  ABRIDGED: We study the Star Formation History (SFH) of 210 galaxies members
of 55 Hickson Compact Groups (HCG) and 309 galaxies from the Catalog of
Isolated Galaxies (CIG). The SFH traces the variation of star formation over
the lifetime of a galaxy, and yields consequently a snapshot picture of its
formation. Comparing the SFHs in these extremes in galaxy density, allows us to
determine the main effects of Compact Groups (CG) on the formation of galaxies.
We fit our spectra using the spectral synthesis code STARLIGHT and obtain the
stellar population contents and mean stellar ages of HCG and CIG galaxies in
three different morphological classes: early-type galaxies (EtG), early-type
spirals (EtS), and late-type spirals (LtS). We find that EtG and EtS galaxies
in HCG show larger contents of old and intermediate stellar populations as well
as an important deficit of the young stellar population, what clearly implies
an older average stellar age in early galaxies in HCG. For LtS galaxies we find
similar mean values for the stellar content and age in the two samples.
However, we note that LtS can be split into two subclasses, namely old and
young LtS. In HCG we find a higher fraction of young LtS than in the CIG
sample, in addition, most of these galaxies belong to groups where most of the
galaxies are also young and actively forming stars. The Specific Star Formation
Rate (SSFR) of spiral galaxies in the two samples differ. EtS in HCG show lower
values of the SSFR, while LtS peak at higher values when comparing with their
counterparts in isolation. We have also measured shorter Star Formation Time
Scale (SFTS) in HCG galaxies, indicating that they have less prolonged star
formation activity than CIG galaxies.
