Is the Sun really hot? The question is, on the face of it, almost insane. No-one could possibly doubt that the sun is the only source of external heat on earth. And, certainly, the part that we see, the sun's photosphere, is some 5,800 degrees Kelvin. The solar corona, which extends into space, may be as hot as one million degrees Kelvin. But what exactly is underneath this hot atmosphere? The explanation universally accepted without question is that it must be an even hotter mass of hydrogen gas, fusing into helium and other elements at temperatures of 15 million degrees...