The core is the hottest part of the sun, and it has a temperature of more than 15 million degrees Kelvin. The second hottest part of the sun is its outermost layer, the corona, which has a temperature in excess of 2 million degrees Kelvin.
The sun's core is where the sun's primary power source, nuclear fusion, takes place. As hydrogen nuclei fuse to form helium nuclei, about 0.7 percent of the fused mass is released as energy. This energy is then radiated to the convection layers that lie between the core and the sun's visible surface, the photosphere. Scientists do not yet know why the corona is so much hotter than the photosphere, which has a temperature of about 6,000 degrees Kelvin.