durbin-e bâlon-bord, teleskop-e ~
Fr.: télescope porté par ballon
A remotely guided or automatic telescope carried to high altitudes by a balloon.
→ balloon astronomy; borne "a past participle of bear," from O.E. beran "bear, bring, wear," from P.Gmc. *beranan (O.H.G. beran, Goth. bairan "to carry"), from PIE root *bher-; "to carry;" compare with Av./O.Pers. bar- "to bear, carry," bareθre "to bear (infinitive)," bareθri "a female that bears (children), a mother," Mod.Pers. bordan "to carry," Skt. bharati "he carries," Gk. pherein, L. fero "to carry." → telescope.
Fr.: rapport baryon-photon
The → baryon number compared with the number of photons in the → Universe. The baryon-photon ratio can be estimated in a simple way. The → energy density associated with → blackbody radiation of → temperature T is aT4, and the mean energy per photon is ~kT. Therefore, the number density of blackbody photons for T = 2.7 K is: nγ = aT4/kT = 3.7 x 102 photons cm-3, where a = 7.6 x 10-15 erg cm-3 K-4 (→ radiation density constant) and k = 1.38 x 10-16 erg K-1 (→ Boltzmann's constant). The number density of baryons can be expressed by ρm/mp, where ρm is the mass density of the Universe and mp is the mass of the → proton (1.66 x 10-24 g). → CMB measurements show that the baryonic mean density is ρm = 4.2 x 10-31 g cm-3 (roughly 5% of the → critical density). This leads to the value of ~ 2 x 10-7 for the number density of baryons. Thus, the baryon/photon ratio is approximately equal to η = nb/nγ = 2 x 10-7/3.7 x 102 ~ 5 x 10-10. In other words, for each baryon in the Universe there is 1010 photons. This estimate is in agreement with the precise value of the baryon-photon ratio 6.14 x 10-10 derived with the → WMAP. Since the photon number and the baryon number are conserved, the baryon-photon ratio stays constant as the Universe expands.
A radioactive isotope of carbon, whose nucleus contains 6 protons and 8 neutrons; also called → radiocarbon. 14C is naturally produced in the atmosphere when a neutron created by a cosmic ray hits the nucleus of an atom of nitrogen-14. The nucleus absorbs the neutron and ejects a proton, thereby transforming itself into 14C. It decays back to nitrogen, with a half-life is 5730 years, after emitting an electron (146C → 147N + e- + νe). See also → radiocarbon dating.
carbon-enhanced metal-poor star (CEMP)
setâre-ye kamfelez-e karbon bolandidé
Fr.: étoile pauvre en métaux enrichie en carbon
A star that presents very low → iron → abundances [Fe/H] < -4 but an → anomalous richness in carbon. CEMP stars have been defined as a subset of → metal-poor stars that exhibit elevated [C/Fe] ≥ +1.0. It has been recognized that ~15-20% of stars with [Fe/H] < -2.0 are carbon enhanced. This fraction rises to 30% for [Fe/H] < -3.0, to 40% for [Fe/H] < -3.5, and ~75% for [Fe/H] < -4.0. This increasing trend of CEMP-star frequency with declining [Fe/H] is confirmed by the observation of many thousands of CEMP stars (Daniela Carollo + ApJ 2014, 788, 180). See also → extremely metal-poor star (EMPS)
com-, col-, con-, cor-, co-
ham- (#), han- (#), hâ- (#), ha- (#)
Fr.: com-, col-, con-, cor-, co-
Prefix denoting "together; with; joint; jointly". It is sometimes used for intensification as in complete, complain, convince.
M.E., from O.L., classical L. form cum "together, together with," Gk. koinos "common," from PIE *kom- "beside, near, by, with."
