Here's a breakdown:

* Gravity: The immense mass of a star creates a strong gravitational force, pulling all the star's material inwards.

* Radiation pressure: Nuclear fusion reactions in the core of the star release a tremendous amount of energy, which is carried outwards by photons. These photons exert a pressure on the surrounding material, pushing outward.

* Gas pressure: The heat from the nuclear fusion also increases the temperature and pressure of the gas inside the star, providing additional outward pressure.

This balance between inward gravity and outward pressure is what keeps a star stable. If the inward force of gravity were stronger, the star would collapse. If the outward pressure were stronger, the star would expand.

Important Note: In the later stages of a star's life, the balance can be disrupted, leading to instability and ultimately the star's death.