Left panel: comparisons of the evolutionary tracks for models with different initial masses in the theoretical HR diagram for a metallicity Z around 0.00004, i.e., corresponding to the metallicity of the galaxy I Zw 18. The code MESA models from Choi et al. (2016) have [Fe/H] = −1.5 and no internal magnetic fields. The code STERN with an internal dynamo was used by Szécsi et al. (2015; we note that they computed a 26 M_⊙ model). Right panel: evolution of the N surface enrichment as a function of the surface equatorial velocity for the 9 (black) and 60 M_⊙ rotating models (red). The meaning of the line styles are the same as in the left panel.