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Summary of Post-Main-Sequence Evolution of Sun-Like Stars

0. Summary of Post-Main-Sequence Evolution of Sun-Like Stars. Core collapses; outer shells bounce off the hard surface of the degenerate C,O core. Formation of a Planetary Nebula. Fusion stops at formation of C,O core. C,O core becomes degenerate. M < 4 M sun. 0.

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Summary of Post-Main-Sequence Evolution of Sun-Like Stars

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  1. 0 Summary of Post-Main-Sequence Evolution of Sun-Like Stars Core collapses; outer shells bounce off the hard surface of the degenerate C,O core Formation of a Planetary Nebula Fusion stops at formation of C,O core. C,O core becomes degenerate M < 4 Msun

  2. 0 The Remnants of Sun-Like Stars: White Dwarfs First example: Sirius B (Astrometric binary; discovered 1862) • M ≈ 1 M0 • L ≈ 0.03 L0 • Te ≈ 27,000 K • R ≈ 0.008 R0 • r ≈ 3x106 g/cm3

  3. 0 White Dwarfs Degenerate stellar remnant (C,O core) Extremely dense: 1 teaspoon of WD material: mass ≈ 16 tons!!! Chunk of WD material the size of a beach ball would outweigh an ocean liner! Central pressure: Pc ~ 3.8*1023 dynes/cm2 ~ 1.5x106 Pc,0 for Sirius B

  4. 0 Thin remaining surface layers of He and H produce absorption lines; DB (Broad He abs. lines) DA (Broad H abs. lines) (ZZ Ceti Variables; P ~ 100 – 1000 s) Low luminosity; high temperature => Lower left corner of the Herzsprung-Russell diagram.

  5. 0 Degenerate Matter Dx ~ n-1/3 Heisenberg Uncertainty Principle: (Dx)3 (Dp)3 ~ h3 => (Dp)3min ~ n h3 Non-degenerate matter (low density or high temperature): Number of available states Electron momentum distribution f(p) e-E(p)/kT Electron momentum p

  6. 0 Degenerate Matter Dx ~ n-1/3 Heisenberg Uncertainty Principle: (Dx)3 (Dp)3 ~ h3 => (Dp)3min ~ n h3 Degenerate matter (High density or low temperature): Fermi momentum pF = ħ (3p2ne)1/3 Electron momentum distribution f(p) e-E(p)/kT Number of available states Electron momentum p

  7. 0 Degeneracy of the Electron Gas in the Center of the Sun

  8. 0 The Chandrasekhar Limit The more massive a white dwarf, the smaller it is. RWD ~ MWD-1/3 => MWD VWD = const. (non-rel.) WDs with more than ~ 1.44 solar masses can not exist! Transition to relativistic degeneracy

  9. 0 Temperature and Degree of Degeneracy as a Function of Radius in a White Dwarf

  10. 0 Cooling Curve of a White Dwarf Nuclei settling in a crystalline structure, releasing excess potential energy

  11. 0 White Dwarfs in Binary Systems X-ray emission T ~ 106 K Binary consisting of WD + MS or Red Giant star => WD accretes matter from the companion Angular momentum conservation => accreted matter forms a disk, called accretion disk. Matter in the accretion disk heats up to ~ 1 million K => X-ray emission => “X-ray binary”.

  12. Nova Explosions Hydrogen accreted through the accretion disk accumulates on the surface of the WD • Very hot, dense layer of non-fusing hydrogen on the WD surface Nova Cygni 1975 • Explosive onset of H fusion • Nova explosion

  13. Recurrent Novae In many cases, the mass transfer cycle resumes after a nova explosion. T Pyxidis R Aquarii → Cycle of repeating explosions every few years – decades.

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