A study of star formation is carried out on 35 giant extragalactic H ii regions ( GEHRs ) in seven galaxies using optical photometric data in BVR broad bands and in the emission line of H \alpha .
Interstellar extinction , metallicity and nebular contributions to the broad bands are estimated using spectroscopic data on these objects .
Dimensionless diagrams involving B - V and V - R colors and the flux ratio of Balmer line to B band continuum are used to study star formation .
The cluster colors indicate reduced extinction towards stellar continuum compared to the values derived from Balmer lines for the ionized gas .
The frequency of detection of classical young ( t < 3 Myr ) regions with only one burst of star formation is found to be low as compared to young regions with an accompanying population rich in red supergiants from a previous burst ( t \sim 10 Myr ) .
Reduced extinction towards cluster stars , destruction of ionizing photons and the existence of older population , often spatially unresolvable from the younger population , all conspire to make the observed Balmer line equivalent widths low in a majority of the GEHRs .
A scenario of star formation is suggested which explains many of the observed properties of GEHRs , including the core-halo structure , reduced extinction for the radiation from stars as compared to that from the nebular gas , non-detection of young single burst regions and the co-existence of two populations of different ages .