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Young’s modulus

Y

yardangs Streamlined elongated hills oriented parallel to the wind and created by erosion of adjacent friable material. The hills are aerodynamically shaped by the wind and resemble inverted boat hulls. The highest and widest part of the ridge is about 1/3 of the distance from the upwind to the downwind side. The downwind side usually has a gently tapering bedrock surface or may have a sand “tail”. Yardangs range in size from about 1 to 200 m in height and a few meters to a few kilometers in length. They typically form in linear clusters and have been identified on both Earth and Mars.

year Unit of time equal to one revolution of the Earth about the sun. “A tropical year” is a measure of mean solar time, specifically the time between successive passages of the vernal equinox by the mean sun (365.242119 days of mean solar time). A “sidereal year” is a measure of sidereal time, namely the period between successive passages through the hour circle of a fixed star (365.25636556 days of mean solar time). An “anomalistic year” is the mean interval between passages of the Earth through perihelion (365.25964134 days of mean solar time). Calendar years approximate tropical years and consist of 365 days plus 1 day added every leap year. A “Besselian year” is defined as the period between successive passages by the mean sun of the right ascension 18h 40m and is, hence, independent of an observer’s location. A “Julian year” consists of 365.25 days of mean solar time. Some types of years refer to longer periods, e.g., the “platonic year” is the period of the precession of the equinoxes (25725 tropical years relative to the fixed ecliptic).

yellow line A coronal line observed at 5694 Å resulting from a forbidden transition in highly ionized calcium atoms (Ca XV). Important for the study of coronal structures at temperatures of order 3.5 MK.

Yerkes classification scheme of galaxies A classification scheme of galaxies, conceived by W.W. Morgan and based on the central concentration of light, which closely correlates to the stellar population of a galaxy. The concentration class of a galaxy is indicated with the same letter used for stellar spectral types, but written in lower case (i.e., k for ellipticals and a for Sc spirals whose spectrum is dominated by K giants and A stars, respectively). A second parameter describes the “form family” of galaxies: S indicates a spiral; B a barred spiral; E an elliptical; I an irregular; R a galaxy having rotational symmetry without prominent spiral arms (an S0 galaxy according to the Hubble scheme); and D indicates an elliptical galaxy with an extended envelope. A third symbol (a number from 0 to 7) describes the apparent flattening of a galaxy. For example, M31 is classified as kS5. Albeit no longer widely in use, the Yerkes scheme is still used to denote supergiant elliptical galaxies often found at the center of clusters and groups of galaxies (cD galaxies).

Yohkoh (“sunbeam”) A Japanese satellite launched in August 1991 designed to study the dynamics, energetics, and morphology of solar flares. Yohkoh is comprised of four scientific instruments: the Soft X-ray Telescope (SXT) for imaging in the 3 to 60 Å wavelength range, the Bragg Crystal Spectrometer (BCS) for spectroscopic observations in the 1 to 5 Å range, the Hard X-ray Telescope (HXT) for imaging at energies above 14 keV, and the Wide Band Spectrometer (WBS) for spectroscopic observations over a wide range of energies from soft X-rays to γ -rays.

Yoshida jet See jet.

Young’s modulus The ratio Y = T /( l/l) for a material under compression or tension. Here T is the stress [N/m2], and l/l is the strain, the fractional length extension or compression. Hence, Young’s modulus has dimensions of stress, [N/m2].

Young’s modulus

In an elastic isotropic

material,

stress σ

is proportional to strain ε:

σ = Y ε.

The quantity Y is a property of the

material and is called Young’s modulus.

© 2001 by CRC Press LLC

Yukawa coupling

Yukawa coupling Particle physics theories predict that fermions will acquire their masses through the Higgs mechanism, by means of a so-called Yukawa term in the Lagrangian, where fermions couple with a Higgs field φ. Specifically, a fermion ψ is said to have a mass m when the Lagrangian density that describes its dynamics contains the term

Lm =

¯

 

mψψ .

