Star clusters grow via accretion of gas particles (panel (a)) and hierarchical merging with less massive clusters (panel (b)). Depending on the locally determined Bondi–Hoyle–Lyttleton accretion rate, gas particles within the accretion radius are tagged to be accreted (circles with green edge colors). Additionally, less massive clusters that are gravitationally bound to the massive cluster are merged into it (magenta circle). The mass growth from accretion and merging can lead to a variety of outcomes (panel (c)): if the accreted material is sufficiently distinct in metallicity from any of the current stellar populations, a new population forms within the cluster (blue star); otherwise, it contributes to one of the existing populations (blue star within the green star). Similarly, if the stellar populations coming from the less massive cluster are sufficiently distinct, they will be added as new (light magenta star); otherwise they will be absorbed into an existing population (magenta star within the green star). The masses, ages, and metallicities of the stellar populations determine the amount of mass, yields, energy, and radiation emitted from the star cluster.