As this is the core temperature of a star with 1.5 solar masses, the upper main sequence consists of stars above this mass. (newworldencyclopedia.org)
From this point, the brightness and surface temperature of stars typically increase with age. (newworldencyclopedia.org)
Images in ultraviolet, blue, and visual light were combined for this picture, so that the colors roughly correspond to the surface temperatures of stars in M15. (maa.clell.de)