What describes a protostar?

A protostar is indeed best described as a star in free-fall collapse. During the early stages of stellar formation, a region within a molecular cloud undergoes gravitational collapse. As this material falls inward due to gravity, it begins to heat up and accumulate at the center, leading to the formation of a protostar. This process is characterized by the object being in a state of contraction, where the temperature and pressure are not yet sufficient to initiate nuclear fusion.

In contrast, a fully formed main-sequence star refers to a more advanced stage in stellar evolution where hydrogen fusion occurs in the star's core, signifying that it has completed its initial contraction phase and reached a stable state. A nuclear burning phase describes later stages in a star's life cycle, when it has moved on from protostar status and instead is fusing heavier elements after exhausting hydrogen. Lastly, a white dwarf is the remnant core of a star that has exhausted its nuclear fuel, representing a much later stage in stellar evolution after it has shed its outer layers.

Thus, the protostar phase is crucial as it marks the transition from a dense, cold region of gas and dust to a hot, contracting body that will eventually become a star capable of sustaining nuclear fusion.