Illustration taken from Elizabeth Brown's article 'A Few Notes on the Sunspots of June 1891' in 'The Journal of the Astronomical Asociation' (London, Vol 1, 1891) edited by Edward Walter Maunder, the prominent British solar astronomer. Sunspots are relatively cool areas on the Sun's surface, the photosphere. 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.
© Science Museum / Science & Society Picture Library