A pair of stars bound together by their mutual gravitation, and orbiting their common centre of mass. Such a system is distinct from an optical double, which is not gravitationally bound.
A visual binary is resolved visually or photographically, whereas an astrometric binary is detectable by irregularities in the proper motion of the one visible star or the photocentre (2). Direct evidence for a companion is provided by the eclipses in an eclipsing binary, and by the Doppler shift of spectral lines in a spectroscopic binary.
The orbital periods of binaries range from minutes to thousands of years. In a close binary, the separation is comparable to the diameter of the stars. Such systems are subdivided according to the amount by which each component fills its Roche lobe, giving rise to detached, semidetached, and contact binaries. The last two categories include interacting binaries, in which mass transfer occurs. Many binaries are also variable stars, the most notable being the various forms of cataclysmic binary, Type I supernovae, and certain variable X-ray sources.
Subjects: Astronomy and Astrophysics.