What happens when the core of a protostar surpasses 18 million degrees?

When the core of a protostar surpasses 18 million degrees Celsius, nuclear fusion of hydrogen into helium begins in earnest. This marks the transition from a protostar to a main sequence star, as the core temperature becomes high enough to sustain the fusion reactions that release energy. The resulting energy creates an outward pressure that balances the inward gravitational pull, stabilizing the star and allowing it to enter a prolonged phase of hydrogen burning. This process initiates the star's main sequence phase, where it will spend the majority of its life.