Stars from Adolescence to Old Age (Astronomy)

Recall cards on how stars age and die. Life on the main sequence: the zero-age main sequence, the small fraction of mass turned into energy by fusion, and how sharply lifetime depends on mass, from an O star lasting a million years to an M dwarf lasting hundreds of billions. The path off the main sequence: core contraction, shell hydrogen fusion, and the swelling into a cool, luminous red giant like Betelgeuse. Reading stellar ages off star clusters: the globular, open, and association types, and the main-sequence turnoff that dates a cluster and even limits the age of the universe. And the later stages: the triple-alpha process and helium flash, the layered carbon-oxygen core, the ejection of a planetary nebula, and, for massive stars, fusion up an onion-shell structure that ends at iron.