Binary star



A binary star is a star system consisting of two stars orbiting around their common center of mass. Systems of two, three, four, or even more stars are called multiple star systems. These systems, especially when more distant, often appear to the unaided human eye as a single point of light, and are then revealed as double (or more) by other means.

The term double star is often used synonymously with binary star; however, double star can also mean optical double star. Optical doubles are so called because the two stars appear close together in the sky as seen from Terra; they are almost on the same line of sight. Nevertheless, their "doubleness" depends only on this optical effect; the stars themselves are distant from one another and share no physical connection.

If components in binary star systems are close enough they can gravitationally distort their mutual outer stellar atmospheres. In some cases, these close binary systems can exchange mass, which may bring their evolution to stages that single stars cannot attain. Examples of binaries are Sirius and Cygnus X-1 (Cygnus X-1 being a black hole). Binary stars are also common as the nuclei of many planetary nebulae, and are the progenitors of both novae and type Ia supernovae.