Figure 11-24 Further Stages in the Evolution of the Sun This diagram shows how the luminosity of the Sun (a 1-M_⊙ star) changes over time. We use different scales for the final stages because the evolution is so rapid. During the AGB stage there are brief periods of runaway helium fusion, causing spikes in luminosity called thermal pulses. All red giants exhibit a slow oscillation in brightness due to their rhythmic “breathing” in and out, and one-third of them are also affected by additional slower and mysterious changes in their luminosity.