Common envelope
![A series of snapshots in the life of a binary star before mass transfer and during its common envelope evolution. The binary has a mass ratio M1/M2=3. The black line is the Roche equipotential surface. The CoM is the centre of mass of the binary system. (a) Shows the two stars with the relatively unevolved primary on the right (mass M1 in red) and the secondary on the left (mass M2 in orange). (b) Shows that as the primary evolves it grows in size. (c) Roche-lobe overflow: the primary fills its Roche lobe and transfers mass to the secondary. (d) The material cannot be accreted onto the secondary so it swells to fill the both Roche lobes. (e) A common envelope forms around both stars. Adapted by permission of the author from Fig. 1 of Izzard et al. (2012).[1]](/Images/godic/202501/05/Common_envelope.svg2528.png")
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.