1. Thermal pressure: As the star's core heats up due to nuclear fusion reactions, the energy released generates an enormous amount of heat and pressure. This thermal pressure pushes the outer layers of the star outward, counteracting the gravitational force that pulls the star inward.
2. Radiation pressure: When nuclear fusion reactions occur in the core, they release not only heat but also high-energy photons (gamma rays). These photons exert radiation pressure as they travel outward, pushing against the inner layers of the star and contributing to the outward pressure that opposes gravity.
In addition to these two main factors, the rotation of a star can also generate a small amount of centrifugal force that helps to balance the gravitational forces. However, thermal pressure and radiation pressure are the dominant contributors to the outward pressure that keeps a star in hydrostatic equilibrium.
