Fr.: fusion avec gaz
A merger between → gas-rich galaxies. Wet mergers may lead to enhanced star formation, trigger → active galactic nuclei, and transform a → disk galaxy into an → elliptical galaxy. The larger the → redshift, the wetter mergers should be.
Fr.: principe de Weyl
The → world lines of galaxies form in the 4D space-time a bundle of non-intersecting → geodesics orthogonal to a series of space-like hyperstructures (e.g. Narlikar 2002, An Introduction to Cosmology, 3rd Edition, Cambridge Univ. Press). Expressed differently: The world lines of galaxies, or "fundamental particles," form (on average) a space-time filling family of non-intersecting geodesics converging toward the past (Rugh & Zinkernagel, 2010, astro-ph/1006.5848). The statement is sometimes denoted postulate, assumption, or hypothesis. The importance of Weyl's principle is that it asserts that cosmic matter moves according to certain regularity requirements. See also → cosmological principle.
First introduced by the German mathematician Hermann Weyl (1885-1955) in 1923 in his Raum, Zeit, Materie; → principle.
The star δ Canis Majoris, magnitude 1.84. It is an F8 supergiant 1800 light-years away. Among bright stars, Wezen is one of the most distant and luminous. Its luminosity is 50,000 times that of the Sun. Other designations: Alwazn, Wesen, HR 2693, HD 54605.
From Ar. Al-wazn (
Vazn, from Ar. Al-wazn, as above.
vâl, nahang (#)
1) Any of the larger ocean mammals, excluding the porpoises and dolphins,
that breathe through a blowhole on the top of their head and have front flippers,
no hind limbs, and a flat horizontal tail.
M.E., O.E. hwæl, cf. O.S. hwal, O.N. hvalr, Swed. val, M.Du. wal, Du. walvis, O.H.G. wal, Ger. Wal, probably cognate with L. squalus and Pers. vâl, as below.
1) Mod.Pers. vâl, wâl, related to Mid.Pers. kar (mâhig) "whale (fish);"
Av. kara- "a mythological fish;" also Mod.Pers. kuli "a kind of fish;"
cf. L. squalus "a kind of large sea fish;" PIE (s)kwalo-
"a large fish."
Fr.: pont de Wheatstone
An device consisting of four → resistances in series, used to determine the value of an unknown electrical resistance when the other three resistances are known.
Named after Charles Wheatstone (1802-1875), British physicist, who extensively used the circuit (1843) but was not its inventor. Such an arrangement of four resistances was first used by Samuel Hunter Christie (1784-1865) in 1833; → bridge.
1) A solid disk or a rigid circular ring arranged
to turn around an axle passed through the center.
M.E. whel(e), O.E. hweol, hweogol, from PIE *k(w)e-k(w)lo- "wheel, circle;" cf. Gk. kyklos "circle, wheel;" L.L. cyclus; Mod.Pers. carx "wheel;" Av. caxra- "wheel;" Skt. cakra- "wheel, circle;" Rus. koleso "wheel."
Carx "wheel," akin to wheel, as above.
Fr.: équation de Wheeler-DeWitt
In → quantum gravity, an equation that describes the → wave function of the → Universe. It is an adaptation of the → Schrodinger equation but includes the curved space attributes of → general relativity.
Named after American theoretical physicists John Archibald Wheeler (1911-2008) and Bryce Seligman DeWitt (1923-2004).
1) carxidan, gardidan; 2) carxeš, gardeš
Fr.: 1) tourbillonner, tournoyer; 2) tourbillon, tournoiement
M.E. whirlen, from O.N. hvirfla "to whirl," akin to O.E. hwyrflung "turning, revolving."
A powerful water current moving rapidly in a circular manner, as that produced by the meeting of opposing currents, sucking down anything that comes within. → Whirlpool galaxy.
From whirl "to turn around," from M.E. whirlen, from O.N. hvirfla "to whirl," cognate with O.E. hweorfan "to turn;" + → pool "a body of water."
Whirlpool galaxy (M51)
kahkešân-e Gerdâb (#)
Fr.: galaxie du Tourbillon
A large → spiral galaxy of type Sc seen → face-on in the constellation → Canes Venatici and lying about 31 million → light-years away. Also known as M51 and NGC 5194. It is interacting with a small irregular galaxy NGC 5195.
