The Sun is a main-sequence star fueled by nuclear fusion in its core. The process of expansion of the Sun is a natural consequence of its evolution and is primarily driven by changes in the nuclear reactions occurring within its core.
The Sun's core primarily consists of hydrogen gas, which undergoes fusion to form helium through a process called the proton-proton chain reaction. In this reaction, hydrogen nuclei (protons) combine to form helium nuclei, releasing tremendous amounts of energy in the form of light and heat. This energy counteracts the gravitational forces trying to collapse the star.
Over time, the hydrogen in the core of the Sun is gradually depleted as it undergoes fusion, converting it into helium. As the hydrogen fuel decreases, the core shrinks under the influence of gravity, causing an increase in pressure and temperature. The increased temperature leads to a rise in the rate of fusion reactions.
The higher fusion rate produces more energy, which creates an outward pressure that pushes against the inward pull of gravity. As a result, the outer layers of the Sun expand, causing it to become larger in size. This phase of expansion is known as the red giant phase.
During this phase, the Sun's outer layers expand to a point where they extend beyond their previous boundaries. The expansion can cause the Sun's outer layers to grow several times larger than its original size. However, it's important to note that when we talk about the Sun's expansion, we are referring to its outer layers and not its core, which remains relatively stable.
The expansion of the Sun eventually leads to the loss of its outer layers, which are expelled into space, forming a planetary nebula. The remaining core of the Sun, known as a white dwarf, gradually cools and fades over billions of years.
It's worth mentioning that the Sun is currently in the main-sequence phase, where it is fusing hydrogen into helium in its core. The expansion of the Sun into a red giant is predicted to occur in about 5 billion years from now, as it exhausts its hydrogen fuel.