Colour-magnitude diagram of NGC 330 focused on the turn-off region. Hα emitters are marked in red. An isochrone of non-rotating stars is plotted in blue (see Appendix B for model details). Green and red isochrones depict critically rotating stars viewed pole-on and equator-on, respectively, as derived from a simple model of critical rotators (see Appendix A). The solid purple isochrone represents critically rotating stars viewed at the mean inclination angle when the rotation axis is randomly oriented (51.8°), and for the dotted purple isochrone, 0.25m_F814W has been added to simulate the decretion disc. The isochrone age is 30 Myr, the distance modulus is μ = 18.8 mag, and the reddening of E(B–V) = 0.1 mag. Data are from Milone et al. (2018).