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The unexpected deviation from the thermal distribution in the extreme ultraviolet (EUV) and shorter wavelengths (not shown) in the solar spectrum constitutes the celebrated solar corona problem discovered by Walter Grotrian in 1939. The green curve shows the wavelength dependence of the 11-year solar cycle, indicating a threshold-like effect around visible UV energies. The non-thermal part in short wavelengths is much more pronounced in young sun-like stars.
Image credit: Judith Lean/NRL Washington.