Here's why:

* Fusion and Energy: Fusion reactions in the core of a star release tremendous energy, which pushes outward against gravity. This outward pressure is what keeps the star stable.

* Heavier Elements: As the star fuses lighter elements (like hydrogen) into heavier ones (like helium, carbon, oxygen, etc.), the fusion process becomes less efficient. This means less energy is produced, leading to a decrease in outward pressure.

* Gravitational Collapse: With less outward pressure, the star's core starts to collapse under its own gravity. This collapse causes the outer layers of the star to expand dramatically, leading to a red supergiant.

* Red Color: The expanded outer layers cool down, making the star appear red.

The red supergiant stage is a temporary phase in the life of a massive star. Eventually, it will end its life in a spectacular supernova explosion.