Chapter 14

1. Chapter 14 • Our Star

2. p. 111

3. Ideas About the Sun’s Energy Production • Pre-19th century • Fire due to wood or coal • Ruled out after we knew the actual size and distance to the Sun • 19th Century • Gravitational Contraction • Could maintain energy output for 25 million years • Ruled out when we discovered that the earth was far older than 25 million years

4. So Why Does the Sun shine? • E=mc2 • Conversion of matter into energy through nuclear fusion • For fusion to occur, the Sun must generate extreme temperature internally without flying apart

5. Nuclear Fission vs. Nuclear Fusion Fusion Small nuclei stick together to make a bigger one. (Example: the Sun, stars) Fission Big nucleus splits into smaller pieces. (Example: nuclear power plants)

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

8. The energy comes from the fact that the Helium nucleus has slightly less mass (0.7%) than the sum of two protons and two neutrons. It appears as kinetic energy and the energy of the gamma-ray photons

9. Proton-Proton Cycle Need large proton speed ( high temperature) to overcome Coulomb barrier (electromagnetic repulsion between protons). Basic reaction: 4 1H 4He + energy 4 protons have 0.048*10-27 kg (= 0.7 %) more mass than 4He. T ≥ 107 K = 10 million K • Energy gain = m*c2 = 0.43*10-11 J per reaction. Sun needs 1038 reactions, transforming 5 million tons of mass into energy every second, to resist its own gravity.

10. 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. The solar thermostat keeps burning rate steady.

11. Gravitational Equilibrium Contraction of the sun due to its own gravity provided the energy that heated the core as Sun was forming. Once nuclear fusion began, it generated pressure that halted the contraction. The Sun exists is a state of gravitational equilibrium (also called hydrostatic equilibrium

12. 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.

13. Calculations show that the Sun has enough hydrogen in its core to maintain gravitational equilibrium for 10 billion years. • We will investigate what happens after that in a later chapter

14. Sun’s Basic Structure Insert TCP 6e Figure 14.3

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

16. Radiation Zone: Energy transported upward by photons until they reach a region where the temperature has dropped to about 2 million K. Photons get absorbed there.

17. Convection Zone: Absorbed photons heat the gas at the bottom of the convection zone. Energy is transported upward by rising hot gas

18. Photosphere: Visible surface of Sun ~ 5800 K

19. Chromosphere: Middle layer of solar atmosphere ~ 104–105 K

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

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

22. Radiative Diffusion Most of the energy from the Sun works its way out from the core as photons. They execute a random walk to make their way out. It takes thousand of years for energy liberated in the core to get to the top of the radiation zone.

23. Follow a single gamma ray as it heads toward the surface. The ray is scattered by electrons in the core until it reaches cooler gas The cooler gas absorbs the gamma photon and emits two x-ray photons, each of less energy than the gamma photon The x-rays migrate toward cooler regions where they are absorbed and emitted as still longer-wavelength photons. This process repeats until the single gamma in the core has resulted in about 1800 photons of lower energy. Energy Transport in the Sun

24. Convection Zone Photons absorbed at the plasma near the surface heats the plasma and results in a convection zone (rising hot gas) taking energy to surface.

25. The Photosphere: Granules • Bright central regions are hotter than the edges so they appear brighter. • Doppler shifts show that hot gas is rising in the center and sinking at the edges • Each granule is about the size of Texas • They last about 10-20 minutes

26. Supergranuleswhich are a little over twice Earth’s diameter, include about 300 granules each. These supergranules are regions of very slowly rising currents that last a day or two. They appear to be produced by larger currents of rising gas deeper under the photosphere. Supergranules

27. Below the photosphere, the gas is dense and hot and therefore radiates a continuous spectrum of light. Atoms in the photosphere absorb photons of specific wavelengths—producing the absorption lines you see. Photosphere Spectrum

28. The chromospherelies above the photosphere. Visible to unaided eye only during a total eclipse Pink color due to red, blue, and violet Balmer emission lines of hydrogen. Chromospherre

29. The chromosphere produces an emission spectrum. Atoms in the lower chromosphere are ionized, and atoms in the higher layers of the chromosphere are even more highly ionized. Can be used to determine the temperature of various parts of the chromosphere. Chromosphere Spectrum

30. Just above the photosphere, the temperature falls to a minimum of about 4,500 K and then rises rapidly to the extremely high temperatures of the corona. Temperature Profile

31. The outermost part of the sun’s atmosphere is called the corona, after the Greek word for crown. You can see the inner parts of the corona during a solar eclipse. The Solar Corona

32. Chronograph • Observations made with specialized telescopes called coronagraphson Earth or in space can block the light of the photosphere and record the corona out beyond 20 solar radii. • Such images reveal that magnetic fields link the sunspots with features in the chromosphere and corona.

