Because the atmosphere absorbs most of the ultraviolet radiation from outer space, observation of the sky in the ultraviolet became possible only with the advent of rockets. The first attempts to photograph the ultraviolet spectrum of the Sun with equipment aboard balloons, made in the twenties of the twentieth century were unsuccessful. Positive results were obtained only in 1946 with a camera aboard a rocket.
The importance of ultraviolet astronomy is linked first of all to the fact that the hottest and massive stars preferentially emit their energy in this wavelength band, exciting the surrounding interstellar matter, whose study is, in turn, indispensable to know the conditions in which stars develop. Moreover, many spectral lines related to chemical elements of extreme diffusion in the Universe (hydrogen, helium, oxygen, carbon, nitrogen, etc..) are emitted in the ultraviolet, so that the examination of their characteristics and properties – essential for the construction of a model of the chemical structure of our Galaxy and the others – can only be performed in those particular electromagnetic frequencies.
In general, the emissive sources in the ultraviolet are located in thermal intervals ranging from 104 to 106 K. As mentioned, the first astronomical space observation in ultraviolet (with the Sun as target) dates back to 1946 with the launch of a rocket at 80 km altitude, above the absorbing layers of the atmosphere; in 1955 the first ultraviolet extrasolar source was observed in the constellation Vela.
Ultraviolet spectroscopy did not begin until 1960. However, it developed rapidly in the following years thanks to the use of appropriate optical materials (quartz, calcium fluorides, lithium fluorides), the treatment of optics (aluminization of surfaces) and the introduction of sensitized photographic emulsions. The adoption of the microchannel plate then allowed a real qualitative leap in ultraviolet astronomical investigation. The apparatus consists of a mosaic of small glass tubes (a few decimillimeters in section) closely flanked and contained in a vacuum chamber whose front wall – the one on which the telescopic image is dropped – is maintained at a potential difference of several thousand volts with respect to the rear wall made photosensitive by a layer of phosphors. Each photon incident on the front face of the plate generates, within the tubes, a cascade of electrons, which in turn is multiplied and collimated, by the electric field, in a light machine on the phosphor-treated face. A directly facing photographic camera records the phosphoric image; but this task is also performed by CCD image-matrix receptors.
In any case, the celestial ultraviolet below 91 nm is precluded, because it is absorbed by interstellar hydrogen. In 1968 the orbiting observatory OAO-2 was launched by NASA, followed (1973) by the OAO-3 Copernicus. ESA, the European Space Agency, is responsible for the placement in orbit of TD-1 (1972), DB-2 (1975) and IUE (International Ultraviolet Explorer, 1978). The latter satellite has proved particularly valuable for continuity of operation and amount of data collected. In June 1992, it was replaced by the EUVE satellite (Extreme UltraViolet Explorer) programmed to investigate the emissive properties of the electronic crowns of giant stars, flare stars, atmospheres of white dwarfs and stars with solar activity.
In 1999, the FUSE (Far Ultraviolet Spectroscopic Explorer) satellite, which specializes in high-resolution far-ultraviolet spectroscopy, was put into orbit by NASA.