What happens to the temperature of a star as it transitions from the protostar phase to the main sequence phase?

During the transition from the protostar phase to the main sequence phase, a star undergoes significant changes primarily due to gravitational contraction and the onset of nuclear fusion. As a protostar forms, it accumulates matter and experiences increasing gravitational pressure, which causes its core to heat up.

As the core temperature rises, it eventually reaches the critical threshold where hydrogen fusion can begin. This process marks the transition to the main sequence phase. The initiation of nuclear fusion releases a vast amount of energy, further increasing the temperature of the star.

Consequently, the temperature of a star increases significantly as it evolves from the protostar stage, where it is still contracting and heating, to the main sequence, where stable nuclear fusion occurs and the star has reached a balance between gravitational collapse and the outward pressure from fusion energy. This dramatic temperature increase is a key hallmark of a star's evolution into a more stable and mature phase of its life cycle.