A sunspot is a region of the Sun’s photosphere that is cooled by a very strong magnetic field, ranging between 1 and 4 kilogauss. The larger the sunspot, the stronger the magnetic field. In comparison, the Sun’s average photospheric field strength is around 1 gauss, and the Earth’s surface field strength is around 0.5 gauss. The strength of the magnetic field at any point on the Sun can be accurately determined by measuring the degree that spectral lines are split due to the Zeeman effect. Under the influence of a strong magnetic field, individual spectral lines in a hot gas will be split into several adjacent lines at slightly different wavelengths. The greater the distance (in wavelength) between the sublines, the stronger the magnetic field.
A sunspot is magnetically cooled, then, to a temperature that is 2,300 to 5,000° F cooler than the surrounding photosphere. Since a cooler gas emits less light, the sunspot appears dark against the hotter and brighter photosphere. It is a contrast effect. Large sunspots have the “coolest” temperatures.
Every once in a great while, a really large sunspot forms. The area covered by a sunspot is usually given in units of “millionths of the Sun’s Earth-facing hemisphere”. Here are the 10 largest sunspots recorded since 1874.
| Rank | Month | Active Region | Size (10-6) |
| 1 | Apr 1947 | 14886 | 6132 |
| 2 | Feb 1946 | 14417 | 5202 |
| 3 | May 1951 | 16763 | 4865 |
| 4 | Jul 1946 | 14585 | 4720 |
| 5 | Mar 1947 | 14851 | 4554 |
| 6 | Jan 1926 | 9861 | 3716 |
| 7 | Jan 1938 | 12673 | 3627 |
| 8 | Mar 1989 | 5395 | 3600 |
| 9 | Feb 1917 | 7977 | 3590 |
| 10 | Jul 1938 | 12902 | 3379 |
How big would an Earth-sized sunspot be? Just 84 millionths of the Sun’s area on its Earth-facing hemisphere. Far smaller than the giant sunspots listed above!