binary star or binary system, pair of stars that are held together by their mutual gravitational attraction and revolve about their common center of mass. In 1650 Riccioli made the first binary system discovery, that of the middle star in the Big Dipper's handle, Zeta Urase Majoris. True binary stars are distinct from optical doubles—pairs of stars that lie nearly along the same line of sight from the earth but are not physically associated. Binary stars are grouped into three classes. A visual binary is a pair of stars that can be seen by direct telescopic observation to be a distinct pair with shared motion. A spectroscopic binary cannot be seen as two separate stars, even with the most powerful telescopes, but spectral lines from the pair show a periodic Doppler effect that indicates mutual revolution. Some lines indicate motion toward the earth while others indicate motion away; later, as the stars revolve around in their orbit, this pattern reverses. An eclipsing binary has the plane of its orbit lying near the line of sight, and shows a periodic fluctuation in brightness as one star passes in front of the other. The more massive star (A) of a binary is called the primary, and the less massive (B) is called the secondary; e.g., Sirius A and Sirius B are the primary and secondary components of the Sirius system. It seems likely that more than two-thirds of the stars in our galaxy are binary or multiple (a system of more than two stars moving around their mutual center of mass), since many stars within 30 light-years of the sun are binary or multiple. The masses of the components of a spectroscopic binary can be determined from the observed motions and Newton's law of gravitation; binary stars are the only stars outside the solar system for which masses have been directly determined. Binary stars are thus important indicators from which the masses of all similar stars can be deduced. Measurements of the masses of some of the visual binary stars have been used to verify the mass-luminosity relation. Although most binary stars have distance between them, the components of W Ursae Majoris binaries are actually in contact with each other, their mutual gravity distorting their shapes into teardrops. There are binary systems in which one member is a pulsar: PSR 1913+16, for example, has an orbital period of 7 hr 45 min; in this case the other star is also a neutron star. The orbit period decreases as the system loses energy in the form of gravitational waves; used as a clock to measure the effect of the curvature of space-time on the binary's orbit, such a system confirms Einstein's theory of general relativity.