galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas and dust, and dark matter.[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 just a few thousand (103) stars to giants with one hundred trillion (1014) stars,[3] each orbiting their galaxy's own center of mass. Galaxies can be categorized according to their visual morphology, including elliptical,[4] spiral, and irregular.[5] Many galaxies are believed to have supermassive black holes at an active center. The Milky Way's central black hole, known as Sagittarius A*, has a mass four million times that of our Sun.[6] As of May 2015, EGS-zs8-1 is the most distant known galaxy, estimated to be 13.1 billion light-years away and to have 15% of the mass of the Milky Way.