Here's why:

* Large: Red giants are significantly larger than our Sun, often having diameters hundreds of times greater.

* Cool: They are cooler than our Sun, with surface temperatures ranging from 3,500 to 5,000 Kelvin (compared to our Sun's 5,500 Kelvin).

* Bright: Despite being cooler, they are very luminous because of their immense size. Their large surface area radiates a vast amount of energy, making them appear bright in the sky.

While they appear red, their color is actually more orange. However, the term "red giant" is widely used, even though it's not entirely accurate.