1. Lecture Slides CHAPTER 11: Our Star: The Sun
Understanding Our Universe
SECOND EDITION
Stacy Palen, Laura Kay, Brad Smith, and George Blumenthal
Prepared by Lisa M. Will, San Diego City College

2. Our Star: The Sun
Describe the structure of the Sun’s interior and atmosphere.
Explain how energy is produced and transported by the Sun.
Understand solar activity cycles.

3. Solar Interior We only see the outer layers of the Sun, a G2 star.
Physics tells us about the interior.
The structure of the Sun is caused by a balance between forces due to pressure and gravity.
Figure 11.2 (a)

4. Solar Interior: Hydrostatic Equilibrium
The Sun must be in balance to exist for billions
of years.
Hydrostatic equilibrium: outward pressure = inward force of gravity at every point in the Sun.
Figure 11.2 (a) 4

5. Solar Interior: Sun’s Surface
Density, pressure, and temperature decrease away from the center of the Sun.
Energy production peaks in the center – the core of the Sun.
Figure 11.2 (a)

6. Energy Production Protons: positive electrical charge.
Recall: Nuclei may consist of protons and neutrons.
Protons: positive electrical charge.
Neutrons: no electrical charge.
Protons are kept apart by electric repulsion.
Figure 11.3

7. Energy Production: Nuclear Fusion
The strong nuclear force binds the nucleus together.
Fusion requires ramming protons together at high speed (i.e., at high temperature).
Fusion also requires high densities to insure collisions.
Figure 11.3 7

8. Energy Production: Nuclear Fusion (Cont.)
Figure 11.3 (a) 8

9. Energy Production: Nuclear Fusion (Cont.)
They get close enough for the strong nuclear force to overpower electric repulsion
Figure 11.3 (b) 9

10. Energy Production: Nuclear Fusion (Cont.)
Figure 11.3 (c) 10

11. Energy Production: Sun’s Lifetime
The Sun has been around a long time, about 4.6 billion years.
The Sun must therefore generate a lot of energy over a long time.
Source: nuclear fusion of hydrogen to helium in the core of the Sun.
Fusion takes place in the core, where it is hot and dense enough. 11

12. More facts about the Sun
The core of the Sun extends from the center to about 20–25% of the Sun’s radius.
The Sun’s radius is 109 times bigger than the Earth’s radius!
The core of the sun has a temperature of about 15.7 million Kelvin!
The core of the sun has a density of 150 g/cm3 (but the average density of the Sun is only about 1.4g/cm3 ). Note that the density of water is 1g/cm3.

13. Energy Production: The Proton-Proton Chain13

14. Fusion process: proton-proton chain.
Net result: 4 hydrogen nuclei turn into 1 helium nucleus, plus neutrinos, positrons and gamma rays! Positrons are just like electrons but have positive charge. Neutrinos have mass but have almost no interaction with matter—they easily go right through the Earth!
Why is energy produced?

15. Energy Production: Hydrogen Burning Process
Mass of 4 H nuclei is slightly greater than 1 He nucleus. It’s about 0.7% greater.
Thus, a little bit of mass is converted into energy as 4 H nuclei fuse into 1 He nucleus.
Relativity: mass and energy are equivalent: E = mc2
Difference in mass is released as energy.
This nuclear fusion process is often called hydrogen burning.
This happens for all main-sequence stars. 15

16. Class Question In which layer of the Sun does nuclear fusion occur?
Convective Zone
Core
Corona
Radiative Zone
Correct Answer: B

17. Energy Transport: Radiation Zone
Radiative zone: layer just outside of the core of the Sun.
Radiative transfer: photons travel from hotter to cooler regions.

18. Energy Transport: Convection Zone
Convection: rising/falling of hot/cool gas.
Convective zone: layer in between the radiative zone and the surface of the Sun. 18

19. Energy Transport
We can see evidence for the convective zone by looking at the surface of the Sun.
Convection is visible as bubbling of the surface.
Bubbles are large – the size of countries on Earth! 19

20. Energy Transport: Helioseismology
Helioseismology: sound waves move through the Sun, making surface and interior waves.
Doppler shifts give the speed of wave motion. 20

21. Energy Transport: Interior of the Sun
Speeds depend on the Sun’s composition and the depth of the convection zone.
Observations agree with models of the solar interior. 21

22. Surface
Photosphere: layer where light is emitted into space = apparent surface of the Sun.
Average temperature: 5800 K
Limb darkening: because we look through less material at the edges, it appears darker.
Figure 11.12 22

23. Surface (Cont.)
Figure (a) 23

24. Surface (Cont.)
Figure (b) 24

25. Atmosphere
Atmosphere: where the density drops very rapidly with increasing altitude.
The solar atmosphere is much less dense than the atmosphere of the Earth.

