Galaxies are massive, gravitationally bound systems consisting of stars, an interstellar medium of gas and dust, and dark matter. Typical galaxies range from dwarfs with as few as ten million stars up to giants with one trillion stars, all orbiting a common center of mass. Galaxies can also contain many multiple star systems, star clusters, and various interstellar clouds. The word Galaxy is derived from the Greek word for milk, a reference to our own Milky Way.