What prevents a collapsing Protostar from being classified as a true star during its initial phase?

Answer

It is not generating energy through sustained nuclear fusion in its core.

A protostar represents the embryonic stage of stellar evolution, characterized by intense gravitational contraction of gas and dust material gathered from a giant molecular cloud. Although this contraction generates significant thermal energy, heating the core considerably, this heat alone does not constitute true stellar life. The critical threshold for becoming a true star is the initiation of sustained nuclear fusion in the core, specifically the process where hydrogen nuclei combine to form helium. Until this thermonuclear ignition occurs, the object shines only due to the release of gravitational potential energy being converted into heat, meaning it lacks the sustained internal furnace characteristic of a Main Sequence star.

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