More than half of all have one or more close companion stars, to which it is tied by . When there are only two stars in orbit around each other they are called a binary star. When there are three or more they are called a multiple star.
The closer the stars are together the faster they move. The time it takes for them to go round each other varies from days to years. Some binaries are so close that gravity pulls gas from one to the other, forming an accretion disc around the smaller star in a binary system. This releases s.
A may also be formed by binary stars.
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