Stellar Evolution

The HR diagram in the left frame illustrates the central principle of stellar evolution that more massive stars evolve faster. Note that in the young cluster NGC 2264 only the hottest and most luminous stars have collapsed all the way to the main sequence (the color index range for stars on the main sequence from about 0.1 to -0.3 corresponds to spectral classes A5 - B0). These stars are hot and luminous because they are massive, causing their fuel to be burned at a high rate.