The core of the sun has the highest temperature, approximately 10 million Kelvin, as a result of the incessantly thermonuclear fusion processes that produce the energy the sun relies on. In general, the temperature decreases from the core to the photosphere and then increases towards the corona; however, the abnormally high temperature of the corona (~1 million kelvin) is still a mystery. One interesting thing is that some colleagues compare the sun to fried ice cream, indicating the solar corona is much hotter than the solar surface, but this is not very accurate because the core of the sun is the hottest.