The Milky Way stellar halo out to 40 kpc: squashed, broken but smooth

We introduce a new maximum-likelihood method to model the density profile of blue horizontal branch and blue straggler stars and apply it to the Sloan Digital Sky Survey Data Release 8 photometric catalogue. There are a large number (similar to 20 000) of these tracers available over an impressive 14 000 deg(2) in both Northern and Southern Galactic hemispheres, and they provide a robust measurement of the shape of the Milky Way stellar halo. After masking out stars in the vicinity of the Virgo overdensity and the Sagittarius stream, the data are consistent with a smooth, oblate stellar halo with a density that follows a broken power law. The best-fitting model has an inner slope alpha(in) similar to 2.3 and an outer slope alpha(out) similar to 4.6, together with a break radius occurring at similar to 27 kpc and a constant halo flattening (i.e. ratio of minor axis to major axis) of q similar to 0.6. Although a broken power law describes the density fall-off most adequately, it is also well fitted by an Einasto profile. There is no strong evidence for variations in flattening with radius, or for triaxiality of the stellar halo.