Stellar Population and Galaxy Evolution
		� �Metal� abundance during time � �Metals� are elements heavier than He � A given star�s atmospheric abundance is approx. fixed at birth � Interstellar metal abundance grows with each new generation of stars � Red giants and supernova eject new heavy elements into interstellar gas � Orbits during time � A given star�s orbit is approx. fixed at birth � just plows through gas � Orbits of gas clouds evolve with time since they can collide � Orbits get more circular and disk-like with time