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Elementary Fusion

  1. Oct 11, 2007 #1
    How do 4 Hydrogen atoms combine to form 1 Helium atom?

    What I'm looking for is the sub-atomic process. So, here come 4 Hydrogen atoms racing towards each other and they are at such a high speed and temperature that something different happens. Something different than if they were going slower or at a lower temperature.

    I'm guessing there are unseen particles that are interacting and changing and some flying off and energies being exchanged left and right. I'm just wondering specifics.
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  3. Oct 11, 2007 #2


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  4. Oct 12, 2007 #3


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    The interiors of stars are very hot and the hydrogen and other elements exists as a plasma, i.e. the electrons are free of the nuclei. The protons (nuclei of H atom) fuse upon collision and with subsequent decay for form a deuteron. Deuterons exist in a low concentration compared to protons, so the deuterons more likely fuse with protons to for He-3 than with other deuterons.

    Here is another site which discusses pp fusion process - http://csep10.phys.utk.edu/astr162/lect/energy/ppchain.html
  5. Oct 12, 2007 #4
    jtbell - Looking at that link and going through the detailed steps and the "quark transformation" page, I realized that what I am wondering about is how and why the quarks decide to change. Why do the quarks decide to change? They seem to indicate that at a certain temperature (energy level) these changes take place; but do they know beyond that, on a smaller scale what is happening? Maybe my question goes further than science has currently discovered and therefore cannot be answered yet.

    Astronuc - Why does it show time in the PP chain? For example [tex]10^{9}[/tex] years to fuse two Hydrogen atoms (just protons really) together. Then 1 second to fuse with another Proton, and then [tex]10^{6}[/tex] years to form Helium-4. Is that just saying that on average it takes it that long; but it doesn't need to be that long? So could the whole cycle could occur quickly (within a few seconds) if conditions were right?

    And what are the Alpha particles? It looks like it simply shows two Hydrogen atons (protons) being expelled. So does that mean we can consider two of the Hydrogen atoms (or protons) as a catalyst, like it discusses on the CNO cycle page?
  6. Oct 12, 2007 #5


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    The cross-section (probability) that two protons will fuse and produce a deuteron is relatively low. A hotter or denser star would have higher rate of proton fusion - hence it would have a shorter life. The proton-deuteron cross-section is higher, but it is a short intermediate step.

    The higher the temperature and plasma density the greater the reaction rate. Under the 'right' conditions the reaction rate increases.

    Hydrogen is the fuel and is consumed. In the CNO cycle, the CNO are effectively catalysts. The alpha particles (2p, 2n), which are also the nuclei of He-4 atoms, are simply the final product of proton-based fusion. He-4 can fuse under higher temperatures.

    In the pp-chain, d and He-3 are intermediate steps.
  7. Oct 12, 2007 #6
    Astronuc - Thanks for the info. So I'm assuming they're making those time measurements off of the values associated with our own sun. That makes sense.

    Thanks again for the info. I'm still trying to grasp how things actually "fuse". How and why (what conditions) the quarks make the mad dash and cause a seemingly constructive destruction which ultimately gives light and life to us simple beings.
  8. Oct 12, 2007 #7


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    I don't know off-hand of any reference on the physics of fusion at the quark level. That would be interesting.

    Basically we have p + p -> d (pn) or at the quark level.

    (uud)+(uud) -> (uud)(udd), so the question is what happens in fusion to make a u transform to d.

    Here is something of a description - http://hyperphysics.phy-astr.gsu.edu/hbase/astro/procyc.html#c4
  9. Oct 12, 2007 #8
    Yeah, I read that description. I guess science has not yet figured out completely why the down and up shoot off using a Boson to make a positron and neutrino. That would be interesting to find out. How many sub-atomic particles are there that they've found now anyway? Last I remember I heard there were over 100. It looks like wikipedia divides them up into different types. It would be interesting to see what is exactly happening between the quarks. Though that may introduce further particles, which are even smaller. Do they believe there is an end to how small a particle is?
  10. Oct 12, 2007 #9


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    The process [itex]u \rightarrow d + e^+ + \nu_e[/itex] is closely related to other processes like [itex]d \rightarrow u + e^- + {\bar \nu}_e[/itex] which causes nuclear beta decay, and [itex]{\bar \nu}_e + u \rightarrow d + e^+[/itex] which has been studied at high energies using (anti)neutrino beams on hydrogen or nuclear targets at particle accelerators. They are all understood using the exchange of W bosons in the standard model of the weak interaction, which has been widely accepted for over thirty years. Using this model we can calculate interaction probabilities as a function of energy, etc.

    Of course, nothing in physics has been "figured out completely," because all theories have fundamental assumptions which cannot be explained in the context of the theories themselves, and must await "deeper" theories for their explanation.

    Well, if the p+p collision provides enough energy to make the process energetically feasable, then there is a certain probability (cross-section) for it to actually happen, which I assume can be calculated from the standard model, at least in principle.

    A somewhat similar process would be nuclear [itex]e^+[/itex] decay, which uses the same quark transformation. In this case the energy comes from the difference in mass between the initial and final nuclear states.
    Last edited: Oct 12, 2007
  11. Oct 13, 2007 #10


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    This is what I was trying to say.

    I suspect that protons have a higher probability of scattering than fusion, but then it is interesting that the cross-section for p+d is orders of magnitude greater than p+p.
  12. Oct 13, 2007 #11
    This is just a consequence of which interaction mediates the two reactions. The p+d reaction is totally electromagnetic, while p+p has to proceed through the weak force. But, the strength of the weak force is suppressed by (approximately) the square of the mass of the W boson.
  13. Oct 15, 2007 #12
    jtbell - I guess that's true. Nothing has been completely figured out, and maybe nothing can be completely figure out. I'm sure there are people who take philosophical stands on that.

    So am I correct in assuming that they can calculate the prbability of such a quark interaction; but cannot really say why it happens yet? That's what it sounds like. It would be interesting to see the next ("deeper") theory. I love physics; but don't know too much.

    Astronuc - Are you saying the probability curve for p+d is greater than p+p? Either way, where can I find more info on that?

    Parlyne - So the actual force involved in causing this process involves the electromagnetic force for p+d; but not for the p+p?
  14. Oct 15, 2007 #13
    You can often tell which force is mediating an interaction by what is produced. The p+p reactions looks like [tex]p + p \rightarrow d + e^+ + \nu_e[/tex]. There are several hints that this must be a weak interaction. First, there are no particle/anti-particle pairs or photons in the products. This suggests that the boson mediating the interaction must be charge, implicating a W. More conclusively, though, one of the products is a neutrino. Neutrinos only interact under weak interactions. And, finally, this involves the flavor change of a single quark (up to down), which can only happen under weak interactions.

    The p+d interaction, however, has none of these caveats. All that needs to happen is a net loss of energy to allow binding. [tex] p + d \rightarrow ^3He + \gamma[/tex] This same process, I suppose, could also be mediated by Z. And, it's probably worth noting that the strong force is involved inasmuch as it's what provides the potential energy which allows bound states in the first place.
  15. Oct 15, 2007 #14
    Parlyne - Wow, interesting stuff. Is there a site that tells you what to look for to tell what kind of force it is? I'd like to know more about that. I'd like to study the forces and see what all the fuss is about as far as trying to unify them. I think it will come down to patterns which can be expressed as a function. The function itself will not explain the force; but it will be a kind of guide as to where the forces fall, and then with that guide we might find patterns for forces which we haven't noticed before.
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