binary star
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binary star
binary star
(binary) A pair of stars that are revolving about a common center of mass under the influence of their mutual gravitational attraction. In most cases the stars may be considered to be moving in elliptical orbits described by Kepler's laws. Binary and multiple stars are very common in the Galaxy: in a recent survey of 123 nearby sunlike stars over half (57%) were found to have one or more companions. Studies of the orbital motions in binaries are important because they provide the only direct means of obtaining stellar masses. Optical determination of orbits is only possible if the components are sufficiently far apart to be distinguished (see visual binary). In an astrometric binary one component is too faint to be observed directly; the presence of this unseen component is inferred from perturbations in the motion of the visible component.Orbital motion in spectroscopic binaries is revealed by variations in radial velocity. Most spectroscopic binaries are close binaries, in which the components are too close together to be seen separately. Stars in a close binary are often distorted into nonspherical shapes by mutual tidal forces. If the two stars are physically separate, it is a detached binary system; in a semidetached system gas is drawn off one star on to the other; a contact binary consists of two stars sharing gas. In the latter two cases the gas flow from one star to the other (see equipotential surfaces; mass transfer) profoundly alters the evolution of the stars (see W Ursae Majoris stars; W Serpentis star; Algol variables). When one star is a compact white dwarf or neutron star, the infalling gas powers novae outbursts and X-ray binary systems.
The orbital planes of binaries are randomly oriented and only a minority of systems are eclipsing binaries, most of which are also spectroscopic binaries. See also cataclysmic variable; common envelope star; RS Canum Venaticorum star; symbiotic star.