Rao is a fictional star in the DC Comics Universe. It is the red giant (in some continuities, red dwarf) that the planet Krypton orbited. The title also refers to a supervillain of the same name and same Kryptonian etymology
"Rao" is also the name of the star's deity, who was worshiped as a god of light and life. As such the name is sometimes invoked as a Kryptonian exclamation.
History
In early Superman comics, Krypton's star was unnamed and was given no effect on Superman's powers, which were attributed first to greater evolution, and then to a combination of innate powers and Earth's lower gravity. Starting in 1960, the fact that Earth's Sun was yellow while Krypton's was red became the explanation for Superman's powers, with sunlight fueling them like the charge of a battery. In the standard Silver Age continuity existing up until Crisis on Infinite Earths, Rao's red star radiation actively suppressed the superhuman abilities of Kryptonians, as their powers only worked in the radiation of a yellow star. The post-Crisis version created by John Byrne stated that Kryptonians absorbed solar energy, with the dim output of Rao being just enough to sustain them, and a yellow star, such as the Sun, producing enough energy to "supercharge" a Kryptonian metabolism to levels of power not seen in their native environment, though it takes years for Clark Kent to build up enough energy to reach the level of power he displays as Superman. The 2004 graphic novel Superman: Birthright corroborates this, although in recent years, other writers have depicted red-star radiation matching that of Rao as once again actively shutting down Kryptonian powers, for as long as an individual is exposed to it.
References to Rao began to creep into Superman's speech in the 1970s under the editorship and policy of Julius Schwartz, with Superman occasionally exclaiming "Great Rao!" instead of his standard "Great Krypton!".
In Super Friends #47 (August 1981), Superman reveals that "Rao" is also the Kryptonian name for God. This was subsequently confirmed in "mainstream" DC continuity of the time in the 1982 miniseries Phantom Zone.
Sandman: Endless Nights
In the Sandman graphic novel Endless Nights, there is a story with Rao as the personification of the Kryptonian sun. The story hints that the Kryptonian race and Kal-El's survival was no coincidence, but a making of Rao.
In that story, Despair of the Endless speaks with Rao some billion years ago:
Think about it, Rao. Wouldn't bringing life onto a planet that is inherently unstable add to the beauty of the life? If at any moment, it could explode...Truly it would only be perfectly beautiful, a perfect piece of art, if one single life-form escaped. To remember, to mourn, to despair.
The Last God of Krypton
In Superman: The Last God of Krypton, a trade paperback by Walter Simonson, Greg Hildebrandt and Tim Hildebrandt, when Krypton was a primitive planet, the gods including Rao fathered men to worship them and live in peace.
The Goddess of Ice Cythonna however could bear no children and took to attacking the men She tried to persuade Rao to marry her, intending to use him to birth elementals to attack men. Rao saw through Cythonna's trickery and battled her. This was later referred to as "The Wars of Fire and Ice". Cythonna lost, and was imprisoned by Rao in another dimension. She managed to escape, and started to look for Kal-El, the last Kryptonian and a direct descendant of Rao himself.
She initially sought to mate with Kal-El, initially thinking of him as a god, but decided to kill him after finding out that he was just a mortal and wanting to take revenge on Rao, who had abandoned Krypton millennia before it exploded.
Superman and Cythonna held a final battle in the sun, resulting in Superman's victory due to the power boost he received from fighting so close to his own power source, imprisoning Cythonna in the sun's core, due to its gravity.
Krypton Chronicles
It is described in detail in E. Nelson Bridwell's Krypton Chronicles that later on in Kryptonian history, Rao went from being the star to a much more cosmic, monotheistic concept, which is "he who ignited the sun".