How to explain the existence of heavier elements in Population I stars and in Population II stars contain lighter one?

Question

Recently I read Astrophysics Notes where I found a statement that young stars are classified as Population I stars and relatively older one as Population II stars. Population I stars contain heavier elements or "Metals" - that means elements other than helium and hydrogen. While Population II contain older stars and they contain Hydrogen and Helium - lighter elements more in abundance.My doubt is that, stars begin their journey from Hydrogen in nebula. In course of time due to nuclear reactions more and more heavier nuclei are created,and hence Population I stars which should contain lighter elements in much abundance and the older Population II stars should contain heavier elements. Whether the explanation of the Notes is wrong? or any other plausible explanation? Please clarify.

Answer 1

Population I stars are young stars. They contain heavy elements for the following reason: The older stars die and produce heavier elements in their death.

When a large star dies, it explodes. This explosion creates heavy elements. These heavy elements then combine with gas and form a new younger star. Therefore the young star will contain the heavy elements.

Now over time a star does create heavier elements inside of it as it ages on its way to death, HOWEVER, this stops once a star creates iron. A Pop 2 that made iron will explode.

(Not all stars can get to this point as they have to be super-massive to produce iron. A star that does not produce iron, will stop at some element short of iron, and just fizz out, and the heaviest element it will ever contain will be something like Calcium or Aluminum.)

When a Pop 2 does so it creates elements that are heavier than iron.

Now this is the important point to understand: 2 seconds before that star exploded, the heaviest element it had was iron. After the star exploded it no longer contains those elements because, well, it exploded. Those heavy elements now go on to form a brand new and young Pop 1 star.

Hence why young stars have heavy elements, and old stars dont.