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

2. Stellar Evolution Stars evolve because they shine
Life cycle depends on MASS (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