The Sun is a main-sequence star, currently in a stable phase of its life cycle where it fuses hydrogen into helium in its core through a process called nuclear fusion. The expansion of the Sun is a gradual and natural consequence of its evolution.
As hydrogen in the core is converted into helium through nuclear fusion, the composition of the core changes. This process releases an enormous amount of energy in the form of light and heat, which creates an outward pressure. This pressure counteracts the force of gravity that is trying to collapse the star.
Over time, as more hydrogen is converted into helium, the core of the Sun gradually contracts under the influence of gravity. However, the outer layers of the Sun, which are not yet affected by the core's contraction, expand as the overall structure adjusts to maintain equilibrium.
The expansion of the Sun leads to an increase in its radius and a decrease in its surface temperature. This expansion occurs over millions to billions of years and is known as stellar evolution. As the Sun expands, it becomes a red giant, a phase in which it will eventually engulf the inner planets, including Earth.
The expansion of the Sun is driven by the balance between the energy released through nuclear fusion and the gravitational forces acting on the star. This balance is delicate, and as the supply of hydrogen in the core decreases, the fusion reactions become less efficient. Eventually, the Sun will exhaust its hydrogen fuel and undergo further changes, leading to its eventual evolution into a white dwarf.