Binary Star

Binary Star, pair of stars linked by gravitation, orbiting around their mutual centre of mass. Orbital periods, which can range from minutes for very close binaries to many thousands of years for widely separated pairs, are governed by the distance apart of the stars and their individual masses. Observation of the orbits of binary stars is the only direct method that astronomers have of “weighing” stars. In the case of very close pairs, their mutual gravitational attraction can distort the stars’ shapes, and gas can flow from one star to the other in a process termed mass transfer (see Variable Star: Interacting Binary Stars). Many stars are seen to be double through telescopes, but very close pairs are revealed only when their light is studied by spectroscopy; the spectra of two stars are then seen, and their orbital motion can be deduced from the differing Doppler effects in the spectra. Such a pair is known as a spectroscopic binary. Surveys show that most stars in the sky are binaries or even multiples (that is, they have not just two but three or more gravitationally linked components). Occasionally, the two stars may pass in front of each other as seen from Earth, producing an eclipsing binary. In most cases, the members of a binary system are thought to have been born together, although sometimes one star may be captured by the gravity of another in areas of high star density such as star clusters. See Gravitational Wave; X-Ray Astronomy.