binary star

binary star, Albireo

The binary star Albireo (Beta Cygni), which consists of two stars orbiting around each other every 7,300 years at a mean distance of about 400 billion miles (650 billion km).


Apastron: Greatest separation in a binary.

A binary star is two stars in orbit around their common center of mass and held together by their mutual gravitational pull. Surveys have shown that one star in every two or three is a member of a binary or multiple star system. The two components of a binary system each move in an elliptical orbit around their common center of gravity. The further apart they are, the slower they move. Their orbital periods may be measured in tens, hundreds, or even millions of years.


The point in the orbital motion of a binary star system when the two stars are closest together is called the periastron; when they are furthest apart, it is known as the apastron.

Physical double stars are binaries, whereas optical doubles are not. A visual binary is one whose components can be resolved visually or photographically. An astrometric binary gives itself away by a regular wobble in the motion of a visible component. In a spectroscopic binary, the apparent separation is so small that the presence of two stars can only be deduced from regular changes in the Doppler shift of the stars' spectral lines. In a close binary the separation is comparable to the diameter of the stars.


The question of whether planets in a binary star system could conceivably support life has been widely discussed. See also habitable planets of binary stars.