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A site on a lattice on which there is no atom present. [D89]


(a) A space containing gas below atmospheric pressure. A perfect vacuum contains no matter at all, but for practical purposes soft (low) vacuum is usually defined as down to about 10-2 pascal, and hard (high) vacuum as below this. Ultra-high vacuum is lower than 10-7 pascal. [DC99]
(b) Roughly speaking the vacuum is a space devoid of matter, but this definition suffers from the ambiguity of the word "matter." Particle physicists therefore define the vacuum as the state of lowest possible energy density. The vacuum is not simple, since the inherently probabilistic nature of quantum theory implies that unpredictable events, such as the chance materialization of an electron and its antiparticle, the positron, can occur at any time. Such pairs have a fleeting existence of perhaps 10-21 seconds, and then annihilate into nothingness. In addition, particle theories suggest that there are Higgs fields which have nonzero values in the vacuum. [G97]
(c) The state of minimum energy (or ground state) of a quantum theory. It is the quantum state in which no real particles are present. However, because of Heisenberg's uncertainty principle, the vacuum is actually seething with virtual particles which constantly materialize, propagate a short distance and then disappear. [CD99]
(d)Any physical system will settle into the lowest-energy sta te it can, which in particle physics we call its vacuum state. For most systems, this is the state where the fields making up the system are zero, but theorists hypothesize that for systems containing Higgs fields, the lowest energy occurs when the Higgs field takes on a constant value different from zero. The value of the Higgs field in that system is called its vacuum expectation value. [K2000]

Vacuum Expectation Value

The value of the Higgs field (a constant value different from zero) is called a system's Vacuum Expectation Value. [K2000]

Vacuum Fluctuation

An unpredictable event occurring in the vacuum as a result of the inherently probabilistic nature of quantum theory. Particles can materialize in conjunction with their antiparticles, and fields can undergo fluctuations in their values. (see Vacuum) [G97]

Vacuum Genesis

Hypothesis that the universe began as nothingness, from which matter and energy arose by a process analogous to the appearance of virtual particles from a vacuum. [F88]

Vacuum Higgs Value

The value that the Higgs field(s) have in the vacuum. (see Higgs Fields) [G97]


Also valence band or valence electrons; the electrons in the outermost orbit. [McL97]

Valence Band

The highest completely filled energy band of a solid. In an insulator or semiconductor empty states in the valence band can carry an electric current as positively charged `holes'. [D89]

Valence Electron

In an atom, an electron in an incompletely filled (usually outer) shell, available for chemical bonding to form a molecule. [DC99]

Van Allen (Radiation) Belts

(a) Two doughnut-shaped belts in the Earth's magnetosphere (inner belt some 3000 km above the surface; outer belt, 18,000-20,000 km above the surface), where many energetic charged particles from the solar wind are trapped in Earth's magnetic field. The energy of the particles is highest in the inner belt. [H76]
(b) Two toroidal zones of high radiation in Earth's upper atmosphere, above the equator, caused by the trapping of charged particles in the magnetosphere. The outer zone is composed chiefly of electrons, the inner of protons. [A84]

van Biesbroeck's Star

A very faint (Mv = 18.6; Mbol = 13.12), nearby (parallax 0".168, about 8 pc distant) dM5e star of very low mass (0.07 Msmsun). Temperature about 2250 K. (Gliese 752b, BD+4°4048B) [H76]

van der Waals Equation

An equation of state for real gases. For one mole of gas the equation is

(p + a / Vm2) (Vm - b) = RT ,

where p is the pressure, Vm the molar volume, and T the thermodynamic temperature. a and b are constants for a given substance and R is the gas constant. The equation gives a better description of the behavior of real gases than the perfect gas equation (pVm = RT).
The equation contains two corrections: b is a correction for the non-negligible size of the molecules; a/Vm2 corrects for the fact that there are attractive forces between the molecules, thus slightly reducing the pressure from ideal. [DC99]

van der Waals Forces

(a) Attractive forces existing between molecules. These forces are the ones giving the pressure correction in the van der Waals' equation. They are much weaker than chemical bonds and act over short range (inversely proportional to the seventh power of distance). They are caused by attraction between dipoles of molecules. For atoms or molecules without permanent molecular dipole moments, the attractive forces result from attractions between nucleus-electron dipoles (called dispersion forces). [DC99]
(b) The relatively weak attractive forces operative between neutral atoms and molecules. [H76]

van Maanen's Star

A white dwarf 4 pc distant; density 4 × 105 g cm-3. [H76]


