What transition marks the end of the protostar phase and the beginning of main-sequence life?

Answer

The outward pressure from thermonuclear reactions perfectly balances the inward crush of gravity

The transition occurs when the immense outward pressure generated by sustained nuclear fusion precisely counterbalances the inward force of gravity, causing the object to stop contracting and stabilize its core temperature.

What transition marks the end of the protostar phase and the beginning of main-sequence life?
star formationastrophysicsprotostarsstellar physicsheat generation