  Using cosmological hydrodynamical simulations, we investigate the effects of
hierarchical aggregation on the triggering of star formation in galactic-like
objects. We include a simple star formation model to transform the cold gas in
dense regions into stars. Simulations with different parameters have been
performed in order to quantify the dependence of the results on the parameters.
We then resort to stellar population synthesis models to trace the color
evolution of each object with red-shift and in relation to their merger
histories. We find that, in a hierarchical clustering scenario, the process of
assembling of the structure is one natural mechanism that may trigger star
formation.
  The resulting star formation rate history for each individual galactic object
is composed of a continuous one ($\leq 3 \rm{M_{\odot}/yr}$) and a series of
star bursts.
  We find that even the accretion of a small satellite can be correlated with a
stellar burst. Massive mergers are found to be more efficient at transforming
gas into stars