A silvery transition element used in alloy steels.
Symbol: V; m.p. 1890°C; b.p. 3380°C; r.d. 6.1 (20°C); p.n. 23; r.a.m. 50.94. [DC99]

Vapor Pressure

The pressure exerted at a particular temperature by a vapor. When a liquid or solid evaporates, molecules are continuously escaping from the surface at a rate that increases rapidly with temperature; they exert a vapor pressure. Those striking the surface tend to re-enter the liquid or solid, so that eventually a state of dynamic equilibrium is reached in which the number of molecules returning to the surface per second is the same as the number leaving it. The vapor now exerts its equilibrium pressure for that temperature, the saturated vapor pressure (SVP). The value of this depends only on the temperature, and is independent of the volume (unless all the vapor condenses or all the liquid evaporates). [DC99]

Variable-Mass Theory

A theory of Hoyle and Narlikar in which the masses of fundamental particles are assumed to vary with time in a manner that precisely accounts for the Hubble redshift law. [Silk90]

Variable Star

(a) A star whose luminosity changes over periods of time; there are many reasons and many types. Periods vary widely in length and even regularity. Novae and supernovae are classed as variables. The present brightest variable star is Betelgeuse (Alpha Orionis). [A84]
(b) A star whose light varies. Some variables vary simply because they consist of two stars, one of which eclipses the other; Algol is the most famous example. Other variables, however, vary because the stars themselves actually change in brightness; the most famous are the Cepheids, RR Lyraes, and Miras, all of which pulsate. [C95]
(c) A star that varies in luminosity. The first variable discovered in a given constellation has the letter R preceding the name of the constellation. Then S, . . . , Z. Then RR, RS, . . . , Rz, SS, . . . , Sz, . . . , ZZ. Then AA, . . . , AZ (the letter J is never used), BB, . . . , BZ, . . . , QQ, . . . QZ. The next variable (the 335th) is given the designation V335. [H76]


Very Broad Line Region


A quantity which has both magnitude and direction, such as the spin of a magnetic atom in the Heisenberg model. [D89]

Vector Boson

Force-carrying particles of nature. Three vector bosons are responsible for the weak nuclear force. By admitting the photon on an equal footing it is possible to create a unified electroweak theory. As a result of symmetry-breaking processes, however, this photon remains massless while the three other vector bosons pick up mass. [P88]

Vector Meson

Also called the intermediate vector boson. [H76]

Vector Space

A set of elements (called Vectors) for which a binary operation of vector addition is defined, such that u1 + u2 is a vector if u1 and u2 are vectors: and a binary operation scalar multiplication is defined, such that cu is a vector if u is a vector and c is a scalar (a real number or a complex number, according to specification of the kind of vector space); and a standard collection of conditions governing these two operations is satisfied. [D89]

Vector Translation

The small theoretical precession of the axis of an orbiting body due to the gravitational influence of its primary. This effect is predicted by general relativity, but so far it has not been observed. [H76]


The brightest star in the constellation Lyra and the fifth brightest star in the night sky. Vega is a white A-type main sequence star 25 light-years away. [C95]

Veil Nebula

see Cygnus Loop. [H76]

Vela Pulsar

A pulsar about 400-500 pc distant, probably associated with the Vela supernova remnant. Period 0.0892 seconds. (PSR 0833-45) [H76]

Vela Satellites

A sequence of satellites launched to monitor possible violations of the nuclear test ban treaties. The system consists of four satellites in a circular orbit around the Earth with a radius of 120,000 km. The Vela satellites have detected cosmic gamma-ray bursts (q.v.). [H76]

Vela Supernova Remnant

A gaseous nebula in the middle of the Gum Nebula, the remnant of a Type II supernova whose light reached Earth about 10,000-30,000 years ago. It consists of bright filaments that form a D-shaped ring in Halpha and a rough circle in the ultraviolet. It includes the Vela X, Y, and Z radio complexes and is a strong X-ray source. [H76]

