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We investigate the star formation history (SFH) as a function of radius inM101 using archival HST/ACS photometry. We derive the SFH from the resolvedstellar populations in five 2' wide annuli. Binning the SFH into time framescorresponding to stellar populations traced by H$\alpha$, far ultraviolet(FUV), and near ultraviolet (NUV) emission, we find that the fraction ofstellar populations young enough to contribute in H$\alpha$ is 15%-35% in theinner regions, compared to less than 5% in the outer regions. This provides asufficient explanation for the lack of H$\alpha$ emission at large radii. Wealso model the blue to red supergiant ratio in our five annuli, examine theeffects that a metallicity gradient and variable SFH have on the predictedratios, and compare to the observed values. We find that the radial behavior ofour modeled blue to red supergiant ratios is highly sensitive to both spatialvariations in the SFH and metallicity. Incorporating the derived SFH intomodeled ratios, we find that we are able to reproduce the observed values atlarge radii (low metallicity), but at small radii (high metallicity) themodeled and observed ratios are discrepant.

The Massive Star Population in M101. II. Spatial Variations in the Recent Star Formation History