<p>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?</p>

CC BY-SA 3.0
2018-01-28T10:16:19.157Z
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
CC BY-SA 3.0
2018-01-28T10:54:31.563Z
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
CC BY-SA 3.0
2018-01-28T13:26:01.963Z
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
<supernova><star-formation><stellar-evolution><nebula>
Doubt about star formation
soumya
# Doubt about star formation

(Asked by: username_0 on 2018-01-28)

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?

**Tags:** supernova, star-formation, stellar-evolution, nebula
> **Comments:**
> **username_1**: 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.  
> **username_2**: 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  
> **username_3**: 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.  

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