1. Our Star

2. A Closer Look at the Sun Our goals for learning
Why was the Sun’s energy source a major mystery?
Why does the Sun shine?
What is the Sun’s structure?

3. Why was the Sun’s energy source a major mystery?

4. Scientists knew the Sun had existed and shone for millions of years.
Could calculate the Sun’s daily energy output
Could not work out what fuel source could provide so much power for so long

5. Is it on FIRE? … NO!
Chemical Energy Content
~ 10,000 years
Luminosity
Is it CONTRACTING? … NO!
Gravitational Potential Energy
~ 25 million years
Luminosity
It can be powered by NUCLEAR ENERGY! E=mc2
Nuclear Potential Energy (core)
~ 10 billion years
Luminosity

6. Why does the Sun shine?

7. Composition of the Sun The Sun is a plasma - an extremely hot gas
The atoms’ electrons are ionized - stripped away from the nucleus
The Sun is made mostly of:
Hydrogen, the smallest element (~70%)
Helium, the 2nd smallest element (~30%)
Atom Ion
Atom Ion

8. Fusion occurs in the innermost part of the Sun because the weight of upper layers compresses then crushes lower layers

9. Gravitational contraction:
Provided energy that heated core as Sun was forming. Intense heat eventually forced fusion.
Contraction stopped when fusion began

10. Stable Star
Sun’s core fuses hydrogen into helium, releasing energy into the outer layers of the Sun.
The energy causes gas expansion, pushing outward against the gravity pulling inward. This expansion is called thermal pressure.
When the pressures balance, a star is in gravitational equilibrium.

11. What have we learned? Why was the Sun’s energy source a major mystery?
Chemical and gravitational energy sources could not explain how the Sun could sustain its luminosity for more than about 25 million years
Why does the Sun shine?
The Sun shines because gravitational equilibrium keeps its hydrogen core hot and dense enough to release energy through nuclear fusion.

12. 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?

13. How does nuclear fusion occur in the Sun?

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

15. Fusion occurs in the innermost part of a star because the weight of upper layers compresses then crushes lower layers

16. High temperatures cause particles to smash together
High temperatures cause particles to smash together. Enables nuclear fusion to happen in the core

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

18. The Complete Fusion Reaction
Proton-proton chain is how hydrogen fuses into helium in Sun

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

20. Solar Thermostat
The energy released from fusion causes the sun’s gases to expand (thermal expansion).
The balancing act between thermal expansion pushing out and gravity contracting inward, forces the temperature to stay constant.

21. Solar Thermostat If temperature falls because fusion rate drops :
If temperature rises because fusion rate increases:
Thermal pressure drops, gravity gains
=>core contracts and heats back up
Thermal pressure increases over gravity.
=>core expands, which causes core to cool down

22. Solar Thermostat
Therefore, gravitational equilibrium, causes thermal equilibrium, as long as a star’s core continues to fuse.
Sun is estimated to do this for another 5 billion years!

23. © 2014 Pearson Education, Inc.
What is the net fusion reaction that produces energy in the core of the Sun?
4 hydrogen nuclei form 1 helium nucleus plus energy.
2 hydrogen nuclei form 1 helium nucleus plus energy.
6 hydrogen nuclei form 1 helium nucleus, 1 carbon nucleus plus energy.
3 hydrogen nuclei form 1 helium nucleus plus energy.
4 hydrogen nuclei form 1 helium nucleus, 1 carbon nucleus, plus energy.
Answer: a) 4 hydrogen nuclei form 1 helium nucleus plus energy.
© 2014 Pearson Education, Inc. 23

24. What balances the inward push of gravity inside the Sun?
the rigidity of the solid central core
electron degeneracy pressure
thermal pressure of the gas
neutron degeneracy pressure
Answer: c) thermal pressure of the gas
© 2014 Pearson Education, Inc. 24

25. What have we learned? How does nuclear fusion occur in the Sun?
The core’s extreme temperature and density are just right for nuclear fusion of hydrogen to helium through the proton-proton chain
Gravitational equilibrium acts as a thermostat to regulate the core temperature because fusion rate is very sensitive to temperature

26. What is the Sun’s structure?

27. } } Sun Contains Core Radiation Zone Convection Zone Photosphere
Chromosphere
Corona
Solar wind }
interior
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 }
Atm.

