What self-regulating state defines the stability of a star during its main sequence phase?

Hydrostatic equilibrium

The extended period where a star burns hydrogen steadily in its core is maintained by a precise, dynamic balance known as hydrostatic equilibrium. This state requires that the immense inward gravitational force, which constantly attempts to crush the star, must be perfectly counteracted by the outward pressure. This outward pressure is generated by the intense thermal energy and radiation resulting from the core's ongoing nuclear fusion reactions. This system is inherently self-regulating: if fusion slightly decreases, gravity causes minor contraction, increasing temperature and reigniting fusion; conversely, if fusion spikes, expansion lowers the core density and temperature, dampening the reaction rate until the perfect balance is mechanically restored.