Figure 19-8: Stages in the Evolution of the Sun This diagram shows how the luminosity of the Sun (a 1-M_⊙ star) changes over time. The Sun began as a protostar whose luminosity decreased rapidly as the protostar contracted. Once established as a main-sequence star with core hydrogen fusion, the Sun’s luminosity increases slowly over billions of years. The post–main-sequence evolution is much more rapid, so a different timescale is used in the right-hand portion of the graph.

(Adapted from Mark A. Garlick, based on calculations by I.-Juliana Sackmann and Kathleen E. Kramer)