Topic summary
Common envelope
In astronomy, a common envelope (CE) is gas that contains a binary-star system. The gas does not rotate at the same rate as the embedded binary system. A system with such a configuration is said to be in a common-envelope phase or undergoing common-envelope evolution.
During a common-envelope phase, the embedded binary system is subject to drag forces from the envelope, which cause the separation of the two stars to decrease. The phase ends either when the envelope is ejected to leave the binary system with a much smaller orbital separation, or when the two stars become sufficiently close to merge and form a single star. A common-envelope phase is short-lived relative to the lifetimes of the stars involved.
Evolution through a common-envelope phase with ejection of the envelope can lead to the formation of a binary system composed of a compact object with a close companion. Cataclysmic variables, X-ray binaries, and systems of close double white dwarfs or neutron stars are examples of systems of this type that can be explained as having undergone common envelope evolution. In all these examples, there is a compact remnant (a white dwarf, neutron star, or black hole) which must have been the core of a star that was much larger than the current orbital separation. If these systems have undergone common-envelope evolution, then their present close separation is explained. Short-period systems containing compact objects are sources of gravitational waves and Type Ia supernovae.
Predictions of the outcome of common-envelope evolution are uncertain.
A common envelope is sometimes confused with a contact binary. In a common-envelope binary system, the envelope does not generally rotate at the same rate as the embedded binary system; thus, it is not constrained by the equipotential surface passing through the L2Lagrange point. In a contact-binary system, the shared envelope rotates with the binary system and fills an equipotential surface.