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Yes, the atomic hydrogen is probably mostly left over from the Big Bang. [Edited to add: Not sure how much that is true and how much present-day atomic hydrogen is the result of recombination.] And, yes, ${\rm H}_{2}$ does get dissociated by high-energy photons -- and also by cosmic rays, which can penetrate dense, dusty clouds that block most of the high-...


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This is one of those questions that is easy to state but complicated to answer - and this won’t at all be a complete answer, but mostly a quick outline of some important factors to consider and terms you might search for in order to learn more. The question of why the interstellar medium (ISM) has the structure it does is a long-standing one, and one that a ...


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