1. EXAM II Monday Oct 19 th (this coming Monday!) HW5 due Friday midnight

2. Our Sun

3. M sun = 333,000 x M earth 98 % H and He 2 % others

4. What makes it glow so? Can’t be burning chemically, it’d burn out in 10,000 years! Can’t be due to Kelvin Helmholtz heating, it’d be only 25 My old! It’s HOT  Nuclear fusion

5. Nuclear physics 101

6. p + Proton 1 H (1+) n Neutron Recall: the nuclei of elements are made of Neutral, aids in nucleon binding (strong force)

7. Mostly 1 H + at high temps and pressure. Fuse to make 4 He with a release of energy “Fusion” The sun creates energy (in its youth) by fusing H into He Hydrogen plasma Background of free electrons

8. Light elements can release energy when fused 2H2H + release of energy Nuclear force binds them when they’re close enough together 1H1H common in sun! rare Binding decreases the net mass  energy

9. Light elements can release energy when fused 2H2H Neutron unstable when alone Stable when bound Energy decays in about 15 mins. rare

10. Neutron decay is reversible: + ‘energy’If + ‘energy’Then

11. What if we try fusing two hydrogen nuclei? 1H1H common in sun! 1H1H ?

12. What if we try fusing two hydrogen nuclei? 1H1H common in sun! 1H1H + release of energy Borrows some binding energy … 2H2H Converting to a neutron

13. The sun creates energy (in its youth) by fusing H into He 1H1H 1H1H rare 4 He A four particle collision, two of which are rare when isolated! A very unlikely scheme

14. The proton-Proton chain How our sun makes He !

15. energy

17. The proton-proton chain Energy release Annihilates with plasma electron to make a  -ray photon Escapes the sun (2% total energy

18. The net result: back into circulation

19. Why must it be hot to start fusion?

20. + + Coming in from far away with this velocity (temperature) Two protons colliding… Long-range electrostatic repulsion Strong force is short range – no nuclear attraction yet

21. + +

22. + + STOP Distance of closest approach

23. + +

24. + +

25. + + Remember: temperature of a gas is just related to the average kinetic energy of the gas particles

26. Temperature T Distance of closest approach

27. Temperature T Range of strong force (attractive) Distance of closest approach Minimum temperature for p-p fusion ~13.6 ×10 6 K !

28. How it got started ….

29. Gravitational Compression: Cool hot Kelvin-Helmholtz heating

30. Gravitational Compression: Cool really hot! fusion! Kelvin-Helmholtz heating T > 13.6 ×10 6 K

31. Core regulation ! (negative feedback system)

32. The sun in equilibrium (a big gas ball) Gravitational equilibrium Thermal equilibrium

33. Ball of gaseous hydrogen some small volume

34. P - pressure T - temperature n - density Ball of gaseous hydrogen

35. Hydrostatic or Gravitational equilibrium:

36. Three forces must balance at each point ….

38. 1: Weight of mass shell itself

39. 2: Combined Weight of all gas above

40. 3: Pressure exerted by the gas below

41. Thermal equilibrium:

42. Thermal energy generated (fusion) For T to remain constant here … Heat in = Heat out Heat flow

43. Thermal energy generated (fusion) = energy radiated from surface

44. Two major mechanisms of heat flow (in stars): 1) convection 2) radiative diffusion

45. Convection heat sink heat source

46. hot cool Convection heat sink heat source

47. Convection hot expand less dense cool contract more dense heat sink heat source

48. Convection hot expand less dense cool contract more dense gravity heat sink heat source

49. heat sink Convection hot expand less dense cool contract more dense float sink

50. heat source heat sink cools and contracts heat and expands ready to go again Convection

52. the steady-state situation: heat sink heat source convection cells T, P and  at every point is constant in time. Fusion, compression Matter and energy into space

53. Heat/Light source (fusion) Relatively ‘transparent’ Relatively ‘opaque’ Mostly ions Mostly 1 H atoms p + and e - Radiative diffusion

54. photon Relatively ‘transparent’ Relatively ‘opaque’ Atomic absorption and re-emission: Build up of heat e - scattering

56. pressure temperature density For any radius Hydrostatic equilibrium Thermal equilibrium Complicated model of equilibrium solar structure Solution Fusion energy source

57. Fusion core

58. 13.6 ×10 6 K 5,800 K

59. Summaries of Solar Interior:

60. Fusion Core:

61. Mass: 94% of all mass inside

62. Density: center 14 × lead 0.3 R lead 0.5 R water 0.9 R 2 × air

63. 0.7 R T ~ 2 MK Opacity: transparent opaque ions atoms

64. 0.7 R T ~ 2 MK Heat Transfer: transparent opaque photons Radiative zone Convective zone 5,800 K Thermal radiation “hundreds of thousands of years”