Low-Mass Stellar Evolution: Observation, Measurement & Evidence

Evolution proceeds from main sequence to subgiant and red-giant phases, then to core helium burning and later shell-burning stages. The outer envelope becomes increasingly extended and is eventually expelled, while the degenerate carbon-oxygen core remains as a white dwarf.

Star clusters contain populations at each predicted stage, and their H-R diagrams show turnoffs and giant branches consistent with age-dependent models. Planetary nebulae surround hot compact remnants, and white-dwarf masses match the expected final states of many ordinary stars.

Core hydrogen depletion removes the original central energy source, allowing the core to contract and heat while hydrogen burns in a surrounding shell. The envelope expands dramatically. Helium ignition later stabilizes the core temporarily, but after helium exhaustion shell burning and strong winds drive further expansion and mass loss.

Cluster H-R diagrams show turnoffs, giant branches, horizontal or red-clump stars, and white-dwarf sequences. Planetary nebulae and asteroseismology supply direct tests of post-main-sequence structure and core-burning state.