Sunspots are cooler than rest of photosphere
The internal temperature of the Sun decreases from the nucleus to the radiative zone, then to the convective zone where the plasma becomes turbulent.
On the surface of the Sun, called the photosphere, the temperature is only about 6400 K, which explains the yellow colour of the solar disk. Regularly, spots appear in this solar photosphere (see figure). These regions appear darker because they are cooler than the rest of the photosphere, typically 4500 K.
Sunspots, associated with intense magnetic fields, often appear in groups and in pairs of opposite magnetic polarities.
Rotation period of the Sun
The sunspot lifetime is typically a few weeks, which allowed to measure the rotation period of the Sun, estimated at an average of 27.2753 days.
But in reality, the Sun does not rotate like a solid body because its speed varies with latitude. The rotation period is 25 days at the equator and 36 days near the poles.
Periodic variation of about 11 years
The appearance of the sunspots is modulated by the solar cycle: the number of spots increases then decreases according to a periodic variation of about 11 years.
This 11-year period corresponds to the average duration between two maximums of activity and is due to the inversion of the magnetic polarities North and South of the Sun.
It is therefore only after about 22 years, known as the Hale cycle, that the magnetic poles of the Sun return to their original configuration. Very low solar activity and very few sunspots were observed from 1645 to 1715 (Maunder minimum) and from 1796 to 1820 (Dalton minimum).