What occurs at the stage of a protostar?

At the stage of a protostar, the key event is the initiation of hydrogen fusion in the star's core. During this phase, a protostar undergoes gravitational contraction, leading to the increase in temperature and pressure in its center. Once the conditions become extreme enough, hydrogen nuclei begin to fuse into helium through nuclear fusion. This process releases a tremendous amount of energy, which is critical for the star to eventually reach the main sequence stage of its life cycle.

The initiation of hydrogen fusion marks a significant transition; it transforms the protostar into a main-sequence star, allowing it to maintain stable energy production for millions to billions of years, depending on its mass. Before this fusion begins, the protostar merely collects mass and energy from its surrounding gaseous environment without producing energy through fusion.

The other options describe events that occur during different phases of stellar evolution, rather than during the protostar stage. The formation of a helium-rich core, the explosion of a red supergiant, and the formation of a neutron star all happen later in the evolutionary timeline of more massive stars after they have exited the main sequence stage.