vacuum Volume of space containing a low pressure gas. A perfect vacuum would contain no particles at all, but this is unreachable in practice. On the Earth, a soft vacuum contains gas at a pressure of 0.01 Pa, a hard vacuum, gas at 10 ~6 Pa. The interstellar medium, away from interstellar gas clouds, contains gas at a pressure of around 10~12to 10~14Pa.

Valles Marineris Series of enormous, interconnected canyons on mars, situated between 30°W and 110°W, just south of the Martian equator. These canyons, produced by extensive faulting of the crust, extend from the crest of tharsis montes to the vicinity of chryse planitia, a distance of 4500 km (2800 mi). In the west the canyon system emerges from Noctis Labyrinthus then trends approximately west-east. The main section of the system comprises a number of parallel, straight-sided chasms, beginning in the west with Ius Chasma and Tithonium Chasma. The central section sees both the widest and deepest expression: here three vast troughs, each 200 km (120 mi) wide and up to 7 km (4 mi) deep, slice through the plains. At the eastern end the canyons pass into chaotic terrain. Ophir Chasma and Candor Chasma are situated to the north of the central canyon complex. The west and east ends of Ophir Chasma are both rather blunted and the trend of the canyon walls is continued laterally by lines of crater chains, indicating an underlying structural control. Spacecraft images reveal the existence not only of erosional debris but also of rhythmically layered deposits, which, by analogy with terrestrial rocks, would be expected to have been laid down in quiet conditions beneath a cover of water. Such deposits are seen generally to form flat-topped hills (mesas) or irregular blocks within the troughs. The mesas and canyon walls are finely stratified by both light and dark materials on a scale of a few metres, indicating a complex geological history. Very recent evidence points to the deposition of the layered strata prior to the large-scale faulting that gave rise to the canyon system.

Valles Marineris This composite Viking image shows the huge, sprawling canyon system of the Valles Marineris (Mariner Valley) on Mars. Ancient dry river channels run northwards from the chaotic terrain at the east (right).

Valhalla Largest multiple-ringed impact basin on Jupiter's satellite callisto. At 4000 km (2500 mi) in diameter, it is in fact the largest such structure in the entire Solar System. A 600-km-wide (400-mi) bright circular patch representing the impact site is surrounded by a series of concentric fractures.

Van Allen belts radiation belt regions in the terrestrial magnetosphere in which charged particles are trapped as they spiral along the magnetic field and bounce up and down between reflection points located towards the magnetic poles. These regions were discovered in 1958 by James Van Allen on analysis of Geiger counter data from the explorer 1 satellite. The inner region of the Van Allen belts is located between 1000 and 5000 km (600 and 3000 mi) above the equator; it contains protons and electrons, which are either captured from the solar wind or originate from collisions between upper atmosphere atoms and high-energy cosmic rays. The outer belt region is located between 15,000 and 25,000 km (9000 and 16,000 mi) above the equator, but it curves downwards towards the magnetic poles. This region contains mainly electrons from the solar wind. The Van Allen belts are a potential hazard to Earth-orbiting spacecraft, since the radiation levels are high enough to have an adverse effect on the electronic subsystems and on-board instrumentation, especially following a geomagnetic storm. Similar radiation belt regions of trapped, charged particles have been discovered around Jupiter, Saturn, Uranus and Neptune.

Van Allen belts Earth is surrounded by radiation belts, which contain energetic particles trapped by the magnetic field. The inner belt dips low over the South Atlantic, where its particle population can present a hazard to satellites.

Van Allen, James Alfred (1914- ) American physicist who discovered (1958) the radiation belts surrounding Earth. He has spent most of his career (1951- ) at the University of Iowa, investigating Solar System radiation and magnetic fields. After World War II, he used captured German V-2 rockets to measure cosmic rays, inventing the 'rockoon' method of reaching far greater altitudes than previously possible by launching rockets first carried to high altitude by balloons. An organizer of the international geophysical year, he analysed data from the early Explorer spacecraft to map two torus-shaped regions of radiation above the Earth's equator now named the van allen belts. More recently, he and his colleagues have made detailed studies of solar-terrestrial interactions. In all, Van Allen has served as principal investigator for 24 different space missions.

Van Biesbroeck, George (1880-1974) Belgian-American astronomer (born Georges Achille van Biesbroeck), an observer of double stars at Yerkes Observatory (1917-64). He moved to the United States when Belgium was invaded in World War I. At Yerkes, he used the 40-inch (1-m) telescope to carefully re-observe 1200 double stars identified at Lick Observatory by William Joseph Hussey (1862-1926). During his long career, Van Biesbroeck preferred computing and refining orbits of known pairs to seeking new doubles. He discovered three new comets (1925, 1936, 1954) and the faint companion of the star BD +4° 4048, with an absolute magnitude of only +19. Named Van Biesbroeck's Star, this was for many years the least luminous star known.

Vandenberg Air Force Base US military launch facility about 240 km (150 mi) north-west of Los Angeles, operated by the US Air Force Space Command's 30th Space Wing. It began as a US Army training centre in 1941 but from 1957 was developed as the nation's first space and ballistic missile operations and training base under the Air Force. In 1958 it was renamed to honour General Hoyt S. Vandenberg. The base supports west-coast launches for the NATIONAL AERONAUTICS AND SPACE ADMINISTRATION (NASA) and commercial contractors as well as the Air Force and the Department of Defense. All US spacecraft destined for near-polar orbit are launched from Vandenberg.

