Roche-lobe overflow

In a close binary, the gravity of both stars and the system's orbital motion shape the effective gravitational potential. Around each component lies a bounded region called its Roche lobe. In the usual idealized circular, co-rotating model, the lobes meet at the inner Lagrange point, L1. Their shape depends on the masses and separation of the components; they are often described as roughly teardrop-shaped. A Roche lobe is a useful boundary for describing whether material near a star remains associated with it, rather than a solid surface.

When a star's surface expands to its Roche-lobe boundary, material in its outer layers can flow through the L1 region toward the companion. The donor is then said to overflow its Roche lobe. Depending on the system, transferred gas may strike the companion directly, form an accretion disk, or be partly expelled. If neither star fills its lobe, the binary is termed detached; if one does, it is often called semi-detached. If both components fill their lobes, their outer layers may form a shared envelope.

Mass transfer alters the masses and sometimes the orbit of both stars, so a binary can evolve differently from two isolated stars. In a white-dwarf binary, accreted hydrogen-rich material may accumulate and ignite in a thermonuclear nova. Under some conditions, accretion can also be associated with a Type Ia supernova, though this is not the outcome of every such system. In compact binaries containing a neutron star or black hole, accretion can produce X-rays. The stability and duration of transfer depend on the donor's structure, the mass ratio, and how material and angular momentum move through or leave the system.

Astronomers use Roche-lobe concepts to interpret interacting binaries and model when mass transfer begins. The lobe is not directly a visible surface; its size and shape are inferred from orbital and stellar parameters. Many calculations approximate a complex three-dimensional equipotential by a volume-equivalent radius. Research continues to refine predictions, including how a star's extended atmosphere can transfer material even before its photosphere reaches the conventional lobe boundary.