What energy conversion drives initial heating as nebulae contract into a protostar?

Gravitational potential energy into kinetic energy

The process of star birth begins when gravity causes localized regions within a cold, diffuse nebula to contract, pulling material inward toward a central clump. As this immense quantity of matter falls toward the gathering center, the energy stored in its position relative to the center—its gravitational potential energy—is converted into the energy of motion, which is kinetic energy. This kinetic energy manifests locally as intense heat. This heating process is crucial because it continues relentlessly until the core reaches the necessary density and extreme temperatures required to finally ignite sustained nuclear fusion reactions, thus marking the official birth of the star.