Binary Star System

A binary star system consists of a pair of stars that orbit about their common center of mass and are held closely together by their mutual gravitational attraction. The orbital periods of binaries range from minutes to hundreds of years. By convention, the star that is nearest the center of mass in a binary star system is called the primary, while the other (smaller) star of the system is called the companion.

Binary star systems can be further classified as visual binaries, eclipsing binaries, spectroscopic binaries, and astrometric binaries. Visual binaries are those systems that can be resolved into two stars by an optical telescope.  Eclipsing binaries occur when each star of the system alternately passes in front of the other, obscuring, or eclipsing, it and thereby causing their combined brightness to diminish periodically. Spectroscopic binaries are resolved by the Doppler shift of their characteristic spectral lines as the stars approach and then recede from the Earth while revolving about their common center of mass. Astronomers use variations in radial velocity to analyze the orbital motion of the stars in a spectroscopic binary, because these stars are generally too close to be seen separately. Frequently, the two stars in such a closed binary are distorted into non-spherical shapes by the action of mutual tidal forces. In a closed binary system the separation distance of the two stars might be comparable to their diameters. Astronomers subdivide closed binary star systems as detached, semidetached, or contact binaries. Semidetached and contact binaries include interacting binaries in which mass transfer occurs from the companion star to the primary star. In an astrometric binary one star cannot be visually observed, and its existence is inferred from the irregularities and perturbations in the motion of the visible star of the system.

Binary star systems are more common in the Milky Way galaxy than is generally realized. Perhaps 50 percent of all stars are contained in binary systems. The typical mean separation distance between members of a binary star system is on the order of 10 to 20 astronomical units.

The first observed binary star systems revealed their “binary characteristic” to astronomers through variations in their optical emissions. The X-ray binary was discovered during the 1970s by means of space-based X-ray observations. In a typical X-ray binary system, a massive optical star is accompanied by a compact, X-ray-emitting companion that might be a neutron star or possibly even a black hole. An interacting binary star system is one in which mass transfer occurs, generally from a massive optical star to its compact, cannibalistic, X-ray-emitting companion. The binary pulsar, first observed in 1988, reveals its existence by an apparent change in the pulse period of the pulsar as it orbits its companion. Examples of binary systems are Algol (an eclipsing binary), Sirius, and Cygnus X-1(of which one member is probably a black hole).

Questions to Ponder

• Can binary or triple systems have stars with different ages?
• What are the key features in the motion/orbit of binary stars?