page_title: Common envelope
section_title: Formation
hierarchical_section_title: Common envelope / Formation
context_page_description: In astronomy, a common envelope 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 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 lifetime 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 which can be explained as having undergone common envelope evolution.
context_section_description: A common envelope is formed in a binary star system when the orbital separation decreases rapidly or one of the stars expands rapidly.
  The donor star will start mass transfer when it overfills its Roche lobe and as a consequence the orbit will shrink further causing it to overflow the Roche lobe even more, which accelerates the mass transfer, causing the orbit to shrink even faster and the donor to expand more.  This leads to the run-away process of dynamically unstable mass transfer. In some case the receiving star is unable to accept all material, which leads to the formation of a common envelope engulfing the companion star.
caption: Stages in the life of a binary system as a common envelope is formed. The system has mass ratio M1/M2=3. The black line is the Roche equipotential surface. The dashed line is the rotation axis. (a) Both stars lie within their Roche lobes, star 1 on the left (mass M1 in red) and star 2 on the right (mass M2 in orange).  (b) Star 1 has grown to nearly fill its Roche lobe.  (c) Star 1 has grown to overfill its Roche lobe and transfer mass to star 2: Roche lobe overflow.  (d) Transferred too fast to be accreted, matter has built up around star 2.  (e) A common envelope, represented schematically by an ellipse, has formed. Adapted from Fig. 1 of Izzard et al. (2012).[6]