  Deep color magnitude diagrams extending to the main sequence provide the most
direct measure of the detailed star formation history in a stellar population.
With large investments of observing time, HST can obtain such data for
populations out to 1 Mpc, but its field of view is extremely small in
comparison to the size of Local Group galaxies. This limitation severely
constrains our understanding of galaxy formation. For example, the largest
galaxy in the Local Group, Andromeda, offers an ideal laboratory for studying
the formation of large spiral galaxies, but the galaxy shows substructure on a
variety of scales, presumably due to its violent merger history. Within its
remaining lifetime, HST can only sample a few sight-lines through this complex
galaxy. In contrast, a wide field imager could provide a map of Andromeda's
halo, outer disk, and tidal streams, revealing the spatially-dependent star
formation history in each structure. The same data would enable many secondary
studies, such as the age variation in Andromeda's globular cluster system,
gigantic samples of variable stars, and microlensing tracers of the galaxy's
dark matter distribution.
