Figure 11-14 The Zero-Age Sun and Today’s Sun Over the past 4.56 × 109 years, much of the hydrogen in the Sun’s core has been converted into helium, the core has contracted a bit, and the Sun’s luminosity has gone up by about 40%. These changes in the core have made the Sun’s outer layers expand in radius by 6% and increased the surface temperature from 5500 K to 5800 K.