Galaxy

A galaxy is a physical system of stars, planetary systems, stellar remnants, substellar objects, interstellar gas, dust, and dark matter bound together by gravity. The word is derived from the Greek galaxias (γαλαξίας), meaning 'milky', a reference to the Milky Way galaxy that contains the Solar System. Galaxies, averaging an estimated 100 million stars, range in size from dwarfs with less than a... (Wikipedia)