binary star

A pair of stars in orbit around each other, held together by their mutual gravitational attraction. About half of all "stars" are in fact binary or multiple, though many are so close that the components cannot be seen individually. The presence of more than one star is inferred from the appearance of the combined spectrum. The two components in a binary system each move in an elliptical orbit around the common centre of mass. The further apart they are, the slower they move. Pairs in which the separation is great enough for the two stars to be distinguished, or "split", in a telescope often have orbital periods as long as 50 or 100 years. Such pairs are called visual binaries. If one star is much fainter than the other, its presence may be revealed only by the obvious orbital motion of its brighter companion. Pairs of this type are called astrometric binaries. As the members of a binary system move in orbit, their velocities towards or away from the Earth change in a regular repeating pattern. Through the Doppler effect, these velocity variations are reflected as wavelength changes in the features of the combined spectrum. Study of such a spectrum can reveal details about the nature of the stars and their orbits. Binary stars recognized as such only by means of spectroscopy are called spectroscopic binaries. Their periods usually lie in the range from a day to a few weeks. Some binary components are so close that the pull of gravity distorts the individual stars from their normal spherical shape. They may exchange material and be surrounded by a common envelope of gas. An accretion disc may develop as material streams towards a compact, spinning star in a binary system. The energy released results in the emission of X-rays. Novae are another consequence of mass transfer in binary stars. If the orbits of a binary pair are oriented in space so that one star has to pass in front of the other as seen from the Earth, the system is described as eclipsing. Such a system is observed to be variable since one star periodically blots out light from the other. The best-known eclipsing binary is Algol.