Glossary W


W VIRGINIS STAR - Another name for Type II Cepheid variable stars.

WADSLEYITE - High pressure polymorph of olivine, β-Mg2SiO4, found on Earth and in some meteorites. It is thought to make up 50% or more of Earth's mantle between depths of 400 and 525 km. Wadsleyite transforms into ringwoodite at high pressure, but the exact pressure depends strongly on composition. At lower pressures, it transforms into olivine. This mineral was synthesized in the laboratory and subsequently observed as inclusions in diamond. It has an unusual structure in which one oxygen ion is incompletely bonded, allowing it to be easily protonated. The structure can accept up to 3 wt. % H2O as hydroxyl at this site, so wadsleyite may constitute a vast reservoir of mantle water.

Image source: http://bowfell.geol.ucl.ac.uk/~lidunka/GlobalGeophysics/Geophysics7%20-%20Deep%20Earth/Earth%20Structure.htm.

WANE - Decrease of the illuminated portion of a object as observed from Earth. The waning moon refers to that portion of the lunar revolution between full and new. Opposite of wax.

WAVE - Propagating disturbance which transmits energy from one point to another without physically transporting the oscillating quantity. A wave is characterized by wavelength and frequency.

WAVE-PARTICLE DUALITY - Principle of quantum mechanics implying that light (and, indeed, all other subatomic particles) sometimes acts like a wave, and sometime acts like a particle, depending on the experiment being performed. For instance, low frequency electromagnetic radiation tends to act more like a wave than a particle; high frequency electromagnetic radiation tends to act more like a particle than a wave.

WAVELENGTH - Distance from one peak of a wave to the next. Wavelength is measured in units of distance. The wavelengths of visible light correspond to ~400-650 nm. Wavelength is an important way to characterize a wave. For light, the shorter the wavelength, the higher the energy of the light wave.

WAX - Increase of the illuminated portion of a object as observed from Earth. The waxing moon refers to that portion of the lunar revolution between new and full, when the illuminated area is increasing. Opposite of wane.

WEAK LENSING - Small distortion and amplification of background galaxy images by foreground mass concentrations. In the weak lensing regime, distortion of an individual image is undetectably small (unlike in strong lensing), and the effect must be detected by searching for correlated distortions of thousands of source images. Weak lensing has been used to map the projected mass distribution of a number of galaxy clusters and used statistically to constrain the masses and sizes of dark galaxy halos.

WEAK NUCLEAR FORCE - Fundamental force which accounts for some particle interactions. The weak nuclear force causes radioactive decay, and, in particular, governs β decay whereby a neutron breaks up into a proton, an electron and an antineutrino.

WEAKLY INTERACTING MASSIVE PARTICLES (WIMPs) - Broad class of particles that were once in thermal equilibrium with the early universe but were "cold," i.e., moving non-relativistically at the time of structure formation. WIMPs have nonzero rest mass and participate only in the weak nuclear interaction. WIMPs are expected to have collapsed into a roughly isothermal, spherical halo within which the visible portion of our galaxy resides, consistent with measurements of spiral galaxy rotation curves. The original WIMP was a heavy Dirac neutrino, now experimentally ruled out. The neutralino predicted by supersymmetric extensions of the Standard Model is the favorite WIMP nowadays. Such particles could fill space and provide gravitational force without any associated luminosity. As such they are a candidate for dark matter. Another candidate WIMP is the axion. Direct detection of WIMPs is possible through their elastic scattering from nuclei. However, contributions of individual nucleons summed coherently yield extremely small WIMP-nucleus cross sections.

WEIRD TERRAIN - Region on the surface of Mercury of oddly rippled features. This feature is probably the result of a strong impact which occurred on the other side of the planet, and sent seismic waves traveling through and around the planet, converging in the weird region.

Image source: http://physics.uoregon.edu/~jimbrau/BrauImNew/Chap08/FG08_25.jpg.

WHITE DWARF - Remnant of a star with mass <8 Msun. White dwarfs have masses <1.4 Msun (the Chandrasekhar mass) and are supported by electron degeneracy pressure. White dwarfs have radii ~Rearth (<0.02 Rsun) and densities ~105-6 g/cm3. No nuclear fusion or gravitational contraction occurs in white dwarfs, they shine by residual heat. The surface has a very high temperature and radiates mainly in the ultraviolet. White dwarf varieties are defined by the elements that dominate their surfaces as revealed by their spectra. However, if material is accreting onto the surface of a white dwarf from a companion star, the spectral will reflect this fact. Nearly all white dwarfs have C-O cores. Three varieties (DA, DB and DO) have nearly pure surfaces of either H or He; PG 1159 stars appear to have partially exposed cores. White dwarfs with mixtures of elements on their surfaces receive compound classifications. For example, DAB stars contain H and He, whereas DAO stars have H and ionized He.

