What is a protostar?

A protostar is accurately described as an early stage in the formation of a star, before nuclear fusion begins. In this phase, a region of dense gas and dust in a molecular cloud accumulates material through gravitational attraction. As this material falls inward, it heats up, forming a protostar. It is crucial to note that during this stage, the temperature and pressure in the core of the protostar increase, but it has not yet reached the conditions necessary for nuclear fusion to start. This process eventually leads to the formation of a main-sequence star once fusion ignites.

The other options do not correctly describe a protostar. For instance, being an advanced stage of a star's life cycle pertains to later phases, such as the main sequence or even the red giant stages. Additionally, a protostar is not a type of planet; planets form from different processes in a stellar system. Lastly, the description of a dying star emitting high-energy radiation refers to evolved stars, like supernovae, and does not apply to protostars. Understanding these distinctions helps clarify the unique characteristics that define a protostar in stellar evolution.