In the right image we compare blow-up views of the sunspot in optical light (left side) and radio waves (right side). The optically dark region in the middle of the sunspot is known as the umbra (Latin for "shadow"): it is dark because it is cool. We believe that it is cool because the strongest magnetic fields rise up at the center and somehow suppress heat from the Sun's interior from emerging as effectively here. The strong magnetic fields make the sunspot a bright source of radio waves, as you can see in the right panel.
The radio images were made with the Very Large Array radiotelescope in New Mexico, operated by the National Radio Astronomy Observatory.