If you happen to be enjoying a sunny day, thank the bright surface of the Sun, known as the photosphere. At a piping hot temperature of about 5,800 K, the photosphere provides nearly all the sunlight Earth receives. But for all its glorious radiance, the photosphere isn't the hottest part of the Sun. That award goes to the diffuse outer atmosphere of the Sun known as the corona, which has a temperature of more than a million Kelvin. Parts of the corona can be as hot as 20 million Kelvin, which is hotter than the Sun's core. Of course, the big mystery is why the corona is so hot.