The transformation of a main sequence star into a red-giant star in the core-and- shell model

As a star is evolving from the hydrogen to the helium burning stage, i. e. from the sun-like state to the state of a red-giant star, the temperature of its core increases whereas the volume decreases. At the same time the temperature of its shell decreases while the volume of the shell increases. This behaviour of the core and of the shell is explained qualitatively by a simple model. The star is considered to be composed of two homogeneous subsystems: the active core where the heat production takes place and the shell which plays the part of a thermal insulator. Each of these subsystems is stabilized by a feedback mechanism which is working thanks to the negative heat capacity of both systems. The transformation of a main sequence star into a red-giant star in the core-andshell model H. Hauptmann, F. Herrmann and K. Schmidt Abteilung für Didaktik der Physik, Universität, 76128 Karlsruhe, Germany