Supernova explosions enrich the interstellar medium with metals. The next generation of stars form out of the metal-enriched interstellar gas. Successive generations get more metal rich. Higher Metal Content in Later Generations. Chemical Evolution only affects populations .
Fusion occurs in the deep interiors of stars.
Except for CNO elements, surface composition remains effectively unchanged over a stars life. Once a star forms, its chemical composition is mostly fixed for life. Metal content gives us a clue to the formation history of populations of stars. Massive Pop II stars go supernova and enrich the gas with metals. Low-mass Pop II stars are still around today.