Dear student,
Binary star or binary system, pair of stars that are held together by their mutual gravitational  attraction and revolve about their common center of mass. In 1650  Riccioli made the first binary system discovery, that of the middle star  in the Big Dipper's handle, Zeta Urase Majoris. True binary stars are  distinct from optical doubles—pairs of stars that lie nearly along the  same line of sight from the earth but are not physically associated.  Binary stars are grouped into three classes. A visual binary is a pair  of stars that can be seen by direct telescopic observation to be a  distinct pair with shared motion. A spectroscopic binary cannot be seen  as two separate stars, even with the most powerful telescopes, but  spectral lines from the pair show a periodic Doppler effect that  indicates mutual revolution. Some lines indicate motion toward the earth  while others indicate motion away; later, as the stars revolve around  in their orbit, this pattern reverses.
 

















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