  The recent discovery of double main sequences in the young, massive star
cluster NGC 1856 has caught significant attention. The observations can be
explained by invoking two stellar generations with different ages and
metallicities or by a single generation of stars composed of two populations
characterized by different rotation rates. We analyzed the number ratios of
stars belonging to both main-sequence components in NGC 1856 as a function of
radius. We found that their number ratios remain approximately unchanged from
the cluster's central region to its periphery, indicating that both components
are homogeneously distributed in space. Through a comparison of the loci of the
best-fitting isochrones with the ridge lines of both stellar components, we
found that both multiple stellar populations and rapid stellar rotation can
potentially explain the observed main-sequence bifurcation in NGC 1856.
However, if NGC1856 were a young representative of the old globular clusters,
then the multiple stellar populations model would not be able to explain the
observed homogeneity in the spatial distributions of these two components,
since all relevant scenarios would predict that the second stellar generation
should be formed in a more compact configuration than that of the first stellar
generation, while NGC 1856 is too young for both stellar generations to have
been fully mixed dynamically. We speculate that the rapid stellar rotation
scenario would be the favored explanation of the observed multiple stellar
sequences in NGC 1856.
