Chapter 14 Our Star

1. Chapter 14Our Star

2. 14.1 A Closer Look at the Sun Our goals for learning: • Why was the Sun’s energy source a major mystery? • Why does the Sun shine? • What is the Sun’s structure?

3. Why was the Sun’s energy source a major mystery?

4. Is it on FIRE?

5. Is it on FIRE? Chemical Energy Content ~ 10,000 years Luminosity

6. Is it on FIRE? … NO! Chemical Energy Content ~ 10,000 years Luminosity

7. Is it CONTRACTING?

8. Is it CONTRACTING? Gravitational Potential Energy ~ 25 million years Luminosity

9. Is it CONTRACTING? … NO! Gravitational Potential Energy ~ 25 million years Luminosity

10. Why does the Sun shine?

11. E = mc2 - Einstein, 1905 - Special Theory of Relativity It can be powered by NUCLEAR ENERGY! Nuclear Potential Energy (core) ~ 10 billion years Luminosity

12. Nuclear Fusion of Protons - hydrogen cycle The Sun derives energy from fusion of protons. There are many possibilities, but two detailed cycles were proposed. The hydrogen cycle: H + H 2D (+e–) + + + n2D + H 3He + 3He + 3He 4He + 2 Hnet 4 H = 4He (+ 2e–) + 2+ +2 + 2n + 26.7 MeV Or Enough energy to keep a 60W light bulb on for 7 x 10-14 sec Fusion

13. Gravitational equilibrium: Energy provided by fusion maintains the pressure

14. Weight of upper layers compresses lower layers

15. Gravitational contraction: Provided the energy that heated the core as Sun was forming Contraction stopped when fusion began

16. What is the Sun’s structure?

17. Solar wind: A flow of charged particles from the surface of the Sun

18. Corona: • Outermost layer of solar atmosphere • ~1 million K • Very low density • Only seen during eclipses. • Most visible in X-Ray region

19. Chromosphere: Middle layer of solar atmosphere ~ 104 - 105 K Most readily observed in the ultraviolet region of the EM spectrum

20. Photosphere: Visible surface of Sun ~ 6,000 K

21. Convection Zone: Energy transported upward by rising hot gas

22. Radiation Zone: • Energy transported upward by primarily by X-Ray photons • Process is very slow since photons continually collide with electrons. • Photons take about 100,000 years to reach the surface. • ~10 million K

23. Core: • Energy generated by nuclear fusion (gamma rays). • ~ 15 million K • Density about 100x that of water. • Pressure about 200 billion times that on the surface of the Earth.

24. What have we learned? • Why was the Sun’s energy source a major mystery? • Chemical and gravitational energy sources could not explain how the Sun could sustain its luminosity for more than about 25 million years • Why does the Sun shine? • The Sun shines because gravitational equilibrium keeps its core hot and dense enough to release energy through nuclear fusion.

25. What have we learned? • What is the Sun’s structure? • From inside out, the layers are: • Core • Radiation Zone • Convection Zone • Photosphere • Chromosphere • Corona

26. 14.2 The Cosmic Crucible Our goals for learning: • How does nuclear fusion occur in the Sun? • How does the energy from fusion get out of the Sun? • How do we know what is happening inside the Sun?

27. How does nuclear fusion occur in the Sun?

28. Fusion Small nuclei stick together to make a bigger one (Sun, stars) Fission Big nucleus splits into smaller pieces (Nuclear power plants and atomic bombs)

29. High temperature and pressure enables nuclear fusion to happen in the core. Gravitational contraction ensures that the density is high enough such that collisions will occur at a high enough rate (~1038) per second. Insert TCP 5e Figure 14.6

30. Sun releases energy by fusing four hydrogen nuclei into one helium nucleus

31. Proton-proton chain is how hydrogen fuses into helium in Sun

32. IN 4 protons OUT 4He nucleus 2 gamma rays 2 positrons 2 neutrinos Total mass is 0.7% lower

33. Our Sun by the Numbers

34. Our Sun by the Numbers • About 0.7% of the mass of the fusion process is converted into energy through E = mc2. • About 98% of this energy exists as kinetic energy of the helium nuclei and gamma radiation. • Neutrinos carry off the other 2% of energy. • Overall, about 600 million tons of hydrogen are converted into 596 million tons of helium every second. • Or about 4 million tons of matter is converted into energy. • This is enough energy to supply our needs for about the next 500,000 years.

35. Thought Question What would happen inside the Sun if a slight rise in core temperature led to a rapid rise in fusion energy? A. The core would expand and heat up slightly B. The core would expand and cool C. The Sun would blow up like a hydrogen bomb Solar thermostat keeps burning rate steady

36. Solar Thermostat Rise in core temperature causes fusion rate to rise, so core expands and cools down Decline in core temperature causes fusion rate to drop, so core contracts and heats up

37. How does the energy from fusion get out of the Sun?

38. Energy gradually leaks out of Radiation Zone in form of randomly bouncing photons that collide with electrons.

39. In the Convection Zone (rising hot gas) takes energy to surface

40. Bright blobs on photosphere are where hot gas is reaching surface

41. How we know what is happening inside the Sun?

42. We learn about inside of Sun by … • Making mathematical models • Observing solar vibrations • Observing solar neutrinos

43. Patterns of vibration on surface tell us about what Sun is like inside

44. Data on solar vibrations agree very well with mathematical models of solar interior

45. Neutrinos created during fusion fly directly through the Sun Observations of these solar neutrinos can tell us what’s happening in core

46. Solar neutrino problem: Early searches for solar neutrinos failed to find the predicted number

47. Solar neutrino problem: Early searches for solar neutrinos failed to find the predicted number More recent observations find the right number of neutrinos, but some have changed form

48. What have we learned? • How does nuclear fusion occur in the Sun? • The core’s extreme temperature and density are just right for nuclear fusion of hydrogen to helium through the proton-proton chain • Gravitational equilibrium acts as a thermostat to regulate the core temperature because fusion rate is very sensitive to temperature

49. What have we learned? • How does the energy from fusion get out of the Sun? • Randomly bouncing photons carry it through the radiation zone • Rising of hot plasma carries energy through the convection zone to photosphere • How do we know what is happening inside the Sun? • Mathematical models agree with observations of solar vibrations and solar neutrinos

50. What causes solar activity?