The Galactic spheroid - What is Population II?

Observations of the Population II stars in external spiral galaxies show the domination of a metal-rich stars over metal-poor ones outward from the center for some distance, farther than which the metal-poor component dominates. This apparent contradiction between the properties of Population II in the Galaxy and in external spirals is resolved here by identifying the vast majority of spatially defined Population II stars in the solar neighborhood not with the metal-poor subdwarfs but with stars in the thick-disk component. Spatially defined Population II and metallicity-defined Population II are essentially distinct populations. Implications of this proposal for disk galaxy formation and evolution are discussed. It is shown how detailed information about the element ratios present in spheroid stars can place constraints on the timescale on which the metal-poor system formed and how homogeneous the protogalaxy could have been at this time. 109 references.