The Milky Way's central black hole, known as Sagittarius A*, has a mass four million times greater than the Sun. Many are thought to have supermassive black holes at their centers. Galaxies are categorized according to their visual morphology as elliptical, spiral, or irregular. Supermassive black holes are a common feature at the centres of galaxies.

Most of the mass in a typical galaxy is in the form of dark matter, with only a few percent of that mass visible in the form of stars and nebulae. Galaxies, averaging an estimated 100 million stars, range in size from dwarfs with less than a hundred million stars, to the largest galaxies known – supergiants with one hundred trillion stars, each orbiting its galaxy's center of mass. The word is derived from the Greek galaxias ( γαλαξίας), literally 'milky', a reference to the Milky Way galaxy that contains the Solar System. NGC 4414, a typical spiral galaxy in the constellation Coma Berenices, is about 55,000 light-years in diameter and approximately 60 million light-years from Earth.Ī galaxy is a system of stars, stellar remnants, interstellar gas, dust, and dark matter bound together by gravity.