Life History of Intermediate Mass Stars

Abstract

We shall now discuss the evolution of stars in the intermediate mass range of 2.5–9 $M\odot$. These are located in the upper part of the main sequence (see Fig. 12.1). An essential difference between these stars and low-mass stars like the Sun is the nature of the helium core and its behaviour. In the Sun, for example, the core is in radiative equilibrium. The formation of the helium core (due to the fusion of hydrogen) depends only on the local rate of helium production. This results in the gradual formation of the helium core, starting with zero mass. And once formed, the helium core is in a degenerate state. The mass of the core grows essentially due to hydrogen burning in the shell surrounding the core. Thus the growth of the core occurs over a nuclear timescale, lasting many billions of years. This is why the transformation of the star into a giant is a gradual process, and we can catch the stars in this phase.