common envelope star
common envelope starA hypothetical type of giant star that consists of two stellar cores orbiting within a large common envelope of gas. Because the two cores are hidden within the envelope the star should superficially resemble a normal giant, and as a result no common envelope stars have yet been unambiguously identified. Their existence is strongly attested, however, by evidence for systems just before and just after this stage of evolution. Contact binaries of the W Ursae Majoris type should become common envelope stars when one member expands as a red giant. Again, a close binary system where one member is a white dwarf must have had a common envelope at a previous evolutionary stage when the star now seen as a white dwarf went through its red giant phase: these systems are observed either as cataclysmic variables or in the centers of some planetary nebulae; the nebula's gas is probably the original common envelope, ejected as a result of the orbital motion of the two star cores within it. In some common envelope stars, the two cores may not eject the envelope gas but instead spiral together and eventually fuse into a single core. Angular momentum from the orbital motion would spin up the distended star; some rapidly spinning giant stars, like FK Comae and V Hydrae, may be examples of such coalesced stars.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006