Ultraviolet (UV) light is electromagnetic radiation in the
approximate wavelength range 10 to 400 nm. It has wavelengths
shorter than visible light but longer than x-rays and is
invisible to the human eye. The ultraviolet wavelength range is
broadly divided in order of decreasing wavelength into the near
ultraviolet (NUV; closest to the wavelength of visible light),
the far ultraviolet (FUV) and the extreme ultraviolet (EUV;
closest to the wavelength of xrays). The Sun is a source of
ultraviolet radiation which is harmful to human skin. The
Earth’s ozone layer blocks the majority of the Sun’s uv-
radiation, which is beneficial for us but hampers ground-based
ultraviolet astronomy. Instead, ultraviolet-wavelength
telescopes must be put into space on satellites. Astrophysical
sources of ultraviolet light are hot objects (T ~ 106-108 K)
including young, massive OB stars, evolved white dwarfs,
supernova remnants, the Sun’s corona and gas in galaxy clusters.
Ultraviolet light was discovered by Johann Wilhelm Ritter