1. The Sun

2. Discussion
Why does the Sun shine?

3. Discussion
How do you know the Sun is hot?

4. Infrared radiation
The Sun feels warm because of the infrared radiation it emits.
Anaxagoras (500 – 428 B.C.E.) believed the Sun was a very hot, glowing rock about the size of the Greek peninsula.

5. The setting Sun is red because
The Earth is rotating away from the setting Sun, so it is redshifted.
The setting Sun is cooler at sunset, so Wien’s law says the frequency of maximum emission shifts to lower frequencies, thus appears redder.
At sunset the light has to travel through more of the Earth’s atmosphere, which has lots of absorption lines in the blue portion of the spectrum.
None of the above

6. Solar Data Radius: 109 Earth radii Mass: 333,000 Earth masses
Composition: 74% hydrogen
25% helium
Mean density: g/cm3
Luminosity:  1026 Watts

7. The Sun as a big cosmic light bulb
Suppose every human being on Earth turned on
1000, 100-watt light bulbs. With about 6 billion
people this would only be 6  1014 watts. We
would need 670 billion more Earth’s doing the
same thing to equal the energy output of the Sun.
4X 10^26 watts

8. Discussion
What kind of spectrum does the Sun have?

11. Discussion
Why is there less solar intensity at sea level than there is at the top of Earth’s atmosphere?

12. Discussion
Where do you think that energy goes?

13. Discussion
The Sun releases lots of energy each second, what if it were cooling down over time. How could we tell?

14. Thermal equilibrium
The Sun is not measurably heating up or cooling down.

15. No cooling ember
At the rate that the Sun is emitting energy, the Sun must have been much hotter just a few hundred years earlier, making life on Earth impossible.
The Sun must have an energy source; a way of generating its own heat.

16. Discussion
Given the composition of the Sun, why is it unlikely that it could be heated by the burning of wood or coal?

17. Kelvin-Helmoltz contraction
As things contract gravitationally, they become hotter.

18. Discussion
Why do you think gravitational contraction leads to a temperature increase?

19. Discussion
If the Sun is getting its energy from Kelvin-Helmholtz contraction, what observation could you make to test this? Do you think this is an easy measurement to make? Explain.

20. Hydrostatic Equilibrium
The Sun is not measurably expanding or contracting

21. The age of the Sun
Sedimentary rocks on Earth which were deposited in liquid water are 3.8 billion years old.
Rocks containing fossils are 3.5 billion years old. The Sun must have been shining for at least this long.

22. What energy source can keep the Sun hot for 3.8 billion years?
Burning coal: Sun would last 10,000 years
Kelvin-Helmholtz contraction: if the Sun’s heat
were generated from contraction of the Sun’s mass, it would shine for only 25 million years.

23. E = m c2 Matter is a form of frozen energy.
Energy equals the mass times the speed of light squared.
Matter is a form of frozen energy.

24. The Sun is huge!
A little bit of matter can be turned into a large amount of energy. If the Sun’s mass could be converted to energy it could shine for hundreds of billions of years.
The Sun needs to convert 4.3 million tons of matter to energy every second.

25. The Sun’s Mass is Converted to Energy
4 hydrogen atoms have a mass of  kg 1 helium atom has a mass of  kg
Thus,  kg are converted to energy.

26. Thermonuclear Fusion
The Sun fuses 4 hydrogen atoms together to produce 1 helium atom releasing energy. In the Sun about 600 million tons of hydrogen is converted to helium per second.

27. Discussion
How can we change a positively charged proton from a hydrogen nucleus into a neutral neutron?

28. How does it work?
We need a new form of matter called anti-matter. Antimatter is made up of anti-particles which have the same mass as ordinary particles but opposite charge.
Matter and antimatter will annihilate each other if they come in contact producing energy.

29. Proton-Proton chain
Helium nuclei can be built up one proton at a time in what we call the proton-proton chain.
Normally, two protons will repel each other with the electrostatic force, but if they are smashed together with enough force they can stay together via the strong nuclear force.

