A solar flare is a violent eruption that explodes from a star's photosphere with energies equivalent to tens of millions of hydrogen bombs. Solar flares from the Sun send out streams of highly energetic solar wind that can present a radiation hazard to spacecraft outside of the planetary magnetospheres and can disrupt radio signals on Earth. Solar flares were first observed on the Sun in 1859 by English astronomer Richard Carrington. They have also been observed to varying degrees on other stars in modern times. The frequency of solar flares varies, from several per day when the Sun is particularly "active" to fewer than one each week when the Sun is "quiet." Solar flares may take several hours or even days to build up, but the actual flare takes only a matter of minutes to release its energy. The resulting shockwaves travel laterally through the photosphere and upward through the chromosphere and corona at speeds on the order of 5,000,000 kilometers per hour.
Powerful solar flares are given an X designation that generally runs to X10. Infrequently, X designations run 'off the charts' (see illustration). X20 events that were recorded on 16 August 1989 and 2 April 2001 were outshone by a flare on November 4, 2003 that was the most powerful flare ever recorded in the history of astronomy, estimated at X28. Sunspot Region 486 (shown in the illustration) was the most turbulently active sunspot ever recorded.
There are also M-class and C-class designations for solar flares, though they are smaller than the X-class.
The radiation risk posed by solar flares is one of the major concerns in discussions of manned missions to Mars. Some kind of physical or magnetic shielding will be required.