Figure 12-2 The Structure of an Old High-Mass Star Near the end of its life, a star with an initial mass greater than about 8 M_⊙ becomes a red supergiant. The star’s overall size can be as large as Jupiter’s orbit around the Sun. The star’s energy comes from a series of concentric fusing shells, all combined within a volume roughly the same size as Earth. Thermonuclear reactions do not occur within the iron core, because fusion reactions that involve iron absorb energy rather than release it.