Fr.: naine Y
A type of ultra-cool → brown dwarf with an → effective temperature lower than 500 K. → Near infrared spectra of these objects show deep absorption bands of H2O and CH4. So far only seven brown dwarf candidates belonging to this class have been found, all Y0 subtypes. These objects are very dim, with H magnitudes 19-23. See Cushing et al. 2011 (arXiv:1108.4678). The precise definition of the Y class requires new findings in the future about these objects.
For the choice of the letter Y, see Kirkpatrick et al. 1993, ApJ 406, 701; → dwarf.
ânten-e Yâgi (#)
Fr.: antenne de Yagi
A very familiar antenna array, which is the commonest kind of terrestrial TV aerial to be found on the rooftops of houses. It consists of a single "feed" or "driven element," usually a dipole antenna. The rest of the elements help transmit the energy in a particular direction. These antennas typically operate in the HF to UHF bands (about 3 MHz to 3 GHz), although their bandwidth is typically small. In astronomy Yagi antennas are used as elements in some → radio interferometers. Same as Yagi-Uda antenna.
Named after the Japanese electrical engineer Hidetsuga Yagi (1886-1976); → antenna.
The largest → impact crater on → Ceres after → Kerwan. It is adjacent to another large crater, called → Urvara. Yalode appears to have a series of canyons running from it, in a northwestern direction.
Named after Yalodé, the West African (Dahomeyan) deity of harvest.
Fr.: effet Yarkovski
A phenomenon that causes a slow variation of the orbital elements of asteroids and meteoroids. It takes place because the surface thermal conductivity of these bodies is not negligible and the rotation of the body about its axis shifts the warmest region from midday to the object's afternoon hemisphere. Consequently the temperature distribution is asymmetric with respect to the Sun direction, and the momentum carried off by the photons emitted in the infrared has a net component along the orbital velocity of the asteroid. This causes a decrease or increase of its orbital energy depending on whether the rotation is prograde or retrograde. The bodies therefore spiral either sunward or outward. The secular drift of the semi-major axis of the orbit is estimated to be of the order of 10-4 A.U. per million years for a → near-Earth object with a diameter of 1 km. The effect is unimportant for bodies larger than a few km because of their very large mass per unit area (106 g cm-2 or more) and is especially unimportant for comets that spend little time under intense illumination close to the Sun. Compare with the → Poynting-Robertson effect, which is isotropic. See also → YORP effect.
Named after Ivan Osipovich Yarkovsky (1844-1902), a Russian-Polish civil engineer. Yarkovsky knew nothing of photons and based his reasoning on the → ether concept, but his idea survives the translation to modern physics; → effect.
Fr.: année, an
In general, the time required for the Earth to complete one → revolution. Similarly, the time in which a planet completes its orbit around the Sun. In astronomy a distinction is made between various kinds of years, depending on the reference point used to measure the period of revolution. → anomalistic year; → Besselian year; → calendar year; → eclipse year; → embolismic year; → Galactic year; → Julian year; → leap year; → lunar year; → Platonic year; → sidereal year; → solar year; → Sothic year; → tropical year; → vernal-equinox year.
M.E. yeer; O.E. gēar (cf. O.S., O.H.G. jar, O.N. ar, Goth. jer, Du. jaar, Ger. Jahr); cf. O.Pers. dušiyāra- "evil year, bad harvest, famine" (from duš- "bad," → dys-, + yār- "year"); Av. yārə- "year;" Skt. paryārini- (*pari-yāram "a year long") "cow which has its first calf after a year;" Gk. hora "season, time of a day, year;" L. hornus "of this year;" → hour.
Sâl "year;" Mid.Pers. sâl "year;" Sogd. sarδ "year;" O.Pers. θrad- "year;" Av. sarəd- "year;" cf. Skt. śarád- "autumn;" maybe related to Lith. šilti "to become warm;" L. calor "heat," calere "to become warm;" PIE base *kele- "warm."
M.E. yelou; O.E. geolo, geolu; P.Gmc. *gelwaz (cf. O.S., O.H.G. gelo, M.Du. ghele, Du. geel, Ger. gelb, Swed. gul "yellow"); cognate with Pers. zar "yellow," as below.
Zard "yellow," related to zarr "gold;" Mid.Pers. zard "yellow," zarr "gold;" O.Pers. daraniya- "gold;" Av. zaray-, zairi- "yellow, green," zaranya-, zarənu- "gold;" cf. Skt. hari- "yellow, green," hiranya- "gold;" Gk. chloros "light green," chloe "green shoot;" L. helvus "yellowish, bay;" Rus. zeltyj "yellow;" P.Gmc. *gelwaz, as above.
qul-e zard (#), qulpeykar-e ~ (#)
Fr.: géante jaune
A star that appears in the upper-middle part of the → H-R diagram, to the left of the → red giants. Yellow giants are low-mass evolved stars that are burning their helium, on their path to the → planetary nebula stage. Most yellow giants behave as variable stars, usually because their outer layers pulsate. Periods of these pulsations are usually days or weeks. The Sun after leaving the red giant stage will become a pulsating yellow giant for some 100 million years.
