Proton-proton chain

The proton-proton chain reaction (also known as the PP chain) is one of two fusion reactions by which stars convert hydrogen to helium, the other being the CNO cycle. The proton-proton chain is more important in stars the size of the Sun or less.

The first step involves the fusion of two hydrogen nuclei 1H (protons) into deuterium 2H, releasing a positron as one proton changes into a neutron, and a neutrino. To overcome the electromagnetic repulsion between two hydrogen nuclei requires a large amount of energy, and this reaction takes an average of 10 billion years to complete. It is because of the slowness of this reaction that the Sun is still shining; if it where faster, the Sun would have exhausted its hydrogen long ago.

The neutrinos detected from the sun are signficantly below what the the proton-proton calculations predict resulting in what is known as the solar neutrino problem. Observations of pressure waves in the sun, known as helioseismology have indicated the the pressures and temperatures in the sun are very close to the pressures and temperatures predicted assuming our understanding the proton-proton chain is correct. This has led astrophysicists to believe that the resolution of the solar neutrino problem lies in unexpected behavior of the neutrinos after they are produced.

1H + 1H → 2H + e+ + νe + 0.42 MeV

The positron immediately annihilates with one of the hydrogen's electrons, and their mass energy is carried off by two gamma ray photons.

e+ + e- → 2&gamma + 1.02 MeV

After this the deuterium produced in the first stage can fuse with another hydrogen to produce a light isotope of helium, 3He:

2H + 1H → 3He + &gamma + 5.49 MeV

Finally, after millions of years, two of the helium nuclei 3He produced can fuse together to make the common helium isotope 4He, releasing two hydrogen nuclei to start the reaction again through three different paths called PP1, PP2 and PP3:

PP1:

3He +3He → 4He + 1H + 1H + 12.86 MeV
The complete PP1 chain reaction releases a net energy of 26.7 MeV. PP1 chain is dominant in temperatures of 10-14 million Kelvin. Below 10 million Kelvin, the PP chain does not produce much 4He.

PP2:
   
   
   
       3He + 4He 7Be + &gamma
       7Be + e- 7Li + νe
       7Li + 1H 4He + 4He
PP2 chain is dominant in temperatures of 14-23 million Kelvin.

PP3:
   
   
   
   
       3He + 4He 7Be + &gamma
       7Be + 1H 8B + γ
       8B 8Be + e+ + νe
       8Be 4He + 4He
PP3 chain is dominant if the temperatures exceeds 23 million Kelvin. This chain is not a major source of energy in the sun, but is very important in the solar neutrino problem because it generates the highest energy neutrinos.

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