What physical process generates a force inside a pre-main-sequence star to offset the force of gravity and stop the star from slowly condensing and shrinking, thus producing a stable, non shrinking main-sequence star?
Stars are formed when a cloud of gasses, mainly hydrogen, collapse due to gravity. As more and more gas accumulates the temperature rises. The temperature creates an outward thermal pressure.
When the outward thermal pressure balances the inward gravitational force, the star is said to be in hydrostatic equilibrium and stops collapsing.
Newly forming stars typically go through several phases of gravitational collapse and hydrostatic equilibrium until the core temperature is high enough for fusion reactions to start and the star enters the main sequence.