An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

A space telescope launched by NASA in 2001 which measures the temperature fluctuations in the → cosmic microwave background (CMB) radiation. It creates a full-sky map of the CMB, with a 13 arcminute resolution via multi-frequency observations. WMAP is the first mission to use a → Lagrangian point L2 as its permanent observing station at a distance of 1.5 million km. WMAP completed its prime two years of mission operations in September 2003 and is continuing in 2009 its observations for still several years to come. WMAP's measurements have played a considerable role in establishing the current standard model of cosmology. They are consistent with a Universe that is dominated by → dark energy, with negative pressure or a → cosmological constant. In this model, the age of the Universe is 13.73 ± 0.12 billion years. The current expansion rate of the Universe measured by the Hubble constant, is 70.5 ± 1.3 km·s^-1 Mpc^-1. The content of the Universe consists of 4.56% ± 0.15% ordinary → baryonic matter, 22.8% ± 1.3% → cold dark matter, and 72.6% ± 1.5% of → dark energy, that accelerates the → expansion of the Universe.

WMAP, short for Wilkinson Microwave Anisotropy Probe, in honor of David Todd Wilkinson (1935-2002), who had been a member of the mission's science team.

The depression on the → Sun's → photosphere associated with → sunspots and involving the → Wilson effect. The measured depression values vary between about 700 and 2,000 km.

→ Wilson effect; → depression.

A phenomenon in which the shape of → sunspots flattens as they approach the → Sun's limb due to the → solar rotation. More specifically, when a sunspot approaches the → solar limbs the width of the → penumbra, relative to the → umbra, on the side facing the center of the Sun seems to become narrower than on the side facing the limb. This phenomenon arises from a projection effect, and is due to a geometrical depression (the → Wilson depression) in the layers of constant → optical depth in sunspots (see, e.g., Sami K. Solanki, 2003, Sunspots: An overview, The Astron. Astrophys. Rev., 11, 153).

First noticed by Alexander Wilson (1714-1786); → effect.

The strong correlation between the equivalent width of Ca II → H and K lines of a late-type giant or supergiant star with the absolute visual magnitude of the star.

O. C. Wilson & M. K. Vainu Bappu (1957, ApJ 125, 661); → effect.

A quasi-spherical accretion that is likely to occur in a → high-mass X-ray binary (HMXB) when the optical star of → early spectral class (O-B) does not fill its → Roche lobe, but has a significant → mass loss via → stellar wind. In → close binary systems another accretion regime, → disk accretion, occurs when the optical star overfills its Roche lobe.

→ wind; → accretion.

A function whose value is zero outside a given interval. Applications of window functions include signal filtering and spectral analysis. The various types of windw functions include: → rectangular window, cosine window, triangular window, Gaussian window, Hanning window, and so on.

→ window; → function.

An optical device for producing and analyzing polarized light. It divides incoming unpolarized light into two orthogonal, linearly polarized beams. It consists of two prisms of either quartz or calcite cemented together.

After the English scientist William Hyde Wollaston (1766-1828); → prism.

The least amount of energy required to remove an electron from the surface of a solid, to a point just outside the solid where the electron has zero kinetic energy. See also → photoelectric effect.

→ work; → function.

1) Not in accordance with what is morally right or good: a wrong deed.
2) Deviating from truth or fact; erroneous: a wrong answer.
3) Not correct in action, judgment, opinion, method, etc., as a person; in error (Dictionary.com).
See also: → false, → invalid.

M.E. wrong, wrang, O.E. wrang "twisted, crooked," from O.N. rangr "crooked, wry, wrong;" cf. Dan. vrang "crooked, wrong," Du. wrang "sour, bitter."

Nâdorost, from nâ- "not," → a-, + dorost, → right.

The → determinant of order n associated with a set of n functions, in which the first row consists of the functions, the second row consists of the first → derivatives of the functions, the third row consists of their second derivatives, and so on. For example, If y₁ and y₂ are functions of x, the determinant W(y₁,y₂) = y₁ . y₂^' - y₁^' . y₂ is called the Wronskian of the given function.

Named after the Polish mathematician Józef Hoene-Wroński (1776-1853).

