Sunspots: Cool, Dark and Magnetic Sunspots are relatively dark patches on the Sun’s bright surface. They are regions of concentrated magnetic fields that can last from several hours to several months. Sunspots appear dark because they’re cooler (4000°C) than the plasma surrounding them (6000°C). They tend to develop in groups, with some groups covering areas 20 times the diameter of Earth. A Sunspot’s magnetic field is similar to a horseshoe magnet lying below the Sun’s surface. A Sunspot group has a “north” and “south” magnetic pole (like Earth does).