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I'm not a physicist or have a very good physics background but I've often wondered why there are new stars that are born in the nebula which was created after the parent star has exploded. As I understand, a star (at least those which have enough mass) explodes after it runs out of fuel, or when the core inside the star starts to fuse iron. So how can new stars be born when the nebula does not have any hydrogen in it?

I am probably missing something but this question interests me very much, and I can't seem to find any valuable information which would explain to me the answer to my question.

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New stars are not formed from the nebulae created when a parent star explodes.

In space there is thin interstellar gas and plasma. This gas is buffeted and blown by the solar winds of stars, and the shockwaves of supernovae. The gas is mostly Hydrogen and Helium.

Stars die in two ways. The most common way is for their outer layers to be blown out into space in a fairly gentle way. This process forms a "planetary nebula" The outer layers are formed mostly of hydrogen and helium, but are enriched by other elements. Or stars can die as supernovae. These are much more energetic. Even so, much of the gas blown out is Hydrogen and Helium as it comes from the outer layers of the star, but it will be further enriched by heavier elements. There are different kinds of supernovae with different mixtures of elements.

The elements blown off of dying stars mixes with the interstellar gas, enriching it and compressing it. This mixture of gas is still mostly hydrogen and helium and hydrogen is the main fuel for stars!

If the gas is sufficiently compressed (for example by a supernova shockwave) then its own gravity can start to pull it together, ultimately forming stars.

So stars are not formed from the iron "ashes" of dead stars, but from a mixture of the original Hydrogen fuel that has never been in a star, and the outer layers of stars that are made of "unburnt" hydrogen that was blown off the star as it died.

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    $\begingroup$ So it's like recycling steel? You just keep mixing in more steel until the old tramp elements are diluted? $\endgroup$
    – DKNguyen
    Commented Aug 14, 2022 at 2:43
  • $\begingroup$ It seems to me that your first statement is incorrect and contradicted by the rest of the answer. New stars are formed by the nebulae, plus some other stuff. Can you clarify? $\endgroup$
    – Drake P
    Commented Aug 14, 2022 at 15:57
  • $\begingroup$ No, the stuff in the nebula mixes and enriches the interstellar gas. Stars don't form in the remnant nebula. For example, do you drink toilet water? But you know that that water from your toilet must ultimately go and mix with the rivers and oceans and then it could come back to your tap... but you would not say that the water that comes from your tap comes from the toilet. $\endgroup$
    – James K
    Commented Aug 14, 2022 at 16:14
  • $\begingroup$ @DKNguyen Somewhat. Tho the proportions about which we are talking is something like 10% of the mass in the nebula is going to end up in the star and a further 10-20% of these is going to be fused into heavier elements. Meaning about 98% of the initial hydrogen is still there. This results in the metallicity of the initial nebula not changing a whole lot during the lifetime of a single star. Population 1 ("Young") stars have about 10-100 times the metallicity of Population 2 ("older") stars. At the moment our Sun contains about 2% "metals". This started at about 1.5%. 1/2 $\endgroup$
    – SirHawrk
    Commented Aug 15, 2022 at 7:31
  • $\begingroup$ There was a paper published earlier this year that puts the maximum metallicity of stars that are able to expell more metals at about 15%. After that the metals produced during its death will be trapped inside the white dwarf. Meaning that stars up to this metallicity would behave reasonably similar to the Stars we already know. Tho the higher the metallicity the lower the temperature of a star. 2/2 $\endgroup$
    – SirHawrk
    Commented Aug 15, 2022 at 7:33
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New stars aren't directly born in the exploded remnants of massive stars. Star formation does not occur in newly produced supernova remnants.

Instead what happens is that, over the course of millions of years, the gas in the supernova remnant is mixed into the gas that is already part of the interstellar medium, and which is composed almost entirely of hydrogen and helium. The dilution factor is large, such that after mixing, the gas contains (currently) just 1-2% by mass of elements heavier than helium.

Star formation may then take place if this gas is compressed or otherwise becomes unstable to collapse.

Highly relevant:

How can there be 1,000 stellar ancestors before our Sun?

Parent stars of our Sun - Where are its remains?

How could a supernova seed solar nebula?

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