# Broadband spectrum of Sun

Broadband spectrum is one which is inclusive of all the observed wavelength ranges. In the case of the BB spectra of our Sun, there is a sharp (non-differentiable) kink in the curve in the UV-range (as shown above). What is the significance of this kink?

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The point here is, that the kink you're pointing out is not belonging to the BB-spectrum of the sun. This part of the solar spectrum is a classic example for a broad class of radiation, called nonthermal radiation. The physical origin of this is in general that, every accelerated particle will radiate part of its energy away, with the experienced acceleration a, radiated power P and their relation $P \sim a^2$ in a wavelength-range depending on the exact process.
Spectrae for such processes are derived in every standard textbook about theoretical astrophysics and are given by roughly $\frac{dP}{dt} \sim exp(-\frac{h \nu}{k_BT})$, which should fit the UV-part in the spectrum you gave pretty well. This now also explains the variability of this part of the solar spectrum: As the number of loops and sunspots correlates with solar activity, it is clear that the UV-bremsstrahlung-production should too.