LY).
The area between galaxies is crammed with a tenuous gas (the accumulation medium) having a median density of but one atom per kiloliter. the bulk of galaxies area unit gravitationally organized into teams, clusters, and superclusters. The Milky Way System is an element of the native cluster, that is dominated by it and therefore the spiral nebula and is an element of the Virgo Supercluster. At the most important scale, these associations area unit typically organized into sheets and filaments enclosed by vast voids. the most important structure of galaxies however recognised may be a cluster of superclusters that has been named Laniakea, that contains the Virgo supercluster.