1. The Reactions The Main Sequence – The P – P Chain 1 H + 1 H  2 H + proton + neutrino 2 H + 1 H  3 He + energy 3 He + 3 He  4 H + 1 H + 1 H + energy 4 ( 1 H )  4 He + energy + 2 neutrinos The net result -

2. The Reactions The Main Sequence – The CNO Cycle M > 1.2 Mסּ and T > 17 million K 12 C + 1 H  13 N 13 N  13 C (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.The CNO CYCLECNO cycle 13 C + 1 H  14 N 14 N + 1 H  15 O 15 O  15 N (unstable radioactive decay) 15 N + 1 H  12 C + 4 2 He

3. The Reactions Both the p – p chain and the CNO cycle produce Helium

4. The Reactions The Triple Alpha Process T > 100 million K 3 ( 4 He )  12 C

5. Advanced Nuclear Reaction Stages 12 C + 4 He  16 O

6. Advanced Nuclear Reaction Stages What’s next * “Common” Element Fusion * Helium Capture

7. Advanced Nuclear Reaction Stages T > 500 million K Carbon Fusion to Magnesium 12 C + 12 C  24 Mg

8. Advanced Nuclear Reaction Stages T > 1 billion K Oxygen Fusion to Sulfur 16 O + 16 O  32 S

9. Advanced Nuclear Reaction Stages What’s next * “Common” Element Fusion * Helium Capture Notice from Previous slides: “Common Element Fusion” requires VERY high temperatures

10. 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

11. Advanced Nuclear Reaction Stages Helium Capture to form Oxygen, Neon, Magnesium and Silicon 12 C + 4 He  16 O 16 O + 4 He  20 Ne 20 Ne + 4 He  24 Mg 24 Mg + 4 He  28 Si

12. Advanced Nuclear Reaction Stages Silicon can be broken apart by the high energy photons in the core (photodisintegration). Photon + 28 Si  7 ( 4 He) The Helium produced in the photodisintegration of Silicon drive further reactions

13. Advanced Nuclear Reaction Stages Helium Capture to form Sulfur, Argon, Calcium and Titanium 28 Si + 4 He  32 S 32 S + 4 He  36 Ar 36 Ar + 4 He  40 Ca 40 Ca + 4 He  44 Ti

14. 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) 44 Ti + 4 He  48 Cr 48 Cr + 4 He  52 Fe 52 Fe + 4 He  56 Ni 56 Ni → 56 Co  56 Fe

15. 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.