What event marks the precise moment an object transitions from a protostar to a main-sequence star?

Answer

When the outward pressure from radiating energy balances the inward pull of gravity

The object stops being a protostar and settles onto the main sequence at the precise moment when the outward pressure generated by the newly ignited fusion energy finally balances the inward pull of gravity.

What event marks the precise moment an object transitions from a protostar to a main-sequence star?
protostarstar formationastrophysicsenergy generation