Why will the force from radiation in a star decrease?

The force from radiation in a star decreases primarily due to the reduction in energy output as the star exhausts its nuclear fuel. As a star ages, it moves away from the main sequence phase, leading to changes in temperature and luminosity. Additionally, as the outer layers expand or contract, the effective radiation pressure decreases, resulting in a diminished force. This decline in radiation pressure affects the balance between gravitational collapse and outward force, altering the star's structure and evolution.