yellow hypergiant (YHG)
Fr.: hypergéante jaune
An evolved, → very massive star of spectral type F or G with a very high luminosity (~105 times solar) lying near the empirical upper luminosity boundary in the → H-R diagram (→ Humphreys-Davidson limit). Yellow hypergiants have high → mass loss rates (10-5-10-3 solar masses per year) and are in a short, transitional evolutionary stage. Their evolutionary state is thought to correspond to post-red supergiants rapidly evolving in blueward loops in the H-R diagram. In their post-RSG blueward evolution these stars enter a temperature range (6000-9000 K), called → yellow void, with increased dynamical instability. Their link to other advanced evolutionary phases of massive stars such as → Luminous Blue Variables and → Wolf-Rayet stars is still an open issue in stellar evolution theory. The most famous yellow hypergiant is → Rho Cassiopeiae.
yellow supergiant (YSG)
abarqul-e zard (#)
Fr.: supergéante jaune
A supergiant star of type F and G whose effective temperature is between 4800 and 7500 K. Yellow supergiants are extremely rare, because they represent a very short-lived phase, typically a few tens of thousands of year, in the evolution of → massive stars.
Fr.: lacune jaune
Fr.: réfracteur de Yerkes
The largest → refracting telescope and the last of the great refractors with a lens diameter of 102 cm (f/d = 19), completed in 1897. The lens was ground by American telescope builders Alvan Clark & Sons. Used mainly for both visual and photographic studies of double stars, it is typical of the long-tube refractors traditionally employed in such work.
After Yerkes Observatory; → refractor.
Fr.: système de Yerkes
Same as → Morgan-Keenan classification.
After Yerkes Observatory, where the classification was developed; → system.
1) bâzdeh (#); 2) vâdehi, vâdâd
Fr.: 1) rendement; 2) limite d'élasticité
1a) Chemistry: The quantity of product resulting from a chemical
reaction or process, generally expressed as a percentage of the amount
that is theoretically obtainable.
M.E.; O.E. geldan, gieldan "to pay;" cf. O.S. geldan "to be worth," M.Du. ghelden, Du. gelden "to cost, be worth," O.H.G. geltan, Ger. gelten "to be worth."
1) Bâzdeh "yield, return," from bâz- "anew, again,"
→ re- + deh present stem of dâdan
"to give" (Mid.Pers. dâdan "togive, create;" O.Pers./Av. dā-
"to give, grant, yield," dadāiti "he gives;"
Skt. dadáti "he gives;" Gk. tithenai "to place, put, set,"
didomi "I give;" L. dare "to give, offer," facere
"to do, to make;" Rus. delat' "to do;" O.H.G. tuon, Ger. tun,
O.E. don "to do;" PIE base *dhe- "to put, to do").
noqte-ye vâdahi, ~ vâdâd
Fr.: limite d'élasticite
The point at which the → strain caused by a → stress on a material begins to increase without further increase in the stress. This point marks the end of → elastic deformation and the beginning of → plastic deformation. Same as → elastic limit.
The term first used by George Gamow (1904-1968) in a paper (Physical Review, 1948, coauthored by Alpher and Bethe) to describe the → primordial → substance from which the → chemical elements were formed. It fell into disuse.
M.E. ylem, from O.Fr. ilem "universal matter," from M.L. hyle "matter," from Gk. hule "matter, material, woods," used by Aristotle as perote hule "fundamental matter, raw material."
A metric prefix denoting 10-24. As of 2007, yocto- is the smallest SI prefix to be approved.
From L. octo, Gk. okto "eight," because it is equal to 1/10008.
A device for joining together a pair of draft animals, especially oxen, usually consisting
of a crosspiece with two bow-shaped pieces, each enclosing the head of an animal
M.E.; O.E. geoc "yoke," earlier geoht (cf. O.S. juk, Dan. aag, M.Du. joc, Du. juk, O.H.G. joh, Ger. joch, Goth. juk "yoke"); cognate with Pers. yuq, as below
Yuq "yoke," variants yuj, juh, jut, jot; Mid.Pers. jug, ayoxtan "to join, yoke;" Av. yaog- "to yoke, put to; to join, unite;" cf. Skt. yugam "yoke;" Hittite yugan "yoke;" Gk. zygon "yoke," as above, zeugnyanai "to join, unite;" L. jungere "to join," O.C.S. igo, O.Welsh iou, Lith. jungas O.E. geoc, as above; PIE base *yeug- "to join."
Fr.: monture anglaise à berceau
A form of → English mounting in which the → telescope is suspended inside an inclined fork, supported at both ends, and forming a → right ascension axis parallel to the Earth's → axis. The telescope pivots about the → declination axis inside two parallel forks.
Fr.: effet YORP
A phenomenon in which the rotation rate of a small asteroid changes under sunlight absorption. Photons from the Sun are absorbed by a small body and reradiated in infrared. In the process, two forces influence the object: one from the impact of the photons, providing a tiny push, and the other as a recoil effect when the object emits the absorbed energy. In the YORP effect the body's shape has a more effective role than albedo in altering the spin rate. For small asteroids (< 10 km), YORP can cause measurable changes in rotation rate. The effect can even speed up the rotation leading to disintegration. → Yarkovsky effect.
Short for Ivan Osipovich Yarkovsky, John A. O'Keefe, V. V. Radzievskii, and Stephen J. Paddockk, who developed the explanation; → effect.
A metric prefix denoting 1024.
On the model of → yocto-.