* Gravity: The Sun's immense mass exerts a powerful gravitational force that pulls all the material within it inward. This force is what keeps the Sun from expanding and what ultimately fuels nuclear fusion in its core.

* Radiation pressure: The Sun's nuclear fusion produces a constant outward flow of energy in the form of light and heat. This radiation exerts a pressure outward, counteracting the force of gravity. This pressure is much weaker than gravity, but it plays an important role in maintaining the Sun's equilibrium.

These two forces are in a delicate balance, which is essential for the Sun's stability.