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

A measure of the amount by which a curve, a surface, or any other manifold deviates from a straight line, a plane, or a hyperplane. In particular, The reciprocal of the radius of the circle which most nearly approximates a curve at a given point.
See also:
→ curvature constant, → curvature of space-time, → field curvature, → primordial curvature perturbation.

From L. curvatura, from curvatus, p.p. of curvare "to bend," from curvus "curved," → curve.

Xamidegi, from xamidé "curved," from xamidag "curved" + noun suffix -i.

A parameter occurring in the → Friedmann equations of → general relativity describing the geometry of → space-time. A spatially → open Universe is defined by k = -1, a → closed Universe by k = + 1 and a → flat Universe by k = 0. See also the → Robertson-Walker metric. See also → curvature of space-time.

→ curvature; → parameter.

According to → general relativity, → space-time is curved by the presence of → matter. The curvature is described in terms of → Riemann's geometry. In → cosmological models three types of curvature are considered: positive (spherical, → closed Universe), zero (Euclidean, → flat Universe), and negative (hyperbolic, → open Universe). See also → curvature constant.

→ curvature; → space-time.

A line that deviates from straightness in a smooth, continuous fashion. A line representing a variable on a graph.

From L. curvus "crooked, curved, bent;" cf. Av. skarəna- "round," Gk. kirkos, krikos "a ring;" PIE base *sker- "to turn, bend."

Xam, variant kamân "arc," Mid.Pers. kamân, probably from PIE *kamb- "to bend, crook," cf. Breton kamm "curved, bent."

Construction of mathematical functions whose graphs are curves that "best" approximate a given collection of data points.

→ curve; → fitting.

A plot showing how the → equivalent width of an → absorption line, or the radiance of an → emission line, increases as a → function of the → number of → atoms that produce the line.

→ curve; → growth.

Not straight.

Adj. from → curve.

Consisting of, represented by, or bound by curved lines. → rectilinear.

From → curve + → linear.

1) General: Pointed end. A point of transition. → polar cusp.
2) Either point of a → crescent moon.
3) A steep power-law representing the number density of stars in the central region of a galaxy. Cusps are characteristic of low-mass ellipticals. They are thought to result from the gravitational attraction of a central → supermassive black hole.
4) A peaked concentration of dark matter in the center of galaxies, as predicted by the → cold dark matter (CDM) model of galaxy formation. See also → cusp problem.
5) Math: A tooth-like meeting of two branches of a curve, with sudden change of direction.

L. cuspis "point, spear, pointed end."

Tizé, noun from tiz "sharp, pointed," from Mid.Pers. tēz, tēž, tigr "sharp," O.Pers. tigra- "pointed," Av. taēža-, tighra- "pointed," Skt. taējas- "the sharp edge (of a knife), piercing (flame)", from tij- "to be sharp, to pierce," Gk. stizein "to prick, puncture," stigma "mark made by a pointed instrument," L. instigare "to goad," P.Gmc. *stik- "to pierce, prick, be sharp," O.H.G. stehhan, Ger. stechen "to prick," O.E. stician "to pierce, stab," E. stick "to pierce;" PIE *st(e)ig- "to stick; pointed".

A problem encountered by the → cold dark matter (CDM) model of galaxy formation. The numerical simulations with CDM predict a large concentration of dark matter in the center of galaxies, with a peaked density distribution, in contrast to the real, observed galaxies. See also: → angular momentum catastrophe; → missing dwarfs.

→ cusp; → problem.

To penetrate or divide something, as with a sharp-edged instrument.

M.E. cutten, kytten, kitten; O.E. *cyttan, cognate with O.Swed. kotta "to cut;" O.N. kuti "little knife," or from O.Fr. couteau "knife."

Boridan "to cut off;" Mid.Pers. brin-, britan, brinitan "to cut off," brin "cut, delimitation, determined;" Av. (pairi-) brī- "to shave, shear;" cf. Skt. bhrī- "to hurt, injure," bhrinanti "they hurt."

1) A designated limit beyond which the passage of something must be stopped.
2) A device that cuts off a transmission of photons.

→ cut; → off.

Boré, from bor- present stem of boridan "to → cut" + noun suffix -é.

Filter rejecting all light with wavelengths on one side of the cutoff wavelength.

→ cutoff; → filter.

The electrode voltage which reduces the value of a dependent variable, e.g. anode current, to a specified low value.

→ cutoff; → voltage.

Wavelength at which the transmittance of a filter, or the detectivity of a detector, has fallen to one-half its peak value.

→ cutoff; → wavelength.

A chemical compound that contains the → cyano radical, -CN. Most cyanides are highly toxic.

→ cyano-; → -ide.

A diatomic chemical radical composed of carbon and nitrogen atoms. The triple bonds of C to H leave one electron available, which makes the CN radical very reactive. Organic molecules with the -CN group are potential sources of → prebiotic amino acids. Same as the → CN molecule. The CN radical was first identified by Gay-Lussac, who in 1815 published an extensive study of the derivatives of prussic acid (→ hydrogen cyanide). He showed that the cyano radical remained intact throughout a series of chemical transformations. Also called cyanogen radical.

→ cyano-; → radical.

1) A combining form meaning "blue, dark blue," used in the formation of compound words. Also cyan- before a vowel.
2) A combining form representing → cyanide in the formation of chemical compounds.

From Gk. kyanos "dark blue, lapis lazuli," because of its presence in the Prussian blue, Fe₇(CN)₁₈, a dye which was first accidentally made around 1706, by heating substances containing iron and carbon and nitrogen.

1) A colorless, very poisonous, flammable gas with a smell of bitter almonds. Chemical formula: C₂N₂. It was discovered by Louis Joseph Gay-Lussac (1778-1850) in 1848.
2) A univalent radical, CN, found in simple and complex cyanide compounds. CN exists in → interstellar medium and is one of the main molecules detected in → comets. It has energy levels at 113 and 227 GHz (2.6 and 1.3 mm wavelength) above the ground level.

→ cyano- + → -gen.

An → absorption band of molecular origin characterizing the spectra of → late-type stars (G0 and later, e.g. → S star). → Cyanogen absorption is an important → luminosity criterion for → low-mass stars, and is more pronounced in → giant stars than in → dwarf stars of the same → spectral type.

→ cyanogen; → band.