Stars form on this curved line: the bluer ones are the hotter, brighter, more massive ones. But they also burn through their fuel the most quickly! When a star cluster is first formed, it makes all the different types of stars, from the bright, blue, ultra-massive O-stars to the dim, red, barely-burning-their-fuel M-stars. But after just a few million years, the O-stars are all gone. (Even the Pleiades is out of O-stars!)