How does the support mechanism of a main-sequence star fundamentally differ from that of a protostar?

Answer

Main-sequence stars use a continuous nuclear energy source, while protostars are supported by the finite resource of heat from gravitational shrinking.

The protostar inefficiently supports itself by converting gravitational energy into heat, whereas the main-sequence star is supported by a continuous, self-regulating nuclear energy source (hydrogen fuel).

How does the support mechanism of a main-sequence star fundamentally differ from that of a protostar?
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