Binary star

Binary star, or double star, pair of stars that orbit around their common center of gravity. Though some binaries can be observed separately without a telescope, in most cases the stars are so close together that they appear to be a single star. A great number of binaries are too close to be seen separately by even the most powerful optical telescopes and are detected by studying the spectral lines of the stars.

See also: Black hole; Nova; Pulsar.