COOL TOPICS IN
MODERN ASTRONOMY
(that have something to do
with various kinds of
collisions)
Class 15 : The Sun
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Basic properties of the Sun.
Structure of the Sun.
Nuclear fusion that powers the Sun.
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Collisions of atomic nuclei!
I : Basic properties of the Sun
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Distance: 150 million km.
Radius : 700,000 km.
Mass : 2.01030 kg.
Luminosity : 3.851026 Watts.
Temperature : 5800 K.
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How do we know these facts?
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[See discussion on board]
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Finally, temperature…
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Look at color of “glow”.
As temperature increases, go from…
REDYELLOWWHITEBLUE
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Precise measurements of Sun’s color give
temperature of 5800 K.
 COOL
HOT 
II: Structure of the Sun
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How is the Sun put together?
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Let’s unpeel it!
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Core (< 200,000 km)
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Radiation zone (200,000-500,000 km)
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T = 7 million K.
Energy transported outwards by radiation.
Convection zone (500,000-696,000 km)
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T = 15 million K.
Place where energy is produced (fusion).
T = 2 million K.
Energy transported by convection.
Like boiling water in a pot.
Photosphere (696,000 km)
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T=5800 K.
Place where the Sun becomes transparent.
This is what we see as the “surface”.
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But there’s more…
Chromosphere (thin layer just above
photosphere)
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Corona (surrounds Sun, very large)
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Looks pink!
T = 1 million K.
Very hot gas – can see with X-ray telescopes!
Can only see normally during eclipse.
Solar wind
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Tenuous wind being blown by processes in corona.
Flows into outer space (causes aurorae on Earth).
III : The Sun’s energy source
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Sun powered by nuclear fusion
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Fusion = “merging” of 2 or more atomic
nuclei to form a bigger nucleus.
In Sun, hydrogen nuclei collide and merge
to form helium nuclei.
Actual process in Sun is called the “protonproton chain” (p-p chain).
Often called “hydrogen burning”… not
hydrogen burning as you know it!
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The p-p chain:
1.
2.
3.
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Proton (p) + proton (p)  deuterium (D) +
neutrino () + positron (e+) [& e+ + e-  2]
D + p  3He + 
3He + 3He  4He + 2p
Overall result:
4p  4He + neutrinos + energy
Step 1 is the hardest…
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Like charges (protons in this case) repel,
so they have to smash together very fast!
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Mass of 4 protons: 6.694310-27 kg
Mass of Helium-4: 6.646610-27 kg
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4.7710-29 kg has been lost…
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This is converted into energy according to Einstein’s
formula E=mc2.
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Sun “processes” 600 million tons of hydrogen
per second… 4 million tons per second is
completely converted into energy (0.7%).
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ALL DRIVEN BY SUB-ATOMIC COLLISIONS!
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HOW DO WE KNOW THIS IS
CORRECT?
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Two main lines of evidence:
1.
2.
Can produce these kinds of nuclear
reactions in laboratories (and in bombs)…
know their properties well.
Can observe neutrinos from the Sun!
Sudbury
Neutrino
Observatory
(SNO)