Figure 11-17 Plotting the Ages of Stars (a) This photograph shows a region of ionized hydrogen and the young star cluster NGC 2264 in the constellation Monoceros. The red nebulosity is located about 2600 light-years from Earth and contains numerous stars that are about to begin hydrogen fusion in their cores. (b) Each dot plotted on this H-R diagram represents a star in this cluster whose luminosity and surface temperature have been measured. Note that most of the cool, low-mass stars have not yet arrived at the main sequence. Calculations of stellar evolution indicate that this star cluster started forming about 2 million years ago.