Figure 11-19 Stages in the Evolution of a Star with More than 0.4 Solar Masses (a) During the star’s main-sequence lifetime, hydrogen is converted into helium in the star’s core. (b) When the core hydrogen is exhausted, hydrogen fusion continues in a shell, and the star expands to become a red giant. (c) When the temperature in the red giant’s core becomes high enough because of contraction, core helium fusion begins. (These three pictures are not drawn to scale. The star is about 100 times larger in its red-giant phase than in its main-sequence phase, then shrinks somewhat when core helium fusion begins.)