A binary star system consists of a pair of stars that orbit about their common center of mass and are held closely together by their mutual gravitational attraction. The orbital periods of binaries range from minutes to hundreds of years. By convention, the star that is nearest the center of mass in a binary

star system is called the primary, while the other (smaller) star of the system is called the companion.

Binary star systems can be further classified as visual binaries, eclipsing binaries, spectroscopic binaries, and astrometric binaries. Binary star systems are more common in the Milky Way galaxy than is generally realized. Perhaps 50 percent of all stars are contained in binary systems. Just this month,

NASA discovered the first planet orbiting a binary system, Kepler-16b. Called the “Star Wars Planet” for  its similarity to Tatooine--but unlike Tatooine, Kepler-16b is orbiting beyond the system's habitable zone (where water can exist as a liquid).

The first observed binary star systems were discovered by astronomers through variations in their optical emissions. The Xray binary was discovered during the 1970s by mean s of space-based X-ray observations. In a typical X-ray binary system, a massive optical s tar is accompanied by a compact, X-ray emitting companion that might be a neutron star or possibly even a black hole. An interacting binary star system is one in which mass transfer occurs, generally from a massive optical star to its compact, cannibalistic, X-ray-emitting companion. The binary pulsar, first observed in 1988, reveals its existence by an apparent change in the pulse period of the pulsar as it orbits its companion.