All main sequence stars are in hydrostatic equilibrium, where outward thermal pressure from the hot core is balanced by the inward gravitational pressure from the overlying layers.
As stars use up most of the hydrogen in their cores through thermonuclear fusion, they leave the main sequence and pass into old age.
Once a main sequence star consumes the hydrogen at its core, the loss of energy generation causes gravitational collapse.
Most stars, the Sun included, will turn into red giants or super giants.
Stars below 0.23 solar masses will become white dwarfs.