Running Out of Fuel

After billions of years on the main sequence, a Sun-like star exhausts the hydrogen in its core. Without fusion's outward pressure, gravity compresses the core, heating it further. Meanwhile, a hydrogen shell around the core ignites, pouring out far more energy than the core ever did. The outer layers respond by expanding enormously.

As the star swells, its surface cools from around 5,500°C to roughly 3,500°C, shifting its color from yellow-white to deep red. But the total luminosity skyrockets, because the enormous surface area more than makes up for the lower temperature at each point.