tâjnegâr (#), hurtâjnegâr (#)
An instrument which, when used in a telescope, produces an artificial eclipse, permitting the study of the → solar corona without a total eclipse of the Sun. It was invented in 1930 by the French astronomer Bernard Lyot (1897-1952).
Of, or relating to → coronagraphy.
A technique used to observe a relatively dim object (like an → exoplanet) lying close to an outshining bright source (such as star). This is done by blocking the bright object, in the same way the → solar corona is observed using a → coronagraph.
→ coronagraph + suffix -y.
binâbnegâr bâ pâšeâš-e chalipaayi, ~ ~ ~ xâji
Fr.: spectrographe à dispersion croisée
A spectrograph that utilizes cross dispersion.
The science of forms, properties, and structure of crystals.
L. delphinus, from Gk. delphin-, delphis; akin to Gk. delphys womb, cf. Skt. garbha- "womb; interior."
Dolfin loanword from Gk.
Fr.: Delta Cephée
The prototype of classical → Cepheid variables, which is a pulsating → yellow supergiant. John Goodricke was the first in 1784 to discover its variability. The star shows a quick and sharp rise from minimum to maximum, and slowly declines to its minimum again. The changes in brightness are accompanied by and principally caused by changes in stellar temperature and also by changes in radius. δ Cephei was actually the second Cepheid variable to be discovered. The first one, Eta Aquilae, had been discovered earlier the same year by Edward Pigott. δ Cephei varies with a period of 5.366341 days (or 5 days 8 hours 37.5 minutes) from magnitude 3.48, spectral type F5 Ib in its maximum to magnitude 4.37, spectral type G2 Ib in its minimum. It lies at a distance of 1,340 → light-years.
To make two signals out of phase. For example, to get one signal at its highest peak while the other signal is at its lowest peak; they will be 180 degrees out of phase.
Same as → out of phase.
Past participle of → dephase.
Fr.: physique déterministe
The classical representation of the laws of nature according to which a particular future state (B) will arise from a particular past one (A). In contrast to → quantum physics which deals with the probability for the transition from A to B.
Deterministic, adj. of determinism; → physics.
A device with a restricted aperture, located in an optical system at any of several points, that cuts off marginal light rays not essential to image formation.
From L.L. diaphragma, from Gk. diaphragma "partition, barrier," from diaphrassein "to barricade," from dia- "across" + phrassein "to fence or hedge in."
Miyânban, from miyân "middle, interior, between" (Mid.Pers. miyân "middle," Av. maiδya-, maiδyāna- "medium, middle," cf. Skt. mádhya- "middle, intemediate," Gk. mesos "middle," L. medius "middle," Goth. midjis, O.E. midd "middle," O.C.S. medzu "between," Arm. mej "middle," PIE *medhyo- "middle," , from base *me- "between") + band "barrier, shutter," from bastan "to bind, shut" (Mid.Pers. bastan/vastan "to bind, shut," Av./O.Pers. band- "to bind, fetter," banda- "band, tie," cf. Skt. bandh- "to bind, tie, fasten," PIE *bhendh- "to bind").
Diphda (β Ceti)
Diphda, from Ar. zafda' (
Mid.Pers. wazaγ, vak; Av. vazaγa- "frog," → tadpole orbit.
Phonetics: A → vowel sound produced by a blended sequence of two separate vowels in a single syllable, where the sound begins as one vowel and moves toward another (as in loud, light, and lair).
From M.Fr. diphthongue, from L.L. diphthongus, from Gk. diphthongos "having two sounds," from → di- "double" + phthongos "sound, voice."
Fr.: graphe orienté
Fr.: tomographie Doppler
A technique using a series of Doppler-shifted line profiles at different orbital phases in cataclysmic variable stars (CVs) to image the compact system in the light of a particular emission line. Although developed for CVs, the method has wider applications. An alternative method is → eclipse mapping.
Fr.: céphéide naine
An old name for a class of pulsating variable stars with small variations in amplitude, also called an AI Velae star or delta Scuti star. They lie in the lower part of the Cepheid instability strip.
dwarf spheroidal galaxy (dSph)
kahkašân-e korevâr-e kutulé (#)
Fr.: galaxie sphéroïdale naine
A subtype of dwarf ellipticals (→ dwarf elliptical galaxy), which are companion to the → Milky Way and other similar galaxies. The first example of such objects was discovered by Harlow Shapley (1938) in the constellation → Sculptor. 22 such galaxies are known currently to orbit the Milky Way and at least 36 exist in the → Local Group of galaxies. Nearby → galaxy clusters such as the → Virgo, → Fornax, → Centaurus, and → Coma clusters contain hundreds to thousands of individual dSph galaxies. These galaxies have very low → surface brightnesses, as low as only 1% that of the → sky background. They are also among the smallest, least luminous galaxies known. Most of the radiation from dSph galaxies is emitted by stars in the optical portion of the → electromagnetic spectrum. The lack of strong → emission lines, → infrared, or → radio emission suggests that these galaxies are generally devoid of → interstellar medium. The velocities of stars within dSph galaxies are so high that them must be disrupting. However, the bulk of mass in these galaxies might be undetected. Dynamical models that include → dark matter do adequately explain the → velocity dispersion of the stars in all dSph systems. In the most extreme cases, only 1% of the mass of the galaxy is visible. Many of the Local Group dSph galaxies show evidence for → star formation more recent than 10 Gyr.
Fr.: sphère de Dyson
A hypothetical structure built around a → star by an advanced → civilization to utilize most or all of the → energy radiated by their star. The idea of such a sphere was first formalized and popularized by theoretical physicist Freeman Dyson in 1960, though it was originally put forward by a 1945 science fiction novel. Dyson assumed that the power needs of → intelligent civilizations never stops increasing. He also proposed that searching for the existence of such structures might lead to the discovery of advanced civilizations elsewhere in the Galaxy. Sometimes referred to as a → Dyson shell or → megastructure.
Freeman John Dyson (1923-). His article, entitled "Search for Artificial Stellar Sources of Infrared Radiation," appeared in the 1960 issue of Science, 131 (3414), 1667-1668; → sphere.
early AGB phase
fâz-e AGB-ye âqâzin
Fr.: phase initiale de l'AGB
A fairly long-lived step in the evolution of → low-mass and → intermediate-mass stars when helium burning shifts from the center to a shell around the core. At this phase the stellar luminosity is provided almost entirely by → helium shell burning. The He-shell burning generally adds mass to the growing carbon/oxygen core, resulting in → degenerate matter due to its increasing density.