Cepheid Pulsation & the Period–Luminosity Relation

Classical Cepheid variables are stars caught pulsating: the κ-mechanism, an opacity "valve" in a partially-ionized helium layer, pumps energy into a radial oscillation faster than damping can remove it, so the star's radius and surface temperature swing rhythmically instead of settling to equilibrium. This simulator renders a physically-modelled Cepheid whose pulsation period follows the real period–mean-density relation (P·√ρ̄ ≈ const.), whose radius and temperature oscillate a quarter-cycle out of phase as observed, and whose brightness is computed self-consistently from the Stefan–Boltzmann law — reproducing the star's characteristic asymmetric light curve. Adjust the star's mass and pulsation amplitude, watch the live period, radius, temperature and luminosity readouts, and see how the resulting period and luminosity line up with Leavitt's Law, the period-luminosity relation astronomers still use to measure distances across the universe.