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

1) The act of compacting or the state of being compacted.
2) Geology: The physical process whereby the volume of a fixed mass of sediment reduces either by the weight of overlying deposits or by the pressure of earth movements.

→ compact; → -tion.

1) General: The quality of being compact.
2) For a → compact object, a rough measure of the importance of gravity, defined by a dimension-less quantity which is the ratio between its gravitational radius (→ Schwarzschild radius) and its physical radius. It is expressed as φ = 2 GM / c² R, where M and R are the mass and radius of the object respectively, and G and c are the → gravitational constant and speed of light respectively. For a → black hole  φ is of order unity (by definition), for a → white dwarf it is of order 10^-4, and for a → neutron star  φ is around 0.5.

→ compact + → -ness.

A fraction in which the → numerator or → denominator, or both, contain fractions. For example (3/5)/(6/7). Also called → compound fraction.

→ complex; → fraction.

Same as → complex fraction.

→ compound; → fraction.

In thermodynamics, the quantity Z = pV_m/RT, in which P is the gas pressure, V_m the molar volume, R the gas constant, and T the temperature. The compression factor is a measure of the deviation of a real gas from an ideal gas. For an ideal gas the compression factor is equal to 1.

→ compression; → facteur.

A mapping of an infinite → space-time onto a finite one that may make the far away parts of the former accessible to study. The technique invented by Penrose defines an equivalence class of → metrics, g_ab being equivalent to ĝ_ab = Ω²g_ab, where Ω is a positive scalar function of the space-time that modifies the distance scale making the asymptotics of the physical metric accessible to study.

→ conformal; → compactification.

1a) The act or state of touching or being in immediate proximity, as in a → contact binary.
1b) One of the instances when the apparent position of the edges of the Sun and the Moon cross one another during an eclipse. They are designated as the → first contact, → second contact, → third contact, and → fourth contact. See also → contact binary, → last contact.
2a) (v. intr.) To be in or come into contact.
2b) (v.tr.) To bring or put in contact.

From L. contactus "a touching," p.p. of contingere "to touch," from → com- "together" + tangere "to touch."

Parmâs "contact, touching," stem of parmâsidan "to touch, feel," from *pari-mars-, from Indo-Iranian *pari- "around" (O.Pers. pariy "around, about," Av. pairi "around, over," Skt. pari) + *mars- "to touch; to wipe, rub," Mid.Pers. marz "contact, touching," marzitan "to touch," Mod.Pers. mâlidan "to rub," Av. marəz- "to rub, wipe," marəza- "border, district," Skt. mrś- "to touch," mrśáti; L. mulceo "to caress," margo "edge" (Fr. marge "margin"); P.Gmc. *marko; Ger. Mark; E. mark, margin.

1) A → binary star system in which the two → components are so close that they exchange gases in a complex manner. Their overlapping → gravitational fields form a "peanut" shaped → equipotential surface.
2) Two roughly similar-sized asteroids or cometary nuclei resting on one another, presumably after coming together very gently. Examples are → Comet 67P/Churyumov-Gerasimenko and the asteroid → (486958) 2014 MU69.

→ contact; → binary.

In mathematics, a fraction whose numerator is an integer and whose denominator is an integer plus a fraction whose numerator is an integer and whose denominator is an integer plus a fraction and so on.

→ continued; → fraction.

1) To become smaller, shorter, tighter, as a metal when cooled.
2) To reduce to smaller size by or as if by squeezing or forcing together, e.g. contract a muscle.
→ contraction, → gravitational contraction, → Kelvin-Helmholtz contraction, → length contraction, → Lorentz contraction, → vena contracta.

From M.E., from O.F., from L. contractus, p.p. of contrahere "to draw together," from → com- "together" + trahere "to draw."

Terengidan, variant taranjidan [Dehxodâ] "to contract, become rough and hard, to be squeezed, compressed," Borujerdi terengessa "cramped, tightly dressed," Malâyeri terengidan "to be tightly dressed, cramped in a garment," related to tarang "horse girth, a strap for fastening a load," Proto-Iranian *trng- "to pull tight, squeeze, compress;" PIE base *strenk- "to pull tight, twist; tight, narrow" (cf. L. stringere "to bind or draw tight;" Gk. strangein "to twist;" Lith. stregti "to congeal;" O.E. streccian "to stretch," streng "string;" Ger. stramm, Du. stram "stiff").

An act or instance of contracting; the quality or state of being contracted.
→ gravitational contraction, → Kelvin-Helmholtz contraction, → length contraction, → Lorentz contraction.

Verbal noun of → contract.

