Common envelope

A common envelope is an astrophysical phenomen that occurs in binary evolution. When an evolved donor star with a convective envelope overfills its Roche lobe, it will start mass transfer to the companion star. As a result of the mass loss, the envelope of the donor star may expand so rapidly that it overfills its Roche lobe even more, causing a higher mass-transfer rate and an even faster expansion of the star. The result of this run-away process is thought to be a rapidly expanding envelope of the donor that quickly engulfs the companion star: a common envelope.