  In this paper we investigate the power of spectral synthesis as a mean to
estimate physical properties of galaxies. Spectral synthesis is nothing more
than the decomposition of an observed spectrum in terms of a superposition of a
base of simple stellar populations of various ages and metallicities, producing
as output the star-formation and chemical histories of a galaxy, its extinction
and velocity dispersion. Our implementation of this method uses the Bruzual &
Charlot (2003) models and observed spectra in the 3650--8000 \AA range. The
reliability of this approach is studied by three different means: (1)
simulations, (2) comparison with previous work based on a different technique,
and (3) analysis of the consistency of results obtained for a sample of
galaxies from the SDSS. We find that spectral synthesis provides reliable
physical parameters as long as one does not attempt a very detailed description
of the star-formation and chemical histories. We show that besides providing
excellent fits to observed galaxy spectra, this method is able to recover
useful information on the distributions of stellar ages and, more importantly,
stellar metallicities. Stellar masses, velocity dispersion and extinction are
also found to be accurately retrieved for realistic signal-to-noise ratios. We
apply this synthesis method to a volume limited sample of 50362 galaxies from
the SDSS DR2, producing a catalog of stellar population properties. A
comparison with recent estimates of both observed and physical properties of
these galaxies obtained by other groups shows good qualitative and quantitative
agreement. The confidence in the method is further strengthened by several
empirical and astrophysically reasonable correlations between synthesis results
and independent quantities.