Van Maanen, Adriaan (1884-1946) Dutch-American astronomer who conducted astrometric and spectroscopic studies at Mount Wilson Observatory (1912-46). There, he used the 60- and 100-inch (1.5- and 2.5-m) telescopes to make a comprehensive survey of parallaxes for 500 starfields. He discovered Van Maanen's Star, a nearby 12th-magnitude white dwarf. He also made many measurements of proper motions, among them many Cepheid variables, thus helping to determine the scale of the Galaxy. This led Van Maanen to extend his earlier research on star streaming, carried out with Jacobus KAPTEYN, to determine which stars belong to Double Cluster in Perseus, the Pleiades and Hyades, and other clusters. This work greatly extended our knowledge of the density and luminosity of stars in the solar neighbourhood.

variable star Star whose brightness changes with time. These variations in light may be the result of some inherent feature in a star or its atmosphere, or of some geometrical alignment. The former are termed INTRINSIC VARIABLES; the latter EXTRINSIC VARIABLES. There are about 35,000 known variable stars and at least 15,000 objects that are suspected to vary in light, but have not yet been confirmed. These numbers do not include variables in GLOBULAR CLUSTERS or those in other galaxies. A few variable stars are known by their Greek letters or proper names, for example a Orionis (BETELGEUSE). The system of naming variable stars is to assign the letter R to the first discovered in a constellation; the second becomes S and so on down to Z. Then comes RR to RZ, SS to SZ, and so on. This sequence ends at ZZ, after which comes AA to AZ, BB to BZ and so on down to QZ, but the letter J is never used. After QZ the next variable in any constellation would be the 335th: it becomes V335, and each subsequent variable in that constellation gets a higher number. The letters and numbers are followed by the name of the constellation.

Early discoveries of variables were made visually and a few are still discovered by this method. However, most discoveries result from conventional or CCD photography. Two images taken of the same region of the sky may be placed in a blink or stereo COMPARATOR, which ensures that any difference in a star is soon noticed. CCD images may be compared electronically with a reference image of the same area of sky. The plates, films or images must be taken some time apart and with the same instrument and, if possible, under the same sky conditions.

Variable-star work is a field in which a large contribution is made by amateur astronomers, working in co-operation with central organizations such as the AMERICAN ASSOCIATION OF VARIABLE STAR OBSERVERS (AAVSO), the BRITISH ASTRONOMICAL ASSOCIATION (BAA) or the ROYAL ASTRONOMICAL SOCIETY OF NEW ZEALAND (RASNZ). There are so many variable stars that the professional astronomers can only observe a few. They wish to select those stars that pose special problems, or to observe them at certain phases of their light variations. As a result, they rely on amateurs to provide data that will enable them to select the stars that interest them and to provide light-curves and predictions for the more regular variables so that they can select the phases that they wish to observe. The amateur astronomers are thus able to make a very valuable contribution by consistently following these stars.

The intrinsic variables are divided into many classes, each of which has a number of divisions depending on the way in which the stars change in brightness or because of some other property inherent in them. Broadly speaking, there are seven classes. The first are called ERUPTIVE VARIABLES because of violent processes taking place in their chromospheres and coronae. The resultant flares are often accompanied by shell events, as matter is carried off by stellar winds, and by interaction with the interstellar matter that surrounds the star. Typical examples are the various types of Orion variables. The second class are the pulsating variables, which have fairly regular expansion and contraction of their surface layers. MIRA STARS and the many types of CEPHEIDS belong to this class. The third class consists of rotating stars, with light variations resulting from their axial rotation or from starspots or some feature of their atmospheres caused by a magnetic field. This class includes both intrinsic stars and extrinsic variables. The fourth class comprises the explosive or CATACLYSMIC VARIABLES such as NOVAE, SUPERNOVAE and DWARF NOVAE. The fifth class comprises the ECLIPSING BINARY systems such as the Algols, which are really extrinsic variables. The sixth class consists of X-ray sources in which the variability is in the X-ray radiation. Finally, there is a seventh class, which consists of unique objects that cannot be assigned to any other type. Included in this class are the BL LACERTAE OBJECTS and optically varying QUASARS, which are, of course, extragalactic objects.

The observation of many of the foregoing stars has undergone a change in recent years, with the availability of photoelectric PHOTOMETERS and their associated equipment. It is now possible to obtain precise three-colour data on a wide range of stars. Similarly, the use of CCD equipment has enabled amateurs to reach fainter magnitudes than were possible with purely visual methods. All such observations have been welcomed by the professionals because there are so many variable stars to be observed: in 1786 there were only 12; by 1866 this had grown to 119; by 1907 the number had increased to 1425; the advent of photographic methods of discovery saw numbers grow rapidly thereafter until by 1941 there were 8445 known variable stars. Since then the number has continued to increase rapidly so that today there are tens of thousands of these stars.

To observe variable stars, both professionals and amateurs need detailed maps of small areas of the sky in which the variable is situated and on which both the variable and the surrounding stars are clearly identified, preferably with a sequence of comparison stars of constant brightness. This is often impossible for the simple reason that the magnitudes of the surrounding stars have not been accurately determined. This problem may be overcome in several ways, one of which is to select what appear to be suitable comparison stars from photographic plates and to assign letters to them as symbols. This then enables visual observations to be made pending the determination of accurate magnitudes later. For making a visual estimate of a variable star, the observer compares it with two stars of known, unchanging brightness. There are three principal methods - the argelander step method, the fractional method and the pogson step method.

