= Kappa mechanism
{title2=$\kappa$ mechanism}
= Opacity mechanism
{synonym}
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>.
Back to article page
