An Etymological Dictionary of Astronomy and Astrophysics
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A part of the ultraviolet radiation with wavelengths between 50 and 300 Angstöms.

→ extreme; → ultraviolet.

Ultraviolet radiation in the wavelength range 912-2000 Å. See also → extreme ultraviolet.

→ far; → ultraviolet.

A satellite dedicated to spectroscopic observations of astronomical objects in ultraviolet wavelengths, launched in 1978. It was an international collaboration between → NASA, the → European Space Agency (ESA), and the United Kingdom's Science and Engineering Research Council. It operated until September 1996 and collected over 70,000 spectra. IUE consisted of a 45-cm telescope (f/15) equipped with two spectrographs operating in the ranges 1850-3300 Å and 1150-2000 Å. Each spectrograph had a high-resolution and a low-resolution mode with resolutions of about 0.2 Å and 6 Å respectively.

→ international; → ultraviolet; → explorer.

The longest wavelengths of the ultraviolet region, which are adjacent to the visible, from 200 to 350 nm.

→ near; → ultraviolet.

The part of the electromagnetic radiation beyond the violet end of the visible spectrum with wavelengths approximately in the range 50 Å to 4,000 Å. → extreme ultraviolet; → far ultraviolet.

→ ultra-; → violet.

The study of astronomical objects in the ultraviolet portions of the electromagnetic spectrum, in the waveband 3000 Å to about 10 Å. At these wavelengths, the atmosphere prevents ultraviolet radiation from reaching the Earth surface. Therefore ground-based observatories cannot observe in the ultraviolet. Only with the advent of space-based telescopes has this area of astronomy become available for research.

→ ultraviolet; → astronomy.

A → paradox encountered in the classical theory of → thermal radiation (→ Rayleigh-Jeans law), whereby a → blackbody should radiate an infinite amount of energy at infinitely short wavelengths, in contradiction with what is observed. The problem was solved by Max Planck in 1900, who suggested that, rather than being continuous, the energy comes in discrete parcels called → quanta. The avoidance of the ultraviolet catastrophe was one of the first great achievements of → quantum mechanics.

This problem was first raised by Lord Rayleigh (1842-1919), whereas the term ultraviolet catastrophe was first used by Paul Ehrenfest (1880-1933); → ultraviolet; → catastrophe.

Ultraviolet emission from an object in excess of that expected for a reference. For example, → subdwarf stars show ultraviolet excess with respect to that expected from a star with → solar metallicity at a given → effective temperature. In this case, UV excess results from smaller → line blanketing in → population II stars.

→ ultraviolet; → excess.

A star, such as O types or hot central stars of planetary nebulae, which radiates essentially in the ultraviolet part of the electromagnetic spectrum.

→ ultraviolet; → star.