The main sources of the necessary charts are the Variable Star Sections of the BAA and RASNZ, and the AAVSO. The importance of variable stars is shown by the fact that about one-third of the astronomical literature is concerned with them in some way.

variation Term with several meanings in different contexts. Lunar variation is the second-largest periodic perturbation of the Moon's longitude, caused by the Sun, displacing it from its mean position by ± 39'29".9 with a period of 14.765 days (half of the synodic month). The perturbation, discovered by Tycho Brahe at around 1580, was the first major advance in the knowledge of the Moon's orbit since the discovery of the evection by Ptolemy in about ad 140. The perturbation varies as the sine of twice the longitude difference of the Sun and Moon, and hence it is zero at new and full moons. Thus it does not affect the times of occurrence of eclipses, which allowed it to escape detection for 14 centuries. Annual variation is the annual rate of change of the coordinates of a star due to the combined effects of precession and proper motion. Secular variation is the rate of change per century of the annual precessional change in a star's coordinates. Magnetic variation is the angle between the directions of magnetic north and true north at any particular location on the Earth's surface; its value changes with time.

Varuna Large trans-neptunian object catalogued as asteroid number 20000. It was discovered late in the year 2000, although images of it on photographic plates dating back to 1955 have been identified. Varuna takes 285 years to orbit the Sun, in a low-eccentricity orbit with perihelion at 40.9 AU and aphelion at 45.7 AU. Varuna is particularly notable because its absolute magnitude (H=3.7) indicates that it may be among the largest known asteroids. If its albedo is as low as 0.05, then its equivalent diameter would be about 1100 km (680 mi), which is larger than ceres. If its albedo is 0.25, however, its diameter would be only 500 km (300 mi).

Vastitas Borealis Northern circumpolar plain of mars.

Vatican Advanced Technology Telescope (VATT) Research facility consisting of the 1.8-m (72-in.) Alice P. Lennon Telescope and the Thomas J. Bannan Astrophysics Facility located at the mount graham international observatory. It is operated by the Vatican Observatory Research Group, and observing time is shared with the university of arizona department of astronomy. The telescope has an unusually deep primary mirror whose focal length is equal to its diameter. It was made at the University of Arizona Mirror Laboratory to test new spin-casting and optical polishing techniques that have since been used for mirrors up to 8.4 m (330 in.) in diameter (in the large binocular telescope). The VATT saw first light in 1993.

Vega The star a Lyrae, the fifth-brightest in the sky, visual mag. 0.03 (although hipparcos found it to be delta scuti star - a variable with fluctuations of a few hundredths of a magnitude). It is a blue-white main-sequence star, spectral type A0 V and 50 times as luminous as the Sun, at a distance of 25 l.y. The Infrared Astronomical Satellite detected a disk of gas and dust surrounding Vega, possibly a protoplanetary disk. The star's name comes from of the word waqi', part of an Arabic phrase meaning 'swooping eagle'.

Vega 1 and 2 Two space probes launched by the Soviet Union in 1984 December. Seven months later, on their way to Halley's Comet, they released landers on to the surface and two balloons into the atmosphere of venus. The landers sent back data for just under an hour, while the balloons transmitted for 46.5 hours as they drifted around the planet. Their closest approaches to the comet took place on 1986 March 6 and 9 respectively.

Although the Vegas were used as pathfinders for the giotto spacecraft, they also obtained valuable images of the comet's nucleus and other data on its coma. The flyby distances were 8890 km (5520 mi) and 8030 km (4990 mi) on the sunward side of the nucleus. The probes carried identical payloads of two cameras, infrared spectrometers, gas and dust mass spectrometers, dust impact detectors and plasma analysers. The name Vega was derived from Venera and Galley (the Russian pronunciation of Halley).

Veil Nebula Faint supernova remnant in Cygnus; it is spread over an area about 3° wide, just south of e Cyg. Also known as the Cygnus Loop, the Veil Nebula has three principal sections, each with separate NGC designations: NGC 6960 at the west (RA 20h 45m.7 dec. + 30°43'); NGC 6974 and 6979 in the centre (RA 20h 50m.9 dec. +32°00'); and NGC 6992 and 6995 at the east (RA 20h 56m.8 dec. +31°28'). While quite prominent on long-exposure images, the filaments of the Veil Nebula are rather difficult to observe visually without

velocity of light See light, velocity of the aid of special OIII filters. The supernova that gave rise to the structure is thought to have occurred 30,000 to 40,000 years ago. The nebulosity is expanding at about 100 km/s (60 mi/s). It lies 2500 l.y. away.

Vela See feature article, page 427

Vela Pulsar Strong, short-period radio pulsar, discovered in 1968, and identified with a supernova remnant. It has a period of 89 milliseconds and in 1977 an optical counterpart was discovered that flashes with the same period. The Vela Pulsar occasionally increases its spin rate abruptly; this is called a glitch.