33. Complex coronal spectrum Sunlight reflected from dust particles produces a spectrum with absorption lines just like the sun When sunlight from the photosphere is scattered off free electrons in the ionized coronal gas, it produces a continuous spectrum without absorption lines. Fast moving electrons produce photons with a large Doppler shift smearing out the absorption lines Emission lines of highly ionized gas are superimposed on this continuous spectrum. Higherin the corona means more ionization – implies the temperature rises Coronal Spectrum

34. Just above the chromosphere, the temperature is about 500,000 K. In the outer corona, it can be as high as 2 million K or more. Coronal Temperature Profile

35. Magnetic fields extend from the photosphere to the corona. Turbulence in the photosphere whips the field around As the gas of the chromosphere and corona has a very low density, it can’t resist movement in the magnetic fields. This motion heats the gas Heating the Corona

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

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

38. Mathematical Models • Use basic physics to develop equation to describe the measured properties of the Sun • Using a computer we can calculate the temperature, density, and pressure at any depth within the Sun • From these we predict the rate of fusion can predict many measurable properties of the Sun. • If these models correctly predict the known properties, then we gain confidence that we really do understand what is going on in the solar interior.

39. Helioseismology • Random motions in the sun constantly produce vibrations like resonance in organ pipes. • These are sound waves of very long wave period. • These vibrations are observed by Doppler shifts. • Interpretation requires a lot of data to reconstruct the subsurface features. • GONG, Global Oscillation Network Group, uses telescopes spread around the world to observe the sun continuously.

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

41. Solar Neutrinos • Neutrinos created during fusion fly directly through the Sun. • Observations of these solar neutrinos can tell us what’s happening in core. • Neutrinos interact very rarely with matter. Trillions pass through you every second. So they are very difficult to detect.

42. Raymond Davis filled a 100,000-gallon tank with the cleaning fluid perchloroethylene (C2Cl4). Theory predicted that, about once per day, a solar neutrino would convert a chlorine atom in the tank into radioactive argon. This could be detected later by its radioactive decay. Results showed only one event every 3 days There were two possible explanations We didn’t correctly understand how the sun and stars make their energy. There was something about neutrinos that we did not understand. Searching for Neutrinos

43. Solar Neutrino Problem • Early searches for solar neutrinos failed to find the predicted number, but they were looking only for electron neutrons, the kind produced by fusion in the Sun • We discovered that some neutrinos change into other types (muon and tauneutrinos) in a process called oscillation. • More recent observations find the right number of neutrinos Illustrates the great synergy between astrophysics and subatomic physics

44. This solution to the solar neutrino problem is exciting because neutrinos can’t oscillate unless they have mass. • Neutrinos were long thought to be massless. • However, if they have even a small mass, they are so common their gravity could affect the evolution of the universe as a whole.

45. Solar Activity

46. Solar activity is like “weather”. • Spicules • Sunspots • Solar flares • Solar prominences All these phenomena are related to magnetic fields. They can at times affect our daily lives.

47. Spiculesare small plasma burst from the photosphere into the chromosphere. They are created by periodic sound waves leaving the sun. The plasma is concentrated to a small thread like jet due to strong magnetic fields. that last 5 to 15 minutes Spicules

48. Sunspots • Cooler than other parts of the Sun’s surface (4000 K) • Regions with strong magnetic fields • They appear dark by comparison

49. In the early 17th century, Galileo observed the sun and saw spots on its surface. Day by day, he saw the spots moving across the sun’s disk. These are sunspots. He rightly concluded that the sun was rotating. Observing the Sun

50. Sunspot Rotation http://www.youtube.com/watch?v=U0Lt3SgiEQ8