Chapter 13: Our Star – The Sun

1. Chapter 13: Our Star – The Sun

2. The Sun Today Visible light Hydrogen Alpha light The Sun nearing the peak of its 11-year cycle. This cycle seems to be a low activity one.

3. The Sun in other wavelengths X-Ray Extreme UV Hydrogen Alpha These show what the Sun can look like when it is active

4. Basic Solar Facts Diameter: 1.4 million km Age: 4.5 billion years Mass: 330,000 x Earth Average Density: 1.41 gm/cm3 Distance from Earth: 149.6 million km Average Solar Wind Speed: 3 million km/hr Luminosity: 3.9x1026watts Temperature at surface: 5,770 K Temperature at Core: 15,000,000 K Rotation Period at Equator: 25 Earth days Rotation Period at Poles: 35 Earth days

5. What is the interior of the Sun like? How do we know?

6. We make models based on hydrostatic equilibrium

7. The models tell us the conditions inside the Sun

8. We compare what the models predict to what we can see We observe the Sun in as many different wavelengths to learn as much as possible about how it works

9. Where does the Sun get its energy? To the ancients who believed the Earth was the center of the universe, the Sun was made of quintessence, an element whose property was to glow. The concept of “energy” wasn’t even invented until the late 1600’s.

10. By the 1700’s the best ideal for the source of the Sun’s energy was chemistry If Sun was highest quality chemical fuel (i.e. pure carbon coal) it would exhaust its fuel in less than 10,000 years!

11. Later ideas for source of Sun’s energy: Gravitational Collapse Kelvin-Helmholtz Contraction Whenever anything shrinks it heats up.This could produce the observed solar output for about 25 million years. This was the original source of energy as the Sun was forming.

12. In 1905 Einstein proposed a new way to get energy: from matter The answer came from his famous equation: E = mc2

13. By the 1920’s Sir Arthur Eddington proposed the Sun produced energy by fusion The same source of energy as the hydrogen bomb

14. The Sun converts hydrogen into helium in a multi step process Hans Bethe worked out the details of hydrogen fusion in the 1930’s It starts with two normal hydrogen nuclei fusing to form a heavy hydrogen nuclei: a deuterium. The reaction also produces a positron and a neutrino. When the positron annihilates with an electron two gamma rays are produced

15. The second step uses the product of the first step plus another hydrogen A deuterium and a hydrogen fuse to form a helium-3 plus a gamma ray photon

16. Final step of the Proton-Proton cycle Two helium-3 fuse to form a normal helium plus two hydrogen nuclei

17. Overall Proton – Proton Cycle 4 1H Þ4He + 2e+ + 2g + 2n Releases 4.3x10-12 Joules per helium atom produced The Sun converts600,000,000 tonnes of H into 596,000,000 tonnes of He every second! The difference in mass is the energy produced according to E = mc2. This is only a 0.67% efficient conversion! The Sun has enough hydrogen in its’ core to last another 5 billion years before it runs out Energy is only produced in the core region where the temperature and pressure are high enough Watch ClassAction Proton-Proton Animation in Sun and Solar Energy Module

18. Fusion requires high temperatures to overcome the electric repulsion of protons The electric force between the protons is repulsive. The strong nuclear force between them is attractive but it is a very short range force so they have to get very close

19. The solar neutrino problem Step 1 in the Proton-Proton cycle Early measurements only detected 1/3 as many as predicted by solar models. Either the models were wrong or we didn’t really understand the neutrino

20. Detecting neutrinos is not easy Neutrinos come in three “flavors” and early experiments could only detect one flavor

21. We now know the neutrino changes “flavor” Current experiments can detect all three flavors and we are now finding just as many as our solar theories predict

22. How does the energy get from the core to the surface? Most of the energy released at the surface is in the form of visible photons Most of the energy released in the core is in the form of gamma ray photons Something must happen between the core and the surface!

23. Heat Transfer: Energy can move by one of three methods Conduction: atomic & molecular vibrations in solids. Example…cast iron skillet Convection: large scale motions in liquids and gasses Example…boiling water Radiation: electromagnetic radiation Example…heat lamp

24. Which method works best is determined by pressure, density and temperature

25. Once again, we use hydrostatic equilibrium models to determine which method works best at each layer of the Sun

26. The Solar Interior The models tell us that radiation is the means of heat transport for the first 70% of the way then convection takes over near the surface before going back to radiation at the surface.

27. How can we verify our models for the interior of the Sun? GONG stands for Global Oscillating Network Group

28. Solar Surface Oscillations The surface of the Sun oscillates in many different ways. This shows one of the ways it oscillates (extremely exaggerated)

29. Different ways sound bounces around inside Sun How different waves travel depends on the density and temperature of the gas it is travelling through

30. Internal Structure from Surface Waves

31. Actual Image of Oscillations

32. Internal Differential Rotation

33. The Photosphere The Sun is a ball of gas so there is no surface. The “surface” is a layer of gas that is only about four hundred kilometers thick. The density of the gas has to be just right to emit a good blackbody spectrum. Too dense and the light can’t get out. Not dense enough and not enough light is produced

34. Solar Granulation Each granule is ~1000 km across and lasts a minute or two

35. Granulation Cells are Convection Cells The photosphere is the top of the convection layer of the Sun where convection changes back to radiation

36. Note the decrease in temperature in the photosphere

37. The Chromosphere

38. Spicules

39. The temperature in the chromosphere climbs slowly but then jumps up in the corona

40. The Corona The corona eventually fades into the solar wind. There are other components to the solar wind as well but the corona contributes the main wind.

41. The corona is heated by coronal loops Note the size of Earth for scale

42. The solar wind moves outwards at 1 to 3 million kilometers per hour At those speeds it can take two to six days to reach Earth

43. Eventually the Sun’s magnetic field plows into the galactic field

44. Sunspots

45. A Sunspot Close-up Penumbra Umbra The umbra can be up to two thousand degrees cooler than the surrounding photosphere

46. Under a sunspot Heat flow is stopped by the sunspot like a cork in a bottle. The heat has to go somewhere, though, so it squirts out around the edges

47. The Solar Cycle The number of sunspots increases and decreases with about an eleven year cycle

48. The location of sunspots changes during the solar cycle

49. The cause of sunspots lies in differential rotation Solar Rotation Period versus latitude and depth

50. Sunspots are created by kinking in the Sun’s magnetic field It starts with a global magnetic field but the differential rotation causes it to wrap around and get kinked up after a few rotations