1. Chapter 3, Part2 Nuclear Chemistry CHEM 396 by Dr
Chapter 3, Part2 Nuclear Chemistry CHEM 396 by Dr. Ahmad Hamaed Fall 2014

2. Fusion Process in Stars
The elemental composition of our sun is about 73% hydrogen, 25% helium and 2% carbon, nitrogen, oxygen and other elements (about 70 elements).
Such facts were discovered from the solar spectrum of the sun.
There are many indications to believe that all the elements up to uranium are present in our sun.
Only few nuclear reactions for the formation of these elements and their relative energies will be discussed.

3. Hydrogen Burning to Helium
Helium (4 protons) can be formed by different forms in the sun. All helium formation reactions are exothermic. It should be clear that few of these reactions for stars with m ≤ 1.5 M and T ≥ 2×107K and m ≥ 1.5 M.
M = Msun = ( ± )× 1030 kg
1H1 + 1H H1 + e-1 + e+1 + υ Q = 1.44 MeV
1H1 + 2H He Q = 5.49 MeV
1H1 + 3H He Q = 19.8 MeV
These reactions are called the proton-proton chain

4. Hydrogen Burning to Helium
2H1 + 1n H Q = 6.25 MeV
2H1 + 2H H1 + 1H Q = 4.03 MeV
2H1 + 2H He n Q = 3.27 MeV
2H1 + 2H He Q = MeV

5. Hydrogen Burning to Helium
2H1 + 3H He n Q = MeV
The above equation is the hydrogen fusion reaction which requires tritium (therefore needs to be synthesized prior to the reaction).
2H1 + 3He He H Q = 3.27 MeV

6. Hydrogen Burning to Helium
1H1 + 1H H β+1 + e-1 + υ Q = 1.44 MeV
2H1 + 1H He gamma Q = 5.49 MeV
3He2 + 3He He H Q = 4.03 MeV
Overall 4 1H He β+1+ 2e-1 + 2υ Q = 26.7 MeV
1.4×1010y

7. Other Reactions to Produce Helium
3H1 + 3H He n Q = MeV
3He n He2 + 1n Q = MeV
3He2 + 3He He2 + 21H1 Q = MeV

8. Reactions for Stars with m≤ 1.5 M
3He2 + 4He Be Q = 1.5 MeV
7Be4 + 0β Li3 + υ Q = 1.37 MeV
7Li3 + 1H He Q = MeV
Q = MeV for the sum of the above reactions:
3He2 + 1H1 + 0β He2
7Be4 + 1H B5
8B5 + 0β Be4
8Be He2
Q = MeV for the sum of the above reactions:
7Be4 + 1H β He2

9. Summary of Proton-Proton Chains in Sun-like Stars

10. The proton–proton chain reaction dominates in stars the size of the Sun or smaller.

11. Reaction Cross Sections for some light element fusion reactions at solar Conditions

12. Helium Burning to Iron For Stars with 0.26M˂ m ˂ 1.5M
When the density is about 107 Kg/m3 and the temperature is about 1.5x108 K, helium starts to fuse 3-alpha process or fuses with 8Be to form carbon:
3 4He C6 + υ
8Be4 + 4He C6 + υ Q = 1.37 MeV
Once carbon is formed further reactions with helium can explain the formation of oxygen, neon, and higher elements.

13. Helium Burning to Iron, Cont’d For Stars with 0.26M˂ m ˂ 1.5M
12C6 + 4He O8 + γ
16O8 + 4He Ne10 + γ
20Ne10 + 4He Mg12 + γ
24Mg12 + 4He Si14 (stable)
12C6 (4He2, γ ) 16O8 (4He2, γ) 20Ne10 (4He2, γ) 24Mg12 (4He2, γ) 28Si14

14. Helium Burning to Iron, Cont’d For Stars with m ˃7.5M, at T ≥ 8×108K
12C6 + 12C Mg12
12C6 + 12C Na11 +1H1
12C C Ne10 + 4He2
The above reactions are observed as a carbon flash, the star then either continues to burn carbon or explodes (a supernova) with destruction of most of the star.

15. Helium Burning to Iron, Cont’d For Stars with m ˃15M, at T ≥ 8×108K
Helium burning lasts for a few million years, and the carbon burns to O and Mg. Further fusion by helium capture to form heavy elements, produces 40Ca, 44Ti, 48Cr, 52Fe and 56Ni
28Si Si Ni28
56Ni28 + 0β Co days
56Co β Fe days
56Ni28 (0β-1) 56Co27 (0β-1) 56Fe26 (0β-1)