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Solar flares are intense bursts of electromagnetic radiation from the Sun. 

Scientists separate solar flares into different classes according to their peak brightness in X-ray wavelengths. The least powerful are A-class flares, followed by B, C and M. The highest class of solar flare is X. 

Each class marks a 10x increase in energy from the previous one. For example, an X is 10 times stronger than an M flare.

The class is often followed by a number; flares in classes A to M range from 1 to 9, however X flares have no upper limit.