Cataclysmic variable star

A close binary system in which a white dwarf pulls matter from a companion star, producing variable brightness through accretion and related eruptions.

Roche-lobe overflow: Mass transfer begins when a star expands beyond the region gravitationally bound to it in a binary system. The companion usually feeds the white dwarf by overflowing its Roche lobe.
White dwarf: A dense stellar remnant supported against gravity by electron degeneracy pressure. It is the compact accretor at the center of a cataclysmic variable.
U Geminorum: A dwarf nova in Gemini whose recurring outbursts helped establish the class of cataclysmic variables. Its brightening episodes exemplify disk-instability outbursts.
X-ray binary: A binary system in which a neutron star or black hole accretes matter from a companion and emits strong X-rays. Its compact accretor is usually more massive and more extreme than a white dwarf.
Accretion disk: A rotating disk of gas formed as matter spirals toward a central object. Transferred gas often settles into a disk around the white dwarf before accreting.
Roche lobe: A region around a star in a binary within which orbiting matter remains gravitationally bound to that star. Its boundary determines when the companion can transfer matter to the white dwarf.
SS Cygni: A nearby dwarf nova that undergoes recurring optical outbursts. Its well-observed outbursts provide a classic test of accretion-disk behavior.
Symbiotic star: An interacting binary containing a compact hot star and an evolved giant, often surrounded by ionized gas. Its giant donor and extended nebula differ from the close, usually compact-donor configuration here.
Dwarf nova: A cataclysmic variable whose accretion disk periodically brightens after a thermal-viscous instability. Its recurring outbursts arise from changes within the accretion disk.
Mass transfer: The movement of matter from one astronomical object to another, often within a binary system. It is the broader process that powers accretion in these binaries.

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