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B Does fusion occur because you exceeded the strong force?

  1. Aug 12, 2016 #1
    When 2 protons fuse in a star, does the fusion rely on how strong the gravitational pressure is from the star so it gets past the strong force so the nuclei fuse or the temperature because of the pressure or both?
  2. jcsd
  3. Aug 12, 2016 #2


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    Fusion occurs primarily because of two reasons:

    1.) The nuclei get close enough for the strong force to overcome the repulsion each protons feels from the electromagnetic force.

    2.) The weak force turns one of the protons into a neutron, eliminating the repulsion.

    Number two is important because helium-2, helium with two protons and no neutrons, is extremely unstable and simply decays back into two unbound protons almost as soon as it forms. The weak force only turns a proton into a neutron in a tiny fraction of fusion events. Since most fusion events simply decay right back into two protons, the Sun can last for about 10 billion years before running out of fuel.

    Gravity comes into play because it is the driving mechanism that heats and compresses the plasma, which allows for number 1 and 2 above to occur.
  4. Aug 13, 2016 #3


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    Gravity contributes to the hot and dense interior of stars, but not directly to fusion. The interior is hot, so the protons have a high kinetic energy. This allows them to come close to each other randomly. A tiny fraction of those close encounters leads to fusion due to the weak and strong interaction.
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