The accepted explanation for the pulsation of Cepheids is called the Eddington valve,[REF] or κ-mechanism, where the Greek letter κ (kappa) denotes gas opacity.
The mechanics of the pulsation as a heat-engine was proposed in 1917 by Arthur Stanley Eddington[REF] (who wrote at length on the dynamics of Cepheids), but it was not until 1953 that S. A. Zhevakin identified ionized helium[REF] as a likely valve for the engine.