26. Atmosphere: The Solar Spectrum
Cooler outer layers of the Sun absorb some of the light from hotter, deeper layers.
This produces a complex absorption spectrum with more than 70 elements identified. 26

27. Atmosphere: Chromosphere
Chromosphere: layer directly above the photosphere.
Higher temperature than the photosphere.
Gives off a reddish emission-line spectrum due to hydrogen.
Figure 11.14 27

28. Atmosphere: Chromosphere (Cont.)
Figure (a) 28

29. Atmosphere: Chromosphere (Cont.)
Figure (b) 29

30. Atmosphere: Corona Corona: layer above the chromosphere.
Very hot: T = 1 to 2
million K.
Emits X-rays.
Can extend for several solar radii.
This picture was taken during a total solar eclipse (the only time you can see the corona with your eyes)
Figure (c) 30

31. Class Question Rank these layers of the Sun
in the correct order, from coolest to hottest.
Core, Corona, Photosphere
Photosphere, Core, Corona
Corona, Photosphere, Core
Photosphere, Corona, Core
Correct Answer: D

32. Magnetic Field
Sun’s magnetic field affects the structure of the atmosphere.
The solar wind: charged particles flowing away from the Sun through coronal holes, where magnetic field lines extend away from the Sun. 32

33. Magnetic Field: The Solar Wind Streams
Coronal material flows along the magnetic field away from the Sun.
This is the solar wind, which extends about AU.
The Voyager 1 spacecraft is traveling across this boundary. 33

34. Solar Activity: Sunspots
Figure 11.18
Sunspots: cooler areas in the photosphere.
Sunspot structure: dark umbra with surrounding penumbra.
Sunspots are caused by the solar magnetic field. 34

35. Solar Activity: Sunspots (Cont.)
Figure (a) 35

36. Solar Activity: Sunspots (Cont.)
Figure (b) 36

37. Solar Activity: Sunspots (Cont.)
Figure 11.19
The number of sunspots varies as a function of time.
The latitude of sunspots also varies as a function
of time.

38. Solar Activity: Sunspots (Cont.)
Figure (a) 38

39. Solar Activity: Sunspots (Cont.)
Figure (b) 39

40. Solar Activity: 11-year Sunspot Cycle
Sun shows an 11-year sunspot cycle.
Solar maximum: most sunspots and activity.
The Maunder minimum showed a distinct lack of sunspots between 1645 and 1715. 40

41. Class Question Why does the previous plot only date back to 1600?
The Sun did not have sunspots pre-1600.
There were previously too many sunspots to count.
Sunspots were difficult to observe before the
invention of the telescope.
Correct Answer: C

42. Solar Activity: Magnetic Flip
Figure 11.21
The Sun’s magnetic field flips every 11 years => Solar magnetic field shows a 22 year cycle. 42

43. Solar Activity: Magnetic Flip (Cont.)
Figure (a) 43

44. Solar Activity: Magnetic Flip (Cont.)
Figure (b) 44

45. Solar Activity: Solar Prominences and Flares
Prominences: hot rising gas in the chromosphere constrained by magnetic fields.
Solar flares and coronal mass ejections are highly energetic eruptions.
Top Image: Figure (b) 45

46. Solar Activity: Solar Prominences and Flares (Cont.)
Figure (b) 46

47. Solar Activity: Solar Prominences and Flares (Cont.)47

48. Solar Activity: Solar Prominences and Flares (Cont.)
The explosive behavior of the Sun – prominences, flares, coronal mass ejections – is tied to the sunspot cycle.
Activity is greater at solar maximum.
Figure 11.23 48

49. Solar Activity: Solar Prominences and Flares (Cont.)
Figure (a) 49

50. Solar Activity: Solar Prominences and Flares (Cont.)
Figure (b) 50

51. Solar Activity: Solar Prominences and Flares (Cont.)
Figure (c) 51

52. Solar Activity: Changes Over Time
Solar storms can disrupt electric power grids and satellites and cause brilliant auroras. 52

53. Chapter Summary
The Sun’s structure is maintained by hydrostatic equilibrium.
Nuclear reactions converting hydrogen to helium are the source of the Sun’s energy.
The Sun has multiple layers, each with a different densities, temperatures, and pressures.
Sunspot observations led to the discovery of 11- and 22-year cycles in solar activity.

54. Astronomy in Action
Random Walk
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55. Astronomy in Action
Inverse Square Law
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(Requires an active Internet connection)

56. AstroTour
The Solar Core
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57. Nebraska Applet
Proton-Proton Animation
Click the image to launch the Nebraska Applet (Requires an active Internet connection)

58. This concludes the Lecture Slides for
Understanding Our Universe
SECOND EDITION
Stacy Palen, Laura Kay, Brad Smith, and George Blumenthal
Prepared by Lisa M. Will, San Diego City College
This concludes the Lecture Slides for
CHAPTER 11: Our Star:
The Sun
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