BINARY SYSTEM, in astronomy, a system composed of two stars revolving around each other under the influence of their mutual attraction. It is probable that at least one-third of the stars are binary systems. In some cases (visual binaries) a tele scope will show the two components separated. In others (spec troscopic binaries) the duplicity is detected by observing the changing motion in the line of sight of one or both components. (See STAR.)