Sunspots and the Sunspot Cycle Dark features on the Sun have been reported for at least 2000 years. Several sightings per century are contained in ancient Chinese records. Although not the first sightings recorded in Europe, Galileo's telescopic observations in 1610 provided the first details on sunspots, the most conspicuous of a number of transient phenomena to be found in the solar atmosphere. A typical sunspot has a cellular structure with a dark centre, the umbra, surrounded by a grayish filamentary region, the penumbra. Although sunspots emit radiation, the umbra looks dark because it is seen against an even brighter photospheric background, whose temperature is some 1800 K higher.
Sunspots develop in a matter of hours as small pores in the intergranular region of the photosphere.
They grow rapidly, and they generally form in clusters, marking a sunspot group, whose orientation is approximately parallel to the solar equator. Each end of the group is often dominated by a large spot surrounded by smaller spots. The very largest groups may cover up to one-fifth the solar diameter. Sometimes a sunspot group persists for several months, but a typical lifetime is about 1 week. Individual spots in a sunspot group undergo slow changes from day to day while they maintain their association.