* Pressure and Gravity: A star's core temperature is directly related to the pressure within it. The pressure pushes outward, counteracting the inward pull of gravity.
* Nuclear Fusion: The high core temperature allows for nuclear fusion to occur, which generates the energy that keeps the star shining.
* Reduced Fusion: When the core temperature drops, the rate of nuclear fusion slows down. This means less energy is produced.
* Reduced Pressure: The reduced energy output leads to a decrease in pressure pushing outward against gravity.
* Expansion: With less outward pressure, the force of gravity becomes dominant, causing the star to expand.
In simpler terms: Think of the star like a balloon. The hot air inside keeps it inflated. If the air cools down, the balloon will shrink. Similarly, a cooler star core means less pressure, which allows gravity to pull the outer layers inward, leading to a larger radius.
