1. Note that the following lectures include animations and PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode (presentation mode).

2. Chapter 8
The Sun – Our Star

3. Guidepost
The preceding chapter described how we can get information from a spectrum. In this chapter, we apply these techniques to the sun, to learn about its complexities.
This chapter gives us our first close look at how scientists work, how they use evidence and hypothesis to understand nature. Here we will follow carefully developed logical arguments to understand our sun.
Most important, this chapter gives us our first detailed look at a star. The chapters that follow will discuss the many kinds of stars that fill the heavens, but this chapter shows us that each of them is both complex and beautiful; each is a sun.

4. Outline I. The Solar Atmosphere A. Heat Flow in the Sun
B. The Photosphere
C. The Chromosphere
D. The Solar Corona
E. Helioseismology
II. Solar Activity
A. Sunspots and Active Regions
B. The Sunspot Cycle
C. The Sun's Magnetic Cycle
D. Magnetic Cycles on Other Stars
E. Chromospheric and Coronal Activity
F. The Solar Constant

5. Outline (continued) III. Nuclear Fusion in the Sun
A. Nuclear Binding Energy
B. Hydrogen Fusion
C. The Solar Neutrino Problem

6. General Properties Average star Spectral type G2
Only appears so bright because it is so close.
Absolute visual magnitude = 4.83 (magnitude if it were at a distance of 32.6 light years)
109 times Earth’s diameter
333,000 times Earth’s mass
Consists entirely of gas (av. density = 1.4 g/cm3)
Central temperature = 15 million 0K
Surface temperature = K

7. Very Important Warning:
Never look directly at the sun through a telescope or binoculars!!!
This can cause permanent eye damage – even blindness.
Use a projection technique or a special sun viewing filter.

8. The Solar Atmosphere Only visible during solar eclipses
Apparent surface of the sun
Heat Flow
Temp. incr. inward
Solar interior

9. The Photosphere Apparent surface layer of the sun Depth ≈ 500 km
Temperature ≈ 5800 oK
Highly opaque (H- ions)
Absorbs and re-emits radiation produced in the solar interior
The solar corona

10. Energy Transport in the Photosphere
Energy generated in the sun’s center must be transported outward.
In the photosphere, this happens through
Convection:
Cool gas sinking down
Bubbles of hot gas rising up
≈ 1000 km
Bubbles last for ≈ 10 – 20 min.

11. Granulation
… is the visible consequence of convection

12. Chromospheric structures visible in Ha emission (filtergram)
The Chromosphere
Region of sun’s atmosphere just above the photosphere.
Visible, UV, and X-ray lines from highly ionized gases
Temperature increases gradually from ≈ 4500 oK to ≈ 10,000 oK, then jumps to ≈ 1 million oK
Filaments
Transition region
Chromospheric structures visible in Ha emission (filtergram)

13. The Chromosphere (2)
Spicules: Filaments of cooler gas from the photosphere, rising up into the chromosphere.
Visible in Ha emission.
Each one lasting about 5 – 15 min.

14. The Layers of the Solar Atmosphere
Ultraviolet
Visible
Sun Spot Regions
Photosphere
Corona
Chromosphere
Coronal activity, seen in visible light

15. The Magnetic Carpet of the Corona
Corona contains very low-density, very hot (1 million oK) gas
Coronal gas is heated through motions of magnetic fields anchored in the photosphere below (“magnetic carpet”)
Computer model of the magnetic carpet

16. The Solar Wind Constant flow of particles from the sun.
Velocity ≈ 300 – 800 km/s
Sun is constantly losing mass:
107 tons/year
(≈ of its mass per year)

17. Approx. 10 million wave patterns!
Helioseismology
The solar interior is opaque (i.e. it absorbs light) out to the photosphere.
Only way to investigate solar interior is through Helioseismology
= analysis of vibration patterns visible on the solar surface:
Approx. 10 million wave patterns!

18. Cooler regions of the photosphere (T ≈ 4240 K).
Sun Spots
Cooler regions of the photosphere (T ≈ 4240 K).
Only appear dark against the bright sun. Would still be brighter than the full moon when placed on the night sky!

