Why stars inflate to and deflate from red giant dimensions

It is demonstrated that stars become red giants in response to the increasing luminosity being provided by the stellar core, and that the runaway expansion - when it takes place - is triggered by the thermal conductivity in the envelope reaching a maximum and then decreasing. It is shown that the decrease in thermal conductivity at each point in the envelope causes part of the radiative energy flow to be trapped locally, which causes further expansion and further departure from thermal equilibrium. Thermal equilibrium in the stellar envelope is restored by the onset of convection, as the star approaches the Hayashi line. A general criterion for thermal stability is formulated, and it is shown that when it is violated, the envelope undergoes either runaway expansions to, or runaway deflations from, red giant dimensions, depending on the previous evolution.