The Stanford Solar Center states that sunspots form when the sun's magnetic field protrudes through its surface. The sun's magnetic field is composed of tubes that affect the sun’s surface temperature at the point of protrusion. Sunspots exhibit polarity, which creates a pair of positive and negative poles.
Sunspots appear as dark marks, because they release less energy and have a lower temperature than surrounding regions. The darkest region at the center of the sunspot is called the umbra. The penumbra surrounds the umbra and has a lighter color. The average temperature of sunspots is 2240.3 degrees Fahrenheit. Sunspots can be detected easily, because the largest variety have a diameter close to 31,068 miles. Sunspots form in clusters and usually contain about 100 spots in each group. Sunspots can appear within a matter of hours, but sometimes take several months to develop completely. Once a cluster of sunspots develops, the cluster is typically visible for a cycle of 11 years. Samuel Schwabe named this phenomenon the solar cycle based on his observations conducted between 1826 and 1843. Galileo tracked sunspots as they traveled along the sun's surface 200 years before Schwabe measured the solar cycle. This allowed Galileo to conclude the sun takes approximately a month to rotate on its axis.