3. Core He H burning shell

4. Main Sequence Evolution Star brightens significantly (Red Giant)

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

6. 0.1R  1 R  10 R  100 R  ZAMS

7.  Core is collapsing and heating up ◦ He Fusion (Triple Alpha Process)

8.  Threshold Temperature ◦ 10 8 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

9.  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

10.  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

11.  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

12.  All have M = -0.5  Measure m and calculate distance Magnitude Time

13. ZAMS

14.  Periods between 1 and 70 days  Amplitudes of 0.1 to 2 mag

16. H burning shell He burning shell Carbon Core

17.  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

18. ZAMS

19.  Periods several hundred days  Amplitudes several magnitudes MagnitudeTime

20.  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

21. The Ring Nebula The Helix Nebula Wings of a Butterfly Nebula

22. ZAMS

23.  Enhanced stellar wind in giant phases  Planetary Nebula  Chandresakhar Limit ◦ Maximum amount of mass that can be supported by electron degeneracy ◦ 1.4 M 

24.  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

26.  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 

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

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

29. ZAMS Animation