Vela Supernova Remnant supernova remnant (SNR) in Vela (RA 08h 34m dec. -45°45'). It is some 200 l.y. in diameter and about 1600 l.y. away. It is the result of the same supernova that produced the vela pulsar. From the spin-down rate of the latter, the explosion can be dated to about 1000 years ago. The Vela SNR is within the gum nebula. Adjacent to it in the sky, but actually four times more distant, is the Puppis SNR.

velocity Rate of change of position of a body in a specified direction. Velocity has both magnitude ('speed') and direction, and so is a vector quantity.

velocity-distance relation See hubble law

Vendelinus Eroded, ancient lunar crater (16°S 62°E), 165 km (103 mi) in diameter. The lava-flooded floor contains numerous elongated secondary craters from the ejecta of langrenus to the north. Vendelinus appears oblong due to foreshortening, but it is actually more circular in shape.

Venera Series of Soviet venus probes launched 1961-83. Venera 4 was the first spacecraft to send back data during descent through the Venusian atmosphere. The first successful landing on Venus was accomplished by Venera 7 in 1970. Venera 9, the first spacecraft to enter orbit around Venus, also made history by deploying a lander that returned the first picture from the rocky surface. Pictures were also returned from landers on Veneras 10, 13 and 14. The landers collected atmospheric data during descent and, on later missions, used a gamma-ray detector and instruments to detect electrical (lightning) discharges. Veneras 15 and 16, the last in the series, were orbiters equipped with radar to map the surface between 30°N and the north pole at a resolution of 1-2 km (0.6-1.2 mi).

Venus Second planet in the Solar System from the Sun and, apart from the Sun and the Moon, the brightest object in the sky. Venus is one of the terrestrial planets, similar in nature to the Earth but slightly smaller. Like the Earth it has a substantial atmosphere. Since its orbit lies inside that of the Earth, Venus never strays farther from the Sun than 47°. Accordingly, it can only be observed either in the east as a morning object or in the west as an evening star. As a result, like the Moon, it goes through a sequence of phases and at its brightest is actually a thin crescent close to inferior conjunction. Galileo in the 17th century helped to affirm Copernicus' heliocentric theory through his discovery of the phases of Venus.

Venus Centred on longitude 180?, this is a composite view of Venus prepared from Magellan Imaging Radar results in 1991.

All that can be seen optically on Venus is the upper deck of a uniform, unbroken layer of yellowish cloud, the top of which may be at an altitude of about 100 km (62 mi) above the planet's surface. The cloud reflects 76% of the incident sunlight, and Venus has the highest known planetary albedo (0.76). This contrasts with the Earth, which, on average, has only 50% cloud cover and an albedo of about 0.37. Venus' atmosphere is huge, more than 90 times more massive than that of the Earth and is composed primarily of carbon dioxide (Co2). Traces of hydrochloric and hydrofluoric acids, carbon monoxide, nitrogen, water vapour, hydrogen sulphide, carbonyl sulphide, sulphur dioxide (So2), argon, krypton and xenon have also been detected. Venus' atmosphere extends to an altitude of 250 km (155 mi) above the surface, but it possesses only a thermosphere and troposphere in its layered structure. on the day side of Venus, there is a terrestrial-type thermosphere with temperatures increasing from about 180 K at 100 km (62 mi) to about 300 K in the exosphere. The thermosphere does not exist on the night side of the planet, where the temperature falls from about 180 K at 100 km (62 mi) to 100 K at 150 km (93 mi). The transition from day-side to night-side temperatures across the terminator is very abrupt.

The atmosphere is extremely variable in the neighbourhood of the cloud tops in the altitude range of 75-100 km (47-62 mi); diurnal fluctuations of as much as 25 K have been observed at the 95 km (59 mi) level. Below these variable haze layers are the ubiquitous clouds, which occupy a substantial portion of the troposphere. There are three distinct cloud layers in the region of 45-60 km (28-37 mi), with differing particle sizes and concentrations. Sulphuric acid droplets make up the main composition of the clouds of Venus. The droplets are formed from the reaction between H2O and SO2 high in the atmosphere, aided by ultraviolet radiation from the Sun. The sulphuric acid clouds form a layer of varying concentration extending from 40-75 km (25-47 mi), with rain occurring in the lower layers.

The temperature increases steadily from the cloud tops at about 300 K to the surface at 750 K, where there is virtually no wind. Beneath the clouds and on the surface of Venus, the temperatures are the same everywhere. About 90% of the volume of the entire atmosphere lies between the surface and an altitude of 28 km (17 mi). At this level the atmosphere resembles a massive ocean, dense and sluggish in its response to the very weak solar heating. Only 2% of the incident sunlight actually reaches the surface of the planet. The surface pressure on Venus is 90 times greater than that on Earth and the surface temperature is the highest known in the Solar System. This crushing CO2 environment at the surface of Venus is the result of a runaway GREENHOUSE EFFECT. The basic make-up of the Earth and Venus is very similar, but there is now as much CO2 in the atmosphere of Venus as we find in the limestone rocks of the Earth. Because Venus is nearer to the Sun, it receives twice the sunlight than is incident on the Earth, and thus its surface has rapidly heated up by the greenhouse mechanism to its current state. This effect cannot be achieved by the CO2 alone. The small traces of H2O and SO2 are also essential for the efficient greenhouse effect. Venus' surface temperature will not increase further since the atmosphere and surface are in chemical equilibrium.

The weather systems of Venus, at the level of the cloud tops, are strange. Although the planet itself is rotating very slowly, the equatorial clouds have a rotation period of four days, indicating wind speeds of 100 m/s (330 ft/s). Consequently, the cloud tops are moving in a retrograde direction 60 times faster than the surface of the planet. Almost all the solar energy is absorbed in the cloud tops, and this provides the main driving mechanism for the super-rotation of the atmosphere. The cloud tops of Titan and the upper atmosphere of the Earth are the only other regions known to super-rotate.

