Solar disc showing sunspots in false colour, taken from Skylab, 1973.

Sunspots are relatively cool areas on the Sun's surface, the photosphere. Their temperature is about 3800 degrees Kelvin, as opposed to around 5800 degrees on the rest of the photosphere, and they can measure as much as 50,000 kilometres acros. The number of sunspots is greatest at the point in the cycle of solar activity known as the 'solar maximum', which occurs roughly every 11 years. They form when magnetic field lines below the surface become twisted and protrude through the photosphere. Sunspots are closely asociated with solar flares, sudden outbursts of energy which extend far into the Sun's atmosphere. Skylab, launched in 1973, carried the Apollo Telescope Mount which contained eight instruments to study the Sun at various wavelengths.