Sunspot, Vacuum Tower Telescope image. Sunspot (dark area) on the surface of the Sun. Sunspots are areas of magnetic activity that are cooler than the rest of the Sun's surface. They range in size from hundreds to thousands of kilometres across. The darkest regions are the umbras, surrounded by penumbras (bronze regions). The yellow background consists of convection cells called solar granules. This sunspot was imaged with the National Solar Observatory (NSO) Sacramento Peak Vacuum Tower Telescope in 1998.