The surface of Venus is surprisingly varied, which may suggest that the initial geological developments took place before the massive atmosphere evolved into its current state. Images from the Soviet VENERA landers show a stony desert landscape, with outcrops and patches of dark material suggesting some chemical erosion. The subsequent radioactive analyses of Venus' soil suggest the composition is similar to BASALT, but with an unusually high concentration of potassium. Some of the basaltic materials are similar to those found on the terrestrial seabed. About 70% of the surface is covered by huge rolling plains, 20% by depressional regions and the remaining 10% by highlands, which are concentrated in two main areas. A more detailed understanding of the nature and global distribution of the planet's landforms has resulted from studies of the improved-resolution MAGELLAN imagery. Features disclosed by this mission include aeolian dunes, wind-streaks, channels, lava flow lobes, impact craters and outflow material associated with impact ejecta.

Each of the major highlands or continent-like units (Terra) is separated from the others by low lying plains or basins (Planitia). The most extensive is Atalanta Planitia in the northern hemisphere. Atalanta Planitia is about 1.4 km (0.9 mi) below the mean planetary radius, and is about the size of the Gulf of Mexico. Rising from the plains are smaller uplands (Regio), some of which are joined by deep, elongated, steep-sided depressions (Chasma). There are some craters about 25-48 km (16-30 mi) in diameter, suggesting that impacts occurred before the atmosphere reached its current density. The highland areas include mountains (Mons or Montes), plateaux or high plains (Planum) and volcanic areas important in the evolution of the planet and its atmosphere. The major highlands are ISHTAR TERRA, in high northern latitudes, and APHRODITE TERRA, which spans the equator. A third, the most recently discovered, is Lada Terra, revealed by Magellan, which is located largely south of latitude 50°S. Lada Terra is bordered on the north by lowland plains, and isjoined to ALPHA REGIO, some 3500 km (2200 mi) to the north, by a complex of deep rifts. BETA REGIO, another highland area, contains two large shield volcanoes, Rhea Mons and Theia Mons. These tower some 4 km (2.5 mi) above the surface on a faultline that extends north-south. They are similar to the Hawaiian volcanoes and may well be active. Indeed, there has been a noticeable reduction in the measured amount of atmospheric SO2 during the past 10 years, which could be explained by past volcanic eruptions and the associated atmospheric adjustment.

Electrical activity beneath the clouds in the form of lightning has been inferred from measurements made during the descent of Venera 11 and 12. Again this could be caused by volcanic activity.

Venus does not have a magnetic field in spite of possessing a large nickel-iron core. This is due to the very slow rotation of the planet, which is unable to generate a field by dynamo action. However, a very weak magnetic field is induced in the planet's ionosphere through the interaction of the SOLAR WIND; the resulting BOW SHOCK region acts as a buffer between the interplanetary medium and the atmosphere of Venus.

Venus was once described as the twin of the Earth. The reality is otherwise. Our knowledge and understanding, derived from ground-based measurement and numerous spacecraft missions, involving flybys, orbiters, atmospheric probes and the Soviet/French floating balloons from the VEGA mission en route to the HALLEY comet encounter in 1986, reveals a hostile world, utterly different in atmospheric composition, meteorology and surface condition from its terrestrial counterparts.

vernal equinox (spring equinox) Moment at which the Sun appears to cross the CELESTIAL EQUATOR from south to north, on or near March 21 each year, at the FIRST POINT OF ARIES. At this time, the Sun is directly overhead at the equator and rises and sets due east and due west respectively on that day, the hours of daylight and darkness being equal in length. The name is also used as an alternative for the First Point of Aries, indicating the point on the celestial sphere where the ECLIPTIC and the celestial equator intersect at the Sun's ASCENDING NODE, and the zero point for measuring CELESTIAL LONGITUDE. See also AUTUMNAL EQUINOX; EQUINOX

Very Large Array (VLA) One of the world's premier radio observatories, located at an elevation of 2120 m (6970 ft) on the plains of San Augustin about 80 km (50 mi) west of Socorro, New Mexico, and completed in 1980. The VLA is operated by the NATIONAL RADIO ASTRONOMY OBSERVATORY, a facility of the National Science Foundation. The telescope consists of 27 antennae arranged in a huge 'Y' shape with a maximum extent of 36 km (22 mi). Each antenna has an aperture of 25 m (82 ft), weighs 230 tonnes and can be moved along a track to adjust the spacing of the array. The signals are combined electronically, and four different configurations are used - from compact to far-flung. At its highest resolution (with the antennae at their maximum spacing), the VLA has the same resolution as a single dish 36 km (22 mi) across. This corresponds to 0".04 at the maximum operating frequency of 43 GHz. The instrument has the sensitivity of a single telescope 130m (427 ft) in diameter.

