Kappa mechanism

Definition

Consider a pulsation mode of a star and a layer of the star. In the layer, the opacity $\kappa$ depends on the density $\rho$ and the temperature $T$ . The kappa mechanism is the driving of the mode by that layer when the following hold:

During the compression phase of the oscillation, the opacity $\kappa$ of the layer increases.
As a result, the layer absorbs radiative heat during compression and loses it during expansion.
The net work done by the layer on the oscillation over one cycle is positive.

Remarks

Example: in the second helium ionisation zone of a Cepheid, part of the compression work goes into ionising helium. So $T$ rises only a little while $\rho$ rises, the opacity increases on compression, and the three conditions hold. Non-example: in a layer obeying Kramers' opacity law, $\kappa \propto \rho T^{-7/2}$ , the temperature rises strongly on adiabatic compression and $\kappa$ decreases, so condition 1 fails and the layer damps the mode. When the driving comes from the low adiabatic exponent of the ionisation zone rather than from the opacity itself, it is often called the $\gamma$ mechanism; the two act together.

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Stellar pulsation concept
Kappa mechanism definition

Tag	`YNS57N2TT0`
URI	`m2://YNS57N2TT0`
Kind	definition
Writing	W0 stub
MSC	85A15, 76E20
Defines	Kappa mechanism
Uses	Stellar pulsation
Changed	3 November 2026

Kappa mechanism

Definition

Remarks

Reading path

Further reading

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