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

A material composed of two or more → metals, or of a metal or metals with a non-metal, exhibiting characteristic metallic properties. Some examples: → bronze is an alloy of → copper and → tin, brass is an alloy of → zinc and copper, and → steel is an alloy of → iron and → carbon. Alloys have properties which differ from those of their components. Moreover, different component proportions yield alloys with different properties.

From M.F. aloi, from O.Fr. alei, from aleier "to mix, combine," from L. alligare "to bind up," from → ad- "to" + → ligare "to bind."

Âlyâž, loanword from Fr.

The third brightest star in Andromeda and one of the most beautiful double stars in the sky. The brighter star of the pair appears golden yellow or slightly orange; it is a bright (of second magnitude) giant K star. The fainter companion, which appears greenish-blue, is also double.

This star is also known as Almaak, Alamak, Almak, or Almaach, from Ar. Al-'Anaq al-'Ardh "a small animal of Arabia similar to a badger."

A comprehensive treatise, compiled by Claudius Ptolemy of Alexandria, around A.D. 140, that summarized the astronomy, geography, and mathematics of antiquity, and included a star catalogue with data for 1,022 stars.

Almagest, from Ar. Al-majisti, from al "the" + Gk. megiste (suntaxis) "the greatest (composition)," from femenine of megistos, superlative of megas "great."

A book of tables, usually covering a period of one calendar year, that lists the future positions of the Moon, planets, and other prominent celestial objects, together with other useful astronomical data.

M.E. almenak, from M.L. almanach, perhaps from late Gk. almenikhiaka "ephemeris," perhaps of Coptic origin.

Axtar sâlnâmé, from axtar, → star, + sâlnâmé "calendar," from sâl, → year, + nâmé "book."

A small circle on the celestial sphere parallel to the horizon. The locus of all points of a given altitude. Also called altitude circle, circle of altitude, parallel of altitude.

Almucantar, from L. almucantarath, from Ar. almuqantarât, from al- "the" + muqantarât "sundial," from qantarah "arch".

Moqantar, from Ar., as above. Parhun-e farâzâ from parhun, → circle, + farâzâ→ altitude.

A blue star of visual magnitude 1.65 in the constellation → Taurus. Alnath is a giant star of type B7 lying at a distance of about 10.95 → light-years.

Alnath, from Ar. An-nâteh "the butting" (horn), from nath "to butt".

Šâxzan "the butting," from Mod.Pers. šâx zadan "to butt or push with the horns," from šâx "horn" + zadan "to strike, to butt".

The central and brightest of the three stars in → Orion's Belt and the fourth brightest in the whole of → Orion. Alnilam is a blue-white → supergiant of → spectral type B0 Iae with a → visual magnitude of 1.70 and a → luminosity of 375,000 times the → solar luminosity. It lies at about 1,340 → light-years.

Alnilam, from Ar. An-Nizam al-Jawza' (النظام‌الجوزاء) "the Orion's arrangement (of pearls, beads)," from nizam "arrangement" + Jawza' "Orion".

The left hand or easternmost star in → Orion's Belt, which is the fifth brightest in the whole of → Orion with a → visual magnitude of 1.79. Alnitak is a wide visual binary system consisting of components ζ Ori A (HR 1948) and ζ Ori B (HR 1949), currently separated by ~ 2''.4. ζ Ori A is a → close binary system comprising Alnitak Aa and Alnitak Ab. Aa is a hot → blue supergiant of → spectral type O9.5 Iab with an → absolute magnitude of -6.0 and an → apparent magnitude of 2.08. Its mass is estimated as being up to 33 times as massive as the Sun and to have a diameter 20 times greater. It is some 250,000 times more luminous than the Sun, with a surface temperature of about 30,000 K. It is the brightest star of class O in the night sky. Alnitak Ab is a blue → subgiant of spectral type B1 IV with an absolute magnitude of -3.9, an apparent magnitude of 4.3, and a mass of 14 Msun. Ab revolves around Ab with a period of 2,687 days. The system has a 4th magnitude companion, Alnitak B, nearly 3 arc-seconds distant. It is a B0 III type star which orbits Alnitak A every 1,500 years. Alnitak is associated with the → emission nebula  → IC 434 containing the → Horsehead Nebula.A much fainter fourth component, ζ Ori C, is located about 57'' away from ζ Ori Aa (C. A. Hummel et al., 2013, A&A 554, A52, arXiv:1306.0330).

