Red Giant Envelope Expansion

Once a star exhausts core hydrogen, a thin shell of hydrogen keeps burning around an inert, electron-degenerate helium core — and that shell drives the whole red-giant transformation. This simulation tracks the physics behind it: as the shell deposits ash, the degenerate core obeys an inverse mass–radius law and contracts even while gaining mass, and the shell's luminosity (set almost entirely by the core mass via the core-mass–luminosity relation) rockets upward. Because the envelope's surface temperature barely moves, the Stefan–Boltzmann law forces its radius to balloon by a factor of fifty to a hundred to radiate the extra energy away. Scrub or play through the evolutionary track from subgiant to the tip of the red-giant branch, tune the shell-burning rate, and switch to a cutaway view to watch the shrinking core inside the swelling, convective envelope.