1. Chapter 14 Our Star

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

3. Is it on FIRE?

4. Luminosity ~ 10,000 years Chemical Energy Content

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

6. Is it CONTRACTING?

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

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

9. Why does the Sun shine?

10. It can be powered by NUCLEAR ENERGY! Luminosity ~ 10 billion years Nuclear Potential Energy (core) E = mc 2 - Einstein, 1905

11. Weight of upper layers compresses lower layers

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

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

14. Layers of the Sun

15. Radius: 6.9 x 10 8 m (109 times Earth) Volume: Over 1 million Earths. Mass: 2 x 10 30 kg (300,000 Earths)  Density: 1/3 rd Earth’s Luminosity: 3.8 x 10 26 watts

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

17. Corona: Outermost layer of solar atmosphere ~1 million K

18. Chromosphere: Middle layer of solar atmosphere ~ 10 4 - 10 5 K

19. Photosphere: Visible surface of Sun ~ 6,000 K We get spectrum

20. Convection Zone: Energy transported upward by rising hot gas, convective flow.

21. Radiation Zone: Energy transported upward by photons, absorbed and re- emited every few centimeters.

22. Core: Energy generated by nuclear fusion ~ 15 million K

23. The Sun’s structure –From inside out, the layers are: Core Radiation Zone Convection Zone Photosphere Chromosphere Corona

24. Nuclear Fusion in the Sun

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

26. High temperature enables nuclear fusion to happen in the core Strong force is like ‘velcro’.

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

29. IN 4 protons OUT 4 He nucleus 2 gamma rays 2 positrons 2 neutrinos Total mass is 0.007 lower (Agent 007)

30. 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

31. 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

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

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

34. Energy gradually leaks out of radiation zone in form of randomly bouncing (every few cm) and absorbed and re- emitted photons. ‘RANDOM WALK’ takes millions of years!

35. Convection (rising hot gas) takes energy to surface in convective zone.

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

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

38. We learn about inside of Sun by … Making mathematical models—now very accurate for it and all stars. Observing solar vibrations Observing solar neutrinos

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

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

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

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

43. 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—one type morphs equally into three types, one of which is detected.

44. What causes solar activity?

45. Solar activity is like “weather” Sunspots Solar Flares Solar Prominences Coronal Mass Ejections All are related to magnetic fields

46. Sunspots Are cooler than other parts of the Sun’s surface (4000 K) Are regions with strong magnetic fields And come in N-S pairs

47. Zeeman Effect We can measure magnetic fields in sunspots by observing the splitting of spectral lines

48. Charged particles spiral along magnetic field lines

49. Loops of bright gas often connect sunspot pairs, magnetic field lines always do.

50. Magnetic activity causes solar flares that send bursts of X-rays and charged particles into space

51. Magnetic activity also causes solar prominences that erupt high above the Sun’s surface

52. Corona appears bright in X-ray photos in places where magnetic fields trap hot gas

53. How does solar activity affect humans?

54. Coronal mass ejections send bursts of energetic charged particles out through the solar system

55. Charged particles streaming from Sun can disrupt electrical power grids and can disable communications satellites. EMP effect could take our all computers, cell phones, and modern cars. A burn-off of the surface of Earth could even happen—HIGHLY unlikely!

56. How does solar activity vary with time?

57. Number of sunspots rises and falls in 11-year cycle

58. Sunspot cycle has something to do with winding and twisting of Sun’s magnetic field—Babcock’s Theory. Snapping of field lines like stressed rubber bands ends the solar cycle and all sunspots disappear.