Star-to-star light-element abundance variations , know as multiple stellar populations ( MPs ) , are common in almost all Galactic globular clusters .
Recently , MPs have also been detected in a number of massive clusters with ages in excess of 2 Gyr in the Large Magellanic Cloud ( LMC ) , thus indicating that age is likely a control parameter for the presence of MPs .
However , to conclusively confirm this notion , additional studies of star clusters in the LMC ’ s ‘ age gap ’ of 3–6 Gyr are required .
Here , we use Hubble Space Telescope observations to study the 3 Gyr-old cluster NGC 2121 .
Compared with so-called ‘ simple ’ stellar population models , the cluster ’ s red-giant branch exhibits an apparent spread in a specific color index that is sensitive to intrinsic chemical spreads .
The observed spread can be explained by an intrinsic spread in nitrogen abundance of \sim 0.5–1.0 dex .
NGC 2121 has a comparable mass to its younger counterparts without MPs , indicating that cluster mass might not be the dominant parameter controlling the presence of MPs .
The transition phase between the occurrence of clusters with or without MPs seems to occur at ages of 2–3 Gyr , indicating that cluster age may play a dominant role in the establishment of MPs .