VACANCY - Unoccupied sites within a crystal that are usually occupied by an atom. A vacancy may move when a neighboring atom or ion moves to occupy the vacant site (the vacancy shift in the opposite direction to the site that was the source of the atom). The surrounding bonded crystal structure ensures that the neighboring atoms do not collapse around the vacancy. In some materials, neighboring atoms actually move away from a vacancy, because they can better form bonds with atoms in the other directions.
VACUUM - Space entirely devoid of matter (called also, by way of distinction, absolute vacuum). In a more general sense, a space, as the interior of a closed vessel, which has been evacuated to a high degree by a pump or other artificial means.
VACUUM ENERGY DENSITY - Amount of energy per unit volume associated with empty space itself.
VALENCE - Charge on an ion reflecting gained or lost electrons. For example, an iron atom that has lost two electron, Fe2+, is described as having a valence of 2.
VALENCE BAND - Outermost energy band that contains electrons when a solid is in the ground state. An intrinsic semiconductor (or insulator) in its ground state has a completely filled valence band, while the conduction band above the valence band is completely empty.
VALENCE ELECTRON - Electron in the valence shell of an atom, or the valence band of a solid. Valence electrons are more active than non-valence electrons, so valence electrons are responsible for most of an atom's or solid's electrical and chemical properties.
VAN ALLEN BELTS - At least two doughnut-shaped regions of magnetically trapped charged particles high above Earth's atmosphere.
VARIABLE STAR - Star whose luminosity varies over time. Broadly speaking, variable stars are of two types: (1) stars that are intrinsically variable, that is, their luminosity actually changes, for example because the star periodically swells and shrinks; (2) eclipsing and rotating variables, where the apparent changes in brightness are a perspective effect. The first 334 variable stars discovered in a constellation are given a one or two letter code such as R Scuti or UV Ceti. Other variable stars are designated V335, V336, etc. For example, Proxima Centauri is known to variable star astronomers as V645 Centauri.
VELOCITY - Vector measuring the time rate of change of displacement. It is specified by magnitude (the speed) and direction. In general, the velocity is a function of time:
VESTA - Third largest and fourth brightest asteroid; it was discovered in 1807 by Heinrich Olbers and named for the ancient Roman goddess of the hearth. 4 Vesta has a basaltic surface composition and an average density not much less than that of Mars. Evidently lava once flowed here indicating that the interior was at one time molten, heated by short-lived radio-isotopes. A deep impact crater 456 km wide (on a world itself not much over 500 km across!), visible to the Hubble Space Telescope, exposes 4 Vesta's mantle.
Apparently, large chunks of matter were ejected from 4 Vesta during the large impact to form smaller asteroids of similar composition - so-called Vestoids. Some of these bits of 4 Vesta subsequently entered near-Earth orbits, and are thought to be the sources of the howardites, eucrites, and diogenites. Vesta is located in a part of the main asteroid belt that makes it almost impossible for it to send meteorites to us. So there are probably intermediate asteroids, which were once part of Vesta, located in more favorable orbits that provide delivery. This theory has been bolstered by the discovery that the asteroid 1929 Kollaa, based on its reflectance spectrum, was once a part of Vesta, and, moreover, that it moves in an orbit from which meteorites could much more easily be launched Earthward.
VIRGO CLUSTER - Largest and nearest galaxy cluster to the Local Group. The Local Group is often considered to lie on the edge of the Virgo cluster. The Virgo Cluster is ~16 Mpc distant and spans 8° on the sky. Its mass is ~ 1 x 1014 Msun, and it contains ~2,000 galaxies, with many more spiral galaxies than typical for a cluster of this size. There are three clearly identifiable sub-clusters (centered on M87, M86, and M49) and has an irregular X-ray halo, suggesting that the cluster is still forming. The mass of the Virgo cluster is so large that the motions of most galaxies in its neighborhood are influenced by it and drawn towards it. Although the Local Group is current receding from the cluster, the mass of the Virgo cluster is so high that it is expected that the Local Group will eventually slow down and reverse direction, ultimately joining the cluster.
VISCOSITY (η) - Quantity that describes a fluid's resistance to flow. Fluids resist the relative motion of immersed objects through them as well as the motion of layers with differing velocities within them. The SI unit of viscosity is the pascal second [Pa·s], which has no special name. However, the pascal second is rarely used in scientific and technical publications today. The most common unit of viscosity is the dyne second per square centimeter [dyne·s/cm2], which is given the name poise after the French physiologist Jean Louis Poiseuille (1799-1869). 1 Pa·s = 10 poise = 1,000 mPa·s.
VOID - Huge region of space that is unusually empty of galaxies. Voids are not entirely empty, but are underdense and contain far fewer bright galaxies than average.
VOLATILE - Substances which have a tendency to enter the gas phase relatively easily (by evaporation, addition of heat, etc.).
VOLATILE ELEMENTS - Chemical elements that condense (or volatilize) at relatively low temperatures. The opposite of volatile is refractory. Volatile elements can be divided into moderately volatile (Tc = 1230-640 K) and highly volatile (Tc < 640 K). Moderately volatile lithophile elements are Mn, P, Na, B, Rb, K, F, and Zn. Moderately volatile siderophile and chalcophile elements are Au, Cu, Ag, Ga, Sb, Ge, Sn, Se, Te, and S. Highly volatile lithophile and atmophile elements are Cl, Br, I, Cs, Tl, H, C, N, O, He, Ne, Ar, Kr, and Xe. Finally, highly volatile siderophile and chalcophile elements are In, Bi, Pb, and Hg.
VOLATILIZE - Conversion of liquids into gases, usually by heating them.
VOLCANIC - Igneous rock that forms from cooling magma on the surface of a planet or asteroid.