What occurs during hydrogen fusion in a star?

During hydrogen fusion in a star, hydrogen atoms undergo a process where they combine to form helium. This process is fundamental to the star's development and energy production. At the core of a star, extreme temperatures and pressures facilitate nuclear fusion, where hydrogen nuclei (protons) overcome their repulsive forces and fuse together.

The fusion of hydrogen into helium releases an enormous amount of energy in the form of light and heat, which is what makes stars shine. This energy creates outward pressure that balances the gravitational forces pulling inward on the star. It is this continuous process that allows stars to sustain their brightness and existence for millions to billions of years.

The other options focus on different aspects of stellar events that either do not occur during the fusion process or represent later stages in a star's life cycle. For example, splitting hydrogen atoms pertains more to nuclear fission, and a supernova represents a catastrophic end-of-life event, not part of the fusion process itself. Emission of carbon generally occurs in later stages of stellar evolution, particularly in larger stars that have undergone further fusion processes.