The kappa mechanism (κ-mechanism or Eddington valve, helium valve or ionization valve) is the mechanism of a type of instability that can occur in a star when opacity (κ) increases with temperature, and is the mechanism of many pulsating stars, variable stars whose variation is due to pulses in energy transfer from within the star. Stars not like this, in which higher temperature causes opacity to decrease, are stable: the higher temperature from any increase in fusion lowers opacity back to the balance point at which radiative transfer is transferring out all the energy fusion releases, lowering the temperature and fusion rate, thus limiting it. In situations where higher temperature does the opposite, energy (i.e., heat) is trapped, the temperature rises and fusion increases, i.e., positive feedback. This cannot continue indefinitely and eventually some other mechanism such as convection carries a large amount of energy outward. In stars with layers of partially ionized hydrogen or helium (partial ionization zones), this situation can occur, and is the mechanism behind some types of variable stars such as RR Lyrae variables.