Binary stars, also called double stars, are composed of two stars that orbit around a common center of mass. Binary stars are quite common. Astronomers estimate that 85% of the stars in the galaxy are actually systems consisting of two or more stars.

Many double star systems can be detected with a small telescope and a few can even be seen as two stars by the unaided eye. English astronomer William Herschel first determined in 1801 that binary stars actually orbit each other, and are not two unrelated stars that happen to be in the same direction. Double stars that can be observed separately by telescope observation are called visual binary stars. These stars are generally separated by distances of several astronomical units or more. The orbital periods of visual binary stars can be several centuries, so many observations are needed to confirm that they are actually binary...