19. Sun Spots (2)
Active Regions
Visible
Ultraviolet

20. Face of the Sun
Solar Activity, seen in soft X-rays

21. Magnetic Fields in Sun Spots
Magnetic fields on the photosphere can be measured through the Zeeman effect
 Sun Spots are related to magnetic activity on the photosphere

22. Sun Spots (3)
Magnetic field in sun spots is about 1000 times stronger than average.
Magnetic North Poles
Magnetic South Poles
In sun spots, magnetic field lines emerge out of the photosphere.

23. Magnetic Field Lines Magnetic North Pole Magnetic South Pole

24. Image constructed from changing Doppler shift measurements
Star Spots?
Image constructed from changing Doppler shift measurements
Other stars might also have sun spot activity:

25. Reversal of magnetic polarity
The Solar Cycle
After 11 years, North/South order of leading/trailing sun spots is reversed
11-year cycle
Reversal of magnetic polarity
=> Total solar cycle = 22 years

26. Maunder Butterfly Diagram
The Solar Cycle (2)
Maunder Butterfly Diagram
Sun spot cycle starts out with spots at higher latitudes on the sun
Evolve to lower latitudes (towards the equator) throughout the cycle.

27. The Sun’s Magnetic Dynamo
The sun rotates faster at the equator than near the poles.
This differential rotation might be responsible for magnetic activity of the sun.

28. Magnetic Loops
Magnetic field lines

29. The Sun’s Magnetic Cycle
After 11 years, the magnetic field pattern becomes so complex that the field structure is re-arranged.
 New magnetic field structure is similar to the original one, but reversed!
 New 11-year cycle starts with reversed magnetic-field orientation

30. The sun spot number also fluctuates on much longer time scales:
The Maunder Minimum
The sun spot number also fluctuates on much longer time scales:
Historical data indicate a very quiet phase of the sun, ~ 1650 – 1700: The Maunder Minimum

31. Magnetic Cycles on Other Stars
H and K line emission of ionized Calcium indicate magnetic activity also on other stars.

32. Relatively cool gas (60,000 – 80,000 oK)
Prominences
Relatively cool gas (60,000 – 80,000 oK)
May be seen as dark filaments against the bright background of the photosphere
Looped Prominences: gas ejected from the sun’s photosphere, flowing along magnetic loops

33. Eruptive Prominences (Ultraviolet images)
Extreme events (solar flares) can significantly influence Earth’s magnetic field structure and cause northern lights (aurora borealis).

34. Coronal mass ejections
Space Weather
~ 5 minutes
Solar Aurora
Sound waves produced by a solar flare
Coronal mass ejections

35. Coronal Holes X-ray images of the sun reveal coronal holes.
These arise at the foot points of open field lines and are the origin of the solar wind.

36. Energy generation in the sun (and all other stars):
Energy Production
Energy generation in the sun (and all other stars):
Binding energy due to strong force = on short range, strongest of the 4 known forces: electromagnetic, weak, strong, gravitational
Nuclear Fusion
= fusing together 2 or more lighter nuclei to produce heavier ones.
Nuclear fusion can produce energy up to the production of iron;
For elements heavier than iron, energy is gained by nuclear fission.

37. Energy Generation in the Sun: The Proton-Proton Chain
Basic reaction:
4 1H  4He + energy
Need large proton speed ( high temperature) to overcome Coulomb barrier (electromagnetic repulsion between protons).
T ≥ 107 0K = 10 million 0K
4 protons have 0.048*10-27 kg (= 0.7 %) more mass than 4He.
Energy gain = Dm*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.

