One of a pair of stars moving in orbit around their common centre of mass. Observations show that most stars are binary, or even multiple; an example is the nearest star system to the Sun, Rigil Kent (Alpha Centauri). A spectroscopic binary is a binary in which two stars are so close together that they cannot be seen separately, but their separate light spectra can be distinguished by a spectroscope. Another type is the eclipsing binary, a double star in which the two stars periodically pass in front of each other as seen from Earth. When one star crosses in front of the other, the total light received on Earth from the two stars declines. The first eclipsing binary to be noticed was Algol, in 1670, by Italian astronomer Geminiano Montanari.
The binary system Rigil Kent, for example, consists of a star almost identical to the Sun with another star about a third as bright closer to it than Neptune is to the Sun (that is, closer than 4.4 billion km). Each of these stars appears to describe an ellipse about the other in about 80 years. A third, much fainter star, Proxima Centauri, is too far away to disturb their mutual orbit appreciably.
The study of binary star systems has provided the only reliable information about the masses of stars. For a few stars it has also yielded direct measures of their dimensions, shapes, and effective temperatures.