A relatively dark area on the surface of the sun consisting of a dark central umbra surrounded by a penumbra, which is intermediate in brightness between the umbra and the surrounding photosphere. Sunspots are often nearly circular with a typical dimension of 20 000 km. The strongest solar magnetic fields, up to 4000 gauss, are found within the umbra. Sunspots usually occur in pairs with opposite magnetic polarities. They have a lifetime ranging from a few days to several months. Their occurrence exhibits approximately an 11-year period (the sunspot cycle). See relative sunspot number.