The mass of unresolved young star clusters derived from spectro-photometric data may well be off by a factor of 2 or more once the migration of massive stars driven by mass segregation is accounted for .
We quantify this effect for a large set of cluster parameters , including variations in the stellar IMF , the intrinsic cluster mass , and mean mass density .
Gas-dynamical models coupled with the Cambridge stellar evolution tracks allow us to derive a scheme to recover the real cluster mass given measured half-light radius , one-dimensional velocity dispersion and age .
We monitor the evolution with time of the ratio of real to apparent mass through the parameter \eta .
When we compute \eta for rich star clusters , we find non-monotonic evolution in time when the IMF stretches beyond a critical cutoff mass of 25.5 \mathrm { ~ { } M \mbox { $ { } _ { \odot } $ } } .
We also monitor the rise of color gradients between the inner and outer volume of clusters : we find trends in time of the stellar IMF power indices overlapping well with those derived for the LMC cluster NGC 1818 at an age of 30 \mathrm { ~ { } Myr } .
We argue that the core region of massive Antennæ clusters should have suffered from much segregation despite their low ages .
We apply these results to a cluster mass function , and find that the peak of the mass distribution would appear to observers shifted to lower masses by as much as 0.2 dex .
The star formation rate ( SFR ) derived for the cluster population is then underestimated by from 20 to 50 per cent .