In astronomy, a common envelope (CE) or a common envelope event (CEE) is a short-lived (months to years) phase in the evolution of a binary star in which the larger of the two stars (the donor star) has initiated unstable mass transfer to its companion star. A typical donor star that causes a common envelope is a giant star, which has a large convective envelope and a compact, often degenerate core.
Common envelope event begins when by whatever reason a binary orbit begins to decay or one of star 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.
The donor's core does not participate in the expansion of the stellar envelope and the formation of the common envelope, and the common envelope will contain two objects: the core of the original donor and the companion star. These two objects (initially) continue their orbital motion inside the common envelope. However, it is thought that because of drag forces inside the gaseous envelope, the two objects lose energy, which brings them in a closer orbit and actually increases their orbital velocities. The loss of orbital energy is assumed to heat up and expand the envelope, and the whole common-envelope phase ends when either the envelope is expelled into space, or the two objects inside the envelope merge and no more energy is available to expand or even expel the envelope. This phase of the shrinking of the orbit inside the common envelope is known as a spiral-in.
A common envelope is sometimes confused with a contact binary. The former indicates the dynamically unstable process described above, with a typical timescale of years, whereas a contact binary is a stable configuration where the two stars touch or have merged to share their gaseous envelopes, with a typical timescale of millions to billions of years.
Observational manifestations 
Common envelope events are difficult to observe. Their existence has been mainly inferred indirectly from presence in the Galaxy of binary system that can not be explained by any other mechanism. Observationally CEEs should be brighter than typical novae but fainter than typical supernovae. The photosphere of the common envelope should be relatively cool—at about 5,000 K—emitting a red spectrum. However its large size should lead to a large luminosity—on the order of that of a red supergiant. A common envelope event should begin with a sharp rise in luminosity followed by a few months long plateau of constant luminosity (much like that of type II-P supernova) powered by the recombination of hydrogen in the envelope. After that the luminosity should decrease rapidly.
Several events that resemble the description above have been observed in past. These events are called luminous red novae (LRNe). They are subset of a broader class of events called intermediate-luminosity red transients (ILRTs). They have relatively slow expansion velocities of 200–1000 km/s and total radiated energies are 1038 to 1040 J.
The possible CEEs that has been observed so far include:
- M85 OT2006-1, possible ejection of the whole envelope.
- V1309 Sco, a possible star merger.
- M31 RV
- V838 Mon
See also 
- Wheeler, J. Craig (2007). Cosmic Catastrophes: Exploding Stars, Black Holes, and Mapping the Universe (2nd ed.). Cambridge University Press. p. 75. ISBN 0-521-85714-7.
- Ivanova, N.; Justham, S.; Nandez, J. L. A.; Lombardi, J. C. (2013). "Identification of the Long-Sought Common-Envelope Events". Science 339 (6118): 433–435. doi:10.1126/science.1225540. PMID 23349287.