From Birth to Death (Dust to Dust) Star Lives From Birth to Death (Dust to Dust)

Stellar Evolution Stars evolve because they shine Life cycle depends on MASS (composition plays secondary role) More massive => faster evolution => shorter lifetime

Hertzsprung-Russell Diagram A “snapshot” of stars at different life stages Most stars are “middle age” => main sequence (fuse hydrogen to helium in cores) Main sequence a mass sequence

H-R Diagram As stars evolve, internal changes drive external changes in luminosity, size, and surface temperature Star’s point on the H-R diagram changes position Sequence of points traces a star’s evolutionary track

Evolutionary Track Depends on mass! Low (about 1), middle (5 to 10), high (above 20) General trend: protostar => pre-main-sequence => main sequence => red giant (supergiant) => planetary nebula (supernova) => corpse

Fusion Reactions Hydrogen => helium => carbon => ends with silicon fusing to iron Each stage requires higher core temperature (to ignite the reaction); depends on mass (gravitational potential energy)

Sun-like Star Protostar: Gravitation energy, convection Pre-main-sequence: Transition to fusion in core Main sequence: Fuse hydrogen to helium in core (PP chain)

Sun-Like Star Red giant: Hydrogen burning in shell around core (convective) Helium fusion (to carbon) in core Red giant (fusion in shells again!) Planetary nebula: Blows off outer layers Earth vaporized!

Observations! H-R diagrams of clusters Assume born at same time, same composition; only mass varies More massive stars evolve faster (mass-luminosity relation) Turnoff points from main sequence differ; indicate relative age; models give actual ages

Nucleosynthesis “Waste” products are fuels for the next fusion reaction (stops at iron) Convection brings some of these fusion products to surface (red giant phase) Fusion products blown off into space by stellar winds