Krypton's red sun is a giant star which is vastly larger than Earth's yellow sun. Under it, Kryptonians have no superhuman abilities; they are stripped of their powers and rendered effectively powerless under it.
It is possible that the red sun was located near Krypton and was destroyed as a result of the war with General Zod. Kyla Willowbrook mentioned that a constellation used to form the face of a wolf, the animal that Skinwalkers take the form of, that the eye of the wolf is missing and that the star has been destroyed; this star may have been Krypton's sun.
Under a red sun, Kryptonians are returned to their natural, powerless state due to the fact that their species was nourished by the red sun, Rao. After prolonged exposure to yellow sunlight, Kryptonians, when reintroduced into a red sunlight environment, may seem weakened, as Clark Kent and Kara were as seen in Apocalypse.
The cloned Kryptonians who were created by the Orb received their powers under a red sun instead of a yellow sun due to their genetic material being irradiated with blue kryptonite by Jor-El. This is the reverse effects of a natural Kryptonian who receives and stores powers like a battery under a yellow sun, with the clones doing the same but under a red sun.
- Season Two, Skinwalker – Kyla Willowbrook tells Clark that Naman's homeworld is a missing star, or rather its solar system's sun.
- Season Three, Relic – Jor-El tells Louise McCallum that, where he comes from, the sunsets last for hours.
- Season Seven, Lara – Kara remembers her father sending her away from Krypton and remembers its landscape and dusty red skyline from its sun.
- Season Seven, Sleeper – When looking at the satellite images Chloe Sullivan picked from the computer, Clark looks at the vortex and sees a red sun with a bright dot beside it, which is Krypton.
- Season Seven, Apocalypse – Clark travels back through time to Krypton moments before its destruction; there, the red sun strips him of his powers. At the end, Krypton explodes with Kal-El's ship escaping and making its three-year trip to Earth.
- Season Nine, Kandor – During flashbacks of Krypton, the red sun can be seen many times.
- Season Nine, Pandora – In the future, Zod's solar tower re-creates red sun radiation effectively, turning Earth's yellow sun red.
In the Comics
In DC Comics, Krypton's red sun, named Rao, is a red dwarf that the planet Krypton orbited. Before Krypton exploded, Rao's red sun radiation suppressed the superhuman abilities of Kryptonians, as their powers only worked in the radiation of a yellow sun.
References to Rao began to creep into Superman's lingo in the 1970s under the editorship and policy of Julie Schwartz, with Superman occasionally exclaiming "Great Rao!" instead of his standard "Great Krypton!" Worship of Rao and Gods associated with him is the only religion known to have existed on Krypton. Rao is hailed as the creator deity of Krypton and the Kryptonian people. According to the Kryptonian mythology, Rao is the first being to be born out of the void before the universe. Feeling lonely, he sought to bring order to the chaos from which he was born, and thus created the universe. After countless eons of ordering the cosmos, Rao created Krypton, a planet meant to be a jewel in his creation.
- In both Superman: The Movie and Superman Returns, Rao is a red supergiant (as opposed to that of a red dwarf star in the comics) that undergoes a supernova explosion, causing Krypton's destruction by means of the shockwaves emitted by the dying star (in the comic series, Krypton was destroyed by the planet's unstable core).
- In the comics, the name of Krypton's sun, Rao, is also the name of ancient Kryptonian scrolls, the Scrolls of Rao.
- An inconsistency with modern astronomy and the destruction of Krypton in Smallville is Kyla Willowbrook's claim that the wolf constellation's eye is no longer seen in the constellation. Since Clark had to travel through 23 galaxies to get to the Milky Way, it would have taken the light from the destruction of Rao millions of light-years to reach Earth. Clark's ship would have had to travel through the universe at around 19 times the speed of light. However, it was confirmed in Season Seven's Apocalypse that Clark's journey to Earth took three Earth years (factoring in the theory of relativity, the actual journey may have taken less time). Based on Kyla's story, the destruction of the star in the constellation would have taken place long before prehistoric humans inhabited Earth. Since the beginning of Clark's journey and destruction of Krypton would have been around the Earth year 1986, Kyla's claim of the missing star, according to science, astronomy and physics, would not have been Krypton.