Figure 20-1: The Post–Main-Sequence Evolution of a 1-M_⊙ Star These H-R diagrams show the evolutionary track of a star like the Sun as it goes through the stages of being (a) a red giant star, (b) a horizontal-branch star, and (c) an asymptotic giant branch (AGB) star. The star eventually evolves into a planetary nebula (described in Section 20-3).