Here's why:

* Low Temperature: Red giants are relatively cool compared to other types of stars, hence their reddish hue.

* High Luminosity: They are very large, with diameters often exceeding the Sun's by a factor of 100 or more. This large surface area allows them to radiate a tremendous amount of energy, making them highly luminous.

How do they form?

Red giants form when stars like our Sun exhaust their core hydrogen fuel. They then start fusing hydrogen in a shell around the core, causing the star to expand and cool.

Example: Betelgeuse in the constellation Orion is a well-known red giant.