Kappa opacity mechanism

The kappa opacity mechanism is the driving mechanism behind the changes in luminosity of many types of pulsating variable stars.

Note

Sometimes, this is called Eddington Valve

Kappa (\(\kappa\)) here means the opacity of the stellar atmosphere, i.e. how easy it is for light to pass through.

The higher the opacity, the more the gas absorbs the light and the hotter it gets.

So suppose there was an increase in the opacity somehow. The gas will absorb more energy and heat up and expand. Usually though, the temperature increase causes opacity to decrease, allowing energy to escape more rapidly, so it will eventually reach equilibrium.

However, sometimes, opacity increases with temperature, we get the driving force behind many, many types of pulsating variable stars. This only occurs for a few specific elements.