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A pair of stars in physical contact or surrounded by a common envelope of matter. As stars evolve, they expand. Where two expanding stars are close to each other in a binary system, mass from each is drawn towards the other by the mutual gravitational attraction, until a point of contact is reached. Mass transfer between the stars may take place, and the cores of the two stars can share a common envelope as expansion continues. Such close binary stars often show eclipses as one star crosses in front of the other. The distorted shape of the light curve of star systems such as W Ursae Majoris can be explained if the two stars are in contact. See also: Roche lobe. |