A binary star, often called a double star, is a star system in which two stars linked by their mutual gravity orbit around a central point of mass. Binary stars are quite common. A recent survey of 123 nearby Sun-like stars showed that 57 percent had one or more companions.
English astronomer William Herschel (1738–1822) made the first discovery of a true binary system in the 1700s. He observed the motion of a pair of stars and concluded that they were in orbit around each other. Herschel's discovery provided the first evidence that gravity exists out-side our solar system.
Herschel discovered more than 800 double stars. He called these star systems binary stars. His son, John Herschel (1792–1871), continued the search for binaries and catalogued over 10,000 systems of two or more stars.
Astrometric binary: Binary system in which only one star can be seen, but the wobble of its orbit indicates the existence of another star in orbit around it.
Eclipsing binary: Binary system in which the plane of the binary's orbit is nearly edgewise to our line of sight, so that each star is partially of totally hidden by the other as they revolve around a common point of gravity.
Mass: The quantity of matter in the star as shown by its gravitational pull on another object.
Radiation: Energy in the form of waves or particles.
Spectroscopic binary: A binary system that appears as one star producing two different light spectra.
Spectrum: Range of individual wavelengths of radiation produced when light is broken down by the process of spectroscopy.
Visual binary: Binary system in which each star can be seen directly, either through a telescope or with the naked eye.
Several kinds of binary stars exist. A visual binary is a pair in which each star can be seen directly, either through a telescope or with the naked eye. In an astrometric binary, only one star can be seen, but the wobble of its orbit indicates the existence of another star in orbit around it. An eclipsing binary is a system in which the plane of the binary's orbit is nearly edgewise to our line of sight. Thus each star is partially or totally hidden by the other as they revolve.
Sometimes a binary system can be detected only by using a spectroscope (a device for breaking light into its component frequencies). If a single star gives two different spectra (range of individual wavelengths of radiation), it is actually a pair of stars called a spectroscopic binary.
A binary star may be a member of one or more of these classes. For example, an eclipsing binary may also be a spectroscopic binary if it is bright enough so that its light spectrum can be photographed.
The only accurate way to determine a star's mass is by studying its gravitational effect on another object. Binary stars have proven invaluable for this purpose. The masses of two stars in a binary system can be determined from the size of their orbit and the length of time it takes them to revolve around each other.