ultraviolet astronomy

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ultraviolet astronomy,

study of celestial objects by means of the ultraviolet radiationultraviolet radiation,
invisible electromagnetic radiation between visible violet light and X rays; it ranges in wavelength from about 400 to 4 nanometers and in frequency from about 1015 to 1017 hertz.
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 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 layerozone layer
or ozonosphere,
region of the stratosphere containing relatively high concentrations of ozone, located at altitudes of 12–30 mi (19–48 km) above the earth's surface.
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, oxygen, and nitrogen makes UV radiation difficult to observe from ground-based telescopes, high-altitude balloons, sounding rockets, and orbiting observatoriesobservatory, orbiting,
research satellite designed to study solar radiation, electromagnetic radiation from distant stars, the earth's atmosphere, or the like. Because the atmosphere and other aspects of the earth's environment interfere with astronomical observations from the
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 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 ObservatoryOrbiting Solar Observatory
(OSO), series of eight orbiting observatories (see observatory, orbiting) launched between 1962 and 1971 by the National Aeronautics and Space Administration (NASA) to study the sun in the ultraviolet and X-ray wavelengths filtered out by the earth's
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 (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 ObservatoryOrbiting Astronomical Observatory
(OAO), series of four orbiting observatories (see observatory, orbiting) launched between 1966 and 1972 by the National Aeronautics and Space Administration (NASA) to provide astronomical data in the ultraviolet and X-ray wavelengths filtered
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 (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 cometHalley's comet
or Comet Halley
, periodic comet named for Edmond Halley, who observed it in 1682 and identified it as the one observed in 1531 and 1607. Halley did not live to see its return in 1758, close to the time he predicted.
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 during its 1986 approach, provided data about the UV reflectivity of the major planets, and contributed to the understanding of quasarsquasar
, one of a class of blue celestial objects having the appearance of stars when viewed through a telescope and currently believed to be the most distant and most luminous objects in the universe; the name is shortened from quasi-stellar radio source (QSR).
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; 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 [ROentgen SATellite] and the Hubble Space TelescopeHubble Space Telescope
(HST), the first large optical orbiting observatory. Built from 1978 to 1990 at a cost of $1.5 billion, the HST (named for astronomer E. P. Hubble) was expected to provide the clearest view yet obtained of the universe from a position some 350 mi (560 km)
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ultraviolet astronomy

(ul-tră-vÿ -ŏ-lĕt) Astronomical observations in the waveband between the Lyman limit of atomic hydrogen at 91.2 nm and the Earth's atmospheric absorption cut-off at about 320 nm (see Lyman series; atmospheric windows). The waveband between the Lyman limit and the X-ray band starting at about 12 nm is called the extreme ultraviolet (EUV or XUV – see XUV astronomy). Strong atomic and molecular absorption in the Earth's atmosphere requires UV observations to be carried out at balloon altitudes (about 30 km) for the 250–320 nm waveband and on rockets or satellites, at higher altitudes, for wavelengths below 250 nm. The UV band includes many of the resonant absorption and emission lines of most of the more abundant chemical elements, including hydrogen in both its atomic and molecular form. Such resonance transitions (to and from atomic ground states) are very important for the determination of the physical and chemical properties of astronomical sources, such as the interstellar gas, the outer atmospheres of stars, and the gaseous regions of active galaxies, in which the excitation is such that the electrons in atoms or molecules are mainly in the ground state.

Most ultraviolet astronomy has been carried out using satellite instrumentation either to survey the sky at low spatial and spectral resolution or to observe individual preselected sources at high resolution. The OAO–2 satellite, launched in 1968, carried a telescope that made a partial sky survey at 100–300 nm, while the TD-1 satellite carried an experiment that surveyed the whole sky between 135 and 274 nm, observing over 30 000 ultraviolet stars. OAO–3, renamed Copernicus after launch in 1972, carried a reflecting telescope and high-resolution (about 0.02 nm) grating spectrometer that obtained spectra of many stars and interstellar regions in the wavelength range 90–300 nm over the following 8 years. Copernicus was restricted to observations of hot stars brighter than about 6th magnitude and concentrated on studies of the interstellar gas and the stellar winds of hot luminous stars. The Astronomical Netherlands Satellite (ANS), launched in 1974, provided UV photometric observations of many thousands of stars in the waveband 155–320 nm.

In Jan. 1978 the International Ultraviolet Explorer (IUE) satellite was launched into a geosynchronous orbit over the mid-Atlantic. This satellite is operated like a ground-based observatory with astronomers carrying out observations at two stations in the USA and in Spain. IUE carries a 45-cm aperture ultraviolet telescope feeding two echelle grating spectrographs, which cover the waveband 115–320 nm at about 0.01 nm resolution. IUE is much more sensitive than any previous UV satellite and has allowed observations of stars and active galaxies as faint as 17th magnitude. It has provided important data on a wide variety of astronomical fields: there have been studies of the Solar System, of stars and the interstellar medium in our own Galaxy and the Magellanic Clouds, and of external galaxies, Seyfert galaxies, and quasars.

It has been found that an extensive hot gaseous halo surrounds the Galaxy and the Magellanic Clouds, and that the Sun is located in a warm low-density ‘hole’ in the interstellar medium. Extensive mass loss from hot stars, first studied in high-luminosity objects with Copernicus, has been discovered to exist also in low-luminosity hot stars such as O and B subdwarfs and the central stars of planetary nebulae; in many cases the mass loss rates are too high to be accounted for by current theory. Many stars have been found to show considerable variations in their mass loss. The outer chromospheres and coronae of the cool stars have been observed in the UV with IUE for the first time, allowing the densities and temperature distributions in these atmospheric regions to be mapped and shedding light on their energy balance. Because of the observatory nature of IUE it has been possible to respond quickly to the occurrence of transient events and the UV spectra of several novae and supernovae have been obtained. IUE observations of SN 1987A in the LMC showed that the star is surrounded by a nitrogen-rich circumstellar shell at a radius of 0.5 light-years, providing the first direct evidence for a supernova progenitor having gone through the red giant evolutionary phase prior to its explosion. IUE worked successfully for 18 years.

The NASA Hubble Space Telescope (HST), launched 1990, is providing extensive new UV observations. It carries a 2.4-meter primary telescope and several cameras and spectrographs operating in the UV waveband 115–330 nm (as well as coverage at visible wavelengths) at greater sensitivity than IUE. The spherical aberration of the HST primary mirror initially limited its performance. Most of the scientific programs originally envisaged were carried out, however, with the satellite providing important new UV spectroscopy and imaging of hot stars, late-type stellar chromospheres and coronae, the interstellar medium, galaxies and clusters of galaxies, quasars and active galactic nuclei, and Solar System objects. The corrective optics package COSTAR, emplaced in Dec. 1993, brought the HST back to full capability.

ultraviolet astronomy

[¦əl·trə′vī·lət ə′strän·ə·mē]
Astronomical investigations utilizing observations carried out in the spectral region from approximately 350 to 90 nanometers.