Evolutionary models from Stasińska et al. (2001) overplotted on the OB-I diagram. From left to right, we have scenarios 1, 2, and 3 as described in the text (see Sect. 4.3). For all plots, the grey contours represent the FADO-SDSS sample; the black line is our separation line given by Eq. (2); the black arrow shows how the star-forming regions evolve with time; the blue markers and dashed line represent the model with 0.05 Z_⊙; the green markers and solid line represent the model with 0.2 Z_⊙; the red markers and dot-dashed line are the model with 0.4 Z_⊙; and the cyan markers and solid line represent the model with Z_⊙. The first marker of each metallicity is always on the top right of the OB-I diagram, and each subsequent marker represents a 2 Myr step for scenario 1 and a 1 Myr step for scenarios 2 and 3. The white triangle inside some markers represents the point where massive stars fade: 14 Myr for scenario 1 and 5 Myr for scenarios 2 and 3.