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Ultraviolet Astronomy

Ultraviolet astronomy

Matter in the universe emits radiation (energy in the form of subatomic particles or waves) from all parts of the electromagnetic spectrum. The electromagnetic spectrum is the range of wavelengths produced by the interaction of electricity and magnetism. The electromagnetic spectrum includes light waves, radio waves, infrared radiation, ultraviolet radiation, X rays, and gamma rays.

Ultraviolet astronomy is the study of celestial matter that emits ultraviolet radiation. Ultraviolet waves are just shorter than the violet end (shortest wavelength) of the visible light spectrum. This branch of astronomy has provided additional information about stars (including the Sun), galaxies, the solar system, the interstellar medium (the "empty" space between celestial bodies), and quasars.

Words to Know

Electromagnetic radiation: Radiation that transmits energy through the interaction of electricity and magnetism.

Gamma rays: Short-wavelength, high-energy radiation formed either by the decay of radioactive elements or by nuclear reactions.

Infrared radiation: Electromagnetic radiation of a wavelength shorter than radio waves but longer than visible light that takes the form of heat.

Quasars: Extremely bright, starlike sources of radio waves that are the oldest known objects in the universe.

Radiation: Energy transmitted in the form of subatomic particles or waves.

Radio waves: Longest form of electromagnetic radiation, measuring up to 6 miles (9.7 kilometers) from peak to peak.

Ultraviolet radiation: Electromagnetic radiation of a wavelength just shorter than the violet (shortest wavelength) end of the visible light spectrum.

Wavelength: The distance between two troughs or two peaks in any wave.

X rays: Electromagnetic radiation of a wavelength just shorter than ultraviolet radiation but longer than gamma rays that can penetrate solids and produce an electrical charge in gases.

An ultraviolet telescope is similar to an optical telescope, except for a special coating on the lens. Due to Earth's ozone layer, which filters out most ultraviolet rays, ultraviolet astronomy is impossible to conduct on the ground. In order to function, an ultraviolet telescope must be placed on a satellite orbiting beyond Earth's atmosphere.

Information collected by ultraviolet telescopes

Beginning in the 1960s, a series of ultraviolet telescopes have been launched on spacecraft. The first such instruments were the eight Orbiting Solar Observatories placed into orbit between 1962 and 1975. These satellites measured ultraviolet radiation from the Sun. The data collected from these telescopes provided scientists with a much more complete picture of the solar corona, the outermost part of the Sun's atmosphere.

The Orbiting Astronomical Observatories (OAO) were designed to provide information on a variety of subjects, including thousands of stars, a comet, a nova in the constellation Serpus, and some galaxies beyond

the Milky Way. Between 1972 and 1980, OAO Copernicus collected information on many stars as well as the composition, temperature, and structure of interstellar gas.

The most successful ultraviolet satellite to date was the International Ultraviolet Explorer (IUE) launched in 1978. The IUE was a joint project of the United States, Great Britain, and the European Space Agency. With very sensitive equipment, the IUE studied planets, stars, galaxies, nebulae, quasars, and comets. It recorded especially valuable information from novae and supernovae. Although intended to function for only three to five years, the IUE operated until September 30, 1996, making it the longest-lived astronomical satellite.

The IUE was succeeded by the Extreme Utraviolet Explorer (EUEV), which was launched on June 7, 1992. The EUEV was designed to extend the spectral coverage of the IUE by being able to observe much shorter wavelengths. A third ultraviolet satellite, the Far Ultraviolet Spectroscopic Explorer (FUSE), was launched on June 24, 1999. This satellite also was designed to look farther into the ultraviolet (meaning to shorter wavelengths) than the IUE. With FUSE, astronomers hope to study high-energy processes in stars and galaxies in addition to exploring conditions in the universe as they existed only shortly after the big bang (theory that explains the beginning of the universe as a tremendous explosion from a single point that occurred 12 to 15 billion years ago).

[See also Electromagnetic spectrum; Galaxy; International Ultra violet Explorer; Telescope ]

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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 [ROentgen SATellite] and the Hubble Space Telescope.

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