Spectral sequence for luminous supergiants between BO and B1, illustrating the extreme change in the Si III/Si IV ratios. The main He I lines against which the Si lines are compared for luminosity classification are indicated, as well as the He II 4686 Å line, which decreases in strength as luminosity increases. Based on the shape of the Balmer lines, one would be tempted to assume that the two Orion stars are significantly less luminous than the other three, although this effect might instead be related to weaker mass loss. Compare to Fig. A.1 for the same sequence at lower luminosity.