The process that drives the pulsations of many types of variable star, including Cepheid variables and RR Lyrae stars. It is named from the Greek letter kappa (κ‎), the symbol for the coefficient of absorption (or opacity) of stellar material. In an ionization zone within a star, any small, chance rise in density produces increased opacity and thus increased absorption of energy from the stellar interior. This causes heating and expansion of the layer, which overshoots its rest position, leading to a drop in pressure, density, and temperature (and opacity). The effect then reverses, setting up an oscillation and causing the star’s outer layers to pulsate.