Transcript Opening music - CAPSTONE 2010

Nuclear Reactions in Stars
CAPSTONE Lecture 9
July 15, 2010
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How nuclear reactions work
• Plausible that Sun shines by nuclear reactions.
• Nuclei fully ionized
• Temp must be high enough that thermal motions lead to collisions,
overcoming the electrostatic repulsion of charge nuclei for each other.
(the more charge on the burning nuclei, the higher the temp. must be)
• High enough temp to burn the easiest element (H) is reached at center
of collapsing cloud we discussed yesterday.
• 1H++1H+ 2H+1 + e+ + . (proton-proton cycle) (1)
• First product is D (2H), deuterium.
• Equation is electrically and energetically neutral.
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Reaction chain
• 2D+ + 1H+ 3He+2 + (2)
• Energy given up by fusion goes into kinetic
motion, that heats the gas, or photons that ionize
other atoms, releasing more energy that heats the
gas.
• 3He+2+3He+24He+2+1H++1H+

 runs twice, then (2) runs twice, 6 H are used
up. Two come back in (3).
 et 4 H to make one He. Energy released is the
mass difference between one He and 4 H.
• If H runs out, fusion stops, gravity takes over, star
collapses.
• Energy from interior keeps the surface hot.
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CNO cycle
• Proton proton runs at 106 K (first stars)
• If heavy elements already exist and the temperature
is higher (15 to 30x106K, to overcome large positive
charge on heavy nuclei)
• 12C+6 +1H+13N+7 + 
• 13N+7 13C+6 + e+ + 
• 13C+6 + 1H+ 14N+7 + 
• 14N+7 +1H+15O+8 + 
• 15O+8 15N+7 + e+ + 
• 15N+7 + 1H+ 12C+6 + 4He+2
• Takes, again, 4 H to make one He, so energy
released is the same as for the proton-proton
reaction.
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Carbon and missing nuclei
• Where does carbon come from?
• It is not made in the Big Bang, which makes He, but
does not last long enough to make C.
• He4+He4Be8, but this is not stable.
• There are no mass 5 or mass 8 nuclei.
• To make C requires 3 He nuclei at the same time
• This is highly improbable, takes long time.
• Only happens in stable burning fusion over millions of
years
• Fred Hoyle predicted the probability this could happen,
William Fowler confirmed (1983 Nobel Prize).
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Production of heavy elements
• When H burns out, star collapses, heats
up at center, until a new reaction turns
on.
• 95keV
+4He+2+4He+28Be+4+endot
hermic)
• 4He+2+ 8Be+4 12C+6 +  (net, 7.3 MeV released)
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Efficiency in hotter stars
• Net, three helium nuclei (alpha particles) produce one
carbon nucleus.
• The mass difference between three helium nuclei and one
carbon nucleus is 8.1 MeV, or 2.7 MeV per helium
nucleus.
• The rest mass energy of helium is 4x109 eV or 4 GeV
(approximately, since the proton has rest mass 1 GeV.)
• Therefore, the efficiency of this reaction in producing
nuclear energy is 0.000675, or 1/10 as efficient as the
burning of hydrogen to make helium.
• Stars burning helium cannot sustain the star in a stable
state for very long, partly because of this low energy
efficiency.
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Ages in the HR diagram
• Globular clusters, HR diagrams missing
hot stars
• Older clusters have turn off points lower
on main sequence (less massive, MS
stars).
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