A binary star is a star system consisting of two stars orbiting around their common barycenter. 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 eye as a single point of light, and are then revealed as double (or more) by other means. Research over the last two centuries suggests that half or more of visible stars are part of multiple star systems.
Binary star systems are very important in astrophysics because calculations of their orbits allow the masses of their component stars to be directly determined, which in turn allows other stellar parameters, such as radius and density, to be indirectly estimated.
This also determines an empirical mass-luminosity relationship (MLR) from which the masses of single stars can be estimated. Binary stars are often detected optically, 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 or astrometry.
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 well known 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.