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

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.

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.

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.

A crater about 70 km in diameter in Western Australia, considered to be the oldest recognized → meteorite impact structure on Earth. A precise age of 2 229 ± 5 million years is derived from shocked zircon and monazite crystals in the rocks. The age coincides, within uncertainty, with temporal constraint for the youngest Palaeoproterozoic glacial deposits. Numerical impact simulations indicate that a 70 km size crater created by the impact in a continental glacier could release between 8.7 × 10¹³ to 5.0 × 10¹⁵ kg of H₂O vapor instantaneously into the atmosphere. These results provide new estimates of impact-produced H₂O vapor abundances for models investigating termination of the Paleoproterozoic glaciations, and highlight the possible role of impact cratering in modifying Earth's → climate (Erikson, T.M. et al., 2020, Nature communications, 21 January).

The Yarrabubba structure is located on the Yilgarn Craton in Western Australia (lat. 27° 11'S, long. 118° 50'E), approximately 100 km southeast of the township of Meekatharra; → crater.

In general, the time required for the Earth to complete one → revolution (approximately 3.154 × 10⁷ seconds). 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."

The primary color between green and orange in the visible spectrum; an effect of light with a wavelength between 5700 and 5900 Å. → yellow giant; → yellow supergiant.

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.

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; → giant.

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; → hypergiant.

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.

→ yellow; → supergiant.

A temperature range (6000-9000 K) in the → H-R diagram occupied by → yellow hypergiants in their post-RSG blueward evolution, where high → mass loss episodes occur.

→ yellow; → void.

The final product obtained from the processing of uranium ores. It is a coarse powder, a mixture of uranium oxides, with about 80% U₃O₈. It has a pungent odor and melts at approximately 2878 °C. The yellowcake produced by most modern mills is actually brown or black, not yellow; the name comes from the color of the concentrates produced by early mining operations due to impurities from ammonium diuranate. Yellowcake must be converted into → uranium hexafluoride (UF₆) before it can be enriched, the process that makes the sort of uranium used by nuclear power plants or bomb-makers (→ uranium enrichment). The uranium hexafluoride is heated to become a gas and loaded into cylinders. When it cools, it condenses into a solid.

→ yellow; cake, M.E., from O.Norse kaka "cake," from which also derive M.Du. koke, Du. koek, Ger. Kuchen.

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.

Same as → Morgan-Keenan classification.

After Yerkes Observatory, where the classification was developed; → system.

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.
1b) Nuclear physics: 1) The number of → daughter atoms produced by the decay of one atom of a → parent radioactive element. For example, the → fission of one atom of ²³⁸U produces an average 0.0063 atoms of ¹³⁶Xe, denoted: Y (¹³⁶Xe)₂₃₈ = 0.0063.
1c) A measure of the destructive energy of a nuclear explosion, expressed in kilotons of the amount of → T.N.T. that would produce the same destruction.
1d) → quantum yield.
2) → yield point.

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").
2) Vâdehi, vâdâd, verbal noun from vâdâdan "to surrender," from vâ-, → de-, + dehi, dâd, from dâdan, as above.

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.

→ yield; → point.

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 (dictionary.com).
→ yoke mounting.

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."

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.

→ yoke; → mounting.