The Sun and Hydrogen into Helium Fussion?

  1. Hi, i'm just after a definitive answer on how Hydrogen fuses into Helium in the Sun?

    Some places say the Sun is so hot that they overcome their same charge repulsivness and fuse together while other places say that our sun is not hot enough for that to happen and that they just get close enough to allow them to fuse from Quantum Tunnelling.

    It's been the same with the science guys they get on radio stations i've heard some say that our Sun is so hot that they just smash into each other and fuse while other stations they say the Sun simply isn't hot enough and it is Quantum Tunnelling that enables the fussion.

    So which is it.... is our Sun hot enough for Hydrogen to physically fuse into Helium or is it Quantum Tunnelling that enables the fussion? Obviously the Sun still has to be hot enough to get the Hydrogen close enough for Quantum Tunnelling to happen but is it Quantum Tunnelling thats causing the fussion or is the Sun hot enough that Fussion is happening without any Quantum Tunnelling?
     
  2. jcsd
  3. Drakkith

    Staff: Mentor

    The truth is that BOTH happen. Statistically some ions should have high enough velocities on impact to overcome their repulsion and fuse, even if the average temperature is well below what should be required. If the ions don't have quite enough energy they can still sometimes tunnel through it due to quantum effects.

    Also, hydrogen fuses with itself to form Deuterium in the first part of the proton-proton chain reaction, not helium. This actually relies on the weak interaction as well, making the process even more unlikely to occur since one of the protons must decay into a neutron. If this process wasn't so rare the Sun would have used up its hydrogen long ago. See here for more: http://en.wikipedia.org/wiki/Proton-proton_chain_reaction
     
  4. As Drakkith suggests, both aspects play a role. The threshold temperature for nuclear fusion due to collisions alone (at the densities approximately that of the sun's core) is about [itex]10^8[/itex] K. QM tunneling decreases this to about [itex]10^7[/itex] K, which is the temperature of the sun's core. If it weren't for tunneling, virtually no fusion reactions would take place.
     
  5. So the Sun isn't hot enough for fusion to take place if there was not Quantum Tunnelling happening, thank you thats the kind of answer I was looking for.
     
  6. Drakkith

    Staff: Mentor

    The average temperature in the core is not, but there will be ions that have a high enough energy to fuse due to the statistical nature of the ion energies. IE some will be more than the average and some will be less.
     
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