The Dichotomy of the Halo of the Milky Way

We summarize evidence that the halo of the Milky Way comprises two different, and broadly overlapping, stellar components. The two structures exhibit different chemical compositions, spatial distributions, and kinematics. These results were obtained through an analysis of more than 20,000 calibration stars from the Sloan Digital Sky Survey (SDSS). The duality of the stellar halo directly impacts galaxy formation models, for the Milky Way and other large spirals.