The study of celestial bodies using their X-ray emission. X-ray astronomy deals mainly with Galactic and extragalactic phenomena involving very high-energy photon emissions, covering a band of energies between 0.1 keV and 500 keV. The research field includes: → X-ray binaries, → cataclysmic variables, → pulsars, → black holes, → dark matter, → active galaxies, → galactic clusters → X-ray transients. The Earth's atmosphere absorbs most X-rays coming from outer space. X-ray astronomy therefore requires observations to be done above atmosphere. The first rocket flight which successfully detected a cosmic source of X-ray emission was launched in 1962 by an American research group. A very bright source was detected that they named → Scorpius X-1. Since then several dedicated X-ray astronomy satellites have been launched, among which: Uhuru, INTEGRAL, ROSAT, Rossi X-ray Timing Explorer (RXTE), → Chandra X-ray Observatory, and → XMM-Newton, which have contributed to important advances in astronomy.

→ X-ray; → astronomy.

The diffraction of X-rays by the atoms or ions of a crystal. The wavelength of X-rays are comparable to the size of interatomic spacings in solids. Since the atoms in a crystal are arranged in a set of regular planes, crystals serve as three-dimensional diffraction gratings for X-rays. Planes of repetition within the atomic structure of the mineral diffract the X-rays. The pattern of diffraction thus obtained is therefore used to identify minerals by bombarding them with X-rays.

→ X-ray; → diffraction.

A member of a class of isolated, radio-silent → pulsars with peculiar properties. They show a purely thermal spectrum at X-ray energies with no evidence for a high-energy, power-law component often detected in other → isolated neutron star classes. The X-ray luminosity is 10³¹ - 10³² erg s^-1, fully consistent with surface blackbody emission with temperatures ~ 40-100 eV and (radiation) radii of a few kilometers, as derived from X-ray spectral fits. With the only exception of RX J1856.5-3754, broad absorption features have been found in all XDINSs. These features have energies ~ 300 - 700 eV, equivalent widths of ~ 50 - 150 eV and, as in the case of RX J0720.4-3125, may be variable.

→ X-ray; → dim; → isolated; → neutron; → star.

A region of → molecular clouds whose chemistry is affected by → X-rays from neighboring sources.

→ X-ray; → dissociation; → region.

A colorless, odorless, tasteless, chemically un-reactive gas, belonging to the → inert gas group, occurring in exceedingly minute amounts in the air; symbol Xe. → Atomic number 54; → atomic weight 131.29; → melting point -111.9°C; → boiling point -107.1°C. Xenon was discovered spectroscopically in 1898 by William Ramsay and M. W. Travers, who obtained it by fractional distillation of an impure sample of liquid → krypton. The lightest → isotopes of xenon (¹²⁴Xe and ¹²⁶Xe) are produced during → supernova explosions; intermediate-mass isotopes (¹²⁷Xe, ¹²⁸Xe, ¹²⁹Xe, ¹³⁰Xe, ¹³¹Xe and ¹³²Xe) are produced during the → Asymptotic Giant Branch phase of evolved low- and intermediate-mass stars; the heaviest isotopes (¹³⁴Xe and ¹³⁶Xe) are produced during the → merger of → neutron stars.

From Gk. xenon, neuter of xenos "strange," introduced by the discoverers.

A European Space Agency's satellite, launched on 10 December 1999 and designed for the observation of → X-rays emitted by astronomical objects. The satellite carries three very advanced X-ray telescopes. The three corresponding European Photon Imaging Cameras (EPIC) are sensitive over the energy range 0.2 keV to 12 keV. Other instruments on-board are two reflection grating spectrometers which are sensitive below about 2 keV, and a 30 cm diameter → Ritchey-Chretien optical/UV telescope. The telescope moves in a highly elliptical orbit, traveling out to nearly one third of the distance to the Moon and enabling long, uninterrupted observations of faint → X-ray sources. The original mission lifetime was two years, it has now been extended for further observations until at least 2010. Among recent results obtained using XMM-Newton one can mention an intermediate-mass black hole of over 500 solar masses in the galaxy ESO 243-49 (Nature 460, 73, 2009) and broad line emission from iron K- and L-shell transitions in the active galaxy 1H 0707-495 (Nature 459, 540, 2009). See also → X-ray astronomy.

XMM, from "X-ray Multi-Mirror;" Newton, in honor of Sir Isaac Newton, → newton.

An electrically neutral subatomic particle that along with → W boson mediates the → weak nuclear force. Like the photon, the Z boson is its own antiparticle.

→ boson.

A star that has arrived on the → horizontal branch after leaving the → red giant branch. It begins → helium burning in its core.

→ zero; → age; → horizontal; → branch; → star.

The removal of non-significant zeroes from a number. For example, replacing 531.2300 by 531.23.

→ zero; → suppression.

Same as → Alnitak.

Zeta (ζ), Gk. letter in the → Bayer designation scheme.