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Binaries, multiples, variables and clusters

A binary system of O-type stars, stars with both high temperature and high luminosity (brightness)More than half of stars (60%) in the Milky Way Galaxy exist as systems of two or more stars. Solitary stars—like our Sun, for example—are therefore on the unusual side. Double stars, known as binary stars, or binaries, are the commonest type of star system. They stay in pairs because of the force of each other’s gravity, orbiting around one another. A number of other star systems contain more than two stars. Mintaka, for example, in the constellation of Orion, consists of three stars, while Castor in Gemini has six. 

Two binary stars with similar mass and elliptical orbits orbiting around a common barycentre (indicated by a red cross).


Two stars bound together by the pull of their gravity form a binary system. They orbit around a shared balance point, also known as a barycentre. Its position is determined by the stars' relative masses. In a visual binary, two separate stars can be made out, while in a spectroscopic binary, the two stars appear as a single star.
Some binaries are so close together that gases are exchanged between them, and they may even share a common atmosphere. Called interacting binaries, these often appear as variable stars.

Algol, a star, lies in the constellation of Perseus—actually inside the head of Medusa, the serpent-haired Gorgon slain by Perseus, a hero in ancient Greek mythology. As an eclipsing binary, Algol “winks” every 2.87 days when one of the stars blots out the other. Because of this sinister behaviour (as it seemed to early stargazers), Algol became known as the Demon Star.

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