Roche lobe

Summary

Consider a binary star on a circular orbit. The Roche lobe of a star is the region around it in which material is bound to that star rather than to its companion. Its boundary is a critical surface of equal effective potential in the frame that rotates with the binary. This surface passes through the inner Lagrange point. A star that grows larger than its Roche lobe pours gas through the inner Lagrange point onto its companion. This mass transfer drives the evolution of close binaries, cataclysmic variables and X-ray binaries.