A galaxy is bound together by the mutual gravitational attraction of all of its parts. All orbit in the same general direction. Orbit speeds similar at a given radius. Wide ranges of orbital speeds. No Blue M-S stars Random elliptical orbits in all directions.
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.