Generally speaking, a main-sequence star is __________ than it was during the time it was a protostar.

A main-sequence star represents a stable phase in a star's life cycle where it engages in hydrogen fusion at its core. During the protostar stage, a star is accumulating mass and temperature as gravitational forces compress the surrounding gas and dust.

As a protostar evolves into a main-sequence star, the core temperature rises significantly, allowing hydrogen nuclei to begin fusing into helium. This process releases a substantial amount of energy, leading to increased temperatures on the surface and greater luminosity. Thus, a main-sequence star is indeed hotter than it was during its protostar phase.

However, it's important to clarify that while a main-sequence star is hotter and brighter, the comparison between the two phases is relative. A protostar can possess significant luminosity due to its gravitational contraction but is generally less luminous than a fully realized main-sequence star that is in a stable balance between gravitational collapse and outward pressure from nuclear fusion.

The correct choice reflects the general trend in a star's lifecycle where the transition from protostar to main-sequence involves a significant increase in both temperature and brightness, resulting in a star that is hotter and brighter than it was previously.