A binary star is a pair of stars that are so close together in the sky that they appear to be closely associated with one another. Some binary stars, called apparent binaries, are merely close together because of our point of view from Earth; they have nothing to do with one another physically. When two stars that are physically associated together make a binary star system, however, the two stars orbit each other around a single center of gravity.

Physically associated binary stars are further divided into categories. A visual binary is a pair in which each star can be observed distinctly, either through a telescope or with the unaided eye. An astrometric binary is a pair in which the two stars cannot be distinguished visually, but the wobble of one star’s orbit indicates the existence of another star in orbit around it. An eclipsing binary is a pair in which the plane of the stars’ orbits is nearly edgewise to our line of sight; the stars take turns being partially or totally hidden by one another. A spectroscopic binary is a pair in which two stars can be detected by Doppler shifts or other spectral indicators from spectroscopic measurements.

There are also multiple star systems, which may have three or four stars orbiting one another around a single center of gravity, although they are rarer and less likely to be in a long-term stable orbit.