= Common envelope
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A <common envelope> is a phase of binary interaction in which the two stellar cores orbit within shared gas. It can arise when <Roche-lobe overflow> becomes unstable or when a companion is engulfed by an expanding giant. Motion through the gas transfers orbital <energy> and <angular momentum> to the envelope, causing the orbit to shrink.
If enough energy and momentum reach gas that can escape, envelope ejection can leave a much closer <binary star>; otherwise the cores can merge. The <common-envelope energy formalism> estimates the final separation by comparing released orbital energy with envelope binding energy, but its efficiency parameters compress complicated gas dynamics and energy transport. The outcome therefore depends on the donor's structure and the actual coupling to escaping material, rather than orbital energy being automatically available with unit efficiency.
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