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. //

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?
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