Fig. 1. a) Observed CMD of I Zw 18 as obtained by photometric reduction of F555W and F814W broadband images of HST/WFPC2. The diagram contains 247 objects with photometric error σ < 0.2 mag in both bands.
b) Synthetic CMD corresponding to the simulation of a recent starburst started 10 Myr ago and still ongoing (Padova tracks with α = 1.5).
c) Synthetic CMD computed for a single-burst model with an exponentially decreasing star formation activity started 0.5 Gyr ago and with an e-folding time of 0.3 Gyr (Padova tracks with α = 1.5).
d) Synthetic CMD obtained considering a two-burst model with the first burst from 0.5 Gyr to 50 Myr ago and the second one from 30 to 5 Myr ago (Padova tracks with α = 1.5)