Image source: http://www.daviddarling.info/encyclopedia/W/whitedwarf.html.

WIDMANSTÄTTEN PATTERN - Characteristic cross-hatched pattern visible on the surface of octahedrites and pallasites after polishing and etching with nital (nitric acid in solution with ethanol). It is due to an intergrowth of crystals of Ni-rich taenite and Ni-poor kamacite. It is named after its discoverer, Austrian mineralogist Alois von Beckh Widmanstätten (1754-1849).

Image source: http://meteorites.wustl.edu/id/metal.htm.

WIEN'S DISPLACEMENT LAW - Relationship stating that wavelength at which a blackbody emits its maximum quantity of radiation is inversely proportional to its absolute temperature. Where the wavelength (λ) is given in nanometers and T in Kelvins:

WILKINSON MICROWAVE ANISOTROPY PROBE (WMAP) - NASA Explorer mission launched in June 2001 to map the temperature fluctuations of the CMB radiation with much higher resolution, sensitivity, and accuracy than the earlier COBE mission (see http://map.gsfc.nasa.gov/). WMAP is named in honor of David Wilkinson of Princeton University, my freshman physics professor and (incidentally) a world-renown cosmologist and WMAP team member who died in September 2002.

Image source: http://en.wikivisual.com/index.php/WMAP.

WMAP's main science goals are to: (1) improve the precision of the measurement of various cosmological parameters; (2) shed light on the process by which galaxies and other structures formed in the universe; and (3) more accurately deduce the epoch at which the first objects formed in the universe. WMAP produced a map of the CMBR, the temperature of which ranges from 2.7251 to 2.7249 K. This map shows the state of the universe about 380,000 years after the Big Bang with the tiny variations reflecting the earliest lumps and bumps in the universe -- seeds for galaxies and stars. WMAP data indicate that the universe is made of 4% ordinary matter, 23% of dark matter, and 73% of dark energy), and that the age of the universe is 13.7 ± 0.2 Ga. Lastly, the data also reveal that the first generation of stars ignited only 200 million years after the Big Bang, much earlier than many scientists had expected.

Image source: http://map.gsfc.nasa.gov/media/080997/index.html.

WINONAITE (WIN) - Class of primitive achondrite, named for the Winona meteorite, which was found in 1928 in a stone cist in a pueblo near Winona, Arizona. The circumstances of the find, documented by Major Lionel Brady, geologist at the Museum of Northern Arizona at the time, suggest that the pueblo builders venerated the meteorite as a sacred object after they had actually seen it fall. Winonaites are composed largely of fine-grained Mg-rich pyroxene (En91-99), with some olivine (Fa1-8), plagioclase (An8-25), troilite, and Ni-Fe metal. Winonaites are probably better classified as "metachondrites." Recent research suggests that both the winonaites and the IAB group iron meteorites originated on the same parent body - a partially differentiated asteroid that was disrupted just as it began to form an Fe core and a silicate-rich crust. This disrupting impact mixed silicates into molten Ni-Fe metal forming the silicated IAB irons, and mixed olivine-rich residues of partial melts into unmelted silicates, forming the winonaites. A few winonaites have anomalous characteristics, however, suggesting that they may have a different origin.

Image source: http://tin.er.usgs.gov/meteor/get_original_photo.php?recno=5629996.

WOLF-RAYET STAR - Massive stars at an advanced stage of stellar evolution, losing mass at a very high rate (right). They have masses typically >25 Msun and brief lifetimes. About 220 are known in our own Galaxy out of an estimated 1,000-2,000 such objects; most are hidden by dust. Wolf-rayet stars have average temperatures >25,000 K and luminosities up to 106 Lsun. Powerful stellar winds driven by intense radiation pressure, eject ~10 Msun per million years at speeds up to 3,000 km/s. These speeds result in broad emission lines in the spectra.

Image source: http://upload.wikimedia.org/wikipedia/commons/b/be/Wolf_rayet2.jpg.

These stars probably descend from O stars that have lost their H envelopes, revealing a He core. They may be subclassified into 2 main types: WN stars dominated by He and N  and nitrogen emission lines, but containing some C, and  WC stars lacking N and dominated by He, C, and O emission lines. About 50% of Wolf-Rayet stars occur in binary systems; proposed companions include another Wolf-Rayet star, or a black hole or neutron star. The only confirmed Wolf-Rayet companions so identified are other massive stars. Wolf-Rayet stars probably end their lives as either Type Ib or Type Ic supernovae.