XUV astronomyThe study of objects beyond the Solar System that emit radiation bridging the gap between X-rays and ultraviolet radiation (the XUV region), i.e. from about 12 nm to the hydrogen Lyman limit at 91.2 nm (see Lyman series). The high opacity of the interstellar gas at these wavelengths prevents observations beyond about 10 parsecs from 91.2 to about 50 nm, extending to about 200 parsecs at 12 nm. The first partial sky survey in the XUV was carried out in the Apollo-Soyuz mission in 1975, in which were found a number of intense sources, including HZ43 and Feige 24 (both hot white dwarf stars) and Proxima Centauri.
The first deep all-sky survey in the XUV band was carried out by the UK Wide Field Camera (WFC) on the ROSAT satellite, launched in 1990. The WFC provided sky-survey observations into two photometric bands covering the range 6–30 nm, yielding over a thousand sources. A further XUV sky-survey was conducted by the NASA EUVE satellite, launched in 1992, providing observations in the waveband 7–76 nm. Most of the XUV sources found in these surveys are nearby hot white dwarfs, late-type stellar chromospheres/coronae, and active stars. A few extragalactic sources have also been detected. Later EUVE observations provided the first resolved XUV spectra of a number of bright cosmic sources. See also X-ray catalogs.