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Hydrogen Fusion Cycle in Stars: Storage/Transport/Consumption?

  1. Dec 21, 2012 #1
    I have always been wondering how in an active star the H/He fusion cycle is "managed" for lack of a better word, a little like a logistical analogy really.


    i) Where in the star is the yet-to-be-fused H 'stored'?
    ii) Where in the star is the He result of fusion 'stored'?
    iii) Is the H 'transport' mechanism between location i) above and the location of the actual fusion process continuous and smooth or are there irregularities in 'supply' of H? What is the nature of this transport mechanism?
    iv) Does the volume of the 'stored' He end-product of fusion infringe on the location of the fusion process itself as the star uses up more and more of its H?

    Thanx in advance for your replies,

  2. jcsd
  3. Dec 21, 2012 #2
    As you may guess, there are no pipes and valves in the star core. Every ingredient for every p-p chain / CNO cycle is present in the core of, say, our Sun. The key point is that the cross-section for igniting fusion is very small, so that only a minute fraction of hydrogen is actually converted into helium.

    There is a reason for this. The Sun is able to sustain itself for billions of years. Suppose you increase the temperature at the core, making more fusion energy output. Then, the energy would cause increase in Sun's radius, decreasing the pressure and temperature at the core, thus cooling fusion site back to a "normal" state. On the other hand, if you decrease the core temperature, the star would shrink, causing more energy production in the core.
  4. Dec 21, 2012 #3

    Initially when all you have is hydrogen, I can understand your explanation. Subsequently however, what happens as we advance in time with the p-p chain/CNO cycle and accumulate more and more quantities of He?

    Where does all this He go in the star's bowels and do we get to a point in time where all this accumulated He fusion product will impede the 'orderly' supply of hydrogen to the core?

    Finally, what is the transport mechanism involved in getting the hydrogen to the core if it impeded by the He product of the fusion cycle?

    Last edited: Dec 21, 2012
  5. Dec 21, 2012 #4
    The fraction of hydrogen converted to helium each day is very small. The sun is essentially a big ball of viscous churning very hot plasma. Nothing stays in one place for long and there is constant mixing and diffusion, so I would think that the hydrogen and helium would be at equilibrium all the time. I'd guess that helium would be concentrated in the core as it is denser, but don't know how much.

    So the two elements are always present everywhere in the Sun. There is no need to transport anything or have any sort of a cycle.
  6. Dec 21, 2012 #5
    Helium fusion does not require hydrogen. See here and here.

    Your other question is very good indeed. The helium produced by hydrogen fusion is stored in the core. As the star grows older, it builds up this helium "storage" in the very centre of it. Now the thing is, it is supported against an outside material trying to "push" it inwards by a degenerate force. Then, as the star runs out of hydrogen fuel, the temperature increases (refer to my previous post), and eventually the whole He core ignites (degenerate helium core is less prone to temperature instabilities, therefore you literally explode the core), producing helium flash. It will happen to the Sun in a few billion years, turning it into a red giant.
  7. Dec 21, 2012 #6
    Whether or not there is a "supply" process varies widely between stars, and also changes with time as hydrogen is exhausted.

    In Sun, there is NO supply process of hydrogen. The surface of Sun is churning mass of hot gas, creating granulation, magnetic fields and sunspots - but it is only about 200 000 km deep, and holds less than 1/50 the mass of Sun.

    The interior of Sun is hot - but stagnant. The heat slowly conducts out of the core by repeated radiation.

    Hydrogen fusion takes place over somewhat extended region of the core of the Sun. And there is no supply - all the hydrogen Sun is fusing now or ever has fused has been right where it now fuses ever since the Sun formed.

    Since fusion is unequally distributed in SunĀ“s core - it is fastest at the centre, and slower but still significant slightly outside the centre - the helium has built up over the evolution of Sun. In the centre of Sun, about 50 % of original hydrogen has fused by now. Going outwards, you would encounter places where 25 % of hydrogen has fused, then 5 % etc.

    All of the helium has remained in place. There is no mixing in central Sun, and neither diffusion nor gravitational settling has been significant over milliards of years.

    The outer, mixed layer of Sun is so cold, even in its bottom, that it has not undergone any significant amount of fission.

    As the concentration of He in the Sun increases, the core contracts, heats up - and the remaining hydrogen fuses at an increasing rate.

    When the hydrogen is completely exhausted in the core, there shall still be no "transport" of H into core. Rather, the He core shall produce no energy, but the layer of hot hydrogen OUTSIDE the He core shall go on fusing, and at an increasing rate. The hydrogen is not "transported" except by overall shrinking of the core of the Sun - the belt where fusion goes on at important rate propagates outward to there the hydrogen is and has always been.
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