In some cases, however, it is not possible to have such a term, and the corresponding fermions would then appear massless (the symmetry rules obeyed by the total Lagrangian are responsible for this). Instead, a coupling such as

L = ¯

m f ψφψ

(the Yukawa coupling) with f the coupling constant, may always be set. When the Higgs field gets its nonzero vacuum expectation value |φ| in a phase transition, the above coupling reduces to a simple mass term, and thus the fermion acquires a mass proportional to |φ| .

This same mechanism also allows the possibility of fermionic currents along topological defects in the form of zero modes. See cosmic phase transition, fermionic zero mode, Higgs mechanism.

© 2001 by CRC Press LLC

Zwicky compact galaxies

Z

Zeeman effect The splitting of spectral lines into groups of closely spaced lines in the presence of a strong magnetic field. The Zeeman effect occurs in the spectra of sunspots and stars. This effect demonstrates the existence of magnetic fields in celestial bodies, and since the degree of the splitting depends on the magnitude of the magnetic fields, it allows measurement of the field to be made.

Zeldovich process A process of energy extraction from a rotating black hole via incident electromagnetic or gravitational radiation. When radiation hits a black hole, part of it is absorbed and part is carried away. Normally, the absorbed part has positive energy and the amplitude of the transmitted wave is reduced. However, in the case of cylindrical waves, the absorbed wave may carry negative energy, in which case the transmitted wave is amplified. This is the variant with electromagnetic radiation of the Penrose process. See Penrose process.

zenith The point directly overhead an observer, as defined by the local horizontal. Technically, there are three zenith directions: geocentric zenith, the direction directly away from the Earth’s center; geodetic zenith, the direction of the local upwards normal from the Earth ellipsoid; and astronomical zenith, the direction of the upwards normal from the geoid. Normally, when used without another qualifier, zenith refers to either the geodetic or astronomical zenith, which are within a few arc seconds of each other. See also altitude, nadir.

zenith angle The angle from the vertical, or zenith, to a given direction. The zenith angle is the complement of the altitude.

zero mode See fermionic zero mode.

zero up-crossing The point where the water surface in a water wave profile crosses the zero line and is trending upward. Used in the definition of wavelength.

zodiac The apparent path of the sun through the background stars over the course of the year; the intersection of the plane of the ecliptic with the celestial sphere.

zodiacal light A band of diffuse light seen along the ecliptic near the sun immediately after sunset or before sunrise. It is created by sunlight reflecting off the interplanetary dust particles, which are concentrated along the ecliptic plane. To see the zodiacal light, you must have very dark skies; under the best conditions it rivals the Milky Way in terms of brightness. The material within the zodiacal light is slowly spiraling inward toward the sun, due to the Poynting– Robertson Effect; hence, it must be continuously replaced by asteroid collisions and the debris constituting comet tails. The zodiacal light is the name given to the band of light seen close to the sun while gegenschein is the term applied to the same band of light located 180◦ from the sun.

zonal In the direction parallel to the equator, i.e., east-west. See also meridional.

zooplankton Animal forms of plankton.

Z-string See embedded defect.

Zulu Time See Universal Time.

Zwicky compact galaxies Galaxies originally defined by F. Zwicky as distinguishable from stars on the Palomar 1.2 m Schmidt telescope plate and with angular diameter between 2 and 5 sec of arc. “Compact” is, in astronomy, a loose synonym of “not fully resolved”. The notion of compactness of a galaxy therefore depends on the resolving power of the observational equipment, although compact galaxies have, in general, high surface brightness and sharp borders. F. Zwicky circulated seven lists of compact galaxies including about 200 objects in the mid-1960s. Most of them are of blue color and show emission lines in their spectra.

© 2001 by CRC Press LLC

Zwicky compact galaxies

Zwicky’s compact galaxies have turned out to

active galaxies. For example, the object I Zw 1

be a rather heterogeneous class, which included

(the first object of Zwicky’s first list) is a nearby

star-forming dwarf galaxies, as well as several

quasar.

© 2001 by CRC Press LLC

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