Kappa mechanism ($\kappa$ mechanism)

The kappa mechanism drives stellar pulsation when compression raises the opacity in an appropriate stellar layer. Energy transported outward is temporarily trapped, increasing heating during the compressed phase. If the phase of that heating is suitable, the gas does positive net mechanical work over an oscillation and the pulsation grows. Expansion subsequently reduces the blockage and allows energy to escape.

Partial ionization layers are important because their thermodynamic response can supply this behaviour. In Cepheid variables, helium-related ionization zones participate in the driving. An increasing opacity alone is insufficient: the layer's heat-exchange time must also be comparable to the pulsation period, and driving must exceed damping elsewhere. That restriction helps produce an instability strip rather than pulsation throughout the Hertzsprung-Russell diagram.