An Etymological Dictionary of Astronomy and Astrophysics
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فرهنگ ریشه شناختی اخترشناسی-اخترفیزیک

M. Heydari-Malayeri    -    Paris Observatory

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Number of Results: 30 Search : limit
Balmer limit
  حد ِ بالمر   
hadd-e Bâlmer

Fr.: limite de Balmer   

The wavelength in the blue end of the → Balmer series, at 3646 Å, near which the separation between successive lines decreases and approaches a → continuum.

Balmer; → limit.

canonical upper limit
  حد ِ زبرین ِ جرم   
hadd-e zabarin-e jerm

Fr.: limite supériure canonique   

A physical upper mass limit near 150 Msun assumed for the stellar → initial mass function (Kroupa et al. 2012, arXiv:1112.3340).

canonical; → upper; → limit.

central limit theorem
  فربین ِ حد ِ مرکزی   
farbin-e hadd-e markazi

Fr.: théorème central limite   

A statement about the characteristics of the sampling distribution of means of → random samples from a given → statistical population. For any set of independent, identically distributed random variables, X1, X2,..., Xn, with a → mean μ and → variance σ2, the distribution of the means is equal to the mean of the population from which the samples were drawn. Moreover, if the original population has a → normal distribution, the sampling distribution of means will also be normal. If the original population is not normally distributed, the sampling distribution of means will increasingly approximate a normal distribution as sample size increases.

central; → limit; → theorem.

Chandrasekhar limit
  حدِ چاندراسکهار   
hadd-e Chandrasekhar (#)

Fr.: limite de Chandrasekhar   

A limiting mass of about 1.44 Solar masses that the theory predicts a non-rotating → white dwarf can attain without collapsing to become a → neutron star or a → black hole. Over this → critical mass, the degeneracy pressure will be unable to bear the load of the bulk mass.

Named after Subrahmayan Chandrasekhar (1910-1995), Indian-born American astrophysicist who, with William A. Fowler, won the 1983 Nobel Prize for Physics for his research on white dwarfs; → limit.

confusion limit
  حد ِ پشش   
hadd-e pašeš

Fr.: limite de confusion   

The → fluctuations of the → background  → sky brightness below which astronomical → sources cannot be → detected individually. The confusion limit is reached when the density of sources brighter than the → root mean square  → noise becomes high enough within the area of the resolution element.

confusion; → limit.

diffraction-limited
  کران‌مند به پراش   
karânmand bé parâš

Fr.: limité par la diffraction   

The quality of an → optical system that is capable of producing images with angular resolution as small as the theoretical limit of the → Airy disk.

diffraction; limited, adj. of → limit.

Karânmand "bounded, limited," from karânboundary + -mand possession suffix; parâš diffraction.

Eddington limit
  حد ِ ادینگتون   
hadd-e Eddington (#)

Fr.: limite d'Eddington   

The theoretical upper limit of → luminosity at which the → radiation pressure of a light-emitting body would exceed the body's → gravitational attraction. A star emitting radiation at greater than the Eddington limit would break up. The Eddington luminosity for a non-rotating star is expressed as: LEdd = 4πGMmpcσT-1, where G is the → gravitational constant, M the star mass, mp the → proton mass, c the → speed of light, and σT the → Thomson cross section. It can also be written as LEdd = 4πGMcκ-1, where κ is the → opacity. In terms of solar mass, the Eddington limit can be expressed by: LEdd = 1.26 × 1038 (M/Msun) erg s-1. See also → rotational Eddington limit.

Named after Arthur Stanley Eddington (1882-1944), prominent British astrophysicist; → limit.

elastic limit
  حد ِ کشایند   
hadd-e kešâyand

Fr.: limite d'élasticité, ~ élastique   

The smallest → stress beyond which a → solid body can no longer return to its original shape. The material ceases to obey → Hooke's law. Also called → yield point.

elastic; → limit.

Greisen-Zatsepin-Kuzmin limit (GZK)
  حد ِ گریسن-زاتسپین-کوزمین   
hadd-e Greisen-Zatsepin-Kuzmin

Fr.: limite de Greisen-Zatsepin-Kuzmin   

A theoretical limit of approximately 6 × 1019  → electron-volts for the energy of → cosmic rays above which they would lose energy in their interaction with the → cosmic microwave radiation background photons. Cosmic ray protons with these energies produce → pions on blackbody photons via the Δ resonance according to: γCMB + p → p + π0, or γCMB + p → n + π+, thereby losing a large fraction of their energy. These interactions would reduce the energy of the cosmic rays to below the GZK limit. Due to this phenomenon, → Ultra-high-energy cosmic rays are absorbed within about 50 Mpc.

Named after Kenneth Greisen (1966), Physical Review Letters 16, 748 and Georgiy Zatsepin & Vadim Kuzmin (1966), Journal of Experimental and Theoretical Physics Letters 4, 78; → limit.

Humphreys-Davidson limit
  حد ِ همفریز-دیویدسون   
hadd-e Humphreys-Davidson

Fr.: limite de Humphreys-Davidson   

An empirical upper → luminosity boundary in the → H-R diagram. It consists of two sections, a sloping part and a horizontal part. The sloping part, which decreases with decreasing → effective temperature, corresponds roughly to the → Eddington limit. The horizontal part is the temperature-independent upper luminosity limit for late-type → hypergiants. It is thought that → massive stars above the Humphreys-Davidson limit encounter an → instability, possibly due to the opacity-modified Eddington limit, and experience high → mass loss episodes which prevent their evolution to cooler temperatures. → Luminous Blue Variable stars are examples of this high mass loss phase.

