NASA image of sunspots
Sunspots at solar maximum

The visible surface of the Sun is called the photosphere. Sunspots are darker, cooler regions on the bright, hot photosphere. Whilst the photosphere has a temperature of around 6000°C, sunspots are between 3000°C and 4000°C.

Sunspots move across the surface of the Sun, expanding and contracting, reaching up to 50,000 km in diameter. They appear in pairs: one is a magnetic north pole, the other is a magnetic south pole. Magnetic field lines exit the surface of the Sun through one sunspot, and enter through the other. These strong magnetic fields prevent the process of convection.

Convection currents transfer heat, from the hot interior of the Sun, to the outer layers. In sunspot regions, convection cannot transport heat to the surface, and so sunspots are much cooler than their surroundings. Solar flares and huge solar storms (called coronal mass ejections) originate from sunspot regions.

Diagram of convection cells
Convective transport
Credit: I, Eyrian
Galileo studied sunspots in 1610, and made a series of observations to track their movement. Using sunspot tracks as evidence, he concluded that the Sun rotates once every month. However, with modern technology, scientists have discovered that sunspots rotate around the Sun at different rates.
Sunspots closer to the solar equator rotate approximately every 22 days, whereas sunspots near the poles rotate every 35 days. This is because the Sun is not a perfect sphere: it is actually an 'oblate spheroid'. The Sun is wider around its equator than its poles, giving a shape similar to the Earth.