The discovery began with astronomer Henry Draper, who was a pioneer in stellar spectroscopy. When he died, his widow donated money to the Harvard College Observatory. There are hired a group of "computers", who studied the stellar spectra. One of employees classified the spectra by the of their . She ordered them . This scheme proved to shedding any understanding on the nature of stars. Annie Jump Cannon realized that she could reorganize the system into a more natural order. This left us with OBAFGKM. are the hottest and are the coolest