1) A numerical factor that, by multiplication or division, translates one unit or value into another.
2) In → molecular cloud studies, a factor used to convert the → carbon monoxide (CO) line intensity to → molecular hydrogen (H₂) → column density; usually denoted X_CO = I(CO) / N(H₂). This useful factor relates the observed CO intensity to the cloud mass. A general method to derive X_CO is to compare the → virial mass and the ¹²CO (J = 1-0) luminosity of a cloud. The basic assumptions are that the CO and H₂ clouds are co-extensive, and molecular clouds obey the → virial theorem. However, if the molecular cloud is subject to ultraviolet radiation, selective → photodissociation may take place, which will change the situation. Moreover, molecular clouds may not be in → virial equilibrium. To be in virial equilibrium molecular clouds must have enough mass, greater than about 10⁵ solar masses. The way → metallicity affects X_CO is a matter of debate, and there is no clear correlation between X_CO and metallicity. Although lower metallicity brings about higher ultraviolet fields than in the solar vicinity, other factors appear to be as important as metallicity for the determination of X_CO. In the case of the → Magellanic Clouds, X_CO(SMC) = 14 ± 3 × 10²⁰ cm^-2 (K km s^-1)^-1, which is larger than X_CO (LMC) = 7 ± 2 × 10²⁰ cm^-2 (K km s^-1)^-1. An independent method to derive X_CO is to make use of the gamma ray emission from a cloud. The flow of → cosmic ray protons interacts with interstellar low-energy hydrogen nuclei in clouds creating neutral → pions. These pions quickly decay into two gamma rays. It is therefore possible to estimate the number of hydrogen nuclei and hence the cloud mass from the gamma ray counts. Such a gamma-ray based conversion factor is estimated to be 2.0 × 10²⁰ cm^-2 (K km s^-1)^-1 for Galactic clouds, in good agreement with the result obtained from the virial method. However, the gamma ray flux is not well known in general, so this method is uncertain as well. See, e.g., Fukui & Kawamura, 2010 (ARAA 48, 547).

→ conversion; → factor.

A spiral-shaped density enhancement formed around a star when fast stellar winds collide with slower material. This large-scale wind structure can extend from the stellar surface to possibly several tens of stellar radii. The CIRs can be produced by intensity irregularities at the stellar surface, such as dark and bright spots, magnetic loops and fields, or non-radial pulsations. The surface intensity variations alter the radiative wind acceleration locally, which creates streams of faster and slower wind material. CIRs are responsible for the → discrete absorption components seen in some ultraviolet → resonance lines of → hot stars (S. R. Cranmer & S. P. Owocki, 1996, ApJ 462, 469).

→ corotate; → interaction; → region.

A quantity, denoted a(t), which describes how the distances between any two galaxies change with time. The physical distance d(t) between two points in the Universe can be expressed as d(t) = R(t).x, where R(t) is the → scale factor and x the → comoving distance between the points. The cosmic scale factor is related to the → redshift, z, by: 1 + z = R(t₀)/R(t₁), where t₀ is the present time and t₁ is the time at emission of the radiation. The quantity (1 + z) gives the factor by which the → Universe has expanded in size between t₁ and t₀. It is also related to the → Hubble parameter by H(t) = R^.(t)/R(t), where R^.(t) is the time → derivative of the scale factor. In an → expanding Universe the scale factor increases with time. See also the → Friedmann equation.

→ cosmic; → scale; → factor.

The reciprocal force between two or more → charged particles according to → Coulomb's law.

→ coulomb; → interaction.

To cause to be → inactive; remove the → effectiveness of (Dictionary.com).

→ de-; → activate.

A fraction expressed by using → decimal representation, as opposed to a vulgar fraction. For example, 2/5 is a vulgar fraction; 0.40 is a decimal fraction.

→ decimal; → fraction.

The ratio between the D/H value in → water and in → molecular hydrogen, as expressed by:
f = [(1/2)HDO/H₂O]/[(1/2)HD/H₂] = (D/H)_H2O/(D/H)_H2.
When f> 1, there is → deuterium enrichment.

→ deuterium; → enrichment; → factor.

The difference between the deuterium (D)/hydrogen (H) → abundance → ratio in an object with respect to that representing a standard or mean value for that type of objects. Same as → isotope fractionation of deuterium. In the gas phase chemistry many of the D fractionation reactions produce an excess of D atoms relative to → hydrogen atoms. Deuterium fractionation in → interstellar cloud cores, → protostars, and → Solar System bodies is frequently used to infer important aspects of their physical and chemical histories. For example, the → deuterium enhancement in the Earth's sea water, with respect to the cosmic abundance, has been interpreted as being due to → enrichment by → comet-like → planetesimals colliding with the young Earth.

→ deuterium; → fractionation.

A problem encountered in astronomical spectroscopy, which consists of a loss of light from some wavelengths due to → atmospheric dispersion. In simple terms, differential refraction means that at nonzero → zenith distances an object cannot be simultaneously placed at the same position within a → slit at all wavelengths. This problem becomes more important for increasing → airmass, larger → spectral range, and smaller → slitwidths. To remedy this drawback, the slit should always be oriented along a direction perpendicular to the horizon, since differential refraction occurs in that direction.

→ differential; → refraction.