Editing Binary star (section)

{{Short description|System of two stars orbiting each other}}
{{For|the hip hop group|Binary Star (hip hop group)}}{{Distinguish|Double star}}[[File:Sirius A and B Hubble photo.jpg|upright=1.2|thumb|right|The well-known binary star [[Sirius]], seen here in a [[Hubble Space Telescope|Hubble]] photograph from 2005, with Sirius A in the center, and [[white dwarf]], Sirius B, to the left bottom from it]]
A '''binary star''' or '''binary star system''' is a [[Star system|system]] of two [[star]]s that are [[gravity|gravitationally]] bound to and in [[orbit]] around each other. Binary stars in the night sky that are seen as a single object to the naked eye are often resolved as separate stars using a [[telescope]], in which case they are called ''visual binaries''. Many visual binaries have long orbital periods of several centuries or millennia and therefore have orbits which are uncertain or poorly known. They may also be detected by indirect techniques, such as [[spectroscopy]] (''spectroscopic binaries'') or [[astrometry]] (''astrometric binaries''). If a binary star happens to orbit in a plane along our line of sight, its components will [[eclipse]] and [[transit (astronomy)|transit]] each other; these pairs are called ''eclipsing binaries'', or, together with other binaries that change brightness as they orbit, ''photometric binaries''.

If components in binary star systems are close enough, they can gravitationally distort each other's outer stellar atmospheres. In some cases, these ''close [[binary system]]s'' can exchange mass, which may bring their [[stellar evolution|evolution]] to stages that single stars cannot attain. Examples of binaries are [[Sirius]], and [[Cygnus X-1]] (Cygnus X-1 being a well-known [[black hole]]). Binary stars are also common as the nuclei of many [[planetary nebula]]e, and are the progenitors of both [[nova]]e and [[type Ia supernova]]e.