Very Large Telescope (VLT) Largest optical telescope in the world, consisting of four 8.2-m (27-ft) reflectors located at PARANAL OBSERVATORY and operated by the EUROPEAN SOUTHERN OBSERVATORY (ESO). The four separate instruments, known as Unit Telescopes, are named Antu (completed in 1998), Kueyen (1999), Melipal (2000) and Yepun (2001). The names are those of the Sun, the Moon, the Southern Cross, and Venus, respectively, in the local Maopuche language. They can be operated either independently or together; in combination they have an effective aperture of 16.4 m (54 ft). When operated in a mode called VLTI, they are used with three 1.8-m (72-in) outrigger telescopes to carry out optical INTERFEROMETRY. The telescopes of the VLT have mirrors made of a single piece of a glass-ceramic material as opposed to the segmented mirrors at the W.M. KECK OBSERVATORY. Unique among the new instruments (not least because it was built in a non-ESO nation) is the OzPoz robotic spectrograph feed supplied by the ANGLO-AUSTRALIAN OBSERVATORY in 2002. ESO is also investing in multi-conjugate ADAPTIVE OPTICS for the VLT, partly to demonstrate the viability of the OVERWHELMINGLY LARGE TELESCOPE.

Very Large Telescope Shown here are the four lined 8m (312 in.) instruments of the Very Large Telescope (VLT) at the European Southern Observatory, Cerro Paranal in the Atacama Desert. Working together as an interferometer, these instruments will deliver resolution equivalent to that of a single 16 m (624 in.) telescope.

Very Long Baseline Array (VLBA) Series of ten radio antennae spread across the United States and its overseas territories, from St Croix in the Virgin Islands to MAUNA KEA OBSERVATORY, Hawaii. Their signals are recorded separately on tape and later combined at the Operations Centre in Socorro, New Mexico. Each antenna is 25 m (82 ft) in diameter, and the maximum separation of the array is 8600 km (5400 mi), giving it an angular resolution of 0".001. Rather than supplanting the VERY LARGE ARRAY, it is complementary to it, providing astronomers with a finer scale of resolution. Construction of the VLBA began in 1986, and the first observation with the complete array was made in 1993. It is operated by the NATIONAL RADIO ASTRONOMY OBSERVATORY with funding from the National Science Foundation.

very long baseline interferometry (VLBI) Combining of RADIO TELESCOPES separated by thousands of kilometres to act as a RADIO INTERFEROMETER in order to observe the same astronomical object with a resolving power of 0".0002. The radio telescopes for VLBI are located around the world, many in the United States, Europe and Australia, plus several in other countries. Several radio telescopes in Europe can also combine together to form the European VLBI Network (EVN). VLBI can study structure in quasar jets, and it showed the superluminal motions in the jet of the quasar 3C 273 and the active galaxy M87. Knowledge and understanding of radio sources has been revolutionized by VLBI. Intercontinental VLBI has also used measurements of a known quasar to track the positions of the telescopes, monitoring the steady drift of the continents and the wandering of the poles.

Very Small Array (VSA) Ground-based instrument designed to map the cosmic microwave background radiation on angular scales of about 1°. It has 14 antennae with apertures of 0.32 m (13 in.) on a single tip-table mounting located at an elevation of 2385 m (7820 ft) at the Observatorio del Teide of the INSTITUTO DE ASTROFISICA DE CANARIAS (IAC) in Tenerife. The project is a collaboration between the MULLARD RADIO ASTRONOMY OBSERVATORY, JODRELL BANK OBSERVATORY and the IAC.

Vesta Large MAIN-BELT ASTEROID; number 4. Its size, albedo and location, near the inner edge of the main belt, mean that Vesta is the brightest of all such asteroids and may just attain naked-eye visibility. Images obtained with the HUBBLE SPACE TELESCOPE show Vesta to be roughly spherical, with a very large depression around its southern pole, which may be a vast impact crater. Like PALLAS, Vesta has a high average density (c.3.9 g/cm3), which may be due to a metallic interior similar to nickel-iron meteorites, despite its surface reflectivity indicating a stony composition. Its mass is about 3.0 X 1020 kg (0.41% the mass of the Moon). Several smaller asteroids have similar orbits to Vesta (see HIRAYAMA FAMILY) and this group, formed from the fragmentation of an earlier body, is thought to be the source of the METEORITES known as eucrites, howardites and diogenites.

video astronomy Use of video cameras for astronomical imaging. The large image scales produced by a CCD offer dramatic detailed views of the Sun, Moon, planets and brighter planetary satellites. Unlike the long integration times possible with cooled CCD imagers, conventional video cameras are limited to short exposure times comprised of two consecutive fields combined to produce a stream of continuous pictures at 25 or 30 frames per second. This fast frame ability makes video an excellent tool for accurately timing occultation events. The fast exposure time of continuous video also provides an effective way to capture an unpredictable moment of steady seeing. A standard 3-hour videotape can capture over 270,000 individual images, and frame-by-frame playback makes for easy identification of the best images recorded, which can be selected and combined by IMAGE PROCESSING.

Throughout the 1990s, developments in highly light sensitive CCD image sensors have paved the way for convenient astronomical imaging with video cameras. In 1998 a video camera was used on the historic 60-inch (1.5-m) telescope at Mount Wilson by a Boston University team imaging notoriously difficult Mercury. They obtained unprecedented ground-based images of a hemisphere not previously imaged during the Mariner 10 flyby.

Video cameras are also widely used to record fireball and meteor showers. In 1999 members of the INTERNATIONAL OCCULTATION TIMING ASSOCIATION captured video evidence of meteoric impact events on the Moon during the November Leonid meteor shower. To study the radiant of an extremely faint meteor shower, a wide-angle lens is coupled to the input window of an image intensifier, and a video camera then records the image at the output window of the intensifier. Image intensifiers can be used for real-time viewing and recording of many deep-sky objects.