38. The Solar Neutrino Problem
The solar interior can not be observed directly because it is highly opaque to radiation.
But neutrinos can penetrate huge amounts of material without being absorbed.
Early solar neutrino experiments detected a much lower flux of neutrinos than expected ( the “solar neutrino problem”).
Recent results have proven that neutrinos change (“oscillate”) between different types (“flavors”), thus solving the solar neutrino problem.
Davis solar neutrino experiment

39. New Terms sunspot granulation convection supergranule limb
limb darkening
transition region
filtergram
filament
spicule
coronagraph
magnetic carpet
solar wind
helioseismology
active region
Zeeman effect
Maunder butterfly diagram
differential rotation
dynamo effect
Babcock model
prominence
flare
reconnection
aurora
coronal hole
coronal mass ejection (CME)
solar constant
Maunder minimum
weak force
strong force
nuclear fission
nuclear fusion
Coulomb barrier
proton–proton chain
deuterium
neutrino

40. Discussion Questions
1. What energy sources on Earth cannot be thought of as stored sunlight?
2. What would the spectrum of an auroral display look like? Why?
3. What observations would you make if you were ordered to set up a system that could warn astronauts in orbit of dangerous solar flares? Such a warning system exists.

41. Quiz Questions
1. What effect does the formation of negative hydrogen ions in the Sun's photosphere have on solar observations?
a. We can view the Sun's interior through special filters set to the wavelength of the absorption lines created by such ions.
b. Concentrations of such ions form sunspots that allow us to track solar rotation.
c. It divides the Sun's atmosphere into three distinct, easily observable layers.
d. The extra electron absorbs different wavelength photons, making the photosphere opaque.
e. These ions produce the "diamond ring" effect that is seen during total solar eclipses.

42. Quiz Questions
2. What evidence do we have that the granulation seen on the Sun's surface is caused by convection?
a. The bright centers of granules are cooler than their dark boundaries.
b. The bright centers of granules are hotter than their dark boundaries.
c. Doppler measurements indicate that the centers are rising and edges are sinking.
d. Both a and c above.
e. Both b and c above.

43. Quiz Questions
3. Which layer of the Sun's atmosphere contains the cooler low density gas responsible for absorption lines in the Sun's spectrum?
a. The photosphere.
b. The chromosphere.
c. The corona.
d. The solar wind.
e. All of the above.

44. Quiz Questions
4. Which of the following is true about granules and supergranules?
a. They are both about the same size.
b. Granules and supergranules each fade in about 10 to 20 minutes.
c. They are both due to convection cells in layers below.
d. Both a and c above.
e. Both b and c above.

45. Quiz Questions
5. What is revealed by observing the Sun at a very narrow range of wavelengths within the 656-nanometer hydrogen alpha line?
a. The structure of the photosphere.
b. The structure of the chromosphere.
c. The structure of the corona.
d. We can see the electrons make the transition from energy level 3 to level 2.
e. Nothing is seen; all light is absorbed at this wavelength.

46. Quiz Questions
6. What are the general trends in temperature and density from the photosphere to the chromosphere to the corona?
a. The temperature increases and density decreases.
b. The temperature increases and density increases.
c. The temperature decreases and density decreases.
d. The temperature decreases and density increases.
e. The temperature and density remain constant.

47. Quiz Questions
7. What physical property of the Sun is responsible for "limb darkening"?
a. The chromosphere is hotter than the photosphere.
b. The chromosphere is cooler than the photosphere.
c. The lower photosphere is cooler than the upper photosphere.
d. The lower photosphere is hotter than the upper photosphere.
e. Both a and d above.

48. Quiz Questions
8. The spectrum of the corona has bright spectral lines of highly ionized elements. What does this reveal?
a. The corona is a very hot, high density gas.
b. The corona is a very hot, low density gas.
c. The corona is very irregular in shape.
d. The corona extends out to 20 solar radii.
e. Both b and d above.

49. Quiz Questions
9. What heats the chromosphere and corona to high temperatures?
a. Long-wavelength electromagnetic radiation emitted by layers below.
b. Visible light emitted by layers below.
c. Short-wavelength electromagnetic radiation emitted by layers below.
d. Sungrazing comets, giving up their energy of motion as they vaporize in these two layers.
e. Fluctuating magnetic fields from below that transport energy outward.