31. Neutrinos
Neutrinos () are particles that only interact with matter via the weak nuclear force (the force responsible for radioactive decay).
To stop a typical neutrino emitted from the Sun would require 1 light-year (5 trillion miles) of lead.
150 trillion/sec
1/70 years

32. Discussion
Why must matter be so hot, 10 million K, for H to fuse into He?

33. Discussion
If it takes on average 14 billion years to make a deuterium atom, how can the Sun fuse 600 million tons of hydrogen into helium each second?

34. Discussion
How can atoms with more than one proton in the nucleus stay together? Why don’t they just fly apart?

37. Discussion
Fusion keeps the Sun hot, but fusion requires the Sun to be hot. How did the Sun ever get hot enough to start fusion?

38. How do we know thermonuclear fusion is taking place in the Sun?
“We do not argue with the critic who urges that
stars are not hot enough for this process; we tell
him to go and find a hotter place.”
Eddington (1926)
Temp greater than 10 million K, average speed of 2 million mi/hr

39. We can test the theory that the Sun is powered by thermonuclear fusion by:
Modeling the solar interior
Direct observations of solar neutrinos

41. Discussion
Which acrobat would you rather be and why?

43. Discussion
What does this mean for the pressure on the gas as you descend into the interior of the Sun?

44. Pressure increases toward the center of the Sun
To maintain equilibrium, the pressure below each
layer of the Sun must be greater than the pressure
above that layer.

46. Discussion
What happens if you squash a gas?

47. Density increases toward center of the Sun
The Sun is gaseous. If you apply pressure to a
gas is compresses, i.e. it’s density goes up.

48. Temperature increases toward the center of the Sun
As the pressure goes up toward the center of the Sun, the temperature also increases.

49. Fusion only takes place in the Sun’s core
In the inner 1/4 of the Sun’s radius can fusion take place.
Even at 15 million K, it takes on average 14 billion years at a rate of 100 million collisions per second to fuse two protons to produce a deuterium atom.

50. Thermal equilibrium
The Sun is not heating up or cooling down. The temperature of each layer, although different, is constant with time.
Heat generated in the core by thermonuclear fusion must be equal to the energy emitted from the Sun’s surface.

53. Discussion What would happen if the Sun started to
contract? What happens to the density, temperature, pressure, rate of fusion etc? Explain.

54. Discussion
What would happen if the Sun started to expand? What happens to the density, temperature, pressure, rate of fusion etc?

55. Negative feedback
The Sun is stabilized by this negative feedback. Contraction/higher core temperatures lead to increased fusion rates, expansion and cooling.
Expansion/core cooling lead to decreased fusion rates, contraction and heating.

56. Discussion
What happens if all fusion in the Sun ceases?

57. Discussion
If the Sun contracts, it heats up. Why can’t the increased pressure from the contraction alone stop the contraction? Why is thermonuclear fusion required to halt the contraction?

58. Discussion
If the Sun is in thermal equilibrium and it is generating heat in its core via thermonuclear fusion, what must happen to that energy?

59. Heat Transport in the Sun
Conduction – particles transfer energy via collisions
Convection – energy transferred by movement of material from hotter to cooler regions
Radiative Diffusion – energy transferred via photons

60. Discussion
Which would you rather do, put your hand in an oven at 450 degrees F or put you hand on a 450 degree F stove top? Why is there a difference?

61. Radiative Diffusion Radiative zone – inner 71 percent of the Sun’s
Interior where all atoms are ionized.
Takes a photon 170,000 years to reach the
convective zone. Each time a photon is absorbed
it loses energy.
Light that starts off as a gamma ray in core ends up as an x-ray at bottom of convective zone.

63. Convection Convective Zone – outer 29 percent of Sun’s interior.
Bottom of convective zone is cool enough for
heavy atoms to regain electrons and absorb light.

64. Discussion What happens to the bottom layer of the
convection zone as it absorbs light from the
radiative zone and heats up.

65. Why the transition?
The radiative zone is hot enough that most elements are completely ionized. Think about electrons in the mirror.
The bottom of convective zone is cool enough for heavy atoms to regain electrons which can then absorb light and heat up.

