- Common envelope
In astronomy, a common envelope (CE) refers to a short-lived (months to years) phase in the evolution of a binary star in which the largest of the two stars (the donor star) has initiated unstable mass transfer to its companion star. Mass transfer is unstable when the radius of the donor star expands more rapidly or shrinks less rapidly than does the binary orbit. Hence, the donor will start mass transfer when it overfills its Roche lobe and as a consequence the orbit may shrink while the star expands, causing it to overflow the Roche lobe even more, which accerelates the mass transfer, causing the orbit to shrink faster and the donor to expand faster, etcetera. This leads to the run-away process of dynamically unstable mass transfer. The result will be the fast expansion of the donor's stellar envelope, which will then engulf the companion star. Hence the name common envelope.
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. Because of the difference in scale between the envelope and core, the 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 (hence the kinetic energy of the two objects increases, but their potential energy in the binary's gravitational potential decreases more than that, so that the net result is an energy loss). The lost 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.
The common-envelope phase is an important mechanism in close-binary evolution and is still poorly understood. Most astrophysicists model the outcome of a common envelope using the above, cartoonesque scenario. However, as computer power increases, we are beginning to model common envelopes properly, using hydrodynamic codes. Still, it will be some time before we can model a complete common envelope with spiral-in in detail, and even then these computations will be so CPU expensive that we will be able to do them for selected cases, but not for large populations of stars.
An important difference should be pointed out, which is that between a common envelope and a contact binary. Although the two terms look similar and are therefore often confused, 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 in a binary touch, with a typical timescale of millions to billions of years.
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