UV spectrum of single-star BPASS population synthesis models at 1 Myr (left panel) and 2 Myr (right panel) shown in black. The blue spectrum is the contribution of stars with masses in the range 100–300 M_⊙. The red line is the spectrum of the entire population. In each panel the comparison between the normalized spectrum of the population with (red) and without (black) VMS is shown at the bottom..