What exactly is a sunspot? A sunspot is simply a region on the surface of the sun—called the photosphere—that is temporarily cool and dark compared to surrounding regions. Solar measurements reveal that the average surface temperature of the sun is 6000° Celsius and that sunspots are about 1500° Celsius cooler than the area surrounding them (still very hot), and can last anywhere from a few hours to a few months. Sunspots expand and contract as they move across the surface of the sun and can be as large as 80, 000 km in diameter. Sunspots are magnetic regions on the sun with magnetic field strengths thousands of times stronger than the Earth's magnetic field, and often appear in pairs that are aligned in an east-west direction. One set will have a positive or north magnetic field while the other set will have a negative or south magnetic field. The field is strongest in the darker parts of the sunspots—called the umbra. The field is weaker and more horizontal in the lighter part—the penumbra. Overall, sunspots have a magnetic field that is about 1000 times stronger than the surrounding photosphere. Sometimes the sun contains a large number of sunspots, while at other times, few or none are seen. In 1843, the German chemist and amateur astronomer Heinrich Schwabe discovered that there was a fairly regular cycle of change in the number of sunspots and that this cycle lasts about 11 years. The part of the solar cycle with low sunspot activity is referred to as "solar minimum" while the portion of the cycle with high activity is known as "solar maximum" or "solar max." Today, the "sunspot number" is calculated by first counting the number of sunspot groups and then the number of individual sunspots. The sunspot number is then given by the sum of the number of individual sunspots (s) and ten times the number of groups (k), using the formula R=k(10g+s), where k is a variable scaling factor (usually < 1) that indicates the combined effects of observing conditions, telescope, and bias of the solar observers. Since most sunspot groups have, on average, about ten spots, this formula for counting sunspots gives reliable numbers even when the observing conditions are less than ideal and small spots are hard to see. next: The Butterfly Diagram back: History Sunspots and the Solar Max History What exactly is a sunspot? The Butterfly Diagram The Solar Wind References top left: Detailed image of a sunspot. (Image courtesy NASA Marshall Space Flight Center) above: A cluster of sunspots on the surface of the sun. The sunspots change position slightly every day, as seen in this animation. (295 kb) (Image and animation courtesy NASA Marshall Space Flight Center)