Sunspots are dark because they are much cooler than the surrounding portions of the sun, according to Cool Cosmos, a joint project by the National Aeronautic and Space Administration’s Jet Propulsion Laboratory and California Institute of Technology’s Infrared Processing and Analyses Center. These sunspots usually have temperatures of approximately 6,300 degrees Fahrenheit, while the surrounding portions of the sun are approximately 10,000 degrees Fahrenheit.
As explained on SpaceWeather.com, scientists count and catalog these planet-sized cool spots. According to the Marshall Space Flight Center, the abundance of sunspots varies in time. During the late 17th century and early 18th century, sunspot activity was very low. Concurrently, with this reduction in solar spot frequency, the Earth’s temperature dropped significantly. This period is often referred to as the “little ice age,” as many rivers and snowfields remained frozen longer than normal. There is some evidence that similar cool periods occurred in the more distant past as well.
According to Windows to the Universe from the National Earth Science Teachers Association, the sun’s magnetic field causes the sunspots. Once formed, the sunspots may last as briefly as a few hours or as long as a few months. Sunspots typically fluctuate through an 11-year cycle. Even though sunspots appear dark against the surface of the sun, the areas are still almost as bright as the full moon, and if moved away from the sun, they would be luminous.