Astronomical observations carried out in the region of the electromagnetic spectrum with wavelengths from approximately 10 to 350 nanometers. The ultraviolet spectrum is divided into the extreme-ultraviolet (EUV; 10–90 nm), far-ultraviolet (FUV; 90–200 nm), and near-ultraviolet (near-UV; 200– 350 nm). Ultraviolet radiation from astronomical sources contains important diagnostic information about the composition and physical conditions of these objects. This information includes atomic absorption and emission lines of all the most abundant elements in many states of ionization. The hydrogen molecule (H₂), the most abundant molecule in the universe, has its absorption and emission lines in the far-ultraviolet. Thus, ultraviolet observations make it possible to probe a very wide range of physical conditions of matter in the universe, from the very cold gas in dense interstellar regions with temperatures of perhaps 30 K (−406°F) to the hot gas found in supernova remnants and in the coronas of stars and galaxies with temperatures approaching 10⁶ K.