Named after Roberta M. Humphreys and Kris Davidson, who first dealt with this limit (1979, ApJ 232, 409); → limit.

limit
  حد   
hadd (#)

Fr.: limite   

1) General: The final, utmost, or furthest → boundary or → point as to extent, amount, continuance, procedure, etc.
2a) Math.: Of a → sequence, a → number which is approached ever more closely, but never reached, by the successive terms of a convergent infinite sequence.
2b) Of a → variable, a constant C which has the property with respect to some variable V that, as the variable approaches C in value (according to some formula), the numerical difference (C - V) between the constant and the variable diminishes toward 0 but is always greater than 0.

From O.Fr. limite "a boundary," from L. limitem (nom. limes) "a boundary, embankment between fields, border," related to limen "threshold."

Loan from Ar. Hadd "limit, term."

limited
  حدمند   
haddmand

Fr.: limité   

Confined within limits; restricted or circumscribed.

Adj. of → limit.

limiting magnitude
  برز ِ حد   
borz-e hadd

Fr.: magnitude limite   

The faintest magnitude reachable by an instrument.

limit; → magnitude.

lunar ecliptic limit
  حد ِ هورپهی ِ ماه   
hadd-e hurpehi-ye mâh

Fr.: limite écliptique de la Lune   

The farthest distance from a → lunar orbit node within which, if the Moon happens to be at full, a lunar eclipse may occur. The lunar ecliptic limit extends about 12° on each side of the node.

lunar; → ecliptic; → limit.

Lyman limit
  حد ِ لایمن   
hadd-e Lyman

Fr.: limite de Lyman   

The short-wavelength end of the hydrogen Lyman series, at 912 Å. Also called → Lyman continuum. It corresponds to the energy (13.6 eV) required for an electron in the hydrogen ground state to jump completely out of the atom, leaving the atom ionized.

Lyman; → limit.

magnitude-limited survey
  بردید با برز ِ حدمند   
bardid bâ borz-e haddmand

Fr.: relevé limité en magnitude   

A survey in which the observed objects are bighter than a given → apparent magnitude.

magnitude; → limited; → volume.

Newtonian limit
  حد ِ نیوتنی   
hadd-e Newtoni

Fr.: limite newtonienne   

The limit attained by → general relativity when velocities are very smaller than the → speed of light or gravitational fields are weak. This limit corresponds to the transition between general relativity and the → Newtonian mechanics. See also → Newtonian approximation.

Newtonian; → limit.

Oort limit
  حدِ اورت   
hadd-e Oort

Fr.: limite de Oort   

1) The upper limit for the density of all matter in the plane of the Galaxy near the Sun's locality, as calculated from the velocities and distribution of stars in relation to the gravitational field of the Galactic disk. The value is 0.14 solar masses per cubic parsec, or 9.5 x 10-24 g cm3.
2) The outer boundary of the → Oort cloud. The current estimate is about 100,000 → astronomical units from the Sun, which is approximately one third of the distance to the nearest star → Alpha Centauri.

Oort cloud; → limit.

Oppenheimer-Volkoff limit
  حدِ ا ُپنهایمر-وُلکوف   
hadd-e Oppenheimer-Volkoff

Fr.: limite d'Oppenheimer-Volkoff   

The upper bound to the mass of a → neutron star, the mass beyond which the pressure of neutron → degenerate matter is not capable of preventing the → gravitational collapse which will lead to the formation of a → black hole. Modern estimates range from approximately 1.5 to 3.0 → solar masses. The uncertainty in the value reflects the fact that the → equation of state for → overdense matter is not well-known.

Oppenheimer, J.R., Volkoff, G.M., 1939, Physical Review 55, 374. Named after Robert Oppenheimer (1904-1967), an American theoretical physicist, and George Volkoff (1914-2000), a Canadian physicist, who first calculated this limit. Oppenheimer is widely known for his role as the scientific director of the Manhattan Project, the World War II effort to develop the first nuclear weapons at the secret Los Alamos laboratory in New Mexico; → limit.

photon tiring limit
  حد ِ خستگی ِ فوتون   
hadd-e xastegi-ye foton

Fr.: limite par fatigue du photon   

The maximum → mass loss rate of a star when the → wind luminosity equals the total available → stellar luminosity. The mechanical luminosity of the wind at infinity is given by: Lwind = Mdot (v2/2 + GM/R) = Mdot (v2/2 + vesc2/2). For Lwind = L*, the mass loss rate is Mdotmax = 2L*/(v2 + vesc2). Following Owoki & Gayly (1997), Mdottir is the maximum mass loss rate when the wind just escapes the gravitational potential, with v tending toward zero. Mdottir is much larger than typical mass loss rates from → line-driven winds, where the driving lines become saturated with increasing density limiting the wind mass loss rates to about 10-4 Msun yr-1 in even the most luminous stars.

photon; tiring, from tire "to weary; become weary," → tired; → limit.

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