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