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.