Roche-lobe overflow

Roche-lobe overflow occurs in a close binary when a star expands beyond its Roche lobe, allowing gas to flow toward its companion through the inner Lagrange point.

Roche lobe: A region around a star in a binary system within which orbiting matter remains gravitationally associated with that star. Overflow begins when the donor star’s surface reaches the boundary of this region.
Binary star: A system of two stars bound to one another by gravity and orbiting their common center of mass. Roche-lobe overflow requires two stars whose combined gravity shapes the flow.
Algol paradox: The apparent mismatch in Algol’s binary components: the less massive star is more evolved than its companion. Past Roche-lobe overflow transferred mass and reversed which star was more massive.
Stellar wind accretion: Accretion of gas captured from a companion’s outflowing stellar wind. Unlike overflow, this transfer does not require the donor to fill its Roche lobe.
Inner Lagrange point: The point between two orbiting bodies where their gravitational and centrifugal forces balance in the rotating frame. Gas crosses this saddle point to flow from the donor toward its companion.
Roche potential: The effective potential in a frame rotating with a binary, combining gravity and centrifugal effects. Its equipotential surfaces define the regions that become the two Roche lobes.
Cataclysmic variable: A close binary in which a white dwarf accretes matter from a companion, often producing recurring brightness changes. Many systems feed the white dwarf through sustained Roche-lobe overflow.
Bondi–Hoyle–Lyttleton accretion: The gravitational capture of gas by a moving object from a comparatively uniform surrounding flow. This model describes wind capture, rather than gas passing through the inner Lagrange point.
Accretion disk: A rotating disk of gas that spirals inward as it loses angular momentum around a central object. Transferred gas often forms a disk around the receiving star.
Lagrange point: A location in a rotating two-body system where gravitational and centrifugal effects allow an object to remain fixed relative to both bodies. The inner point is one of five balance points that structure binary gas flows.

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