Ham- and ham "together, with; same, equally, even," Mid.Pers. ham-, like L. com- and Gk. syn- with neither of which it is cognate. O.Pers./Av. ham-, Skt. sam-, sa-; also O.Pers./Av. hama- "one and the same," Skt. sama-, Gk. homos-; originally identical with PIE numeral *sam- "one," from *som-. The Av. hąm- (nasal a) appears in various forms: ham-, han- (before gutturals, palatals, dentals) and also həm-, hən-, ha- (Bartholomae, 1772). Variants in Pers. ha- as in (Anâraki) ha-bend, → connect, and (Kurd.) hasûn "to whet, sharpen," and hâ- as in hâ-dâdan, hâ-gereftan, see Dehxodâ.
Fr.: expérience de Davisson-Germer
The experiment carried out in 1927 that confirmed the → de Broglie hypothesis as to the → wave nature of the → electron. It showed that electrons scattering off crystals form a → diffraction pattern. The experimental setup consisted of a → nickle chloride → crystal as → target, an electron gun, and a → detector placed on a graduated circular scale. The intensity of the reflected electrons was measured as a function of angle and electron energy. The observations showed a strong intensity peak at a certain angle. The nickel crystal acted as a → diffraction grating. → Constructive interference occurred at a particular angle, where the peak intensity was observed in accord with → Bragg's law. Interestingly, the intent of the initial experiment was was not to confirm the de Broglie hypothesis. In fact, the discovery was made by accident.
Carried out by American physicists Clinton Davisson (1881-1958) and Lester Germer (1896-1971); → experiment.
karânmand bé parâš
Fr.: limité par la diffraction
giti-ye Eddington-Lemaître (#)
Fr.: Univers d'Eddington-Lemaître
A theoretical model in which the → cosmological constant plays a crucial role by allowing an initial phase that is identical to the Einstein static Universe. After an arbitrarily long time, the Universe begins to expand. The difficulty with this model is that the initiation of galaxy formation may actually cause a collapse rather than initiate an → expansion of the Universe.
Eddington-Sweet time scale
marpel-e zamâni-ye Eddington-Sweet
Fr.: échelle de temps d'Eddington-Sweet
The time required for the redistribution of → angular momentum due to → meridional circulation. The Eddington-Sweet time for a uniformly → rotating star is expressed as: τES = τKH . GM / (Ω2 R3), where τKH is the → Kelvin-Helmholtz time scale, R, M, and L designate the radius, mass, and luminosity respectively, Ω the → angular velocity, and G the → gravitational constant. The Eddington-Sweet time scale can be approximated by τES≅ τKH / χ, where χ is the ratio of the → centrifugal force to → gravity. For the Sun, χ ≅ 10-5 resulting in an Eddington-Sweet time scale which is too long (1012 years), i.e. unimportant. In contrast, for a rotating → massive star χ is not so much less than 1. Hence the Eddington-Sweet circulation is very important in massive stars.
Named after the prominent British astrophysicist Arthur S. Eddington (1882-1944), who was the first to suggest these currents (in The Internal Constitution of the Stars, Dover Pub. Inc., New York, 1926) and P. A. Sweet who later quantified them (1950, MNRAS 110, 548); → time scale.
joft-e elektron-pozitron (#)
Fr.: paire électron-positron
bâl-e parâkaneš-e elektron
A → line broadening phenomenon involving the scattering effect of → free electrons on the → radiation transfer in → stellar atmospheres. The scattering of radiation by free electrons plays an important role in the atmospheres of → hot stars, such as → O-types, early → B-types, and → Wolf-Rayet stars. The first detailed study of electron scattering in Wolf-Rayet stars was by Castor et al. (1970), who used electron scattering to explain the broad emission wings of N IV λ3483 in HD 192163. In → P Cygni stars the explanation of the very extended (almost symmetric) wings on the → Balmer lines as caused by electron scattering was first made by Bernat & Lambert (1978). Hillier (1991) showed that significant reduction in the strength of an electron-scattering wing can be achieved in a model of → clumped wind for a lower mean → mass loss rate. This resulted in a better agreement between observations and theoretical predictions. Electron-scattering wings provide diagnostics regarding the presence of density inhomogeneities in → stellar winds (Münch, 1948, ApJ 108, 116; Hillier, 1991, A&A 247, 455).
