What is the energy source for the early glow of a protostar before sustained nuclear fusion?

Answer

The conversion of kinetic energy during gravitational contraction

A protostar, which is the dense, hot core forming at the center of a collapsing clump, shines brightly, but this light is not yet powered by true stellar energy generation. In this nascent stage, the object's luminosity derives entirely from gravitational contraction. As material continuously falls inward toward the center of mass, the kinetic energy associated with this inward motion is converted into thermal energy (heat) through collisions between particles. This continuous heating and compression of the core causes the temperature to rise dramatically, allowing the object to glow intensely, though it remains distinct from a true star until nuclear fusion ignites.

What is the energy source for the early glow of a protostar before sustained nuclear fusion?

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