Halo stars toward the north field. The upper panel shows the spatial distribution of halo stars in equatorial coordinates, while the two lower panels show the color–magnitude diagrams in a fixed range of decl. (from 42.5 to 44.5 deg) and two ranges of R.A. (left: from 225 to 250 deg; right: from 200 to 250 deg). The orange dotted line in each panel represents an isochrone with age of 11.5 Gyr, metallicity of −1.6, and assumed distance from the Sun of 30 kpc. The black dots with error bars show typical photometric uncertainties as a function of i₀. This figure indicates that substructure exists in the stellar halo beyond 30 kpc, as indicated by the red arrow, and extends over at least 100 deg².