Figure 12-1Post–Main-Sequence Evolution of Intermediate-Mass Stars (a) A typical evolutionary track on the H-R diagram as a star makes the transition from the main sequence to the giant phase. The asterisk (*) shows the helium flash occurring in an intermediate-mass star. (b) After the helium flash, the star converts its helium core into carbon and oxygen. While doing so, its core expands again, decreasing shell fusion. As a result, the star’s outer layers recontract. (c) After the helium core is completely transformed into carbon and oxygen, the core recollapses, and the outer layers expand again, powered up the asymptotic giant branch by hydrogen shell fusion and helium shell fusion.