Binary star is a system consisting of two stars that orbit around their common center of mass (also called center of gravity) under the influence of their mutual gravitational attraction.
Such systems are very common. Astronomers estimate that perhaps half or more of all stars in the Milky Way Galaxy are members of binary or more complex multiple-star systems. Binary stars are sometimes loosely called double stars, but the term “double star” can also include optical doubles, pairs that appear close together in the sky from Earth’s viewpoint yet are not gravitationally bound and lie at very different distances. True binary stars, by contrast, are physically associated and revolve around each other.
