Which is a binary star?

Binary stars are two stars orbiting a common centre of mass. The brighter star is officially classified as the primary star, while the dimmer of the two is the secondary (classified as A and B respectively). In cases where the stars are of equal brightness, the designation given by the discoverer is respected.

Binary pairs can be classified based on their orbit. Wide binaries are stars that have orbits that keep them spread apart from one another. These stars evolve separately, with very little impact from their companions. They may have once contained a third star, which booted the distant companion outward while eventually having been ejected themselves. Close binaries, on the other hand, evolve nearby, able to transfer their mass from one to the other. The primaries of some close binaries consume the material from their companion, sometimes exerting a gravitational force strong enough to pull the smaller star in completely. The pairs can also be classified based on how they are observed, a system that has overlapping categories. Visual binaries are two stars with a wide enough separation that both can be viewed through a telescope or even with a pair of binoculars. Five to 10 percent of visible stars are visual binaries. Spectroscopic binaries appear close even when viewed through a telescope. Scientists must measure the wavelengths of the light the stars emit and determine their binary nature based on features of those measurements.

Eclipsing binaries are two stars whose orbits are at an angle so that, from Earth, one passes in front of the other, causing an eclipse. This feature is based on the line of sight rather than any particular feature of the pair.

Astrometric binaries are stars that seem to dance around an empty space; that is, their companions can not be identified but only inferred. Such a companion may be too dim to be seen or could be hidden in the glare from the primary star.

Stars referred to as double stars are two that appear close together in the sky visually but are not necessarily anywhere near one another in space.