The History of Star Formation in Galaxies
  If we are to develop a comprehensive and predictive theory of galaxy
formation and evolution, it is essential that we obtain an accurate assessment
of how and when galaxies assemble their stellar populations, and how this
assembly varies with environment. There is strong observational support for the
hierarchical assembly of galaxies, but our insight into this assembly comes
from sifting through the resolved field populations of the surviving galaxies
we see today, in order to reconstruct their star formation histories, chemical
evolution, and kinematics. To obtain the detailed distribution of stellar ages
and metallicities over the entire life of a galaxy, one needs multi-band
photometry reaching solar-luminosity main sequence stars. The Hubble Space
Telescope can obtain such data in the low-density regions of Local Group
galaxies. To perform these essential studies for a fair sample of the Local
Universe, we will require observational capabilities that allow us to extend
the study of resolved stellar populations to much larger galaxy samples that
span the full range of galaxy morphologies, while also enabling the study of
the more crowded regions of relatively nearby galaxies. With such capabilities
in hand, we will reveal the detailed history of star formation and chemical
evolution in the universe.
