stellar fusion. fusion: hydrogen to helium our sun is 75% h 2, 24% he, and 1% heavier elements. the...
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Stellar Fusion
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FUSION:hydrogen to helium
• Our Sun is 75% H2, 24% He, and 1% heavier elements.
• The Sun produces energy by FUSION: by combining hydrogen atoms to make helium.
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E = mc2
STEP 1: 1H + 1H -> 2H + +
STEP 2: 2H + 1H -> 3HeSTEP 3: 3He + 3He -> 4He + 2 1H
OVERALL: 4 1H -> 4He + 2+ + energy
How much energy? About 600 million kcal per mole of helium produced! How long would that
“fuel” YOU?
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helium to carbon• While the Sun fuses H to He there is a
balance between gravity pulling the Sun’s contents inward and the energy produced pushing the Sun’s contents outward.
• When most of the Sun’s H fuel is exhausted the balance will tip, and the Sun will contract. This will heat up the core until its hot enough to “ignite” the helium. The Sun will then expand to a new size as it fuses He into C.
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more E = mc2
STEP 1: 4He + 4He -> 8BeSTEP 2: 4He + 8Be -> 12C
OVERALL: 3 4He -> 12C
This process produces less energy than the fusion of H into He. This can be seen in the stars, which are
red giants. “Red hot” is cooler than our Sun’s “yellow hot” … or massive star’s “blue hot”.
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Nuclear Binding Energy
Stars can produce energy by fusion only up to 56Fe … beyond that fusion is
“endothermic.”
Fusion <- -> Fission
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(make elements > 56Fe)
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NebulaeCrab Nebula:
supernova seen
in AD 1054
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Supernovac. 1870 on Earth - the most recent in our galaxy
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Black Hole
X-ray”center”
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artificial fusion
• Produces more energy per pound of fuel than fission
• Creates far less radioactive waste products
• Uses much cheaper fuel (H) than fission (U or Pu)
• So ….. WHY NOT?