There are about a billion stars in our galaxy. The planets from Mercury to Pluto revolve around the star called Sun. A study of our solar system also reveals that there are other planets existing in the outer space that revolve around other stars. The planets revolve around the stars because of gravity. About 80% of the stars in the Milky Way are not single stars but multiples.
Sun is a single star. Two stars in space that are so close to each other that their gravitational interaction causes them to orbit about their common centre of mass are called binaries. Binary stars or double stars are a pair of stars that are closely spaced in space and revolve around one another due to gravitational attraction each has on another.
Binary stars could be classified into three classes –one with a difference in color like that of a reddish primary star and a bluish secondary star. Second class involves a system in which differences in magnitude and color are not significant. Third class involves system in which the fainter star is more red than that other.