A binary system is a system of two objects in space (usually stars, but also brown dwarfs, planets, galaxies, or asteroids) which are close enough that there gravitational movement causes them to circle around each other (orbit) around a shared mass. Some definitions (e.g. that of double planet, but not that of binary star) require that this center of mass is not located within the interior of either object. A multiple system is like a binary system but consists of three or more objects.