What acts as a built-in thermostat in a main-sequence star if the core fusion rate increases slightly?

Answer

Slight expansion cools the core, slowing the reaction

A main-sequence star possesses a built-in regulatory mechanism: if the fusion rate increases, the resulting increased outward pressure causes the star to expand slightly, which cools the core and consequently slows the nuclear reaction back down.

What acts as a built-in thermostat in a main-sequence star if the core fusion rate increases slightly?
physicsastronomyenergynuclear fusionstar