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?

  • $\begingroup$ 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. astronomy.stackexchange.com/questions/6390/… is close, but I think there are clearer explanations. $\endgroup$ – zibadawa timmy Jan 28 '18 at 10:16
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    $\begingroup$ 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. $\endgroup$ – David Hammen Jan 28 '18 at 13:26