A Red Sun is a class M main-sequence star, by far the most common. About 76% of the main-sequence stars in the Solar neighborhood are class M stars. Although most class M stars are red dwarfs, the class also hosts most giants and some supergiants such as VY Canis Majoris, Antares, and Betelgeuse. Krypton's sun Rao is, specifically, a Red Dwarf that sits at the middle of a galaxy. They have a surface temperature of less than 4000 K. These stars are a massive, luminous balls of plasma, held together by gravity, and help sustain life on the planets orbiting them. Young red dwarfs flare constantly but only last for about a billion years. Older stars are more stable. Though they are colossal, they're at least 4 times smaller than a yellow G-type main-sequence star. Krypton's sun is an ancient 13-billion-year-old red dwarf star (3 times older than Earth's), 5 times smaller than Earth's sun, but much cooler, with a surface temperature of only 3317 K, less nourishing, and only 40 percent of Earth's sun's density. Rao appears relatively dim, but colossal on the Kryptonian sky (due to the planet's relative proximity to its star and its narrow habitable zone), and under Rao's light, Krypton's sky usually had an orange hue during the day. The star was worshiped by Kryptonians as Rao - the god of light and life. General Zod named his revolutionary movement, the Sword of Rao, in honor of his native planet's sun.
Effect on Kryptonians
Under their native sun, Kryptonians are only as strong, fast, and durable as a human on Earth, and possess no extraordinary superpowers. While within Black Zero, the atmospheric conditions which contained Rao's radiation, Kal-El was instantly stripped of all of his superpowers, with the artificial Kryptonian atmosphere causing him to have a seizure and convulse, while coughing blood, until he passed out, due to his body never having experienced being under a red sun since his birth (with him having only lived on Krypton a few hours). It was the atmosphere that truly weakened Kal-El, but the red sun environment caused him to lose his power and become equal to humans in weakness.
- In most Superman media, Krypton's red sun is called "Rao", named after the Egyptian sun god Ra.