1. Hydrogen Burning (Proton-proton chain)
1H + 1H → 2H
2H + 1H → 3He
3He + 3He → 4He 1 1 1 1 1 1 2 2 2 2
This process is the longest stage in a star’s life!

2. Hydrogen Burning The most essential nuclear fusion process in stars
The result is the formation of a Helium-4 atom
In stars with a low mass, similar to our Sun, hydrogen burning occurs through the proton-proton chain

3. Nuclear Fusion Step 1
Proton - Proton Fusion

4. Nuclear Fusion Step 1 Proton - Proton Fusion
Start with 2 Hydrogen atoms
Hydrogen has only 1 proton

5. Nuclear Fusion Step 1 Proton - Proton Fusion
Start with 2 Hydrogen atoms
Hydrogen has only 1 proton
As the Hydrogen atoms fuse
1 of the protons becomes a neutron

6. With a nucleus that has 1 proton and 1 neutron
Nuclear Fusion Step 1
Proton - Proton Fusion
Start with 2 Hydrogen atoms
Hydrogen has only 1 proton
As the Hydrogen atoms fuse
1 of the protons becomes a neutron
Finish with 1 Hydrogen atom
With a nucleus that has 1 proton and 1 neutron

7. Deuterium - Proton fusion
Nuclear Fusion Step 2
Deuterium - Proton fusion
Start with Hydrogen -1 and a Hydrogen - 2
Hydrogen -1 is 1 proton nucleus
Hydrogen -2 is 1 proton, 1 neutron nucleus

8. Deuterium - Proton fusion
Nuclear Fusion Step 2
Deuterium - Proton fusion
Start with Hydrogen -1 and a Hydrogen - 2
Hydrogen -1 is 1 proton nucleus
Hydrogen -2 is 1 proton, 1 neutron nucleus
Finished with a Helium - 3
Helium -3 is 2 protons and 1 neutron nucleus

9. Start with Hydrogen -1 and a Hydrogen - 2
Hydrogen -1 is 1 proton nucleus
Hydrogen -2 is 1 proton, 1 neutron nucleus
Finished with a Helium - 3
Helium -3 is 2 protons and 1 neutron nucleus

10. Nuclear Fusion Step 3
Helium - 3 fusion

11. Start with 2 Helium - 3 atoms
Helium -3 is 2 protons and 1 neutron nucleus

12. Start with 2 Helium - 3 atoms Finished with a Helium - 4 atom
Helium -3 is 2 protons and 1 neutron nucleus
Finished with a Helium - 4 atom
Helium -4 is 2 protons and 2 neutrons
And 2 free protons

13. Hydrogen Burning 1H + 1H → 2H 2H + 1H → 3He 3He + 3He → 4He
This process is the longest stage in a star’s life!

14. Hydrogen Burning (Large Stars)
In stars much more massive than our Sun, the Hydrogen-1 and deuterium get used up very fast
Hydrogen burning occurs through the Carbon-Nitrogen-Oxygen Cycle in these stars

15. Hydrogen Burning (CNO Cycle)

16. Fusion of Heavier Elements
Once stars burn through all of their “fuel”, heavier elements begin to be fused in the core
This is how elements as heavy as iron are formed
The fusion of iron takes energy (endothermic) instead of giving off energy (exothermic). Therefore, stars begin to “die” once iron is created.

17. Carbon Burning 12C + 12C → 16O + 4He + 4He + + 12C + 12C → 20Ne + 4He8 2 2 + + + + + + + + + + + + + + + + +
12C + 12C → 20Ne + 4He 6 6 10 2 + + + + + + + + + + + + + + + + + + + + + + + +
12C + 12C → 23Na + 2H 6 6 11 1 + + + + + + + + + + + + + + + + + + + + +

18. Neon Burning 20Ne → 16O + 4He 20Ne + 4He → 24Mg 8 2 20 10 16 8 + 4 2 +12 20 10 + 24 12 4 2 + +

19. Oxygen Burning 16O + 16O → 28Si + 4He 16O + 16O → 31P + 1H + + + + + +

20. Silicon Burning 28Si + 4He → 28S 32S + 4He → 36Ar + + + + + +
A chain of alpha processes
28Si + 4He → 28S + + +
32S + 4He → 36Ar + + +

21. Silicon Burning (cont)
36Si + 4He → 40Ca + + +
40Ca + 4He → 44Ti + + +

22. Silicon Burning (cont)
44Ti + 4He → 48Cr + + +
48Cr + 4He → 52Fe + + +