Explain why a protostar heats up internally as its gravitationally contracts?

As a protostar contracts under gravity, its gravitational potential energy is converted into kinetic energy, increasing the speed and temperature of the particles. This increase in kinetic energy results in collisions that generate heat. The protostar continues to contract and heat up until internal pressures and temperatures are enough to initiate nuclear fusion and establish equilibrium between inward gravitational forces and outward radiation pressure.