Fr.: tourbillon de vent
A whistling sound of descending pitch picked up by radio telescopes under certain circumstances. Whistlers are caused by radio waves from distant lightening flashes, which follow the lines of force of the Earth's magnetic field and are reflected back to Earth by the → ionosphere.
From M.E. whistler,; O.E. hwistlere, from hwistlian "whistle."
Sutzani "whistling," from
M.E. whit(e); O.E. hwit, from P.Gmc. *khwitaz (cf. O.S., O.Fris. hwit, O.N. hvitr, Du. wit, O.H.G. hwiz, Ger. weiß, Goth. hveits); cognate with Pers. sefid, as below.
Sefid, sepid "white;" Mid.Pers. spêt; Av. spita- "white;" cf. Skt. śveta- "white;" Lith. sviesti "to shine," svaityti "to brighten;" cognate with E. white, as above.
sefid kutulé, kutule-ye sefid (#)
Fr.: naine blanche
A compact star of high surface temperature, low luminosity, and high density (105-108 g cm-3), with roughly the mass of the Sun (mean mass ~ 0.6 Msun) and the radius of the Earth (R ~ 0.01 Rsun), representing the end-point of the evolution of all stars with masses less then ~ 5-9 → solar masses. A white dwarf is what remains after the central star of a → planetary nebula fades and becomes cool. The → Chandrasekhar limit of 1.43 solar masses is the highest mass that a white dwarf can achieve before electron → degeneracy pressure is unable to support it. In the → Hertzsprung-Russell diagram, white dwarfs form a well-defined sequence around 8 magnitudes fainter than the main sequence. They are composed of a core of carbon and oxygen nuclei and degenerate electrons surrounded by a thin shell of helium and an outer skin of hydrogen. White dwarf's radiation is the leftover heat from the star's past when its core was an active nuclear reactor. The star slowly cools as heat escapes through the non-degenerate envelope. → white dwarf crystallization; → cooling time; → Mestel theory. The first white dwarf to be discovered was Sirius B, the → companion of Sirius. White dwarfs are divided into several types, according to their spectral features, which depend on the type of → shell burning that dominated as it became a → planetary nebula: → DA white dwarf; → DB white dwarf; → DC white dwarf; → DO white dwarf; → DZ white dwarf; → DQ white dwarf. For a review see Kepler and Brdaley (1995, Baltic Astron. 4, 166).
white dwarf cooling track
râh-e sardeš-e sefid kutulé
Fr.: trajet de refroidissement de naine blanche
In the → Hertzsprung-Russell diagram, the evolutionary track followed by a → low-mass or an → intermediate-mass star when it can no longer produce thermonuclear energy. The track starts at the end of the → horizontal branch to lead the star to a → white dwarf phase.
white dwarf crystallization
bolureš-e sefid kutulé
Fr.: cristallisation de naine blanche
The most important phenomenon occurring during → white dwarf evolution, which results from its cooling. Crystallization is a → phase transition whereby → latent heat is released. At the cooler end of a white dwarf's life (→ cooling time), the → thermal energy of nuclei, which are positively charged ions, becomes small and the effects of electrostatic interaction on the motion of ions become important. The ions repel each other and their distribution will be such that the → Coulomb energy per ion is a minimum. This will cause the ions to form crystal-like lattice structures. As the star cystallizes it releases latent heat, providing an additional energy source that slows the cooling process compared to the → Mestel theory. Once the bulk of the white dwarf is crystalline, heat can travel through the star more easily and the white dwarf cools faster.
surâx-e sefid, sefid-câl
Fr.: trou blanc
A hypothetical opposite of the → black hole, from which particles and light pour out. However, there is a basic problem with white holes. Contrarily to black holes, no physical processes justify their existence. The appearance of a white hole is not due to any cause, it is acausal.
šab-e sefid (#)
Fr.: nuit blanche
The circumstance pertaining to polar latitudes in which when the Sun sets its center does not go beyond 6° below the horizon and the → twilight lasts all the night.
Fr.: bruit blanc
A random signal that has a relatively wide continuous range of frequencies of uniform intensity.
For what cause or reason? On what account? wherefore; on account of which.
M.E., from O.E. hwi, hwy instrumental case of hwæt "who."