Why is the sun`s atmosphere so hot? Spacecraft starts to unravel our star`s mysteries
(PHYSorg.com Space & Earth)

If you ask a child to paint a picture of the sun, you will most likely get a bright yellow circle on a piece of paper. This is actually quite accurate, given that the sun is a ball of hot gas and that its surface (called the photosphere) mostly shines in bright yellow light. The yellow color is determined by the temperature of the photosphere, which is about 5,500°C.