ultraviolet astronomy

ultraviolet astronomy study of celestial objects by means of the ultraviolet radiation they emit, in the wavelength range from about 90 to about 350 nanometers. Ultraviolet (UV) line spectrum measurements are used to discern the chemical composition, densities, and temperatures of interstellar gas and dust, and the temperature and composition of hot young stars. UV observations can also provide essential information about the evolution of galaxies. Because atmospheric interference from the ozone layer , oxygen, and nitrogen makes UV radiation difficult to observe from ground-based telescopes, high-altitude balloons, sounding rockets, and orbiting observatories are employed.

Although attempts to study the sun's UV spectrum from balloons were made during the 1920s, it was not until 1946 that rocket-borne instruments made this possible. Only limited additional progress was made until 1962, when the first Orbiting Solar Observatory (OSO) satellite was launched by the National Aeronautics and Space Administration (NASA). These returned thousands of UV spectra, including the first exteme-ultraviolet (wavelengths below 200 nanometers) observations of the solar corona. Through continuous monitoring of the sun over a 15-year period, this program enhanced our understanding of the solar atmosphere and of the 11-year sunspot cycle.

NASA's Orbiting Astronomical Observatory (OAO) satellites, the first of which was launched in 1966, returned UV data about stars and interstellar gas and dust and the first observations of the powerful UV radiation emitted by certain galaxies. Data from Copernicus (OAO-3), which was launched in 1972, led to the determination of the abundance of deuterium in interstellar matter; it also provided considerable information about the atmospheres of luminous hot stars. The Netherlands Astronomical Satellite (ANS) and the TD-1 satellite performed photometric and spectrophotometric surveys of stars in the UV wavelengths.

The International Ultraviolet Explorer (IUE)—a joint project of the United States, the European Space Agency, and Great Britain—was launched in 1978. In orbit for a decade, it monitored the UV spectrum of Halley's comet during its 1986 approach, provided data about the UV reflectivity of the major planets, and contributed to the understanding of quasars ; its large telescope made possible the first UV observations of objects beyond the Milky Way, permitting the determination of temperature and structural changes of cool stars during their starspot cycles. The Extreme Ultraviolet Explorer (EUVE; 1992-2000) was the first orbiting observatory to focus on that part of the spectrum. In addition to data from these satellites, UV observations have also been made from two satellites launched in 1990 primarily for other purposes, the X-ray astronomy satellite ROSAT [ RO entgen SAT ellite] and the Hubble Space Telescope .

ultraviolet astronomy The study of the Universe in the ultraviolet region of the electromagnetic spectrum, approximately 91.2–350 nm. These wavelengths are largely blocked by the Earth's atmosphere, so observations became possible only with the use of rockets after World War II. Balloons were also used, but the altitude they could achieve allowed observations only in the near ultraviolet, longer than 200 nm.

The Orbiting Astronomical Observatory series of ultraviolet missions commenced in 1968. In 1972 OAO-3, also known as Copernicus, revealed some of the detailed structure of interstellar matter, in particular its patchiness. TD-1A, a European satellite, made an ultraviolet survey from 1972 to 1974 at 135–290 nm. ANS, the Astronomical Netherlands Satellite, made photometric observations of a large number of stars at 155–330 nm, also in the 1970s. Ultraviolet observations were carried out from the Skylab space station and the Voyager interplanetary probes, the latter covering the range 50–170 nm.

Ultraviolet astronomy entered a new era in 1978 with the launch of the International Ultraviolet Explorer (IUE), which obtained tens of thousands of spectra of various objects. Highlights include the discovery of hot haloes of gas surrounding our own and many other galaxies; the monitoring of mass loss by stellar winds in many different types of stars; and the study of the processes operating in novae and X-ray binaries. IUE also observed Halley's Comet and contributed substantially to the understanding of Supernova 1987A.

 The Hubble Space Telescope (HST) has extended the work of IUE, obtaining higher spectral resolution and observing significantly fainter objects. In addition, various ultraviolet telescopes have been carried in the cargo bay of the Space Shuttle. Ultraviolet astronomy has been carried into the extreme ultraviolet by Rosat and the Extreme Ultraviolet Explorer (EUVE). In 1999 the Far Ultraviolet Spectroscopic Explorer (FUSE) was launched to make high‐resolution spectroscopic measurements in the 90–120 nm range.