Common envelope

A short-lived phase of binary-star evolution in which an expanding star engulfs its companion in a shared gaseous envelope. Drag transfers orbital energy to the gas, often shrinking the orbit dramatically.

Orbital energy: The mechanical energy associated with the motion of bodies in orbit around one another. The inspiraling companion releases orbital energy that can help unbind the envelope.
Cataclysmic variable: A close binary in which a white dwarf accretes matter from a companion star. Many proposed formation histories require a common envelope to shrink the progenitor orbit.
Common-envelope efficiency: A parameterization of how effectively released orbital energy unbinds a common envelope. Its value strongly affects predicted post-envelope separations and binary survival.
Stable mass transfer: Binary mass exchange that proceeds without runaway engulfment or rapid orbital collapse. Unlike a common envelope, stable transfer leaves the stars distinct throughout the exchange.
Dynamical friction: A drag force that slows an object moving through a medium by transferring momentum to surrounding material. Drag through the giant star's gas removes orbital energy and drives inspiral.
Type Ia supernova: A thermonuclear explosion that destroys a white dwarf, commonly occurring in a binary system. Common-envelope evolution is one route proposed for producing close white-dwarf progenitors.
Common-envelope formalism: A simplified energy-based prescription for estimating binary orbital change during envelope ejection. This widely used approximation compresses complex gas dynamics into a small number of parameters.
Wind accretion: The capture of gas flowing outward from one star by a companion in a binary system. Wind accretion transfers material without placing both stars inside one envelope.
Envelope ejection: The expulsion of a star's outer gaseous layers into space. The envelope must be expelled for the binary to survive as a compact pair.
Double white dwarf: A binary star system containing two white dwarfs. One or more envelope episodes can bring the two stellar remnants into a tight orbit.

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