W Ursae Majoris stars
W Ursae Majoris stars(er -see mă-jor -iss) (W UMa stars) A class of contact eclipsing binaries that have very short periods amounting to only a few hours and components that are so close that they are grossly distorted by tidal forces into ellipsoidal shapes. Mass transfer occurs between the stars (see equipotential surfaces). The components are approximately equal in brightness, as seen by the equal depths of the minima of the light curves. As with W Serpentis stars the light curves show continuous variation resulting from the distorted stellar shapes and variable surface intensity. The two components are more similar in luminosity than in mass (the ratio of masses can be 12:1), so energy from the more massive star's core is being fed to the companion's photosphere. Resulting chromospheric activity is thought to be responsible for the unusually high X-ray output of these stars.
About 0.1% of all main-sequence stars should be born in close enough doubles to become contact systems: the lower mass F, G, and K stars are the W UMa class while the rare more massive analogs are called SV Centauri stars . In both cases as the more massive component swells to become a giant, its outer layers will surround both stars to create a common envelope star; the final outcome will be a coalesced star.