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

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.

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-.

Being in the first or early stage of existence or evolution; e.g. → young stellar object.

M.E.; O.E. geong "youthful, young," from P.Gmc. *jungas (cf. O.S., O.Fris. jung, O.N. ungr, M.Du. jonc, Du. jong, O.H.G., Ger. jung, Goth. juggs), from PIE base *yeu- "vital force, youthful vigor;" cognate with Pers. javân, as below.

Javân "young;" Mid.Pers. juwân "young, youth;" Arm. yavanak (loaned from Mid.Pers.); Av. yuuan- "youth;" cf. Skt. yuvan- "young, youth;" L. juvenis "young man;" Lith. jaunas "young;" O.C.S. junu, Rus. junyj "young;" cognate with E. young, as above.

Any star that has evolved past the → protostar stage, but has not yet arrived on the → main sequence. There is a variety of YSOs depending on their age, mass, and environment, including → Herbig stars, → T Tauri stars, and, in general, compact infrared sources embedded in molecular clouds.

→ young; → stellar; → object.

A method of producing → interference of light. Two beams of → coherent light are produced by passing light through a very small circular aperture in one screen, then through two small circular apertures very close together in a second screen. On a third screen, behind the second screen, there will be two overlapping sets of waves and, if the light is monochromatic, → interference fringes will appear on the third screen. The experiment can also be performed with a beam of electrons or atoms, showing similar interference patterns. Young's experiment provides an evidence of the → wave-particle duality, as explained by → quantum mechanics. Same as → double-slit experiment.

Named after the English scientist Thomas Young (1773-1829), who originally performed the experiment some time around 1801 in an attempt to resolve the question of whether light was composed of particles (the → corpuscular theory of light); or rather consisted of waves travelling through some → ether. The experiment proved the wave nature of light; → experiment.

A measure of elasticity of a material, defined as the ratio of tensile → stress to tensile → strain, which equals the ratio of compressive stress to compressive strain.

Named after Thomas Young, → Young's experiment.

Same as → paradox of youth.

A soft, malleable, silver-white metallic chemical element; symbol Yb. Atomic number 70; atomic weight 173.04; melting point 819°C; boiling point about 1,194°C; specific gravity about 7.0. It has several radioactive isotopes.

From Ytterby, the name of the Swedish village where the mineral ytterbite (the source of ytterbium) was originally found. It was discovered by the Swiss chemist Jean-Charles Galissard de Marignac in 1878.

A highly crystalline iron-gray metallic chemical element; symbol Y. Atomic number 39; atomic weight 88.9059; melting point about 1,522°C; boiling point 3,338°C; specific gravity about 4.45. It has several radioactive isotopes.

From ytterbite, → ytterbium, which turnd out to be containing two different elements.

The potential function that is associated with the strong, short-ranged force resulting from the exchange of massive particles between two → nucleons in the same atomic nucleus. The potential has the form of V(r) = C. (1/r) exp (-K.r), where r is the distance between the nucleons and C and K are constants, giving measures of the strength and range of the force respectively.

In honor of the Japanese physicist Hideki Yukawa (1907-1981), winner of the 1949 Nobel Prize in Physics; → potential.

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 luminous → FU Orionis object consisting of two young stars in a → binary system, a → Herbig Be star embedded in a dust cocoon and a less massive component located 0.1 arcsecond south-east. It is associated with a very large-scale and high-velocity → bipolar outflow extending in total 3.6 pc (for a distance of 1150 pc) with radial velocities up to -620 km/sec. The outflow is traced by a → jet and at least 15 → Herbig-Haro objects. In 2008 Z CMa showed the largest "outburst" ever reported in the past 90 years.

→ Canis Major.

A member of a class of → low-mass X-ray binary systems containing low-magnetic field → neutron stars. See also → atoll source.

The name derives from the fact that on X-ray → color-color diagrams Z sources usually form a Z shape that is traced on timescales of hours to days; → source

Same as → atomic number.

Z, from the German word Zahl "number, numeral, figure," which was used to specify an element's numerical place in the → periodic table; → number.

The method of using the nebular observations to estimate the stellar ultraviolet radiation and the temperature of the central star in a planetary nebula. The basic assumptions are that the flux from a star could be approximately represented by the Planck function and that the nebula absorbs all the ultraviolet photons from the star which can cause ionization. For each ultraviolet photon absorbed an Hα photon is emitted when the ionized hydrogen subsequently recombines with an electron. Thus the strength of the Hα line is related to the ultraviolet flux of the star. However, modern theoretical work on stellar atmospheres shows that there are important deviations between the emergent fluxes from stars and Planck functions. Moreover, some of the stellar ultraviolet photons may be missed.

Named after the Dutch astrophysicist Herman Zanstra (1894-1972), who first introduced the method in 1927.

The splitting of spectral lines into closely-spaced components when the radiating substance is placed in a strong → magnetic field. Distinction is made between the normal and → anomalous Zeeman effects, and also the longitudinal and transverse Zeeman effects. In the normal → longitudinal Zeeman effect each spectral line is split into two components with frequencies ν ± Δν. In the normal → transverse Zeeman effect un un-displaced line is observed along with a doublet, i.e. three lines in all, with the frequencies ν and ν ±Δν. In the classical theory of the normal Zeeman effect, the motion of an electron in an atom is regarded as the harmonic oscillation of a linear harmonic oscillator. Arbitrary linearly polarized oscillation of the electron can be resolved into two oscillations: one along the magnetic field and the other in a plane perpendicular to this field. The latter can be further resolved into two oscillations, circularly polarized with opposite directions of rotation that occur in the Larmor precession frequency. Classical theory cannot explain the anomalous Zeeman effect. Both effects are accounted for in quantum mechanics as the result of changes in the energy levels of atomic electrons due to the interaction of their → orbital angular momentum and → spin angular momentum with each other and with the external magnetic field. See also → inverse Zeeman effect.

Named after Pieter Zeeman (1865-1943), Dutch physicist who discovered the phenomenon; → effect.

The point on the celestial sphere directly above the observer's head, opposite to the direction in which gravity acts. Opposite of → nadir.

M.E. cenith, from O.Fr. cenith, from M.L. cenit, senit, incorrect transliteration of Ar. samt (سمت) "path, direction," abbreviation of samt ar-ras (سمت‌الرأس), literally "road above one's head."

Sarsu, literally "the way over the head," from sar→ head + su, → direction.

The angular distance of a celestial body from the zenith. The zenith distance is 90° minus the body's altitude above the horizon (i.e. the complement of the altitude) and hence is also known as coaltitude.

→ zenith; → distance