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

29. Radiation Zone:
Energy transported upward by photons

30. Convection Zone:
Energy transported upward by rising hot gas

31. Photosphere:
Visible surface of Sun
~ 6,000 K

32. Chromosphere:
Middle layer of solar atmosphere
~ K

33. Absorption lines due to the Sun’s atmosphere

34. Corona:
Outermost layer of solar atmosphere
~1 million K
Emits x-rays and streamers that follow the magnetic lines

35. Solar wind:
The corona dissipates, becoming a flow of charged particles from the surface of the Sun

36. How does the energy from fusion get out of the Sun?

37. Energy moves randomly around the radiation zone in form of randomly bouncing photons. Eventually it makes it to the edge of the radiative zone and into the convective zone

38. Convection (rising hot gas) takes energy to surface

39. Bright blobs on photosphere are where hot gas is reaching the surface (effect is called granules, or granulation)

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

41. We learn about inside of Sun by …
Making mathematical models
Observing solar vibrations
Observing solar neutrinos

42. Patterns of vibration on surface tell us about what Sun is like inside
The vibrations reflect, reinforce or cancel each other which changes the patterns and frequencies seen at the Sun’s surface.
Download a movie to illustrate solar oscillations from:

43. Data on solar vibrations agree very well with mathematical models of solar interior
Download a movie to illustrate solar oscillations from:

44. Neutrinos created during fusion fly directly through the Sun
Observations of these solar neutrinos can tell us what’s happening in core
John Updike poem - permissions needed ?

45. Which of the following layers of the Sun is farthest from the core?
convection zone
photosphere
chromosphere
corona
Answer: d) corona
© 2014 Pearson Education, Inc. 45

46. What have we learned? What is the Sun’s structure?
From inside out, the layers are:
Core
Radiation Zone
Convection Zone
Photosphere
Chromosphere
Corona
Solar wind

47. What have we learned?
How does the energy from fusion get out of the Sun?
Randomly bouncing photons carry it through the radiation zone
Rising of hot plasma carries energy through the convection zone to photosphere
How do we know what is happening inside the Sun?
Mathematical models agree with observations of solar vibrations and solar neutrinos

48. The Sun-Earth Connection
Our goals for learning
What causes solar activity?
How does solar activity vary with time?
How does solar activity affect humans?

49. What causes solar activity?

50. Solar activity is like “weather”
Sunspots
Solar Flares
Coronal Mass Ejections (CMEs)
Solar Prominences
All are related to magnetic fields

51. Sunspots

52. Sunspots
Are cooler than other parts of the Sun’s surface (4000 K)
Are regions with strong magnetic fields

53. Charged particles spiral along magnetic field lines,
emitting light as they go: x-ray light.

54. Sun at Multiple Wavelengths
Microwave Visible Ultra-violet X-Ray

55. Magnetic field lines pierce the surface
Loops of bright gas often connect sunspot pairs

57. The Corona appears bright in X-ray photos in places where magnetic fields trap hot gas
Wave_fade.mpg photosphere:chromosphere:corona, optical:UV:X-ray

58. Magnetic activity can cause solar prominences that erupt high above the Sun’s surface
The particles associated with the prominance can be flung out into space making a Coronal Mass Ejection

59. Magnetic activity also causes solar flares that send bursts of X-rays into space

60. How does solar activity vary with time?

61. Number of sunspots rises and falls in 11-year cycle

62. Sun in X-Rays over Solar Cycle

63. Sunspot cycle has something to do with winding and twisting of Sun’s magnetic field

64. The magnetic lines erupt out and can snap.
The snapping lines fling particles or energy (or both) out through the solar system
Bursts of charged particles are Coronal Mass Ejections
Bursts of x-rays are Solar Flares

65. Coronal Mass Ejection or Solar Flare

67. How does solar activity affect humans?

69. Solar Flare Headed toward Earth

70. Charged particles streaming from Sun can disrupt electrical power grids and can disable communications satellites

71. Alaska Aurora

72. What have we learned? What causes solar activity?
Stretching and twisting of magnetic field lines near the Sun’s surface causes solar activity.
Activity detectable as sunspots
How does solar activity vary with time?
Activity rises and falls with an 11-year period
How does solar activity affect humans?
Bursts of charged particles from the Sun can disrupt communications, satellites, and electrical power generation

73. Live Solar Observation at the Big Bear Solar Observatory
Daily Space weather and geostorm warning