Roche lobe

A Roche lobe is the region around one star in a binary system bounded by a critical gravitational equipotential surface. Matter inside it is gravitationally associated with that star, while matter crossing the boundary can flow toward its companion.

Roche potential: The effective gravitational and centrifugal potential in a rotating frame containing two orbiting masses. Its equipotential surfaces define the shape of each lobe in the frame rotating with the binary.
Cataclysmic variable: A close binary in which a white dwarf accretes matter from a companion star. The donor commonly loses gas after expanding to fill its Roche lobe.
Binary star: A system of two stars bound to one another by gravity and orbiting their common center of mass. A Roche lobe is defined for each star within such a system.
Hill sphere: The region around an orbiting body where its gravity dominates over tidal effects from a larger body. It describes gravitational dominance in a different orbital setup, not the binary equipotential boundary.
Lagrange points: Locations in a rotating two-body system where gravitational and centrifugal forces balance. The inner point L1 is the saddle through which matter can pass between the lobes.
X-ray binary: A binary system in which a neutron star or black hole accretes matter and emits X-rays. Many systems feed their compact object through gas overflowing a companion’s lobe.
Equipotential surface: A surface on which a potential has the same value at every point. The Roche-lobe boundary is a critical equipotential surface in the rotating frame.
Bondi–Hoyle–Lyttleton accretion: Accretion of gas by a moving object from a surrounding medium flowing past it. Unlike lobe overflow, this process captures gas from an extended ambient flow.
Roche-lobe overflow: Mass transfer that begins when a star expands to fill its Roche lobe. Crossing the critical surface turns the lobe’s geometric boundary into a channel for mass transfer.
Algol paradox: The apparent evolutionary puzzle in Algol-type binaries, where the more massive star is less evolved. Past Roche-lobe overflow can reverse the stars’ mass ranking and resolve the puzzle.

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