ABUNDANCE RATIOS AND GALACTIC CHEMICAL EVOLUTION

The metallicity of stars in the Galaxy ranges from [Fe/H] = -4 to +0.5 dex, and the solar iron abundance is epsilon(Fe) = 7.51 +/- 0.01 dex. The average values of [Fe/H] in the solar neighborhood, the halo, and Galactic bulge are -0.2, -1.6, and -0.2 dex respectively. Detailed ab undance analysis reveals that the Galactic disk, halo, and bulge exhib it unique abundance patterns of O, Mg, Si, Ca, and Ti and neutron-capt ure elements. These signatures show that environment plays an importan t role in chemical evolution and that supernovae come in many flavors with a range of element yields. The 300-fold dispersion in heavy eleme nt abundances of the most metal-poor stars suggests incomplete mixing of ejecta from individual supernova, with vastly different yields, in clouds of similar to 10(6) M-circle dot. The composition of Orion asso ciation stars indicates that star-forming regions are significantly se lf-enriched on time scales of 80 million years. The rapid self-enrichm ent and inhomogeneous chemical evolution models are required to match observed abundance trends and the dispersion in the age-metallicity re lation.