Red Giant
A large, luminous star in a late evolutionary stage that has exhausted core hydrogen fuel and expanded its outer envelope.
Governing formula
Radius R ~ 10–100 R_☉, T_surface ~ 3,000–4,500 K
SI unit
Solar Radii (R_☉) & Kelvin
In depth
When core hydrogen exhausts, the core contracts and heats up while hydrogen shell burning causes outer layers to expand and cool to a reddish color. Later core helium ignition forms the horizontal branch.
Examples in the real world
Aldebaran and Betelgeuse operating as red giant and supergiant stars in the night sky.