Double Star, a pair of closely-spaced stars that to the unaided eye usually appear as a single star. The pair is called an optical double if the two stars seem to be close together only because of their alignment as seen from earth. A true double star is called a binary star, or binary system. It consists of two stars that lie relatively close together in space and revolve around one another because of the gravitational attraction each exerts on the other.
More than half of all known stars belong to binary systems or to multiple-star systems containing binary stars. Astronomers can determine the masses of stars in a binary system whose distance from the earth is known by calculating the orbits of the stars around each other.
When both stars of a binary system can be seen individually in a telescope, the pair is called a visual binary. Often the presence of a second star is discovered by other means. In an astrometric binary, regular variations in the position of a visible star reveal that it is in orbit around a second star. In an eclipsing binary, the two stars pass in front of each other in turn, creating eclipses and periodic dimming of their combined light. A well-known eclipsing binary makes up part of Algol, a multiple-star system in the constellation Perseus. In a fourth type, the spectroscopic binary, a star is known to be in orbit around another star because the lines in its spectrum alternately show motion of approach and recession.