Hα evolution: R (pc) versus log (L Hα) (erg s⁻¹) for six different metallicities and different cluster masses. Cluster masses have been plotted using the same colour coding as that in Fig. 1. Panel (a) on the right shows models computed for a gas electron density of n_e = 10 cm⁻³, while panel (b) on the left uses a value of n_e = 100 cm⁻³.