What is a Binary Star?

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A binary star or a double star is a pair of stars that revolve around each other and are held together by gravity. The closer a pair of stars are, the faster they revolve. Some binary stars are almost touching and go around each other in a few hours, while others, which are separated by hundreds of times the diameter of the solar system, may take a million years to orbit each other. About a quarter of all known stars belong to binary systems. Most of these, particularly the close pairs, must have been born together. In most cases, one star is large, cool and reddish while the other star is small, white and hot. 

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