How is sustained heat generated in a forming star system before nuclear fusion ignites?

Answer

Conversion of gravitational potential energy into heat

As gas and dust particles fall toward the center of the condensing mass, their gravitational potential energy is converted into kinetic energy, which manifests as heat upon particle collision, providing massive heat in the absence of fusion.

How is sustained heat generated in a forming star system before nuclear fusion ignites?
astronomygravitynebulastar formation