1. Ch. 10
Stellar Old Age

3. The Death of a Low-Mass Star: Planetary Nebula
Remnants of stars with ~ 1 – a few Msun
Radii: R ~ light years
Expanding at ~10 – 20 km/s ( Doppler shifts) 
Lifetime < 10,000 years
Have nothing to do with planets!
The Helix Nebula

4. The Formation of Planetary Nebulae
Two-stage process:
Slow wind from a red giant blows away cool, outer layers of the star
The Ring Nebula in Lyra
Fast wind from hot, inner layers of the star overtakes the slow wind and heats it => Planetary Nebula

5. The Cat Eye Nebula:
Approx 3000 LY away
Central star T = 80,000 K
Spectral class O
Mass ~ 1 Msun
Radius ~ 0.65 Rsun

6. The Cat Eye

8. White Dwarfs are supported by electron degeneracy pressure
in a low-mass star, Fusion stops after He -->C and O
Just cools off and fizzles out
Siruis and its white dwraf companion, Sirius B

9. High mass stars : CNO Cycle
H fusion is faster because C, N and O act as catalysts
Same net result: 4 H become 1 He.
No total gain or loss of C, N, O
Question: How does energy produced by CNO cycle compare to PP chain?

10. Death of low-mass star: White Dwarf
White dwarfs are the core, once fusion stops
Some H fusion at surface
Electron degeneracy pressure supports them against gravity
Cool and grow dimmer over time

11. More massive white dwarfs are smaller!
A white dwarf cannot be more massive than 1.4MSun (Chandrasekhar limit)
Nobel Prize 1983

12. A white dwarf can accrete mass from its companion

13. Nova The surface of a white dwarf can fuse H to He
Fusion begins suddenly and explosively: nova
The nova star system temporarily appears much brighter

14. Novae

15. The End…
Some extra slides follow…

16. Summary: Evolution of a Sun-Like Star

18. Running out of H…

19. ‘Helium flash’ ‘Flash’ occurs because matter is degenerate
Core of helium is supported by electron degeneracy pressure
When He ‘ignites’, whole core is ready to fuse He into C

20. Helium burning stars are temporarily stable.

21. After the Helium Flash

22. After He fusion stops in the core…
Variable stars: RR Lyrae
Cepheids

31. Planetary Nebulae
Double-shell burning ends with a pulse that ejects the outer layers into space: planetary nebula
The core left behind is a white dwarf

32. Planetary Nebulae Often asymmetric, possibly due to Stellar rotation
Magnetic fields
Dust disks around the stars
The Butterfly Nebula

33. Earth’s Fate
Sun’s luminosity will rise to 1,000 times its current level—too hot for life on Earth

34. Earth’s Fate
Sun’s radius will grow to near current radius of Earth’s orbit