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When a star runs out of hydrogen, it either goes through supernova or forms white dwarf and planetary nebula. So there should not be much hydrogen left to form a new star in the nebula. So how new stars are formed from these nebulas?

soumya
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  • Basic answer is you are mistaken: non-dwarf stars use up very little of their hydrogen. I think this is answered on the site, but I'm not finding it right now. https://astronomy.stackexchange.com/questions/6390/is-there-enough-hydrogen-left-after-a-star-dies-so-another-star-will-have-enough?rq=1 is close, but I think there are clearer explanations. – zibadawa timmy Jan 28 '18 at 10:16
  • Also https://astronomy.stackexchange.com/questions/13649/where-do-new-stars-get-their-hydrogen-from?noredirect=1&lq=1 and https://astronomy.stackexchange.com/questions/1414/will-new-stars-stop-forming-at-some-point-of-time?rq=1 – ProfRob Jan 28 '18 at 10:54
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    Marked as a duplicate, but all of the answers to all the duplicates miss a key point, which is that star formation is an extremely inefficient process. IIRC, it takes roughly a 100 solar mass gas cloud to form a one solar mass star. The 99% of the cloud that didn't go into forming the star is ejected along the way, available to be used sometime in the future to form stars when/if it (along with other material) reform a collapsing interstellar gas cloud. – David Hammen Jan 28 '18 at 13:26

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