probably produced by its corona, magnetically heated to several million Kelvin. The temperature has likely been stimulated by fast rotation combined with strong convection percolating through the star's outer layers. The soft X-ray sub-coronal X-ray emission is much weaker than the hard X-ray coronal emission. The same behaviour has been measured in other F-class supergiants such as α Persei and is now believed to be a normal property of such stars. Evolution The spectrum of Canopus indicates that it has exhausted its core hydrogen and evolved away from the main sequence, where it spent some 30 million years of its existence as a blue-white star of around 10 solar masses. The position of Canopus in the H–R diagram indicates that it is currently in the core-helium burning phase. It is an intermediate mass star that has left the red-giant branch before its core became degenerate and is now in a blue loop. Models of stellar evolution in the blue loop phase show that the length of the blue loop is strongly affected by rotation and mixing effects inside the star. It is difficult to determine whether a star is currently evolving towards hotter temperature or returning to cooler temperatures, since the evolutionary tracks for stars with different masses overlap during the blue loops. Canopus lies on the warm side of the instability strip and does not pulsate like Cepheid variables of a similar luminosity. However its atmosphere does appear to be unstable, showing strong signs of convection. Canopus may not be massive enough for its fusion chain to reach iron and trigger a core collapse and subsequent supernova, instead eventually becoming a neon-oxygen white dwarf. Cultural significance Canopus was known to the ancient Mesopotamians and given the name NUN-ki and represented the city of Eridu in the Three Stars Each Babylonian star catalogues and later MUL.APIN around 1100 BC. Today, the star Sigma Sagittarii is known by the common name Nunki. Canopus was not visible to the mainland ancient Greeks and Romans; it was, however, visible to the ancient Egyptians. Hence Aratus did not write of the star as it remained below the horizon, while Eratosthenes and Ptolemy—observing from Alexandria—did, calling it Kanōbos. An Egyptian priestly poet in the time of Thutmose III mentions the star as Karbana, "the star which pours his light in a glance of fire, when he disperses the morning dew." Under the Ptolemies, the star