50. Quiz Questions
10. How are astronomers able to explore the layers of the Sun below the photosphere?
a. Short-wavelength radar pulses penetrate the photosphere and rebound from deeper layers within the Sun.
b. Long-wavelength radar pulses penetrate the photosphere and rebound from deeper layers within the Sun.
c. Highly reflective space probes have plunged below the photosphere and sampled the Sun's interior.
d. By measuring and modeling the modes of vibration of the Sun's surface.
e. By observing solar X-rays and gamma rays with space telescopes. These shorter wavelengths are emitted from hotter regions below the photosphere.

51. Quiz Questions
11. What is responsible for the Sun's surface and atmospheric activity?
a. The Sun's magnetic field.
b. Many comets impacting the Sun.
c. Gravitational contraction of the Sun.
d. The Sun sweeping up interstellar space debris.
e. Gravitational interactions between the Sun and the planets.

52. Quiz Questions
12. What is the source of the Sun's changing magnetic field?
a. The differential rotation of the Sun.
b. Convection beneath the photosphere.
c. The Sun's large iron core.
d. Both a and b above.
e. Both a and c above.

53. Quiz Questions
13. What evidence do we have that sunspots are magnetic?
a. The spectral lines of sunspots are split by the Zeeman Effect.
b. Observations show that the north pole and south pole sunspots attract one another and move closer together over time.
c. Observations at far ultraviolet show material arched above the Sun's surface from one sunspot to another.
d. Both a and b above.
e. Both a and c above.

54. Quiz Questions
14. Which active feature in the Sun's atmosphere, seen from a different point of view, corresponds precisely to the dark filaments that are observed with an hydrogen alpha filter?
a. A sunspot.
b. A solar flare.
c. A prominence.
d. A spicule.
e. A coronal hole.

55. Quiz Questions 15. What does a Maunder Butterfly Diagram show?
a. During the 11-year sunspot cycle, the spots begin at high latitude and then form progressively closer to the equator.
b. Between the years 1645 and 1715 the low activity on the Sun correlates with the Little Ice Age.
c. The Sun's magnetic field is simple at the beginning of a sunspot cycle and grows progressively more complex due to differential rotation.
d. Planetary nebulae do not all have spherical symmetry.
e. When a butterfly flaps its wings in Brazil it affects the climate worldwide.

56. Quiz Questions
16. How constant is the solar constant; that is, by how much has the solar constant of 1360 joules per square meter per second been observed to vary over a few years?
a. About 20%.
b. About 10%.
c. About 5%.
d. About 1%.
e. About 0.1%.

57. Quiz Questions 17. How does the Sun maintain its energy output?
a. Gravitational contraction.
b. Fusion of hydrogen nuclei.
c. The impact of small meteoroids.
d. Coal burning in pure oxygen.
e. Fission of Uranium 235.

58. Quiz Questions 18. Why does nuclear fusion require high temperatures?
a. Protons have positive charge, and like charges repel one another.
b. Two protons must get close enough together to overcome the Coulomb barrier.
c. Two protons must get close enough for the strong force to bind them together.
d. Both a and b above.
e. All of the above.

59. Quiz Questions
19. What happens to the neutrinos that are produced in the proton-proton chain?
a. They collide immediately with other particles, thus adding to the gas pressure that supports the Sun against gravitational contraction.
b. They combine with antineutrinos and form a pair of gamma rays.
c. They head out of the Sun at nearly the speed of light.
d. They are blocked by the Coulomb barrier and remain inside the Sun.
e. They spiral out along magnetic field lines to become cosmic rays.

60. Quiz Questions 20. What solved the solar neutrino problem?
a. The discovery that neutrinos oscillate between three different types.
b. The standard model of energy production within the Sun was modified.
c. It was discovered that electron neutrinos do not penetrate rock as easily as expected.
d. Some of the radioactive argon gas was found leaking out of the neutrino detector undetected.
e. The finding that chlorine does not interact with electron neutrinos as predicted.

61. Answers
1. d
2. e
3. a
4. c
5. b
6. a
7. d
8. b
9. e
10. d
11. a
12. d
13. e
14. c
15. a
16. e
17. b
18. e
19. c
20. a