Chapter 12 Stellar Evolution

1. Chapter 12Stellar Evolution

2. Infrared Image of Helix Nebula

3. Mass and Stellar Fate • Low mass stars end life quietly • Massive stars end life violently • Massive - more than 8X M

4. Core-hydrogen burning • Main sequence stars fuse H into He • On main sequence for over 90% of life • Hydrostatic equilibrium - pressure and gravity balance

5. Figure 12.1Hydrostatic Equilibrium

6. Evolution of a sun-like star • Stages 1 - 6 (pre - main sequence) • Stage 7 - main sequence • Stages 8 - 14 (post main sequence)

7. Stages 8 and 9 • Stage 8 - Subgiant branch • Stage 9 - Red Giant branch • H depleted at center, He core grows • Core pressure decreases, gravity doesn’t • He core contracts, H shell burning increases • Star’s radius increases, surface cools, luminosity increases

8. Figure 12.2Solar Composition Change

9. Figure 12.3Hydrogen Shell Burning

10. Figure 12.4Red Giant on the H-R Diagram

11. Stage 10 - Helium Fusion • Red Giant core contracts (no nuclear burning there) • Central temperature reaches 108 K • Fusion of He starts abruptly - Helium flash for a few hours • Star re-adjusts over 100,000 years from stage 9 to 10 • H and He burning with C core - horizontal branch

12. Figure 12.5Horizontal Branch

13. Figure 12.6Helium Shell Burning

14. Stage 11 - Back to Giant Branch • C core contracts (no nuclear burning there) • Gravitational heating • H and He burning increases • Radius and luminosity increases

15. Figure 12.7Reascending the Giant Branch

16. Table 12.1Evolution of a Sun-like Star

17. Figure 12.8LG-Type Star Evolution

18. Figure 12.8RG-Type Star Evolution

19. Death of a low mass star • For solar mass star, core temperature not high enough for C fusion • Outer layers drift away into space • Core contracts, heats up • UV radiation ionizes surrounding gas • Stage 12 - A planetary nebula • (nothing to do with planets)

20. Figure 12.9Planetary Nebulae

21. Other elements • As red giant dies, other elements created in core • O, Ne, Mg • Enrich interstellar medium as surface layers ejected

22. Dense matter • Carbon core shrinks and stabilizes • Core density 1010 kg/m3 • 1000 kg in one cm3 • Pauli Exclusion Principle keeps free electrons from getting any closer together • This is a different sort of pressure

23. Stage 13 - White Dwarf • Red giant envelope recedes • C core becomes visible as a white dwarf • Approximately size of earth, 1/2 mass of sun • White-hot surface, but dim (small size) • Glow by stored heat, no nuclear reactions • Fades in time to a black dwarf - stage 14

24. Figure 12.10White Dwarf on an H-R Diagram

25. Table 12.2Sirius B – A Nearby White Dwarf

26. Figure 12.11Sirius Binary System

27. Figure 12.12Distant White Dwarfs

28. Novae • Plural of nova • Some white dwarfs become explosively active • Rapid increase in luminosity

29. Figure 12.13abNova Herculisa) March 1935b) May 1935

30. Figure 12.13cNova

31. Nova explanation • White dwarf in a binary • Gravitation tears material from companion, forming accretion disk around white dwarf • Material heats until H fuses • Surface burning brief and violent • Novae can be recurrent

32. Figure 12.14Close Binary System

33. Figure 12.15Nova Matter Ejection

34. Evolution of High-Mass Stars • All main sequence stars move toward red-giant phase • More massive stars can fuse C and other heavier elements • Evolutionary tracks are more horizontal • 4 M star can fuse C • 15 M star can fuse C, O, Ne, Mg and become a red supergiant

35. Figure 12.16High-Mass Evolutionary Tracks

36. Evolution of 4 M star • No He flash • Hot enough to fuse C • Can’t fuse beyond C • Ends as a white dwarf

37. Evolution of 15 M star • Rapid evolution • Becomes red supergiant • Fuses H, He, C, O, Ne, Mg, Si • Inner core of iron

38. Figure 12.17Heavy-Element Fusion

39. Figure 12.18Mass Loss from Supergiants

40. Examples in Orion • Rigel - blue supergiant • 70 R, 50,000X luminosity of sun • Originally 17 M • Betelgeuse - red supergiant • 10,000X luminosity of sun in visible light • Originally 12 to 17 M

41. High mass fast evolution • Consider 20 M star • Fuses H for 10 million y • Fuses He for 1 million y • Fuses C for 1000 y • Fuses O for one year • Fuses Si for one week • Fe core grows for less than a day

42. Death of high mass star - 1 • Fe fusion doesn’t produce energy • Pressure decreases at core • Gravitational collapse • Core temperature reaches nearly 10 billion K • High energy photons break nuclei into protons and neutrons - photodisintegration • Reduced pressure, accelerated collapse

43. Death of high mass star - 2 • Electrons + protons  neutrons and neutrinos • Density 1012 kg/m3 • Neutrinos escape, taking away energy • Further collapse to 1015 kg/m3 • Neutrons packed together slow further collapse • Overshoots to 1018 kg/m3, then rebounds • Shock wave ejects overlying material into space • Core collapse supernova

44. Figure 12.19Supernova 1987A

45. Table 12.3End Points of Evolution for Stars of Different Masses

46. Novae and Supernovae • Nova - explosion on white dwarf surface in a binary system • Supernova - exploding high mass star • Million times brighter than nova • Billions of times brighter than sun • Supernova in several months radiates as much as our sun in 10 billion years

47. Types of Supernovae • Type I - very little H • Sharp rise in brightness, gradual fall • Type II - H rich • Plateau in light curve • Roughly half Type I and half Type II

48. Figure 12.20Supernova Light Curves

49. Type II Supernovae • Core collapse as previously described • Expanding layers of H and He

50. Type I Supernovae • Accretion disk around white dwarf can nova • Some material adds to white dwarf • Below 1.4 M (Chandrasekhar mass), electrons support white dwarf • Above 1.4 M, white dwarf collapses • Rapid heating, C suddenly fuses throughout • Carbon-detonation supernova • Also possible for two white dwarfs to merge