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 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) 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

4. The Reactions The P – P Chain vs The CNO Cycle Factors Controlling the Fusion Rates * There must be C, N, or O present for the CNO cycle to occur, so it can only happen for stars where this is true. However, only a very small amount is required, so this condition is often fulfilled. * The reactions have very different temperature dependences, as illustrated in the figure.

5. The Reactions The P – P Chain vs The CNO Cycle At lower temperatures the PP chain dominates, but with rising temperatures there is a sudden transition to dominance by the CNO cycle, which has an energy production rate that varies strongly with temperature. This is why the CNO cycle is more important for heavier stars: their interior temperatures are higher, thus favoring the CNO cycle.

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

7. Advanced Nuclear Reaction Stages Following the Triple-alpha process there are a variety of reactions which may occur depending on the mass of the star. Three general principles influence the roles that these nuclear burning stages may play: * Successive nuclear burning stages, involving more massive nuclei with higher charges, will require increasingly high temperatures to overcome the increased electrical repulsion. * The amount of energy released by each successive reaction stage decreases so that later nuclear burning stages become shorter and shorter. * Once fusion reactions have produced an iron core, further fusion reactions no longer produce energy, but absorb energy from the stellar core. As we shall see this may have a catastrophic effect on the star as it nears the end of its life.

8. Advanced Nuclear Reaction Stages 12 C + 4 He  16 O 16 O + 4 He  20 Ne

9. Advanced Nuclear Reaction Stages T > 500 million K 12 C + 12 C  24 Mg Magnesium

10. Advanced Nuclear Reaction Stages T > 1 billion K 16 O + 16 O  32 S Sulfur

11. Advanced Nuclear Reaction Stages T > 3 billion K Sulfur burning occurs through a series of reactions that produce nuclei near the "iron-peak", that is near 56 Fe on the Periodic Table, the element with the most strongly bound nucleus.