ionization-bounded H II region
nâhiye-ye H II-e yoneš-karânmand
Fr.: région H II bornée par ionisation
nufe-ye Johnson-Nyquist (#)
Fr.: bruit de Johnson-Nyquist
The random fluctuation of voltage across a resistor caused by the thermal excitation of electrons within it, and the dissipation of power associated with these fluctuations. More generally, an intrinsic noise generated by thermal agitation of electrons by all bodies whose temperature is above 0 K. Also called → thermal noise, Johnson noise, or Nyquist noise.
Named after John Bertrand Johnson (1887-1970) and Harry Nyquist (1889-1976) Swedish-born American engineers and physicists, who did important work on thermal noise and information theory. → noise.
Fr.: solution de Larson-Penston
The analytical solution to the → hydrodynamic equations describing the → collapse of an → isothermal sphere. The Larson-Penston solution is → self-similar for a purely dynamical isothermal collapse with spherical symmetry. It corresponds to the collapse prior to the formation of a → protostar, and thus is suitable for the study of → pre-stellar cores. The Larson-Penston solution was extended by Shu (1977) to obtain a whole family of solutions for this problem.
Named after R. B. Larson (1969, MNRAS 145, 271) and M. V. Penston (1969, MNRAS 144, 425), who simultaneously, but independently, did this study.
law of non-contradiction
Fr.: principe de non-contradiction
Same as → principle of non-contradiction.
low-ionization nuclear emission-line region
nâhiye-ye hasteyi bâ xatt-e gosili-ye kamyoneš (#)
Fr.: Noyau de galaxie à raies d'émission de faible ionisation
Same as → LINER.
âzmâyeš-e Michelson-Morley (#)
Fr.: expérience de Michelson-Morley
An experiment performed in 1887 to establish the presence or absence of an → ether, a medium through which light was supposed to travel. The experiment aimed to measure the speed of light coming from different directions. However no → ether drift was found. The null results obtained showed that the ether hypothesis was incorrect. Consequently, the theory of → special relativity, with its hypothesis that the speed of light is the same in all → inertial frames, reconciled the results of the Michelson-Morley experiment with the rest of physics.
→ Michelson interferometer; Michelson received the Nobel Prize in 1907 for his work, the first American to receive the Prize in science. Edward Williams Morley (1838-1923), an American chemist; → experiment.
bonpâr-e giroft-e notron
Fr.: élément de capture de neutron
A → nucleosynthesis process responsible for the generation of the → chemical elements heavier than the → iron peak elements. There are two possibilities for → neutron capture: the slow neutron-capture process (the → s-process) and the rapid neutron-capture process (the → r-process). The s-process is further divided into two categories: the weak s-component and the main s-component. Massive stars are sites of the weak component of s-process nucleosynthesis, which is mainly responsible for the production of lighter neutron-capture elements (e.g. Sr, Y, and Zr). The s-process contribution to heavier neutron-capture elements (heavier than Ba) is due only to the main s-component. The low- to intermediate-mass stars (about 1.3-8 Msun) in the → asymptotic giant branch (AGB) are usually considered to be sites in which the main s-process occur. There is abundant evidence suggesting that → Type II supernova (SNe II) are sites for the synthesis of the r-process nuclei, although this has not yet been fully confirmed. The observations and analysis on → very metal-poor stars imply that the stars with [Fe/H] ≤ -2.5 might form from gas clouds polluted by a few supernovae (SNe). Therefore, the abundances of → heavy elements in → metal-poor stars have been used to learn about the nature of the nucleosynthetic processes in the early Galaxy (See, e.g., H. Li et al., 2013, arXiv:1301.6097).
Fr.: formule de Newton-Leibniz
The formula expressing the value of a → definite integral of a given function over an interval as the difference of the values at the end points of the interval of any → antiderivative of the function: ∫f(x)dx = F(b) - F(a), summed from x = a to x = b.