  In this thesis we have identified two star formation sites for star formation
in galaxies which differ widely in metal content. We conclude that `massive'
star formation that occurs in a high density, high metallicity environment,
like that encountered in circumnuclear star-forming regions, takes place in
systems that comply to the definition of super stellar clusters and that are
arranged in star forming complexes. These complexes have Halpha luminosities,
and therefore masses of ionizing stars, that overlap at the lower end with
those found in HII galaxies, which have a much lower metallicity. The fact that
their ionization structure and the temperature of their ionizing radiation
field are very similar, point to stellar clusters in these two environments
which have the same equivalent effective temperature. This is contrary to what
is expected from stellar evolution models which predict lower stellar effective
temperatures in high metallicity regions.
