...stars of different spectral classes do not arise directly from differences of mass, and that probably the mean masses of the various groups of stars are extraordinarily similar. Also that the surface brightness of the stars diminishes rapidly with increasing redness. Evidence...

...different spectral classes, and between the giant and dwarf stars of the same class, do not arise, (directly at least), from differences in mass. Indeed,...more than one magnitude, from each class to the next. Spectrum B2 A0 A5, dwarf F0 F3 FSandG0 G5. dwarf K0 K5andM" G and G5, giant K0 K5 and M " Observed...