They fly closer to the sun than the rest of us, and there is an incandescence about them.
The emission of visible light from a substance or object as a result of heating it to a high temperature. The color of the light emitted from solids and liquids is a function of their chemical structure and their temperature; the higher the temperature, the more intense and even the distribution of frequencies is (that is, higher temperatures create brighter and whiter light than lower temperatures). Compare fluorescence. See also blackbody radiation.