Figure 12-25 Post–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.