Alnitak, from Ar. An-Nitaq al-Jawza' (النطاق‌الجوزاء) "Orion's Belt," from nitaq "belt" + Jawza "Orion."

The first letter of the Greek alphabet (A, α).

Gk. alpha, from Hebrew or Phoenician → aleph.

An annual → meteor shower that takes place within the boundaries constellation → Capricornus near the star named Alpha. The meteor shower is visible between July 03 and August 15 with the peak occurring on July 30. Alpha Capricornids meteors are bright and often include spectacular colorful → fireballs.

→ alpha; → Capricornus.

Brightest star in the constellation → Centaurus (V = -0.01 magnitude) and third brightest star in the sky; also known as → Rigil Kent. It is a main-sequence star of the same spectral class (G2 V) as the Sun. Actually, Alpha Centauri is a triple-star system, the components being designated A, B, and C. The component C is also called → Proxima Centauri because it is the closest star to the Earth (other than the Sun), at a distance of 4.22 → light-years, but it is too dim to be seen with the naked eye. Components A and B are currently about 4.36 light-years away.

Alpha (α), a Gk. letter of alphabet used in the → Bayer designation; Centauri, genitive of → Centaurus.

A system of three stars, the → close binary Alpha Centauri A (→ spectral type G2 V) and Alpha Centauri B (K1 V), and a small and faint → red dwarf, Alpha Centauri C (M6 Ve), better known as → Proxima Centauri. To the unaided eye, the two main components (AB) appear as a single object with an → apparent visual magnitude of -0.27, forming the brightest star in the southern constellation → Centaurus and the third brightest star in the night sky, after → Sirius and → Canopus. The individual visual magnitudes of the components A, B, and Proxima are +0.01, +1.33, and +11.05, respectively. The masses of A and B are 1.100 and 0.907 Msun, respectively. Their → effective temperatures are (A) 5,790 K and (B) 5,260 K; their luminosities (A) 1.519 Lsun and (B) 0.500 Lsun. The binary members are separated in average by only 23 → astronomical units. They revolve around a common center of mass with a period of about 80 years. Both have a distance of 4.37 → light-years. Proxima Centauri, lying about 15,000 AU apart from AB, is → gravitationally bound to them. It has a mass of 0.1 Msun, a radius of 0.1 Rsun, a luminosity of about 0.001 Lsun, and an → effective temperature of ~ 3,000 K.

→ alpha; → Centaurus; → system.

The radioactive transformation of a nuclide by alpha-particle emission. Also called alpha disintegration.

→ alpha; → decay.

A simple → accretion disk model in which the → angular momentum is transported outward by action of some kind of → viscosity. In this model, first proposed by Shakura & Sunyaev (1973), the turbulent kinematic viscosity is given by ν = α c_s H, where α is a parameter, c_s the sound speed in the medium, and H → scale height. The α parameter controls the amount of → turbulence in the medium whose H and c_s are upper limits for → mixing length and turbulent speed, respectively. Values of α = 10^-3 to 10^-2 yield evolution → time scales that are broadly consistent with the ages inferred for → T Tauri stars. A weak point of this model is the arbitrariness of the choice of the parameter α, which reflects the lack of a rigorous theory of turbulence.

→ alpha, the parameter name; → disk; → model.

The → mechanism that imparts or transfers power to a → telescope so that it can move along the → right ascension direction. See also → tacking.

→ alpha, → right ascension; → drive.

In the → solar dynamo model, the process whereby the → azimuthal magnetic field transforms into a → meridional magnetic field due to the interaction of → convection and → differential rotation. See also → omega effect.

→ alpha; → effect.

A → chemical element synthesized in → massive stars by → alpha particle capture leading to iron before the advent of a → type II supernova. Stable alpha elements are: C, O, Ne, Mg, Si, S, Ar, Ca.

→ alpha; → element.

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.

→ alpha; → element; → knee.

The release of → alpha particles at high velocity from an atom's nucleus as it undergoes radioactive transformation.

→ alpha; → emission.

An atomic nucleus decaying by an → alpha particle emission.

→ alpha; → emitter.