Here's why:

* Low temperature: Red giants have a surface temperature of around 3,500 - 5,000 Kelvin, which gives them their characteristic reddish color.

* High luminosity: They are much larger than the Sun, sometimes hundreds of times bigger. This larger surface area allows them to emit significantly more light energy, making them very luminous.

Red giants are a stage in the evolution of stars like our Sun, where they have exhausted the hydrogen fuel in their core and have begun to fuse helium. This fusion process causes the star to expand greatly and cool down.