A pair of stars revolving about a common centre of mass. In a visual binary the stars are far enough apart to be seen separately by an optical telescope. In an astrometric binary one component is too faint to be seen and its presence is inferred from the perturbations in the motion of the other. In a spectroscopic binary the stars cannot usually be resolved by a telescope, but the motions can be detected by different Doppler shifts in the spectrum at each side of the binary, according to whether the components are approaching or receding from the observer.