Why are Population I and Population II stars compositionally different?

Population I and Population II stars are compositionally different primarily because they formed from different phases of the universe. Population I stars, which are often younger, are found in the spiral arms of galaxies and are composed primarily of heavier elements, also known as metals, due to the enrichment of the interstellar medium from previous generations of stars. This later generation of stars has formed in regions that have undergone significant chemical evolution over time, allowing them to incorporate elements like carbon, oxygen, and iron into their makeup.

In contrast, Population II stars are older and typically found in the halos of galaxies and globular clusters. These stars formed relatively early in the universe's history and have a much lower metallicity, meaning they contain fewer heavy elements. This is because they formed from primordial gas that was largely composed of hydrogen and helium, with little to no contributions from earlier star generations to enrich the material from which they formed.

The differences in their formation phases, reflecting the evolving chemical composition of the universe over time, are key to understanding the distinct characteristics of Population I and II stars.