Post–Main-Sequence Evolution of Low-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 a low-mass star. (b) After the helium flash, the star converts its helium core into carbon and oxygen. While doing so, its core reexpands,
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 reexpand, powered up the asymptotic giant branch by hydrogen shell fusion and helium shell fusion.