* Stars as Metal Forges: Stars are giant furnaces that fuse hydrogen and helium into heavier elements like carbon, oxygen, and even heavier elements like iron. This process is called nucleosynthesis.

* Stellar Evolution: When stars die, they release these heavier elements back into space through supernova explosions or stellar winds.

* Interstellar Medium: These elements become part of the interstellar medium, the gas and dust between stars.

* New Star Formation: New stars are born from the interstellar medium, which means they inherit the heavier elements created by older stars.

* Metallicity: The abundance of elements heavier than hydrogen and helium is referred to as "metallicity" in astronomy.

Therefore, the first generation of stars formed in the early universe had very low metallicity. Over time, as more stars were born and died, the interstellar medium became enriched with metals, leading to higher metallicity in subsequent generations of stars.