The sunspot cycle , in which the number of sunspots varies from low to high and then to low again over eleven years, has been known since at least the eighteenth century. These interactions are primarily related to the rotation of the sun. Sunspots appear as spots on the sun. The very dark central region is known as the umbra and is often surrounded by a less dark halo known ad the penumbra. The umbra is dark because it is cooler around 3,500 degrees Celsius.
Spots occur singly and in groups ranging from two to over one hundred in number.
There is a tendency for spots to occur in pairs. When they do appear in groups they are usually more complex and there also is a leader who is longer lived than the other companion spots. The most exciting and interesting phenomenon is the chromospheric flare. Flares are short lived explosive brightening of plages. Solar flares produce seismic waves in the Sun's interior that resemble those created by earthquakes.