Vela X

A compact radio source about 400-500 pc distant associated with the Vela supernova remnant. It has a nonthermal radio spectrum and is about 20 percent polarized. It is associated with the Gum Nebula, the Vela pulsar, and the X-ray source 2U 0832-45, although the pulsar and the X-ray source are displaced about 0°.7 from the center of the Vela X radio emission. Vela Y and Vela Z are outlying components, also nonthermal, but too weak to exhibit polarization. [H76]

Vela X-1

An eclipsing X-ray source identified with the seventh-magnitude single-lined spectroscopic binary HD 77581 (B0.5 Ib) with a period of 8.96 days. Estimated mass of unseen companion 1.7-15 Msmsun, with a probable value of about 2.6 Msmsun. (3U 0900-40) [H76]


The speed and the direction of an object's motion. [G99]

Velocity Dispersion

The spread of a velocity distribution - that is, how stars move relative to one another. Technically, the velocity dispersion is the standard deviation of the velocity distribution. Stars with similar velocities have a small velocity dispersion, whereas stars with wildly different velocities have a large velocity dispersion. [C95]

Velocity-Distance Relation see Hubble's Law. [H76]
Velocity Field

The velocities of a group of objects with different velocities at different positions of space. [LB90]

Velocity-of-Light Radius The radius of a rotating neutron star at which the rotational velocity of the plasma approaches the velocity of light. (also called velocity-of-light cylinder) [H76]
Velocity Profiles

In radio astronomy, the output response for all filters for a given position of the beam on the source. [H76]

Velocity Space

The subspace of phase space whose coordinates are the velocities in each of the three directions of ordinary space. [H76]

AI Velorum Stars

A class of dwarf Cepheids. They are all RR Lyrae stars with periods shorter than 0.25 days. [H76]

gamma2 Velorum

A triple system (WC8, B1 IV, O9 I) embedded in the Gum Nebula, probably about 400 pc distant. Period 78.5 days. It is the brightest Wolf-Rayet star in the sky (Mv = - 5.6). [H76]

Veneziano Theory

A formula that accounted for the experimental results of the dual resonance model. Nambu discovered that a string theory would reproduce the results of the Veneziano approach. [P88]


Second planet from the Sun. Mass 4.872 × 1027 g; radius of solid surface 6056 km; radius of cloud surface 6100 km. Mean density 5.16 g cm-3. Vesc 10.3 km s-1; surface gravity 8 m s2. Surface temperature (from Venera 8) 743 ± 8 K: temperature of cloud tops about 250 K. Mean distance from Sun 0.7233 AU; orbital period 224.7 days (synodic period 583.9 days); e = 0.0068, i = 3°.39. Rotation period 243.09 ± 0.5 days retrograde (Mariner 10 has established that the cloud tops rotate every 4 hours retrograde). Obliquity 3° R. Orbital velocity 35 km s-1. Radar experiments have established that the surface is somewhat smoother than the Moon, but there are mountains and there is extensive cratering. Atmospheric pressure 92-95 atm. Atmosphere (by volume 1972) 90-95% CO2, remainder primarily N2, traces of water vapor, oxygen, HF, HC1. Maximum elongation 48°. Last transit of Sun was in 1882; next one will be 2004. Venus's rotation period is in synchronism with Earth - that is, at inferior conjunction the same side is always toward the Earth. Albedo 0.76. [H76]

Vernal Equinox

(a) The spring equinox, on or around 21 March. [A84]
(b) The ascending node of the ecliptic on the celestial equator; also the time at which the apparent longitude (see Apparent Place; Longitude, Celestial) of the Sun is 0°. (see Equinox) [S92]
(c) The point of intersection between the ecliptic and the celestial equator, where the Sun crosses from south to north. It is sometimes called the First Point of Aries because several thousand years ago it was in Aries. Because of precession it has now slid west into Pisces and in 200-300 years it will edge into Aquarius. By definition, the vernal equinox is at alpha = 0°, delta = 0°. [H76]


see Radiant [H76]


Apparent direction of gravity at the point of observation (normal to the plane of a free level surface.) [S92]

Very Large Array

VLA A radio telescope scheduled to be built near Socorro, New Mexico, which will consist of 27 antennas, each 82 feet in diameter, distributed along three 13-mile-long arms of a Y-shaped track. According to the NSF, the array will give radio astronomers as much resolution as the 200-inch gives optical astronomers. [H76]

