alpha element knee
zânu-ye bonpâr-e âlfâ
The point in the plot showing → alpha element abundances ([α/Fe]) of a galaxy as a function of the → metallicity ([Fe/H]) where the α-element abundance drops. The metallicity of the turn-over in α-element abundances is linked to the → star formation rate during the early stage of star formation in a galaxy and therefore also depends on the total mass of the system. Higher star formation efficiency leads to higher overall metallicity before the onset of → Type Ia supernova → enrichment, and thus to a knee that is located at higher [Fe/H] values.
Fr.: émission alpha
The release of → alpha particles at high velocity from an atom's nucleus as it undergoes radioactive transformation.
Fr.: émetteur d'alpha
An atomic nucleus decaying by an → alpha particle emission.
Fr.: échelle alpha
Fr.: offset en ascension droite
A short distance from the target, in right ascension, where the telescope is pointed for various purposes.
Fr.: particule alpha
A positively charged particle emitted from the nuclei of certain atoms during radioactive disintegration. The alpha particle has an atomic weight of 4 and a positive charge equal in magnitude to 2 electronic charges; hence it is essentially a helium nucleus.
Fr.: processus α
A class of → nuclear fusion reactions by which stars convert → helium into → heavy elements. Once carbon has been created, through → triple alpha process, in a star's interior, it can then continue to fuse with further → alpha particles to produce progressively heavier elements called → alpha particles. The first stage produces oxygen, followed by neon, magnesium, silicon, sulfur, argon, calcium, titanium, chromium and iron. This is known as the → alpha ladder, with energy released as a photon at each stage.
Alphard (α Hydrae)
The only bright star in the constellation → Hydra, that has a magnitude of about 2 and a reddish color. Alphard is a giant of spectral type K3, and has a → white dwarf→ companion. Alphard is mild barium star probably contaminated by its companion before becoming a white dwarf.
Alphard, from Ar. Al-Frad ash-Shuja' "the solitary of the Serpent," from Frad "solitary" + Shuja' "a species of serpent".
Alphekka (α Coronae Borealis)
Also known as Gemma, the brightest star in Corona Borealis (visual magnitude 2.23). Alphekka is an A type dwarf lying at about 7 → light-years. Actually it has a faint Sun-like (G5 V) companion, that produces an eclipse of the primary every 17.4 days.
Alphekka, from Ar. Nayyir al-Fakkah "the bright of the broken" (ring of star), from Nayyir "bright" + fakkah "broken," from fakk "to disjoin, unloose".
Alpheratz (α Andromedae)
The brightest star in → Andromeda with a visual magnitude of 2.07. Alpheratz is a blue → subgiant star of spectral type B8 IV lying at a distance of about 97 → light-years. It is particularly remarkable because of the unusual strength of mercury and manganese absorption lines in its spectrum.
Other names for this star are Alpherat, Sirrah, or Sirah.
These names derive from Ar. As-Surrat al-Faras
The quality of an → anamorphic system.
râžmân-e ânârixt, ~ ânârixtmand
Fr.: système anamorphique
An optical system whose optical power, and imaging scale, differs in the two principal directions. See also → anamorphosis.
1) Optics: The formation of a distorted image by an
→ anamorphic system.
From Gk. anamorphosis "transformation," noun of action from anamorphoein "to transform," from → ana- "up," + morphe "form" + -sis a suffix forming abstract nouns of action, process, state, condition, etc.
angular momentum catastrophe
negunzâr-e jonbâk-e zâviye-yi
Fr.: catastrophe du moment angulaire
A problem encountered by the → cold dark matter model of galaxy formation. The model predicts too small systems lacking → angular momentum, in contrast to real, observed galaxies. → cusp problem; → missing dwarfs.
Fr.: photométrie d'ouverture
Photometry using a diaphragm to isolate a small sky area, either directly with a focal-plane diaphragm, or with an image processing system.
The point in the orbit of a planet, or other object in the solar system, which is furthest from the Sun.
Aphelion, from L. aphelium, from Gk. → apo- + helios "sun," cognate with L. sol, Skt. surya, Av. hvar-, Mod.Pers. xor, hur, O.H.G. sunna, Ger. Sonne, E. sun; PIE *sawel- "sun".
Apâhur, from Pers. prefix apâ, → apo-, + hur "sun."
Fr.: distance à l'aphélie
fizik-e kârbordi (#)
Fr.: physique appliquée
A set of topics in physics intended for a particular or practical use. Applied physics programs are usually interfaces between pure physics and technology.
The study of the surface features of Mars; the geography of Mars.
Areography, from Gk. Ares "Mars" + → -graphy.
Bahrâm-negâri, from Bahrâm "Mars" + -negâri→ -graphy.
Fr.: sphère armillaire
An ancient instrument, used since ancient times until the Middle ages and later, to determine positions of celestial bodies. It consisted of an assemblage of rings, all circles of the same sphere, designed to represent the positions of the important circles of the celestial sphere.
L. armillarius, from armilla "arm ring, bracelet," from armus "arm" + → sphere.
Zâtolhelaq from Ar. "multi-ringed," from zât "holder, keeper" + helaq "rings," from halqah "ring."