ultraviolet astronomy

The study of electromagnetic radiation from astronomical sources in the wavelength band 10-320 nanometres. Ultraviolet (UV) radiation is strongly absorbed by the Earth's atmosphere, so all observations have to be carried out from satellites. The earliest observations were made during brief rocket flights in the 1940s and 1950s. The first satellite to make systematic ultraviolet observations was the first Orbiting Solar Observatory (OSO-1) in 1962. The highly successful International Ultraviolet Explorer (IUE) was launched in 1978 and continued to operate until 1996. The ultraviolet is often subdivided into the extreme UV (EUV, 10-100 nm), the far UV (FUV, 100-200 nm) and the near UV (NUV, 200-320 nm). The most extreme UV, at the transition to X-radiation in the approximate waveband 6-60 nm, is also known as the XUV. At these wavelengths, the techniques of X-ray astronomy are required, but the rest of the UV band can be observed and analysed by methods similar to those used in the visible part of the spectrum. The main difficulty is the limited range of transparent materials and reflective coatings suitable for use in the UV. Glass, for example, is strongly absorbent, and quartz or fluorite have to be used. In a UV telescope carried in the Astro-1 observatory on board the Space Shuttle, the reflection problem was tackled by the use of the rare metal iridium, which is effective down to wavelengths of 40 nm. Ultraviolet astronomy is important because many of the spectral lines most valuable for analysis, of both atoms and molecules, lie in this waveband. Hotter stars, with surface temperatures in excess of 10,000 K, emit most of their energy in the UV. Even for cooler stars such as the Sun, UV studies are needed for energetic phenomena. The interstellar medium is another important object of study for ultraviolet astronomy though, at wavelengths below 91.2 nm, almost all the UV radiation is absorbed by hydrogen, the most widely distributed element in the universe, making the detection of distant sources difficult at such short wavelengths.