Our Sun is a single star with a system of planets and other cold bodies as travelling companions. If we look around the sky we notice that some stars appear to be quite close to each other. For example, in the constellation Ursa Majoris the star £ Ursae Majoris (Mizar) has a fainter star 80 Ursae Majoris (Alcor) near to it Note, however, that not all stars that appear to us as double (that is. which lie close to each other in the sky) are physically close to each other. Many just appear so because our line of sight to a more distant star just happens to pass very close to a nearby one. In fact, if we could watch such stars, known as optical doubles, for long enough we would see them drift apart.

Most stars in our Galaxy are not solitary, but are bound by the force of gravity to one, and sometimes many, companions. About a half of all star systems are BINARY or MULTIPLE SYSTEMS. In fact if you look at Mizar through a moderate-sized telescope you will find that it consists of two stars. These two stars are physically close to each other and if you were able to watch them for long enough (for much more than a thousand years) you would see them orbiting around one another in the same way that the Earth orbits around the Sun. A system of this type is known as a VISUAL BINARY figure If one star in such a system is too faint to be seen directly, we can sometimes infer its presence through the disturbances caused to the motion of the visible star.Stars with such inferred invisible companions are sometimes known as ASTROMETBIC BINARIES. The existence of the companion to Sirius (Sirius B) was inferred in this way long before techniques improved to the extent that it could be detected Similarly Barnard’s star, is known to have a companion: analysis of the orbit indicates that the mass of the companion is not much larger than that of Jupiter Our Sun is not the only star to have a planet.

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