What is the force that turns a nebula into a protostar?

The force that turns a nebula into a protostar is gravity. As regions within a nebula become denser due to slight fluctuations in density, gravity pulls the surrounding gas and dust inward, causing the material to clump together. This process leads to the formation of a protostar as the collapsing material heats up and begins to accumulate mass. Once the temperature and pressure in the core become sufficient to initiate nuclear fusion, the protostar evolves into a main-sequence star.