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B Gas behaviour in Space

  1. Jul 21, 2017 #1
    What happens to gases in space?

    Do they just dissapear? (Yes yes, yawn) Or can they make up a region of space, and stick together via gravity?

    And what about Jupiter and other gas giants? How do they work, if in space, all gases tend to just shoot out and spread into an even film?
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  3. Jul 21, 2017 #2


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    Gasses in space form structures held together by gravity and resisted by pressure.

    If their self-gravity is high enough to overcome internal pressure, they collapse to form denser structures - such as planets and stars.

    If their internal pressure is too high (e.g. because the gas is hot, and/or there isn't that much of it in the first place), they remain as clouds of plasma, atoms, or molecules.

    The condition for cloud collapse is called the Jeans instability. Wikipedia has a good article on it.
  4. Jul 21, 2017 #3
    But it is possible for there to be a huge region of space comprised purely of gas? (That lasts, outside of longterm redshift, decay, etc)
  5. Jul 21, 2017 #4


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    What does 'huge' mean?

    In any case, that's what the whole universe was like before the first stars formed, so I guess that's huge enough.
  6. Jul 21, 2017 #5
    Yep, good enough for me.

    No I say huge because, again, since the gas expands so much when exposed to zero pressure and vacuum, I would expect it to be a huge area of sparse, but nevertheless "packed" gas. And it would probably be in the middle of nowhere since otherwise it'd get stuck to a planet.
  7. Jul 21, 2017 #6


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    Look up the Jeans instability equations, and specifically Jeans length. It relates the critical radius of a cloud depending on particle mass, gas density, and temperature.
    You can then try plugging in various values and see what is the critical size of the cloud (before it collapses).
    For interstellar molecular clouds, whose eventual collapse triggers stellar formation, and which are extremely tenuous by Earth standards (on the order of ~10^5 particles per cm^3, similar to industrial-grade vacuum, or 'atmosphere' density on the Moon), the clouds can reach hundreds of light-years in size.

    Below is an example of a smaller one, mere 25 ly across, that you can see with your naked eyes on a dark night.
    Stars are already forming in its densest regions, heating up the surrounding gas and dust.
  8. Jul 21, 2017 #7
    The space between the stars is not a hard vacuum though. The interstellar medium is has between one atom in a cubic meter to a million in a cubic centimeter. Hotter gasses also tend to be ionized, so in that case, EM forces can play a role as well as gravity.
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