Visible Infrared Survey Telescope for Astronomy

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See vista visible spectrum Wavelength band that is visible to the human eye. The visible spectrum, sometimes called the 'optical spectrum', extends approximately from 400 to 700 nm. It is subdivided (from long to short wave) by the major visual colours red, orange, yellow, green, indigo and violet. It is flanked by the 'near-ultraviolet' and the 'near-infrared'. The visible spectrum was the first to be explored, and it contains the fraunhofer lines as well as myriad other absorption lines and emission lines. See also electromagnetic spectrum

VISTA (acronym for 'Visible-Infrared Survey Telescope for Astronomy') British project to build a southern-hemisphere 4-m (157-in.) wide-field survey telescope to support the current generation of 8-10-metre class telescopes. The telescope will be located at Paranal Observatory and will be shared with the european southern observatory (ESO) as part of the UK's entry agreement with ESO. It will be operational by 2004. Originally, the telescope was planned to have wide-field imagers in both the visible and infrared wavebands, but the visible-light function will now be carried out by ESO's VST (see paranal observatory), so VISTA will be an infrared-only telescope.

visual binary Gravitationally bound binary star system in which both components can be resolved, sometimes without a telescope. The brighter star is termed the primary, the fainter is termed the companion. When the separation is too large for orbital motion to be observed, they are called common proper motion stars.

If orbital motion can be observed, the orbit of the companion with respect to the primary can be determined. If the distance to the pair is known, the total mass of the binary can be determined from Kepler's third law (see kepler's laws). The individual masses of the components can be determined if the absolute orbit of each star can be measured.

visual magnitude Apparent brightness of an astronomical body as seen by the eye, whose maximum sensitivity is at a wavelength of 550 nm. Such magnitudes are now determined photographically or photoelectrically, using appropriate filters, and are called photovisual magnitudes.

VLA Abbreviation of very large array

VLBA Abbreviation of very long baseline array

VLBI Abbreviation of very long baseline interferometry

VLT Abbreviation of very large telescope

Vogel, Hermann Carl (1841-1907) German astronomer who pioneered the use of spectroscopy to determine stellar diameters and masses, leading to the

discovery of the first spectroscopic binaries. Using the 'reversion spectroscope' invented by Friedrich zöllner, Vogel was the first to calculate the Sun's rotational velocity by measuring Doppler shifts in spectra of its opposite limbs; later he applied the technique to other stars, finding that Algol and Spica were each a spectroscopic binary consisting of stars of nearly equal mass but very different luminosities. In 1876, he documented changes in the spectrum of Nova Cygni as it faded, the first time this had been accomplished. As director of Potsdam Astrophysical Observatory (1881-1907), he supervised Potsdam's participation in the carte DU ciel sky-mapping project.

Vogt-Russell theorem Theorem proposed independently in 1926 by the German physicist Heinrich Vogt (1890-1968) and H.N. russell stating that if a star's mass and chemical composition are known, then all its other properties can be determined by the laws of physics. Since initial chemical composition varies relatively slightly between stars, it is thus principally a star's initial mass that determines its basic structure and evolution.

voids Large areas in the Universe where there are apparently few, if any, galaxies. This is in contrast to areas where large clusters of galaxies reside. See also large-scale structure

Volans See feature article volatile Element or compound that evaporates at a relatively low temperature. All of the noble gases and other constituents of planetary atmospheres, such as hydrogen, nitrogen, methane, ammonia and carbon dioxide, are volatiles. The most significant volatile is water. Sulphur and its compounds are also classed as volatiles. Materials with higher temperatures of evaporation, such as most metals and silicates, are called refractory.

volcanism Eruption of molten material at the surface of a planetary body. magma is usually less dense than the surrounding solid rock, and it tends to rise through any cracks or zones of weakness. Planetary crusts consist of a variety of minerals, so heating these materials does not necessarily lead to a unique melting temperature. Some minerals may melt while others, still solid, may be carried along in a fluidized medium. volatiles, including water, carbon dioxide and sulphur compounds, dissolve in molten silicates at high pressures; they are released at lower pressure near the surface. The mixture's volatile component, viscosity and yield strength will vary between planets and between different rock types on a given planet; thus there are different types of eruption, the extremes being 'quiescent' and 'violent'. On Earth, large volumes of basaltic lava are erupted relatively quiescently from calderas (as in Hawaii), producing gently sloping shield volcanoes, and from fissures (such as in Iceland), producing lava plains. Steeper cones are produced by explosive pyroclastic eruptions of silica-rich material in which the gas pressure shatters the lava and rock into fountains of fragments.

Earth displays a wide variety of volcanic landforms. The volcanic materials reach the surface through conduits and fissures associated with the active margins of tectonic plates and mid-oceanic ridges. Other planets have a variety of volcanic landforms. Mercury appears to show some volcanic features. On the Moon, lavas erupted from fractures, filling the large impact basins (maria) with basaltic magma; some small volcanic domes and lava tubes are also present. Venus appears to have been resurfaced by lava flows of great extent. Mars has several extinct shield volcanoes; olympus mons, with a height of 24 km (15 mi) and a diameter greater than 700 km (430 mi), is the largest volcanic edifice known. Apart from Earth, only io is known to have active volcanoes. Subject to tidal heating produced by Jupiter's

VOLANS (gen. volantis, abbr. vol)

Small, inconspicuous southern constellation, representing a flying fish, south of Carina. It was introduced by Keyser and de Houtman at the end of the 16th century. Its brightest star, p Vol, is mag. 3.8. y Vol is a binary with yellow and pale yellow components, mags. 3.8 and 5.7, separation 14".1.

gravitational pull, Io has numerous volcanoes in nearly constant state of eruption. The products of these eruptions, which are in both pyroclastic and flow form, are unique in their high sulphur content.

In the early Solar System, impacts during accretion produced heat; much of the Moon was probably melted in this manner. Larger planets were melted by conversion of gravitational energy into heat during differentiation. Heating is also produced by natural radioactivity in planetary interiors. See also cryovolcanism

Von Karman, Theodor (1881-1963) Hungarian-American aeronautical engineer and 'father of supersonic flight'. He was one of the first to apply higher mathematics to the new fields of aeronautics and astronautics. In 1930 he became head of the Guggenheim Aeronautical Laboratory of the California Institute of Technology, developing jet and rocket engine designs. Von Karman was one of the founders of the jet propulsion laboratory, responsible for most of NASA's major unmanned space probes.

Voskhod Two modified vostok spacecraft that were used as a stopgap before the introduction of the soyuz. Voskhod 1 (1974 October 12-13) was the first Soviet spacecraft to carry three men. During Voskhod 2, Alexei leonov made the first space walk on 1965 March 18.

Vostok Series of six Soviet manned spacecraft. In Vostok 1, Yuri gagarin became the first man to orbit the Earth on 1961 April 12. Valentina Tereshkova (1937- ), the first woman in space, flew in Vostok 6, 1963 June 16-19. The world solo spaceflight record (4d 23h 6m) is held by Valeri Bykovsky (1934-) in Vostok 5.

Voyager Two national aeronautics and space administration (NASA) spacecraft; they were intended to visit only Jupiter and Saturn, but Voyager 2 went on to complete a 'grand tour' of the four giant outer planets. The two spacecraft were identical except for the more powerful Radioisotope Thermoelectric

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