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The universe is expected to have a time when star formation slowly ceases. However, this seems bizarre as there is always an abundant amount of matter in the Universe. Massive amounts of hydrogen exists in several galaxies, but they are deemed dead. Any gravitational disturbance can send the gas into a protostar, and even if there weren't hydrogen left, enough helium or carbon can turn a star into a star that fuses heavier elements, until the universe is depleted of such and mostly elements heavier than iron.

So why does star formation cease when these is still abundant hydrogen around? Is there some sort of interaction in these hydrogen supplies that prevents star formation? Is the gas kept at a temperature that is too hot for formation? Or have I made a misconception?

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  • $\begingroup$ I suggest to look for star formation conditions. It might be easier. $\endgroup$ – Alchimista Mar 6 at 13:29
  • $\begingroup$ I see the question is close. However in a very general statement the diffuse matter need critical density otherwise the Jean limit can't be reached. That is why planets do not forms by direct collapse from protostar like nebulae. $\endgroup$ – Alchimista Mar 9 at 19:05
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Star formation in a galaxy can end for a number of reasons:

  • Ram pressure affecting a galaxy falling into a cluster can strip away gas, which can end most star formation in the galaxy itself.
  • The winds of massive stars (as well as supernovae shocks) can eject cold gas via radiation pressure.
  • Mergers with other galaxies can also eject gas, although these interactions often lead to an increase in star formation, briefly.

As far as I'm aware, how strongly each of these phenomena contribute is still an open question, but we do know that they either eject gas or heat it up (and therefore make it unsuitable for star formation, which requires cold gas), thereby slowing or even effectively ending star formation in a given galaxy. A gas cloud is supported against gravity by thermal pressure (and occasionally magnetic fields). Clouds that are hot have higher thermal pressures and therefore can resist gravity; cold clouds cannot. Cold, dense clouds are therefore the best places for star formation (more gravity and less pressure).


As for the amount that can be ejected, the exact amount depends on a) the initial gas quantity, b) the mechanism, and c) likely a large number of other free parameters, so I don't think I can answer that for the general case.

Regarding if stars can form with little hydrogen: If hydrogen was ejected, so would other elements, so it's not as if all the hydrogen would be blown away and helium, lithium and others would remain. Hydrogen-poor stars exist, but they're older, evolved stars, not young ones.

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  • $\begingroup$ I was wondering if you could explain why heated gas is unsuitable for star formation. My understanding is that when a star forms and gas becomes more dense it heats up. $\endgroup$ – Moondefender Mar 6 at 12:31
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    $\begingroup$ It must get warm under gravity. If it is already hot, its molecules have escape speed. @Moondefender $\endgroup$ – Alchimista Mar 6 at 13:27
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    $\begingroup$ @Moondefender A gas cloud is supported against gravity by thermal pressure (and occasionally magnetic fields). Clouds that are hot have higher thermal pressures and therefore can resist gravity; cold clouds cannot. Cold, dense clouds are therefore the best places for star formation (more gravity and less pressure). $\endgroup$ – HDE 226868 Mar 6 at 14:40
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Basically because this great amount of matter has many directions , you can see when many stars born in first time has same direction in a galaxy (like to stars ursa mayor), when pass the time, this matter direction change, generating entropy, and is when stars formation cease.

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