This is a photograph of sun spots on the sun. Sun spots are "cool" regions, only 3800 K (they look dark only by comparison with the surrounding regions). Sunspots can be very large, as much as 50,000 km in diameter. Sunspots are caused by complicated and not very well understood interactions with the Sun's magnetic field.
 
 

Nuclear fusion  is a reaction in the sun. Each second about 700,000,000 tons of hydrogen are converted to about 695,000,000 tons of helium and 5,000,000 tons (=3.86e33 ergs) of energy in the form of gamma rays. As it travels out toward the surface, the energy is continuously absorbed and re-emitted at lower and lower temperatures so that by the time it reaches the surface, it is primarily visible light. For the last 20% of the way to the surface the energy is carried more by convection than by radiation.
 
 
 

The surface of the sun is called the photosphere and  is at a temperature of about 5800 K. The area above the photosphere is the cromosphere. The highly rarefied region above the chromosphere ia  called the corona. It extends millions of kilometers into space but is visible only during eclipses . Temperatures in the corona are over 1,000,000K.