  We use the Wide Field Camera 3 onboard the Hubble Space Telescope to obtain
deep, high-resolution photometry of the young (~ 100 Myr) star cluster NGC1850
in the Large Magellanic Cloud. We analyze the cluster colour-magnitude diagram
(CMD) and find that it hosts an extended main sequence turn-off (MSTO) and a
double MS. We demonstrate that these features cannot be due to photometric
errors, field star contamination, or differential reddening. From a comparison
with theoretical models and Monte Carlo simulations, we show that a coeval
stellar population featuring a distribution of stellar rotation rates can
reproduce the MS split quite well. However, it cannot reproduce the observed
MSTO region, which is significantly wider than the simulated ones. Exploiting
narrow-band Halpha imaging, we find that the MSTO hosts a population of
Halpha-emitting stars which are interpreted as rapidly rotating Be-type stars.
We explore the possibility that the discrepancy between the observed MSTO
morphology and that of the simulated simple stellar population (SSP) is caused
by the fraction of these objects that are highly reddened, but we rule out this
hypothesis. We demonstrate that the global CMD morphology is well-reproduced by
a combination of SSPs that cover an age range of ~ 35 Myr as well as a wide
variety of rotation rates. We derive the cluster mass and escape velocity and
use dynamical evolution models to predict their evolution starting at an age of
10 Myr. We discuss these results and their implications in the context of the
extended MSTO phenomenon.
