Chapter 10 Our Star

1. Chapter 10Our Star

2. Why does the Sun shine?

3. Is it on FIRE?

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

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

6. Is it CONTRACTING?

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

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

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

10. 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. • To shine for that long, the Sun must be fairly stable, balancing gravity pulling in with forces pushing out from light and heat.

11. Why does the Sun shine? • The balance of forces in and out is gravitational equilibrium • The Sun shines because gravitational equilibriumkeeps its core hot and dense enough to release energy through nuclear fusion.

12. Weight of upper layers compresses lower layers

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

14. What is the Sun’s structure?

15. What is the Sun’s structure? • From what we can see going outwards… • Photosphere • Chromosphere • Corona These we CAN see – and measure with Visible-light, UV, and X-ray telescopes in orbit & on Earth

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

17. Photosphere: • Visible surface ~ 6,000 K • Seen in Visible Light • Shows • Dark “limb” • Granules • Sunspots! • Absorption lines

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

22. Credit: Chromosphere image courtesy Luc Viatour and Windows to the Universe, http://www.windows.ucar.edu

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

28. The Corona – Visible Light The Corona – X-ray Light

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

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

33. What is the Sun’s structure? • Going *inwards* to layers we cannot see… • Convection zone • Radiation zone • Core These we can’t see – we model with computer simulations, observations of surface features to suggest structure, and observations of neutrinos to hint at fusion reactions taking place in the core.

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

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

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

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

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

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

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

42. 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

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

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

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

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

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

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

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

50. 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.