To produce useful images, detail is important and the number of TV lines a camera produces governs the resolution of the video image. Many amateurs use modified low-light security cameras that output between 400 and 600 lines. The output signal can be recorded directly to videotape or a computer using a video capture interface. Analogue S-VHS video machines are capable of recording up to 400 lines, while digital recorders and computer capture devices are capable of more. To preserve spatial resolution sacrificed in most single-chip colour cameras and maintain optimal signal-to-noise ratios, monochrome cameras produce the best results when used for tri-colour imaging of planets.

As with conventional cooled CCD cameras, video images transferred to computer can be further processed using image-processing software. Since a video camera is an uncooled integrating device, individual frames suffer from some amount of thermal dark noise generated by the camera's electronics. Stacking or layering several selected individual images can produce a smoother, more aesthetically pleasing final picture with improved depth of detail.

vignetting Partial shadowing of an optical image caused by obstructions within the light beam or by optical components too small to pass all the beam. The effect of vignetting is to make some parts of the image dimmer than they should be, but it is not always detected by the user. It is often present in cheap binoculars and can be detected by looking at the bright spot visible in the eyepiece when viewed from a distance. This spot should be circular and evenly illuminated. If it is not, then some vignetting, usually caused by the prisms, is present. The effect is sometimes present when taking photographs through a telescope. It may not be noticed on a photograph of the sky but a terrestrial shot will often be brighter in the middle and darker at the corners.

Viking Mission to Mars consisting of two orbiter-lander spacecraft. Viking 1 was launched by a Titan-Centaur booster on 1975 September 9, and injected into Mars orbit on 1976 June 19. The first month of orbiter operations consisted of locating and verifying a safe landing site for the lander spacecraft. The lander spacecraft separated from the orbiter and landed on the surface on 1976 July 20. Viking 2 was launched on 1975 August 20, injected into Mars orbit on 1976 August 7, and its lander spacecraft touched down on 1976 September 3.

Viking Compiled from images obtained by the Viking orbiters, this composite shows the Amphitrites Patera region of Mars. Radial ridges extend northwards for about 400 km (250 mi) from this old volcano.

Because Mars is the most hospitable of the planets, and because of the long history of debate regarding the possibility of life on Mars (see life in the universe; canals, martian), the landers carried experiments to search for life. All the experiments operated successfully, and no indications of life were found at either landing site (though some have criticized the design of the experiments, claiming the outcome was ambiguous), but this does not preclude life at other locations. Meteorological instruments on the landers regularly reported the weather and monitored surface changes and seasonal effects.

Each orbiter carried two vidicon cameras and each lander carried two facsimile cameras. The orbiters mapped the entire surface of Mars at 150-300 m (500-1000 ft) resolution and selected areas at resolutions down to 8 m (26 ft). In all, they returned more than 55,000 images, including high-resolution pictures of the satellites Phobos and Deimos.

Virgo See feature article

Virgo cluster Rich concentration of galaxies in the direction of the constellation Virgo, although some lie across the border in Coma Berenices. The centre of the

VIRGO (gen. virginis, abbr. vir)

Second-largest constellation and one of the signs of the zodiac, which has been associated with a succession of female deities since Babylonian times. It lies between Leo and Libra and is not particularly prominent, except for its brightest star, spica (or Azimech), mag. 1.0. Porrima is a fine binary with yellow components, both mag. 3.6, separation 2".7, period 168.7 years. There are no bright star clusters or nebulae in Virgo, but there are numerous galaxies, many of which are members of the virgo cluster. These include the 8th-magnitude M49 (NGC 4472) and the 9th-magnitude M87 (NGC 4486), both giant elliptical galaxies (the latter containing the radio source Virgo A); and the sombrero galaxy (M104, NGC 4594), which is not a member of the cluster. The constellation also contains 3c 273, the first quasar to have been discovered (in 1963), mag. 12.9. cluster is about 65 million l.y. away, and is marked by the giant elliptical galaxy M87, also known as the radio source Virgo A, which is ejecting a strong jet of gas. In all there are 16 Messier objects in the cluster; the total number of galaxies in the cluster is about 3000.

virial theorem Theorem concerning the conservation of energy equations that relates the kinetic energy of gas or stars to the gravitational potential energy:

E = PG + KE

where E is the total energy, PG is the gravitational potential energy, and KE is the kinetic energy or energy of motion. It is useful in determining the mass of a system based on its gravitational field and the orbital velocities of stars or gas clouds orbiting around it.

Visible-Infrared Survey Telescope for Astronomy 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'slaws). 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

VOLANS (gen. volantis, abbr. vol) discovery of the first spectroscopic binaries. Using the 'reversion spectroscope' invented by Friedrich zollner, 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.

volcanism The ASTER instrument aboard NASA’s Terra satellite obtained this image of Mount Etna in eruption during 2001 July. Lava flows (red and yellow) can be seen advancing on the volcano’s southern flank.

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 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 Generator (RTG) on Voyager 2, which it was hoped might rendezvous with Uranus and Neptune as well as Jupiter and Saturn. The launch weight of the vehicle and propulsion rocket was in each case 2016 kg, of which the Voyager itself accounted for 792 kg. Ten instruments were carried on each spacecraft.

Voyager The two Voyager spacecraft, now heading into interstellar space, each carry a copy of a gold-plated record of ‘Sounds of Earth’. The records are protected from micrometeorite bombardments by a gold aluminium case, engraved with information showing Earth’s location.

