The origin of the (lighter) elements The Late Stages of Stellar Evolution

1. The origin of the (lighter) elementsThe Late Stages of Stellar Evolution Supernova of 1604 (Kepler’s)

2. High-Mass Stars > 8 MSun Intermediate-Mass Stars Low-Mass Stars < 2 MSun Brown Dwarfs

3. Broken thermostat

4. ‘Helium flash’ Core of helium is supported by electron degeneracy pressure

5. Helium burning stars are temporarily stable.

6. After the Helium Flash

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

9. The Death of a Low-Mass Star: Planetary Nebula Remnants of stars with ~ 1 – a few Msun Radii: R ~ 0.2 - 3 light years Expanding at ~10 – 20 km/s ( Doppler shifts)  Last < 10,000 years Have nothing to do with planets! The Helix Nebula

10. 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

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

20. The Cat Eye

22. 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

23. Summary: Evolution of a Sun-Like Star

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

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

26. 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

27. Life Stages of High-Mass Stars • high-mass stars are similar to low-mass stars: • Hydrogen core fusion (main sequence) • Hydrogen shell burning (supergiant) • Helium core fusion (subgiant) • They are also different.. • H-->He via CNO cycle not p-p chain • Core much hotter • fuse C, O into heavier elements • He core is not degenerate • no He flash! • Lose a lot of mass

28. High-mass stars make the elements necessary for life!

29. Big Bang made 90% H, 10% He – stars make everything else

30. Helium fusion can make only carbon in low-mass stars

31. Helium Capture occurs only in high-mass stars • High core temperatures allow helium to fuse with heavier elements

32. Helium capture builds C into O, Ne, Mg, … Total # of P+N = Multiples of 4! Why?

33. Evidence for helium capture: Higher abundances of elements with even numbers of protons

34. Advanced Nuclear Burning • Core temperatures in stars with >8MSun allow fusion of elements up to iron

36. Si, S, Ca, Fe, etc. can only be made in high-mass stars

37. Multiple Shell Burning • Advanced nuclear burning proceeds in a series of nested shells

38. Fusion releases energy only when the mass of the products < mass of the reactants • Iron is “ash” of fusion: nuclear reactions involving iron do not release energy • Iron-56 has lowest mass per nuclear particle • Highest “binding energy” of all the elements

40. How does a high-mass star die? Iron builds up in core until degeneracy pressure can no longer resist gravity

41. Supernova Explosion • Core degeneracy pressure cannot support degenerate core of > 1.4 Msun • electrons forced into nucleus, combine with protons • making neutrons, neutrinos and LOTS of energy!

42. Collapse only takes very short amount of time (~seconds) Supernova!

43. Energy and neutrons released in supernova explosion cause elements heavier than iron to form, including Au and U

44. Neutron Stars & Supernova Remnants • Energy released by collapse of core drives outer layers into space • The Crab Nebula is the remnant of the supernova seen in A.D. 1054

45. Supernova 1987A • The first visible supernova in 400 years