Stellar Populations and the Star Formation Histories of LSB Galaxies:
  III. Stellar Population Models
  A series of population models are designed to explore the star formation
history of gas-rich, low surface brightness (LSB) galaxies. LSB galaxies are
unique in having properties of very blue colors, low H$\alpha$ emission and
high gas fractions that indicated a history of constant star formation (versus
the declining star formation models used for most spirals and irregulars). The
model simulations use an evolving multi-metallicity composite population that
follows a chemical enrichment scheme based on Milky Way observations. Color and
time sensitive stellar evolution components (i.e., BHB, TP-AGB and blue
straggler stars) are included, and model colors are extended into the Spitzer
wavelength regions for comparison to new observations. In general, LSB galaxies
are well matched to the constant star formation scenario with the variation in
color explained by a fourfold increase/decrease in star formation over the last
0.5 Gyrs (i.e., weak bursts). Early-type spirals, from the S$^4$G sample, are
better fit by a declining star formation model where star formation has
decreased by 40% in the last 12 Gyrs.