Voyager 2 was the first to be launched from Cape Canaveral, on 1977 August 20. Voyager 1 followed on September 5. However, Voyager 1 was travelling on a faster, shorter trajectory and overtook its twin during the crossing of the ASTEROID BELT. On 1979 March 5 Voyager 1 passed Jupiter at a distance of 350,000 km (217,500 mi) and sent back the best images ever obtained of the planet and its moons. Its discoveries included volcanic eruptions on IO, linear features on EUROPA and the existence of a dark, dusty ring.

Voyager 1 then went on to a rendezvous with SATURN on 1980 November 12, at a minimum distance of 124,200 km (77,000 mi). Once again, it sent back high-resolution images of the planet and the satellites TITAN, RHEA, DIONE and MIMAS. The complex nature of the rings was fully revealed, and new data were obtained about the magnetic field, radiation belts and other phenomena. Voyager 1's encounter with Titan bent the spacecraft's path northward, so that it continued on an orbit out of the ECLIPTIC plane at an angle of 35°. Had Titan not been satisfactorily imaged, then Voyager 2 would have been targeted to carry out a survey, so losing the opportunity to rendezvous with either of the outer giant planets.

Voyager 2 passed Jupiter on 1979 July 9 at a distance of 645,000 km (400,000 mi). The results fully complemented those of its predecessor, and showed marked changes in the Jovian atmosphere and the volcanic activity on Io. A small satellite, now known as ADRASTEA, was found close to the rings. Voyager 2 then went on to Saturn, passing the planet on 1981 August 25 at a distance of 101,300 km (63,000 mi). Quite detailed images of the satellites IAPETUS, HYPERION, TETHYS and ENCELADUS were obtained. However, after closest approach to Saturn, the scan platform that carried the camera apparently jammed, so some vital information was lost. The problem solved itself, but subsequently the platform was manoeuvred only at reduced rate.

VULPECULA (gen. vulpeculae, abbr. vul) Small, inconspicuous northern constellation, representing a fox, south of Cygnus. It was named Vulpecula cum Ansere (the Fox and Goose) by Johannes Hevelius in 1687. Its brightest star, a Vul, is mag. 4.4. The brightest deep-sky objects are the COATHANGER (Cr 399, also known as Brocchi's Cluster), an open cluster of about a dozen stars between mags. 6 and 8, and the DUMBBELL NEBULA (M27, NGC 6853), an 8th-magnitude planetary nebula.

After the gravity assist at Saturn, Voyager 2 was directed towards URANUS. The closest approach took place on 1986 January 24, at a distance of 107,000 km (67,000 mi). This part of the mission was particularly important, because comparatively little had been known about Uranus. Many discoveries were made, including ten new moons (including a number of small 'shepherd' satellites near the rings), the weird terrain of MIRANDA and the planet's remarkable, tilted magnetic field.

Voyager 2's 'Grand Tour' was completed with the flyby of NEPTUNE and its large moon TRITON on 1989 August 24. As the spacecraft passed within 4900 km (3000 mi) of Neptune's cloud tops, Voyager 2 obtained remarkable images of giant storm systems on the planet. It also found six new moons and evidence of geyser activity on Triton's icy surface.

Voyager 2's trajectory was radically altered during the Neptune encounter, sending it below the ecliptic plane at an angle of 48°. Both Voyagers are now well beyond the orbit of Pluto and heading out of the Solar System in opposite directions. During this extended Voyager Interstellar Mission, it is hoped that they will determine the location of the heliopause, the boundary of the HELIOSPHERE. With the remote chance that the Voyagers will one day be picked up by some alien race in another solar system, each probe carries a 31-cm (12-in.) gold-plated record containing 'Sounds of Earth', together with information upon how to play it with the cartridge and needle provided.

Vulcan Planet once thought to circle the Sun inside the orbit of Mercury. It was invoked by Urbain LE VERRIER in 1859 to explain the faster than predicted advance of the heliocentric longitude of the perihelion of Mercury. To Le Verrier this implied either the existence of a body roughly the size of Mercury but at half its distance from the Sun, or a similarly placed ring of smaller bodies. Following his announcement in September of that year, Le Verrier had news from a French amateur astronomer, Edmond Lescarbault of Orgres, who reported that in March he had seen an unidentified dark object transit a segment of the Sun. Le Verrier hurried to Orgres and satisfied himself the claim was authentic. He named the object Vulcan and calculated its orbit. More sightings were reported but all turned out to be cases of mistaken identity. Interest revived in 1878, following the observation of mysterious objects near the Sun at the total eclipse of July 29. But the claim degenerated into confusion and controversy, and it was ultimately discarded. There were other false alarms in 1882 and 1900, but in 1915 after a radical reappraisal, which altered our understanding of Newtonian gravitational principles, Albert EINSTEIN neatly resolved the problem with his general theory of relativity.

While Le Verrier's Vulcan has slipped from currency, questions remain. For instance theoreticians have revived the idea of an inner ring of asteroids, or 'Vulcanoids', and have defined a number of possible orbits. The discovery by SOHO of numerous small comets close to the Sun gives credence to the idea. With a better appreciation of Earth-crossing asteroids and interplanetary objects in general, it is now easier to understand why these 19th-century astronomers believed they had observed Vulcan.

Vulpecula See feature article

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