Background image: Albireo is a binary star 380 years distant. The two stars are comparatively far from each other and take about 75,000 years to complete a single orbit. The brighter yellow star is itself a binary star system, but too close together to be resolved. Albireo is the fifth brightest star system toward the constellation of the Swan (Cygnus) and easily visible to the unaided eye.
A binary star consists of two stars in orbit about their common centre of mass. The larger is called the primary star; the smaller the secondary or companion star. It is estimated that about one third of stellar systems in the Milky Way are binary stars.
Binary (also known as 'double' or 'variable') stars were first observed in the early seventeenth century, but it was John Michell who first argued, in 1767, that they must be ‘joined’ since otherwise their close proximity would be highly improbable. William Herschell published catalogues of more than 700 binary stars in 1782-4. In his third catalogue of 1802, he announced the hypothesis that were orbiting each other.
Binary stars are of enormous observational importance, for from their periods and the structure of their orbits their absolute mass can be inferred. This in turn can be related to their spectral characteristics.
Examples of binaries are Sirius and Algol, also known as Beta Persei, an eclipsing binary star. Binary stars are also common as the nuclei of many planetary nebulae and are the progenitors of both novae and type Ia supernovae.