The Sun fuses 600 million tons of hydrogen to helium every second and the helium accumulates like ash in the core. The core gradually shrinks as there’s less hydrogen to burn, but as it shrinks it heats up, and the reaction rate/energy output from the Sun increases and over time the outer envelope of the Sun expands. Interesting stuff. 600 million tons sounds like a lot until you realize the Earth is basically a dot compared to the Sun.