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Star Lives

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.

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Star Lives

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

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

  3. 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

  4. 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

  5. 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

  6. 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)

  7. 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)

  8. 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!

  9. 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

  10. 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

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