  Young star clusters enable us to study the effects of stellar rotation on an
ensemble of stars of the same age and across a wide range in stellar mass and
are therefore ideal targets for understanding the consequences of rotation on
stellar evolution. We combine MUSE spectroscopy with HST photometry to measure
the projected rotational velocities (Vsini) of 2,184 stars along the split main
sequence and on the main sequence turn-off (MSTO) of the 100 Myr-old massive
(10^5 M_sun) star cluster NGC 1850 in the Large Magellanic Cloud. At fixed
magnitude, we observe a clear correlation between Vsini and colour, in the
sense that fast rotators appear redder. The average Vsini values for stars on
the blue and red branches of the split main sequence are ~100 km/s and ~200
km/s, respectively. The values correspond to about 25-30% and 50-60% of the
critical rotation velocity and imply that rotation rates comparable to those
observed in field stars of similar masses can explain the split main sequence.
Our spectroscopic sample contains a rich population of ~200 fast rotating Be
stars. The presence of shell features suggests that 23% of them are observed
through their decretion disks, corresponding to a disk opening angle of 15
degrees. These shell stars can significantly alter the shape of the MSTO, hence
care should be taken when interpreting this photometric feature. Overall, our
findings impact our understanding of the evolution of young massive clusters
and provide new observational constraints for testing stellar evolutionary
models.
