Galaxy

A galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter.^[1][2] The word galaxy is derived from the Greek galaxias (γαλαξίας), literally "milky", a reference to the Milky Way. Galaxies range in size from dwarfs with just a few thousand (10³) stars to giants with one hundred trillion (10¹⁴) stars,^[3] each orbiting its galaxy'scenter of mass. Galaxies are categorized according to their visual morphology as elliptical,^[4] spiral and irregular.^[5] Many galaxies are thought to have black holes at their active centers. The Milky Way's central black hole, known as Sagittarius A*, has a mass four million times greater than the Sun.^[6] As of March 2016, GN-z11 is the oldest and most distant observed galaxy with a comoving distance of 32 billion light-years from Earth, and observed as it existed just 400 million years after the Big Bang. Previously, as of July 2015, EGSY8p7 was the most distant known galaxy, estimated to have a light travel distance of 13.2 billion light-years away.

Approximately 170 billion (1.7 × 10¹¹) to 200 billion (2.0 × 10¹¹) galaxies exist in the observable universe.^[11] Most of the galaxies are 1,000 to 100,000 parsecs in diameter and usually separated by distances on the order of millions of parsecs (or megaparsecs). The space between galaxies is filled with a tenuous gas having an average density of less than oneatom per cubic meter. The majority of galaxies are gravitationally organized into associations known as galaxy groups,clusters, and superclusters. At the largest scale, these associations are generally arranged into sheets and filamentssurrounded by immense voids.