What are double, binary and multiple stars? Can planets orbit a binary star?
Double stars are two stars so close together that they appear as a single star to the unaided eye but telescopes detect them to be two or more stars. They may simply be two stars that lie in the same direction, but where one is much further away than the other – these are optical doubles. Binary stars, unlike double stars, are made of two stars that are genuinely close to each other and bound together by mutual gravitational attraction. In this case the two stars orbit around a common centre of mass. More than half of all stars are in binary systems. Multiple stars are systems made up of three or more stars. In these systems stars are grouped into relatively close pairs with more distant single companions. For example, the Alpha Centauri system is a pair of stars with Proxima a more distant companion. The more complex system that appears as Castor (in the constellation of Gemini) consists of six stars in three separate pairs. The components of binary and multiple star systems are usual
