Kappa mechanism

The kappa mechanism drives stellar pulsations when compression raises opacity, trapping heat, and expansion lowers opacity, releasing it to sustain oscillations.

Opacity: A measure of how strongly matter absorbs or scatters radiation passing through it. Opacity changes control how readily radiation escapes during each pulsation cycle.
Classical Cepheid variable: A luminous pulsating supergiant whose period is correlated with its intrinsic brightness. Partial ionization of helium drives the regular pulsations of classical Cepheids.
Partial ionization: The state in which only some atoms in a gas have lost one or more electrons. Compression increases ionization in the driving layer, changing its opacity and heat capacity.
Epsilon mechanism: A pulsation-driving process in which temperature-sensitive nuclear energy generation feeds energy into stellar oscillations. It drives oscillations through nuclear heat production rather than opacity-controlled radiation flow.
Radiative diffusion: The transport of energy through matter by repeated absorption and re-emission of radiation. The mechanism works by changing radiative energy flow through a star's layers.
RR Lyrae variable: A low-mass pulsating giant star commonly used as a distance indicator. Its radial pulsations are driven primarily by opacity changes in partially ionized helium.
Helium ionization zone: A stellar layer where helium undergoes partial ionization as temperature and pressure vary. Its opacity response supplies the classical driving in many pulsators.
Stochastic excitation: The random excitation of oscillations by turbulent motions, especially convection. It produces modes through fluctuating convective forcing, not a coherent opacity valve.
Stellar pulsation: Periodic expansion and contraction of a star or its outer layers. The kappa mechanism is one physical driver of these oscillations.
Delta Scuti variable: A star showing short-period pulsations, often in multiple acoustic modes. Opacity in the helium ionization zone excites many of its observed modes.

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