Figure 11-23Post–Main-Sequence Evolution (a) The luminosity of the Sun changes as our star evolves. It began as a protostar with decreasing luminosity. On the main sequence today, it gradually brightens. Giant-phase evolution occurs more rapidly, with faster and larger changes of luminosity. Note the change in scale of the horizontal axis at 12 billion years. (b) Model-based evolutionary tracks of five stars are shown on this H-R diagram. In the high-mass stars, core helium fusion ignites smoothly where the evolutionary tracks make a sharp turn upward into the giant region of the diagram.