What energy source exclusively powers a dense Protostar before sustained thermonuclear fusion begins in its core?
Gravitational contraction as surrounding material falls onto it (accretion)
A protostar represents an intermediate stage in stellar evolution, characterized by being hot and dense at its center but not yet a true, self-sustaining star. During this phase, the object's energy output—the heat and light it emits—is entirely derived from the mechanical energy released as material continues to fall onto it from the surrounding cloud envelope, a process known as accretion. It is the conversion of gravitational potential energy into thermal energy during this continual infall and contraction that keeps the protostar hot. Only after further contraction and heating does the core reach the necessary conditions for hydrogen fusion to commence, which marks the transition to a true star on the main sequence.
