  We analyze the stellar and nebular energy distributions of starbursts using
our evolutionary synthesis model PEGASE coupled to the photoionization code
CLOUDY. The originality of this study is to relate the evolution and the
metallicity of the starburst to the past star formation history of the host
galaxy.We compare our model predictions to an observed sample of ~750
starbursts. the most striking feature from line ratio fitting is the decreasing
spread in ionization parameter U at high metallicity Z. The best fits of
emission line ratios are obtained with a combination of a high- and a
low-ionization components. No additional source of ionizing photons --shocks or
hidden AGN -- is needed. Colors and equivalent widths are fitted in coherency
with emission line ratios. An underlying population is needed, even for
small-aperture observations. This evolved population not only reddens the
continuum and dilutes the equivalent width of the emission lines, but also
participates in the ionization process. Its main effect on line ratios is to
maintain a high level of excitation when the burst stops. Models combining
underlying populations typical of Hubble sequence galaxies and instantaneous
starbursts with ages between 0 and 8 Myr agree satisfactorily with all the
data.
