What type of celestial body typically forms after a protostar?

A protostar is an early stage in the formation of a star, and it represents a dense region within a molecular cloud where gas and dust are collapsing under gravity. As the protostar continues to gather mass, its core temperature rises until it reaches the necessary conditions for nuclear fusion to occur. This marks the transition from a protostar to a main sequence star, where hydrogen fusion begins in the core, releasing energy and allowing the star to stabilize on the main sequence of the Hertzsprung-Russell diagram.

The main sequence is where stars spend most of their lifetimes, fusing hydrogen into helium. This process determines the star's properties, such as its luminosity, temperature, and size.

In contrast, the other types of celestial bodies listed are stages or outcomes that typically occur under different circumstances or after different evolutionary paths. A red giant star forms later in a star's life cycle when it exhausts hydrogen in its core and begins to fuse helium and other heavier elements under different conditions. A black hole and a neutron star are remnants of massive stars that have undergone supernova explosions, which occur long after the main sequence phase. Therefore, the correct concept here is that after a protostar evolves, it typically becomes a main sequence star