  We have incorporated star formation algorithms into a hybrid N-body/smoothed
particle hydrodynamics code (TREESPH) in order to describe the star forming
properties of disk galaxies over timescales of a few billion years. The models
employ a Schmidt law of index $n\sim 1.5$ to calculate star formation rates,
and explicitly include the energy and metallicity feedback into the ISM.
Modeling the newly formed stellar population is achieved through the use of
hybrid SPH/young star particles which gradually convert from gaseous to
collisionless particles, avoiding the computational difficulties involved in
creating new particles. The models are shown to reproduce well the star forming
properties of disk galaxies, such as the morphology, rate of star formation,
and evolution of the global star formation rate and disk gas content. As an
example of the technique, we model an encounter between a disk galaxy and a
small companion which gives rise to a ring galaxy reminiscent of the Cartwheel
(AM 0035-35). The primary galaxy in this encounter experiences two phases of
star forming activity: an initial period during the expansion of the ring, and
a delayed phase as shocked material in the ring falls back into the central
regions.
