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Atomic Hydrogen gas cloud to Star

  1. Apr 9, 2013 #1
    Atomic Hydrogen gas cloud to Star!!

    what would be different about stars and their formation if interstellar medium only has atomic gas and nothing else?

    how would it affect stellar interior and energy production?

    My guess is that since atomic hydrogen is less massive than molecular hydrogen.. the contraction of a could would need more mass. and the energy production would start at lower temperature? due to low mass? to less binding energy?

    P.S this is a past exam question trying work through it :/ thanks, B.A
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  3. Apr 9, 2013 #2


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    It would heat up while contracting, first breaking apart the molecules, and then ionising. The energy production in stars happens in the fully ionised interior.
    So, I'd say it wouldn't change much.
  4. Apr 9, 2013 #3
    thanks for the reply

    so if we were to contract a star from atomic hydrogen, would we need more mass or density in terms of particle? because atomic hydrogen is less massive than molecular hydrogen?
  5. Apr 9, 2013 #4


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    Two hydrogen atoms have the same mass as one molecule of H2(obviously). So while in order to create a star of a given mass you'd need twice as many hydrogen atoms as hydrogen molecules, you're still supplying the same mass, and the same number of nuclei(protons) to fuel the fusion. There's no difference.

    Now, molecular clouds are much colder than atomic clouds of intersetellar hydrogen gas, so they're much denser(http://en.wikipedia.org/wiki/Interstellar_medium#Interstellar_matter).
    This means that you can have a smaller(lower in volume) molecular cloud containing the same amount of matter as a larger atomic gas cloud.
    Hot clouds have higher pressure too, so they resist gravity better than cold ones. So it's easier to make a star out of molecular gas, as it'll collapse more readily.

    The end effect should be the same, though. Perhaps fusion would start earlier in an atomic cloud scenario, as the gas is already hot? This I don't know.
    But once the star reaches equilibrium it's pretty much the same either way.
    Last edited: Apr 9, 2013
  6. Apr 9, 2013 #5


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    Heh, I get the feeling that I haven't answered your question at all, at least not at the appropriate level. You obviously know the basics(exam question after all).

    Let's see again, although that's a bit out of my depth, so take it with a grain of salt.

    Atomic gas clouds are hot. If they weren't then molecules would form and we'd be back to square one.
    Using ideal gas law, this means less particles at higher pressure in the same volume.

    Diffuse, less dense clouds collapse less readily than dense ones.
    Note that the fact that H2 is twice as heavy as atomic hydrogen contributes barely a factor of 2 to the density, while temperature differences contribute to as much as five orders of magnitude of difference in density.(according to the wiki article)
    High pressure in the cloud counteracts gravity. The gas can reach equilibrium before it contracts enough to form a dense body and start fusion(would need to do some calculations for that).
    It would take a very long time(again, would be nice to calculate), to radiate all that energy away.

    This would mean that with atomic gas only in the interstellar medium, stars might not form at all for a long time, or only small, long lived stars would form.
  7. Apr 9, 2013 #6
    in terms of concept you answered my question thanks calculation I can deal with :wink:
  8. Apr 11, 2013 #7
    Wait wait wait. So far the replies are given in terms of initial stages of star formation only. But the thing is, if there's only H2 in the cloud and no helium, the fusion mechanism will be totally different! In our sun, the dominant energy production mechanism is p-p chain, and it requires elements, heavier than hydrogen, to be there in the star core. Without heavier elements, two protons would fuse a helium nucleus, which them would beta-decay into a dueteron. So the fusion would still take place, but I think that the energy output must be much smaller, therefore a star would be more compact.
  9. Apr 11, 2013 #8


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    Well, the question doesn't say it's only hydrogen.

    If we were to interpret "only atomic gas" as equivalent to "no metals"(in the astrophysical sense), then it'd still contain helium, and the resultant star would be similar to Population III stars of the early universe.
  10. Apr 11, 2013 #9
    You're right indeed, I missed that!
    But then still, forget about the p-p chain for a second and think about the CNO cycle. They do require metals (in astrophysical sense) to fuse elements through the CNO cycle, therefore in the event of only-atomic-gas-cloud we'll have a large difference in how massive stars (which mostly rely on CNO cycle) fuse helium.

    EDIT: just to be clear, the CNO cycle requires carbon-12, and carbon is "dust" from astronomical point of view. So having gas only is not enough.
  11. Apr 11, 2013 #10


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