An Etymological Dictionary of Astronomy and Astrophysics
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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.

→ alpha process.

→ alpha; → ladder.

A short distance from the target, in right ascension, where the telescope is pointed for various purposes.

→ alpha; → offset.

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.

→ alpha; → particle.

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.

→ alpha;→ process.

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".

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".

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 (السره‌الفرس) "The Horse's Navel," from surrat (سره) "navel" + faras (فرس) "horse". The name refers to the location of the star in the figure of → Pegasus. However, the star is now considered to belong to the constellation → Andromeda.

Ra's-ol-Mosalsalé, from Ar. Ar-Ra's al-Mar'ah al-Musalsalah "The head of the chained woman," from Ra's "head" + Mar'ah "woman" + Musalsalah "chained".

1) By this or that time; previously; prior to or at some specified or implied time.
2) Now; so soon; so early (Dictionary.com).

From M.E. al redy, literally "fully ready," → all, + M.E. redy "ready," from rædig, from O.E. ræde "prompt" + -ig "-y."

Pišnun, literally "prior to now," from piš-, → pre-, + Mid./Mod.Pers. nun "now, at present" (variants aknun, konun, ultimately from Proto-Ir. *hak-nun); Av. nū- "now," nūrəm "now;" O.Pers. nūram "now;" cf. Skt. nú- "now, just, but," nūnám "now, at present, indeed;" Gk. nun "now;" L. nu- "now" (in nu-dis "the day after tomorrow"); Goth. nu "now;" O.E. nu; E. now; PIE base *nu- "now."