Wikipedia:Today's featured article/June 24, 2006

Albireo, a binary system

A binary star is a star system consisting of two stars orbiting around their common center of mass. For each star, the other is its companion star. Recent research suggests that a large fraction of stars are part of systems with at least two stars. Binary star systems are very important in astrophysics: the masses of many single stars are determined by extrapolations made from the observation of binaries. Binary stars are not the same as optical double stars, which appear to be close together as seen from Earth, but are in reality not bound by gravity. Binary stars can either be distinguished optically (visual binaries) or by indirect techniques, such as spectroscopy. If binaries happen to orbit in a plane containing our line of sight, it is possible for them to eclipse each other; these are called eclipsing binaries. The components of binary star systems can exchange mass, bringing their evolution to stages that single stars cannot attain. Examples of binaries are Algol (an eclipsing binary), Sirius and Cygnus X-1 (of which one member is probably a black hole). (More...)

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