  The origin of young star clusters represents a major challenge for modern
stellar astrophysics. While stellar rotation partially explains the colour
spread observed along main-sequence turn-offs, i.e. where stars leave the
main-sequence after the exhaustion of hydrogen in their core, and the multiple
main sequences in the colour-magnitude diagrams of stellar systems younger than
approximately 2 Gyr, it appears that an age difference may still be required to
fulfill the observational constraints. Here we introduce an alternative
approach that exploits the main-sequence turn-on, i.e. the point alongside the
colour-magnitude diagram where pre-main-sequence stars join the main-sequence,
to disentangle between the effects of stellar rotation and age to assess the
presence, or lack thereof, of prolonged star formation in the approximately
40-Myr-old cluster NGC1818. Our results provide evidence for a fast star
formation, confined within 8 Myr, thus excluding age differences as responsible
for the extended main-sequence turn-offs, and leading the way to alternative
observational perspectives in the exploration of stellar populations in young
clusters.
