Begin story of Stellar Evolution…

1. Begin story of Stellar Evolution… • Life on main sequence is very long; review derivation of lifetime, τ, vs. mass, M, for star with luminosity, L: τ ~ M/L = [total mass to “burn”]/[rate of “burning”] = M/M3.5 = ~1/M2.5, so small M is long τ • Eventually we convert enough H to He in core that star begins to change: it starts to collapse slightly and get hotter, which enables it to first burn H in a shell, making core hotter and producing larger L, so star moves OFF main sequence… • The star begins to ascend the red giant branch as initially a sub-giant, with a He core slowly becoming “degenerate” in its center…. Nov. 20, 2007

2. Life as a sub-giant… • Shell burning makes hotter core which makes nuclear burning more efficient (strong temp. dependence) which makes star more luminous • Increased core luminosity expands outer photosphere of star; radius becomes larger; star (now a sub-giant) becomes redder • As shell burning (of H) continues, He “ash” settles in core, which gradually cools (no more burning there) and becomes degenerate: a new state of matter, where electron pressure (all available energy level “states” are filled…) makes a super-dense core, the beginnings of a He White Dwarf (~0.1-.2Msun) • Shell burning extends farther out into star; star becomes more luminous and red – a Red Giant Nov. 20, 2007

3. Life as a Red Giant… • When Sun does this, ~3Gy in future, Earth is “fried” by more luminous Sun (~300X more luminous!): oceans evaporate, trial by fire! • Eventually the degenerate core has become massive enough (~0.4Msun) and hot enough (from surrounding shell burning) that it can ignite its He and “burn” to Carbon and Oxygen by the triple alpha process: He + HeBe; Be + He C; C + He O He burning begins at “tip” of RG branch when “He Flash” occurs in core. Luminosity then decreases when shell expands and Cools, decreasing energy output. Nov. 20, 2007