What must occur internally for a protostar to officially achieve the status of a main-sequence star?

Answer

Stable hydrogen fusion ignites, balancing gravity via outward pressure

The transition to a main-sequence star occurs when the core becomes hot and dense enough to ignite sustained hydrogen fusion, establishing hydrostatic equilibrium where fusion pressure balances gravity.

What must occur internally for a protostar to officially achieve the status of a main-sequence star?
starsstar formationastrophysicsinterstellar clouds