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Stellar Death

Stellar Death. Core. He. H burning shell. The Sun in Five Billion Years. Main Sequence Evolution. Hydrogen is depleted in the last fusion zone Reaction rate falls Core can no longer support its weight core shrinks Temperature in the core increases Extra energy produced to -

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Stellar Death

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  1. Stellar Death

  2. Core He H burning shell The Sun in Five Billion Years

  3. Main Sequence Evolution • Hydrogen is depleted in the last fusion zone • Reaction rate falls • Core can no longer support its weight • core shrinks • Temperature in the core increases • Extra energy produced to - • Begin fusion reaction in next core zone • Lift the envelope • Star brightens significantly (Red Giant)

  4. The Sun as a Main Sequence Star diameter = 1/100 AU The Sun as a Red Giant diameter = 1 AU

  5. 100 R 10 R 1 R 0.1R The Red Giant Branch ZAMS

  6. Back in the Core • Core is collapsing and heating up • He Fusion (Triple Alpha Process)

  7. He Fusion • Threshold Temperature • 108 K = 100,000,000 K • Massive stars do this easily • Core is still ideal gas • Low mass stars (like the Sun) struggle • Core becomes degenerate

  8. Ideal Vs. Degenerate Gas • The ideal gas safety valve • Heat an ideal gas and it expands and cools • Cool an ideal gas and it contracts and heats up • Pressure  Temperature • Degenerate electron gas • Electrons are forced to be very close together and fill every energy state • Pressure and Temperature not related

  9. Fusion in a Degenerate Gas • Extra energy from He fusion causes the core to expand very little • Fusion energy raises the temperature of the core and takes place all over the core • He Flash • End of ascent up the Red Giant Branch • Eventually core does expand enough to become ideal again • Fusion now at much higher temperature

  10. Helium Fusion • Star is quasi-stable • Envelope contracts (luminosity should ) • Temperature increases (lum. should ) • These two effects nearly offset • Horizontal Branch • RR Lyrae Variables • Periods of 0.05 to 1.2 days • Amplitudes of 1 to 2 magnitude

  11. Magnitude Time RR Lyrae Variables • All have M = -0.5 • Measure m and calculate distance

  12. The Horizontal Branch ZAMS

  13. Cepheid Variables • Periods between 1 and 70 days • Amplitudes of 0.1 to 2 mag

  14. Period-Luminosity Relation

  15. Helium Depletion in Core H burning shell Core Carbon He burning shell

  16. Second Giant Phase • He is depleted in the last fusion zone • Reaction rate falls • Core can no longer support its weight • core shrinks • Temperature in the core increases • Carbon fusion threshold 600 million K • Low Mass stars cannot reach this temperature • Envelope expands to supergiant

  17. ZAMS Asymptotic Giant Branch

  18. Magnitude Time Mira Variables • Periods several hundred days • Amplitudes several magnitudes

  19. Planetary Nebula • Core is once again degenerate • He fusion in the shell becomes explosive • Send shocks of energy into the expanded envelope • Recombination energy also drives envelope out

  20. The Ring Nebula Wings of a Butterfly Nebula The Helix Nebula Planetary Nebulas

  21. ZAMS Ejection of the Planetary

  22. Mass Loss • Enhanced stellar wind in giant phases • Planetary Nebula • Chandresakhar Limit • Maximum amount of mass that can be supported by electron degeneracy • 1.4 M

  23. Low Mass Star Fates • Mass < 2.5 M • He ignites in degenerate core (He flash) • Nuclear processing stops at He fusion • Mass between 2.5 and 8 M • He ignites in a non-degenerate core • No He flash required • Nuclear processing stops at He fusion • Mass between 8 and 10 M • C fusion possible

  24. Post Main Sequence Evolution

  25. Fate of the Low Mass Core • Core was collapsing • Can never reach 600 million K for C fusion • Much of the original envelope has left in Planetary Nebula • Material becomes degenerate • Electron Degeneracy Pressure halts collapse • Core must be < 1.4 M

  26. Thin atmosphere of H and He Carbon White Dwarf Mass  1 M Radius  R 

  27. Sirius B • Sirius A • A1V, m = -1.46 • T = 9550 K • Sirius B • m = 8.3 • T = 25,000 K • gives R = 92%R

  28. ZAMS Life History of a Low Mass Star Animation

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