Hertzsprung-Russell diagram tracks of 60 and 100 M_⊙ models until core-He exhaustion. The lines and coloured symbols have the same meaning as in Figs. 1 and 2. The 100 M_⊙ model inflates on the MS until it reaches the H-bump at log(T_eff/K) ∼ 4, after which it loses thermal stability and expands across the H-bump. The growing difference between the surface and nuclear luminosity is magnified in the inset plot.