The kappa mechanism (κ-mechanism or Eddington valve) is an instability that can occur in a star when opacity (κ) increases with temperature. The instability underlies the mechanism of many pulsating stars, variable stars whose variation is due to pulses in energy transfer from within the star. When higher temperature causes opacity to decrease, radiative transfer is more efficient, allowing more energy to escape. In situations where higher temperature does the opposite, energy (i.e., heat) is trapped and the temperature rises, i.e., positive feedback. This cannot continue indefinitely and eventually some other mechanism such as convection carries a large amount of energy out. 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.