  Young star clusters (YSCs) with resolved stellar populations are well suited
for studying star-cluster formation. In most cases, the (pre-main-sequence)
stellar populations found in the YSCs are coeval with an intrinsic age spread
of up to 1Myr. Such observations can be understood as the YSCs having formed in
one burst, which star formation was truncated by stellar feedback. The recent
discovery that the colour-magnitude diagram of the Orion Nebula Clusters (ONC)
contains three well defined age-separated populations appears to shatter this
model. The implication is that the ONC formed in three bursts, with star
formation still on-going in the last burst. We present new observational
results focusing on the three populations in the ONC using OmegaCAM photometry
and Gaia DR2 measurements. We also describe a theoretical model which may
explain these observations by an interplay between stellar feedback and cluster
dynamics.
