What is the primary fuel source for stars during the main sequence phase?

During the main sequence phase, stars primarily rely on hydrogen as their fuel source. In this stage of stellar evolution, hydrogen nuclei (protons) undergo nuclear fusion to form helium nuclei in the core of the star. This fusion process releases a tremendous amount of energy in the form of light and heat, which provides the outward pressure needed to counterbalance the gravitational forces pulling the star's mass inward.

The significance of hydrogen as the primary fuel lies in its abundance in the universe; it is the most common element, making up about 74% of the elemental mass of the universe. This high prevalence makes it the most accessible and efficient fuel for stars during their stable main sequence phase, where they spend the majority of their lifetimes. As the star exhausts its hydrogen fuel over billions of years, it will eventually move to the next phases of stellar evolution, where other elements, such as helium and carbon, may play a role in subsequent fusion processes.