Very Long Baseline Interferometry

VLBI In radio astronomy, a system of two or more antennas placed several hundred or several thousand miles apart, which are operated together as an interferometer. [H76]


An asteroid 500 km in diameter (P = 1325 days, a = 2.361 AU, e = 0.09, i = 7°.1). It is the brightest of all minor planets, at times approaching naked-eye visibility (mag 5.5). Rotation period 5h20m31s.665. (Its spectrum can also be interpreted to mean a rotation period of 10h40m58s.84.) Albedo 0.24. Discovered by Olbers in 1807. [H76]

Vibration Number

Whole number describing the energy in the uniform vibrational motion of a string; the energy in its overall motion as opposed to that associated with changes in its shape. [G99]

Vibrational Energy

Motion of the pair of nuclei in a diatomic molecule along the direction of the internuclear axis (cf. rotational energy). [H76]

Vibrational Pattern

The precise number of peaks and troughs as well as their amplitude as a string oscillates. [G99]

Vibrational Transition

A slight change in the energy level of a molecule due to its vibration. If the possibility of the rotation of the molecule as a whole is disregarded, then one gets from each electronic level a sequence of vibrational levels corresponding to various degrees of vibration of the nuclei around their equilibrium position. These are distinguished by the vibrational quantum number v. [H76]


General name for the class of vacuum tube imaging devices which employ a scanning electron beam to read out the image. [McL97]


A systematic error in the measurement of stellar magnitudes when the object being measured is far off axis. [H76]

Viking Space Probes

Series of 2 US spaceprobes that successfully effected landings on Mars and relayed data back to Earth. [A84]

Violent Galaxy

A type of galaxy differentiated only recently. Violent galaxies include QSOs and exploding galaxies like M82. About 1 percent of the galaxies are classified as violent. Violent galaxies release on the average 1058 ergs of energy, compared with a supernova release of 1049 ergs. Nearest violent galaxy is Cen A. [H76]

alpha Virginis

Spica. [H76]

W Virginis Stars

Population II Cepheids. [H76]

Virgo A

A strong radio source. Optically, it is an elliptical galaxy (M87) with a luminous blue jet about 1500 pc long. It is also an X-ray source (3C 274, Virgo X-1, 2U 1228+12). [H76]

Virgo Cluster

An irregular cluster of about 2500 galaxies (z = 0.004), including the giant elliptical M87 (the galaxy of greatest known mass). [H76]

Virgo Infall

The observed gravitational motion of nearby galaxies toward the Virgo cluster of galaxies, about 50 million light years away. The Virgo cluster represents a strong concentration of mass, a strong departure from a uniform distribution of matter, and it therefore causes galaxies in its vicinity to deviate from the Hubble flow. [LB90]

Virgo Supercluster

An aggregation of galaxies - roughly ten thousand of them - to which the Virgo Cluster and our own galaxy belong. [F88]

Virgo X-1

An X-ray source identical to Virgo A. It is also one of the most powerful extragalactic sources of radiation at infrared wavelengths. (2U 1228+12) [H76]

Virial Theorem

For a bound gravitational system the long-term average of the kinetic energy is one-half of the potential energy. [H76]

Virial Mass

The mass of a cluster of stars or galaxies in statistical equilibrium derived by using the virial theorem that the mean square velocity of all the stars or galaxies in a cluster is proportional to the mass of the cluster divided by its radius. [H76]

Virtual Interaction

Quantum uncertainties in energy make it possible for virtual particles to be constantly created and annihilated during elementary particle interactions. Elementary particles are able to make use of these virtual particles within their interactions. [P88]

Virtual Pairs

Particles and antiparticles that exist for an extremely short time, often as the intermediate stage of a nuclear transition. According to Dirac's theory, the vacuum can be visualized as consisting of a sea of virtual electron-positron pairs that can only be released or separated when sufficient energy is made available. [Silk90]

Virtual Particle

(a) A quantum particle that exists only temporarily, for example while being exchanged between other particles. Because of Heisenberg's uncertainty relation a virtual particle need not satisfy the usual relationship between energy, momentum and mass. [D89]
(b) A particle that exists for an extremely short time in an intermediate stage of a reaction or transition. [H76]
(c) Particles which take part in virtual processes. They are said to be "off mass-shell", meaning that the relation E2 = p2c2 + m02 c4 does not hold. [CD99]
(d) Particles that erupt from the vacuum momentarily; they exist on borrowed energy, consistent with the uncertainty principle, and rapidly annihilate, thereby repaying the energy loan. [G99]
(e) An elementary particle postulated to exist in interactions. For example, it is possible to explain the electromagnetic interaction between two charged particles by assuming that they are exchanging virtual photons. [DC99]

