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A binary star consists of two stars bound by gravity and moving around their shared astronomical barycenter. In a sufficiently isolated pair, a Kepler orbit approximates their relative motion. Combining the orbital period and relative semi-major axis gives the total mass through Kepler's third law, .
A resolved orbit supplies angular information; a spectroscopic binary supplies line-of-sight velocities; an eclipsing binary supplies geometric information through its light curve. Combining these can determine individual masses and stellar radii. Close pairs also exchange matter through binary mass transfer, altering their subsequent stellar evolution and sometimes reversing which component is more massive.