What is the energy source powering a protostar before it officially becomes a true star capable of fusion?

Answer

The energy of gravitational contraction

A protostar generates heat and light exclusively from the release of energy as it contracts under its own immense gravitational pull, not from nuclear reactions.

What is the energy source powering a protostar before it officially becomes a true star capable of fusion?
astronomyforcegravitystar formationinterstellar cloud