Virtual Particle Theory

Theory devised by Stephen Hawking to account for apparent thermal radiation from a black hole (from which not even light can escape). It supposes that space is full of "virtual particles" in a particle and antiparticle relationship, being created out of "nothing" and instantly destroying each other. At an event horizon, however, one particle may be gravitationally drawn into the singularity, and other appear to radiate as heat. [A84]

Virtual Phase

A type of CCD in which only one electrode is physically outside the silicon and is such as to obscure only half of the pixel. A specially doped layer under the transparent part acts as another or virtual electrode. [McL97]

Virtual Processes

Quantum-mechanical processes which do not conserve energy and momentum over microscopic timescales, in accordance with Heisenberg's uncertainty principle. These processes cannot be observed. [CD99]


The internal friction of a fluid or liquid that tends to resist and dissipate its flow. [Silk90]

Visibility Function

The Fourier transformation of a distant radio source, normalized to its value at small antenna spacings. [H76]

Visual Binary Star

see Binary System.

Visual Magnitude

The magnitude determined with the eye. [H76]

Vis Viva Equation

An equation governing the conservation of angular momentum. [H76]


Very Large Array A network of 27 radio telescopes in New Mexico, USA. [McL97]

Vlaslov Equation

A collisionless Boltzmann equation which describes stars moving in regular orbits in an averaged self-contained gravitational field. [H76]

Vlasov-Maxwell Equations

Equations that describe the propagation of radiation in hot, collisionless plasmas. [H76]


Very Long Baseline Interferometry [LLM96]


Very Large Telescope [LLM96]


Very Large Telescope Interferometer [LLM96]

Vogt-Russell Theorem

If the pressure, the opacity, and the energy generation rate are functions of the local values of density, temperature and the chemical composition only, then the structure of a star is uniquely determined by the mass and the chemical composition. (When isothermal cores occur in the interiors of stars, then multiple-valued solutions become possible.) [H76]


Large regions of space without galaxies. [LB90]

Voigt Profile

Profile of a spectral line allowing for the effects of Doppler broadening combined with a Lorentz (damping) profile. [H76]

von Zeipel's Theorem

The surface brightness of a rotating star or a component of a binary at any point on its surface is proportional to the local value of gravity. [H76]


In a planar spin model a vortex is a pattern of spins in which the spin direction rotates by 360 degrees along any path which surrounds the centre of the vortex. The name is taken from hydrodynamics, where it denotes the kind of circular flow pattern that can be observed in water flowing out of a bathtub. The vortices in superfluid helium films are of this sort. [D89]

Vortex Theory

The cosmogony of the solar system by Descartes (1644), who argued that the planets and sun accumulated from matter that moved in a system of vortices extending over all scales. [Silk90]


(a) Pair of unmanned American spacecraft launched in 1977 on missions to Jupiter, Saturn, and beyond. [F88]
(b) Series of US spaceprobes launched to carry out exploration of the outer planets. In 1982 Voyager 2 returned remarkable pictures of Saturn. [A84]

V Velocity

A star's velocity in the direction of Galactic rotation, as measured relative to a nearby star that has a circular orbit. If a star revolves faster than such a star, the V velocity is positive; if it revolves more slowly, the V velocity is negative; and if both revolve at the same rate, the V velocity is zero. The Sun has a V velocity of +12 kilometers per second, so it revolves 12 kilometers per second faster than it would if it had a circular orbit. Since a star on a circular orbit revolves around the Galaxy at 220 kilometers per second, a star with a V velocity of 0 is not stationary; rather, it revolves at 220 kilometers per second. The Sun therefore revolves around the Galaxy at 220 + 12 = 232 kilometers per second. [C95]

VV Cep Stars

A subgroup of composite spectrum stars. One observes a spectrum of a K or M supergiant, showing emissions of hydrogen and [FeII] plus the spectrum of the secondary, which is generally of type B. [JJ95]


The name of a hypothetical planet at one time thought to exist between the Sun and Mercury. [H76]

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