Rao is a fictional star in the DC Comics Universe. It is the red supergiant (in some continuities, red dwarf) that the planet Krypton orbited.
"Rao" is also later written into the Superman mythology as the name of a Kryptonian deity, the personification of their sun, worshipped as a god of light and life. As such the name was sometimes invoked in the comics as a Kryptonian exclamation.
In the early years of Superman comics, the sun of Krypton 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 our 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 sun radiation actively suppressed the superhuman abilities of Kryptonians, as their powers only worked in the radiation of a yellow sun. 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 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-sun 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 lingo 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.