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The Reactions
The Main Sequence – The P – P Chain
1H + 1H 2H + proton + neutrino
2H + 1H 3He + energy
3He + 3He 4H + 1H + 1H + energy
4 ( 1H ) 4He + energy + 2 neutrinos
The net result -
The Reactions
The Main Sequence – The CNO Cycle
M > 1.2 Mּס and T > 17 million K
12C + 1H 13N 13N 13C (unstable radioactive decay)
More massive stars burn hydrogen via a catalytic reaction called The CNO CYCLE. Because the initial step in the CNO Cycle requires a Carbon nucleus (6 p+) to react with a proton it requires higher temperatures and is much more temperature sensitive than the P-P Chain (The energy produced is proportional to T20 for the CNO cycle vs T4 for the P-P Chain). Stars of mass greater than about 1.2 M with core temperatures, Tcore > 17 million K, produce most of their energy by the CNO cycle.
13C + 1H 14N
14N + 1H 15O15O 15N (unstable radioactive decay)
15N + 1H 12C + 4 2He
The Reactions
Both the p – p chain and the CNO cycle produce Helium
The Reactions
The Triple Alpha Process T > 100 million K
3 ( 4He ) 12C
Advanced Nuclear Reaction Stages
12C + 4He 16O
Advanced Nuclear Reaction Stages
What’s next
* “Common” Element Fusion
* Helium Capture
Advanced Nuclear Reaction Stages
T > 500 million K
Carbon Fusion to Magnesium
12C + 12C 24Mg
Advanced Nuclear Reaction Stages
T > 1 billion K
Oxygen Fusion to Sulfur
16O + 16O 32S
Advanced Nuclear Reaction Stages
What’s next
* “Common” Element Fusion
* Helium Capture
Notice from Previous slides: “Common Element Fusion” requires VERY high
temperatures
Advanced Nuclear Reaction Stages
What’s next
* “Common” Element Fusion
* Helium Capture
Since “Common Element Fusion” requires VERY high temperatures, Helium capture is
much more probable in the core of a star
Advanced Nuclear Reaction Stages
Helium Capture to form Oxygen, Neon, Magnesium and Silicon
12C + 4He 16O
16O + 4He 20Ne20Ne + 4He 24Mg
24Mg + 4He 28Si
Advanced Nuclear Reaction Stages
Silicon can be broken apart by the high energy photons in the core (photodisintegration).
Photon + 28Si 7 (4He)
The Helium produced in the photodisintegration of Silicon drive
further reactions
Advanced Nuclear Reaction Stages
Helium Capture to form Sulfur, Argon, Calcium and Titanium
28Si + 4He 32S
32S + 4He 36Ar36Ar + 4He 40Ca
40Ca + 4He 44Ti
Advanced Nuclear Reaction Stages
Helium Capture to form Chromium, Iron and Nickel (unstable to and isotope of Cobalt and then to an isotope of Iron)
44Ti + 4He 48Cr
48Cr + 4He 52Fe52Fe + 4He 56Ni
56Ni → 56Co 56Fe
Advanced Nuclear Reaction Stages
T > 3 billion K
Each reaction produces a nucleus with two more protons.
As a result, elements with an even number of protons are produced.
However, there are enough free protons in the core that single proton capture can occur as well.
Although not as probable as the previous reactions, proton capture will produce elements with an odd
number of protons.