Donor star + Roche lobe
Accretor star + Roche lobe
L1 / mass-transfer stream
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When a star in a close binary system swells to fill its Roche lobe — the tear-drop shaped region around it where its own gravity still dominates over its companion's — its outer gas is no longer gravitationally bound and pours through the inner Lagrange point (L1) toward its partner. This simulator computes the real 3D Roche equipotential surfaces by ray-marching the corotating-frame potential Φ(x,y,z) for any chosen mass ratio, locates L1 by root-finding rather than a lookup table, and integrates the full equation of motion (stellar gravity plus the Coriolis and centrifugal terms of the rotating frame) for every particle in the resulting mass-transfer stream, so the gas genuinely curves from the donor toward the accretor exactly as it does in a real cataclysmic variable or X-ray binary. Adjust the mass ratio, total mass, separation and donor fill factor and watch the Roche lobe shape, the L1 position, the orbital period and the mass-transfer rate respond together.