Why stars inflate to and deflate from red giant dimensions

We demonstrate that a unique physical process is responsible for the runaway expansion of stars to red giant dimensions, as well as for their subsequent recollapse leading to the formation of the so-called blue loops. In response to an increasing luminosity from the core, the stellar envelope expands keeping its thermal equilibrium insofar the envelope thermal conductivity increases. However, expansion implies local cooling, ion recombination, and thus increasing opacity, in such a way that a time comes when further expansion causes a drop of thermal conductivity in the envelope. As the luminosity transferred outwards and radiated away from the surface drops, thermal equilibrium is broken and an increasing fraction of the core luminosity is trapped in the envelope, causing further expansion and further drop of the thermal conductivity: the resulting runaway inflation of the envelope brings the star to the red giant region of the H-R diagram, and thermal equilibrium is not restored until convection penetrates inwards and the whole envelope becomes convective. The reverse process is responsible for the formation of the blue loops. During the early helium-burning phase, the core luminosity decreases and the star descends along the Hayashi track. By contraction the envelope heats up, the heavy ions ionize and the opacity drops. As the inner part of the envelope returns to radiative equilibrium, the envelope departs again from thermal equilibrium, since by contraction the temperature increases, the heavy ions ionize, the opacity drops, the thermal conductivity increases, and so do the radiative energy losses. Thus the envelope catastrophically deflates inside the gravitational potential well of the star. We present detailed analyses of these runaway inflations and deflations, and apply these concepts to achieve a deeper physical understanding of several major features of stellar evolution, including the pre-main-sequence phase, the overall contraction phase, and the formation of first and second blue loops. It is also recognized that the progenitor of the supernova 1987A was describing the so-called second blue loop, being far away from thermal equilibrium, when the explosion caught the star "on the fly" along such loop. The underlying physical mechanism responsible for the blue supergiant structure of the precursor is therefore identified. Finally, a general criterion for stellar thermal stability is formulated, and it is shown that when the criterion is violated either runaway inflations or deflations of the star are produced.