1. Chapter 11: Our Star
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2. 11.1 A Closer Look at the Sun Our goals for learning:
Why does the Sun shine?
What is the Sun's structure?
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3. Why does the Sun shine?
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4. Is it on FIRE?
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5. Chemical Energy Content ~ 10,000 years Luminosity
Is it on FIRE?
Chemical Energy Content
~ 10,000 years
Luminosity
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6. Chemical Energy Content
Is it on FIRE? … NO!
Luminosity
~ 10,000 years
Chemical Energy Content
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7. Is it CONTRACTING?
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8. Gravitational Potential Energy ~ 25 million years
Is it CONTRACTING?
Luminosity
Gravitational Potential Energy
~ 25 million years
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9. Gravitational Potential Energy ~ 25 million years Luminosity
Is it CONTRACTING? … NO!
Gravitational Potential Energy
~ 25 million years
Luminosity
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10. It is powered by NUCLEAR ENERGY!
E = mc2 — Einstein, 1905
It is powered by NUCLEAR ENERGY!
Nuclear Potential Energy (core)
~ 10 billion years
Luminosity
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11. Weight of upper layers compresses lower layers.
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12. Gravitational equilibrium:
Gravity pulling in balances pressure pushing out.
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13. Energy balance:
Thermal energy released by fusion in core balances radiative energy lost from surface.
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14. Gravitational contraction… provided energy that heated the core as the Sun was forming.
Contraction stopped when fusion started replacing the energy radiated into space.
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15. What is the Sun's structure?
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16. Radius: 6.9 ✕ 108 m (109 times Earth) Mass: 2 ✕ 1030 kg
(300,000 Earths)
Luminosity:
3.8 ✕ 1026 watts
Good Sun movies to download:
Images compiled from Owen Gingerich's copy of the first edition of Istoria e Dimostrazioni.
Wave_fade.mpg photosphere:chromosphere:corona, optical:UV:X-ray
C2_1mth.mpg
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17. Solar wind: A flow of charged particles from the surface of the Sun
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18. Corona: Outermost layer of solar atmosphere~ 1 million K
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19. Chromosphere:Middle layer of solar atmosphere ~ 104–105 K
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20. Photosphere:Visible surface of the Sun ~ 6000 K
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21. Convection zone: Energy transported upward by rising hot gas
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22. Radiation zone: Energy transported upward by photons
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23. Core: Energy generated by nuclear fusion ~ 15 million K
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24. 11.2 Nuclear Fusion in the Sun
Our goals for learning:
How does nuclear fusion occur in the Sun?
How does the energy from fusion get out of the Sun?
How do we know what is happening inside the Sun?
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25. How does nuclear fusion occur in the Sun?
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26. Big nucleus splits into smaller pieces.
Fission
Big nucleus splits into smaller pieces.
(Nuclear power plants)
Fusion
Small nuclei stick together to make a bigger one.
(Sun, stars)
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27. High temperatures enable nuclear fusion to happen in the core.
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28. The Sun releases energy by fusing four hydrogen nuclei into one helium nucleus.
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29. The Proton–proton chain is how hydrogen fuses into helium in the Sun.
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30. IN OUT Total mass is 0.7% lower.
4 protons
OUT
4He nucleus
2 gamma rays
2 positrons
2 neutrinos
Total mass is
0.7% lower.
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31. Thought Question
What would happen inside the Sun if a slight rise in core temperature led to a rapid rise in fusion energy?
The core would expand and heat up slightly.
The core would expand and cool.
The Sun would blow up like a hydrogen bomb.
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32. Thought Question
What would happen inside the Sun if a slight rise in core temperature led to a rapid rise in fusion energy?
The core would expand and heat up slightly.
The core would expand and cool.
The Sun would blow up like a hydrogen bomb.
Solar thermostat keeps the rate of fusion steady.
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33. 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.
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34. How does the energy from fusion get out of the Sun?
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35. Energy gradually leaks out of the radiation zone in the form of randomly “bouncing” photons.
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36. Convection (rising hot gas) takes energy to the surface.
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37. Bright blobs on photosphere where hot gas reaches the surface
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38. How do we know what is happening inside the Sun?
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39. We learn about the inside of the Sun by
making mathematical models
observing solar vibrations
observing solar neutrinos
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40. Patterns of vibration on the surface tell us about what the Sun is like inside.
Download a movie to illustrate solar oscillations from
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41. Data on solar vibrations agree with mathematical models of solar interior.
Download a movie to illustrate solar oscillations from
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42. Neutrinos created during fusion fly directly through the Sun.
Observations of these solar neutrinos can tell us what's happening in the core.
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43. Solar neutrino problem:
Early searches for solar neutrinos failed to find the predicted number.
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44. Solar neutrino problem:
Early searches for solar neutrinos failed to find the predicted number.
These careful measurements were awarded the Nobel Prize in physics in 2002.
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45. Solar neutrino problem:
More recent observations find the right number of neutrinos, but some have changed form.
The solar neutrino problem led to the understanding that neutrinos have mass.
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46. 11.3 The Sun–Earth Connection
Our goals for learning:
What causes solar activity?
How does solar activity vary with time?
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47. What causes solar activity?
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48. Solar activity is like "weather" on Earth.
Sunspots
Solar flares
Solar prominences
All these phenomena are related to magnetic fields.
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49. Are cooler than other parts of the Sun's surface (4000 K).
Sunspots…
Are cooler than other parts of the Sun's surface (4000 K).
Are regions with strong magnetic fields.
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50. Zeeman Effect
We can measure magnetic fields in sunspots by observing the splitting of spectral lines.
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51. Charged particles spiral along magnetic field lines.
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52. Loops of bright gas often connect sunspot pairs.
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53. Magnetic activity causes solar flares that send bursts of X rays and charged particles into space.
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54. Magnetic activity also causes solar prominences that erupt high above the Sun's surface.
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55. The corona appears bright in X-ray photos in places where magnetic fields trap hot gas.
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56. Coronal mass ejections send bursts of energetic charged particles out through the solar system.
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57. Charged particles streaming from the Sun can disrupt electrical power grids and disable communications satellites.
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58. How does solar activity vary with time?
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59. The number of sunspots rises and falls in 11-year cycles.
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60. The sunspot cycle has something to do with the winding and twisting of the Sun's magnetic field.
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61. How does solar activity vary with time?
Does solar activity and/or the sunspot cycle affect
the Earth’s climate?
The Maunder minimum ( ) corresponds
to a time of very low temperatures recorded in
Europe and North American known as the “Little
Ice Ages”.
However, clear connections have not been established
and the study of solar activity’s possible effects on
climate remains an active field of research.
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