Sunspots. Several dark sunspot groups seen on the Sun. Sunspots are areas on the Sun's surface which are around 2000 degrees Celsius cooler than their surroundings, causing them to appear darker. The cooling effect is produced by intense magnetic fields in the area. The larger spots seen here have a well-defined dark region (umbra) surrounded by a less-dark region, the penumbra. Sunspots are a sign of an active Sun. The Sun goes through a period of activity every 11 years or so. This is thought to be caused by the interactions between different layers of the Sun, as they rotate at different rates. This photograph was taken in June 1999, before the solar activity maximum in 2000.