The term "binary star" was apparently first coined by Sir William Herschel in 1802 to designate "a real double star - the union of two stars that are formed together in one system by the laws of attraction." At present, binary stars are classified into four types according to their observable properties: visual binaries, spectroscopic binaries, eclipsing binaries and astrometric binaries. Any star can belong to several of these classes, e.g. several spectroscopic binaries are also eclipsing binaries. Another three-category classification is based on the distance of the stars: detached binaries, semidetached binaries and contact binaries.
During the past 200 years a large amount of research has been carried out on binary stars leading to some general conclusions. It is believed that at least 1/4 of all stars are at least binary systems, with as many as 10% of these systems containing more than two stars (ternary etc.) There is a direct correlation between the period of revolution of a binary star and the eccentricity of its orbit, with systems of short period having smaller eccentricity. Binary stars may be found with any conceivable separation, from pairs orbiting so closely that they are practically in contact with each other, to pairs so distantly separated that their connection is indicated only by their common proper motion through space.
In pairs where the two stars are of equal brightness, they are also of the same spectral type. In systems where the brightnesses are different, the fainter star is bluer if the brighter star is a giant star and redder if the brighter star belongs to the main sequence.
Since mass can be determined only from gravitational attraction, and the only stars (with the exception of the Sun) for which gravitational attraction can be determined are the binary stars, these stars are the one group from which the masses of stars may be determined. In the case of a visual binary star, after the orbit has been determined and the stellar parallax of the system obtained, the combined mass of the two stars may be obtained by a direct application of the Keplerian harmonic law. Unfortunately, it is impossible to obtain the complete orbit of a spectroscopic binary unless it is also a visual or an eclipsing binary, so from these objects a determination of mass is impossible except on the basis of a statistical discussion.
In the case of eclipsing binaries which are also spectroscopic binaries it is possible to make a complete solution for the specifications (mass, density, size, luminosity, and approximate shape) of both members of the system.