66. Only hydrogen within core (25%) can fuse.

67. Discussion Will observations of the properties of the
photons emitted by the Sun reveal much
information about the interior of the Sun? Why
or why not?

68. Solar Neutrinos Produced in nuclear reaction that convert neutrons
to protons.
Do not interact much with matter. So How do
we detect them?

69. Neutrino Telescopes HOMESTAKE – 615 ton tank of 378 thousand
litters of cleaning fluid (C2Cl4) buried 1.5 km in a
mine shaft. Once every two to three days a
neutrino would hit a Cl atom and convert it a
radioactive Ar atom.
The cleaning fluid was only loaned and was returned and used.

71. The Solar Neutrino Problem
The HOMESTAKE experiment operated for 25 years and
detected only about 1/3 of the predicted number of
neutrinos.

72. Two ways to fix the Solar Neutrino Problem
Our models of the Sun are wrong.
Our understanding of the neutrino is wrong.

73. Discussion What do we need to do to get less neutrinos from
our model of the Sun?

74. Helioseismology The Sun vibrates like a bell. We can measure
these vibrations at any position on the surface of
the Sun using the Doppler shift. As the surface
moves toward us it is blue shifted, as it moves
away from us it is red shifted.

75. Helioseismology

77. All the local oscillations on the Sun are driven by sound waves that echo back and forth through the Sun.
The speed of sound waves depends on the temperature and density of the material it passes through.
GONG – Global Oscillation Network Group
SOHO – Solar and Heliospheric Observatory

78. Bottom Line The solar oscillations indicate that our current
models of the density and pressure in the Sun’s
interior are correct to 0.2 percent.

79. Super Kamiokande 50,000 tons of purified water buried 1 km in a zinc
mine in Japan.

81. The Outer Layers of the Sun
The Photosphere
The Chromosphere
The Corona
The Heliosphere

82. The Photosphere The “surface” of the Sun. That part of the solar
atmosphere from which most of the light we see is emitted.
The outer atmosphere of the Sun is transparent, we can see through it. The surface we see is the point at which the Sun’s atmosphere
becomes opaque.

84. Properties of the Photosphere
Depth (km)
% of light
Temp (K)
Pressure (atm)
Density (g/cm3 10-7)
99.5
4465
0.0068
0.23
100 97
4780
0.017
0.54
200 89
5180
0.039
1.2
300 64
5840
0.083
2.1
400 4
7610
0.16
3.1

85. Discussion
If each layer of the Sun has a different temperature, how can we say that the Sun has a temperature of 5800 K? How do we get this temperature?

86. Why is the photosphere so opaque?
Hydrogen atoms in the solar atmosphere can
acquire a second electron. This weakly bound
extra electron easily absorbs different
wavelengths of light.

87. Discussion
Why is the edge of the Sun darker than the center?

89. Photospheric Granulation
Mottled appearance – bright areas surrounded
by dark lanes.
Typically 700 to 1000 km in diameter , they
persist for only 5 to 10 minutes.

90. Granulation

91. Discussion What do you think causes the granulation?
What is the difference between the light part
and the dark lanes?

92. Solar Granulation

94. Supergranules As with the granules, hot gas rises in the center,
spreads out and sinks back into the Sun. But
supergranules are much larger, typically 35,000
km in diameter, they move slower, 0.4 km/s, and
last on the order of a day.
Over twice the size of the Earth

95. Supergranules
Speeds are 0.4 km/s hard to see.

96. Sunspots Dark, irregular spots in the Sun’s photosphere.
Can last from hours to months.
Can have diameters as large as 50,000 km.
Often occur in groups of 2 to 100 individual spots.

97. Sunspots Dark, irregular spots in the Sun’s photosphere.
Can last from hours to months.

100. Discussion Why do sunspots appear dark?
Hint: The umbra appears red and the penumbra appears orange when isolated from the rest of the Sun’s surface.

101. Sunspots and Stefan-Bolztmann
Thus, sunspots emit less than a third of the light of the photosphere.

104. Discussion
How can you tell whether or not something is magnetic?

105. The Zeeman Effect A magnetic field interacts with the spin of the
electron in an orbital, causing a single spectral
line to split into two.

107. Iron line