The main driver of change on the main sequence is the change in composition in the core. Over time the fraction of the core that is hydrogen decreases and the

The main driver of change on the main sequence is the change in composition in the core. Over time the fraction of the core that is hydrogen decreases and the fraction of helium increases. This reduces the number density of particles in the core since four free protons have become a single helium nucleus. The decrease in number density causes the core to contract slightly and heat up so that the pressure remains the same. The increased temperature causes the rate of nuclear reactions to increase, which then increases the luminosity. The increased luminosity then causes the outer envelope of the star to "puff up" and the radius to increase. This is not a dramatic process, but over billions of years it has a noticeable effect. The Sun is now 15% larger and about 50% more luminous than when it entered the main sequence. This is actually the source of an open problem in astronomy called the Faint Young Sun Paradox [1]. Because the Sun was much less luminous in the past, the temperature on Earth should have been about 20 K cooler in the past, but geological measurements don't seem to bear that out. Somehow the Earth managed to stay warmer in the past given the same level of solar irradiation. [1]: https://en.wikipedia.org/wiki/Faint_young_Sun_paradox https://en.wikipedia.org/wiki/Faint_young_Sun_paradox