Chapter 10 Our Star

1. Chapter 10Our Star

2. 10.1 A Closer Look at the Sun Our goals for learning: • Why does the Sun shine? • What is the Sun’s structure?

3. Why does the Sun shine?

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. E = mc2 —Einstein, 1905 It is powered by NUCLEAR ENERGY! Nuclear Potential Energy (core) ~ 10 billion years Luminosity

11. Weight of upper layers compresses lower layers

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

13. Gravitational contraction… provided energy that heated the core as the Sun was forming. Contraction stopped when fusion began replacing the energy radiated into space.

14. What is the Sun’s structure?

15. Radius: 6.9  108 m (109 times Earth) Mass: 2  1030 kg (300,000 Earths) Luminosity: 3.8  1026 watts

16. Solar wind: A flow of charged particles (electrons, protons, some helium nuclei) from the surface of the Sun Creates Aurora

17. Corona: Outermost layer of solar atmosphere ~1 million K Seen in X-rays Shows emission lines + faint continuous spectrum

18. Chromosphere: Middle layer of solar atmosphere ~ 104–105 K Seen in UV Light Shows emission lines Features: Prominences, Flares,

19. Photosphere: Visible surface of Sun ~ 6,000 K Seen in Visible Light Shows Absorption lines Sunspots! Cut-away Section of the Sun

20. Convection zone: Beneath Photosphere Creates granules we see Energy transported upward by rising hot gas

21. Radiation zone: Energy transported upward by Gamma Ray & X-ray photons Determine extent based on computer models

22. Core: Energy generated by nuclear fusion ~ 15 million K Generates gamma rays Inner 10% of sun

23. Why does the Sun shine? • Chemical and gravitational energy sources could not explain how the Sun could sustain its luminosity for more than about 25 million years. • The Sun shines because gravitational equilibrium keeps its core hot and dense enough to release energy through nuclear fusion.

24. What is the Sun’s structure? • From inside out, the layers are: • Core • Radiation zone • Convection zone • Photosphere • Chromosphere • Corona These we can’t see – we model with computer simulations These we CAN see – and measure with X-ray, UV, and visible-light telescopes in orbit & on Earth

25. 10.2 Nuclear Fusion in the Sun 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?

26. How does nuclear fusion occur in the Sun? Fusion is the UNITING of light atomic nuclei into heaviernuclei, releasing binding energy in the form of gamma ray radiation and other particles

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

28. High temperatures enable nuclear fusion to happen in the core.

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

30. Proton–proton chain is how hydrogen fuses into helium in Sun

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

32. 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.

33. 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

34. 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 Structure of the Sun

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

36. Energy gradually leaks out of the radiation zone in the form of randomly bouncing photons.

37. Convection (rising hot gas) takes energy to the surface.

38. Bright blobs on photosphere where hot gas reaches the surface

39. How do we know what is happening inside the Sun?

40. We learn about the inside of the Sun by … • making mathematical models. • observing solar vibrations. • observing solar neutrinos.

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

42. Data on solar vibrations agree with mathematical models of solar interior.

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

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

45. 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.

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

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

48. 10.3 The Sun–Earth Connection Our goals for learning: • What causes solar activity? • How does solar activity affect humans? • How does solar activity vary with time?

49. What causes solar activity?

50. Solar activity is like “weather” • Sunspots • Solar flares • Solar prominences All are related to magnetic fields.