How is stellar equilibrium finally reached?

Question

How is stellar equilibrium finally reached?

Answer 1

It's called hydrostatic equilibrium.

The nuclear reactions in the core sustain the temperatures in gas throughout the star. So even though energy is emitted at the surface, the internal temperatures remain constant. So the pressure remains constant and at a level the the force outwards due to pressure exactly balances the force inwards due to gravity.

Up until nuclear fusion starts equilibrium can't be reached because even though the gas increases in temperature, some of the energy is radiated into space, so the gas cools and the pressure drops and gravity wins. Once nuclear fusion starts, the balance is maintained - until the fuel runs out.

Answer 2

You have two opposing forces.  One is the outward push of the nuclear reactions which are taking place in the interior.  The other is the inward pull of gravity.  The star is in equilibrium when these forces are balanced.