A galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter. The word galaxy is derived from the Greek  , 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, each orbiting their galaxy's own center of mass. Galaxies are categorized according to their visual morphology, including elliptical, spiral, and irregular. 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. As of July 2015, EGSY8p7 is the oldest and most distant observed galaxy with a light travel distance of 13.2 billion light-years from Earth, and observed as it existed 570 million years after the Big Bang. Previously, as of May 2015, EGS-zs8-1 was the most distant known galaxy, estimated to have a light travel distance of 13.1 billion light-years away and to have 15% of the mass of the Milky Way.