What is the primary process that occurs in the main sequence stage of a star's life?

During the main sequence stage of a star's life, the primary process occurring is the fusion of hydrogen into helium. This process takes place in the star’s core, where temperatures and pressures are extremely high. Hydrogen nuclei combine under these conditions to form helium nuclei, releasing a significant amount of energy in the form of light and heat. This energy is what powers the star and provides the luminosity that we observe from Earth.

The stability of a star during the main sequence phase is achieved because the energy produced through hydrogen fusion counteracts the gravitational forces attempting to collapse the star inward. This phase can last for billions of years, depending on the star's mass. More massive stars will burn their hydrogen more quickly, leading them to evolve off the main sequence sooner than less massive stars.

Understanding this process is crucial, as it forms the backbone of stellar evolution. The other processes, such as the fusion of helium into heavier elements or the formation of planets, occur later in a star's lifecycle or in different contexts altogether. Therefore, the correct answer highlights the essential process that defines the primary activity during the main sequence stage of a star’s life.