Binary star
A
binary star is a star system consisting of two stars orbiting around their common center of mass. The more massive star is called the
primary and the other is its
companion star,
comes /ˈkoʊmiːz/, or
secondary. Systems of two, three, four, or even more stars are called
multiple star systems. These systems, especially when more distant, often appear to the unaided eye as a single point of light, and are then revealed as double by other means. Research over the last two centuries suggests that half or more of visible stars are part of multiple star systems. The term
double star is often used synonymously with
binary star; however, double star can also mean
optical double star. Optical doubles are so called because the two stars appear close together in the sky as seen from the Earth; they are almost on the same line of sight. Nevertheless, their "doubleness" depends only on this optical effect; the stars themselves are distant from one another and share no physical connection. A double star can be revealed as optical by means of differences in their parallax measurements, proper motions, or radial velocities.