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We characterise the radial density, metallicity and flattening profile of theMilky Way's stellar halo, based on the large sample of 1757 spectroscopicallyconfirmed giant stars from SDSS/SEGUE-2 after excising stars that werealgorithmically attributed to apparent halo substructure (including theSagittarius stream). Compared to BHB stars or RR Lyrae, giants are more readilyunderstood tracers of the overall halo star population, with less bias in ageor metallicity. The well-characterized selection function of the sample enablesforward modelling of those data, based on ellipsoidal stellar density models,$\nu_* (R,z)$, with Einasto profiles and (broken) power laws for their radialdependence, combined with a model for the metallicity gradient and theflattening profile. Among models with constant flattening, these data arereasonably well fit by an Einasto profile of $n=3.1\pm 0.5$ with an effectiveradius $\rm r_{eff} = 15\pm2~$kpc and a flattening of $q=0.7\pm 0.02$; orcomparably well by an equally flattened broken power-law, with radial slopes of$\alpha_{in}=2.1\pm 0.3$ and $\alpha_{out}=3.8\pm 0.1$, with a break-radius of$r_{break}=18\pm1$~kpc; this is largely consistent with earlier work. We find amodest, but significant metallicity gradient within the "outer" stellar halo,$\rm [Fe/H]$ decreasing outward. If we allow for a variable flattening $q =f(r_{GC} )$, we find the distribution of halo giants to be considerably moreflattened at small radii, $q({\rm 10~kpc})\sim 0.57$, compared to $q(>30{\rmkpc})\sim 0.8$. Remarkably, the data are then very well fit by a singlepower-law of index $\rm \sim 4.2\pm0.1$ of the variable$r_q\equiv\sqrt{R^2+(z/q(r))^2}$. In this simple and better fitting model,there is a break in flattening at $\sim 20$~kpc, instead of a break in theradial density function.

The radial profile and flattening of the Milky Way's stellar halo to $\rm 80~$kpc from the SEGUE K-giant Survey