This is the same shell-burning physics as the 3D envelope model, but plotted the way astronomers actually diagnose it: as a track on the Hertzsprung–Russell diagram (surface temperature vs. luminosity) plus an evolution timeline, instead of a rendered star. After core hydrogen exhaustion, a thin hydrogen shell burns around an electron-degenerate helium core. As the shell dumps ash, the core mass Mc grows and — because degenerate matter obeys an inverse mass–radius law — the core contracts even as it gains mass:
R_core/R☉ = 0.013 · (Mc / 0.3 M☉)^(-1/3) (degenerate core, mirror principle)
The shell's luminosity depends almost entirely on the core mass beneath it (Paczyński's core-mass–luminosity relation):
L/L☉ ≈ 1.3 + 210800·(Mc − 0.15 M☉)^4
The envelope's surface temperature barely moves on this track, so from the Stefan–Boltzmann law L = 4πR²σT⁴, almost all of the extra luminosity must escape through a much larger surface:
R/R☉ = √(L/L☉) · (T☉/Teff)²
The left panel is the HR diagram itself — the star's whole future track is drawn as a fixed curve (it only depends on Mc, not on how fast you get there), and a marker slides along it as the evolutionary-stage slider moves. The right panel is a timeline strip showing radius, luminosity, temperature and core mass all plotted against evolutionary stage, with a vertical playhead. Unlike the 3D version's deliberately exaggerated core (drawn ~40× too big so it's visible), the core-radius stat here reports the true, undistorted degenerate-core size — routinely 1:5,000–1:10,000 of the envelope.
- Evolutionary stage slider — scrubs the star from subgiant to the tip of the red-giant branch (helium-flash threshold, Mc ≈ 0.48 M☉).
- Play evolution — advances the stage automatically at the chosen playback speed.
- Shell-burning rate — how fast the core accretes ash from the shell; it compresses or stretches the timeline strip but never changes the fixed HR track.
- HR trail — toggles the fading trail behind the marker as it climbs the giant branch.