Why do we know that at least one generation of stars formed and died before the Milky Way's globular clusters formed?

Question

Quizplus · Accepted Answer

Globular clusters contain heavy elements (elements heavier than hydrogen and helium), which can only be produced in the cores of stars or during supernova explosions. The presence of these elements in globular clusters indicates that at least one generation of stars formed, lived, and died, producing these heavy elements before the globular clusters formed.

Why Do We Know That at Least One Generation of Stars