384912.fig.004
Figure 4: The value of Φm for the Sun increases gradually while fusing H He throughout >95% of its total ~12 Gy lifetime (on the left side of the vertical line to the present and on the right side into the future). Even while on the main sequence for ~11 Gy, the Sun approximately quadruples its luminosity and hence its energy rate density while steadily, yet very slowly, growing internally more complex. Only toward the end of its tenure as a nuclear-burning star does the Sun’s core contract enough to trigger He C fusion, accelerate its internal organization, and cause a rapid rise in Φm by about an order of magnitude.