Low to intermediate-mass stars age in a very different way, through red giant, horizontal branch, asymptotic giant branch (AGB), and then post-AGB phases. Post-AGB evolution generally involves dramatic mass loss, sometimes leaving a planetary nebula, and leaving an increasingly hot exposed stellar interior. If there is sufficient helium and hydrogen remaining, these small but extremely hot stars have an O-type spectrum. They increase in temperature until shell burning and mass loss ceases, then they cool into white dwarfs.
