Sunspots appear as temporary darker regions on the visible surface of the Sun. They are caused by strong magnetic fields blocking the transfer of heat from the solar interior, allowing the region above to cool. Sunspots are characterised by a darker core or umbra surrounded by a lighter penumbra and are often larger than the Earth.
After a long period of relative calm and few sunspots, activity on the Sun is rising towards an expected peak sometime between 2012 and 2014. At the Big Bear Solar Observatory (BBSO) in California, astronomers are commissioning the New Solar Telescope (NST), an instrument with a mirror 1.6-m across.
The NST is already producing remarkable images like the one above, where details as small as 50 km across can be seen on the disk of the Sun, 150 million km away from the Earth, making it the sharpest ever image of a sunspot. The umbra, penumbra and surrounding granulation (marking convection cells) are all clearly visible.