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

The diffraction effects obtained when either the source of light or observing screen, or both, are at a finite distance from diffracting aperture or obstacle. → Fraunhofer diffraction.

Named after Jean Augustin Fresnel (1788-1827), French physicist, a key figure in establishing the wave theory of light. His earlier work on interference was carried out in ignorance of that of Thomas Young (1773-1829), English physician and physicist, but later they corresponded and were allies; → diffraction.

The first step carried out in a → fuzzy logic system during which a → crisp set of → input data are gathered and converted to a → fuzzy set using fuzzy → linguistic variables, fuzzy linguistic terms, and → membership functions.

Verbal noun of → fuzzify; → -tion.

To convert a → crisp set to a → fuzzy set in a → fuzzy logic system.

→ fuzzy; → -fy.

An → isotope separation process using the different diffusion speeds of → atoms or → molecules for separation. This process is used to divide → uranium hexafluoride (UF₆) into two separate streams of U-235 and U-238. Before processing by gaseous diffusion, uranium is first converted from → uranium oxide (U₃O₈) to UF₆. The UF₆ is heated and converted from a solid to a gas. The gas is then forced through a series of compressors and converters that contain porous barriers. Because uranium-235 has a slightly lighter isotopic mass than uranium-238, UF₆ molecules made with uranium-235 diffuse through the barriers at a slightly higher rate than the molecules containing uranium-238. At the end of the process, there are two UF₆ streams, with one stream having a higher concentration of uranium-235 than the other (EVS, a Division of Argonne National Laboratory).

→ gaseous; → diffusion.

A → geochemical classification scheme in which → chemical elements on the → periodic table are divided into groups based on their → affinity to form various types of compounds: → lithophile, → chalcophile, → siderophile, and → atmophile. The classification takes into account the positions of the elements in the periodic table, the types of electronic structures of atoms and ions, the specifics of the appearance of an affinity for a particular → anion, and the position of a particular element on the → atomic volume curve.

Developed by Victor Goldschmidt (1888-1947); → classification.

Any physical theory that unites the strong, electromagnetic, and weak interactions at high energy. It is hoped that GUTs can ultimately be extended to incorporate gravity. → theory of everything.

M.E. graunt, from O.Fr. grant, grand, from L. grandis "big, great," also "full-grown;" unified, p.p. of → unify; → theory.

Negâré, → theory; yegâneš, verbal noun of yegânestan, → unify; bozorg→ great.

The change in the wavelength or frequency of electromagnetic radiation in a gravitational field predicted by general relativity.

→ gravitational; → redshift.

An apparent fissure in the bright clouds of the Milky Way between → Cygnus and → Sagittarius caused by a series of large, dark, overlapping clouds.

→ great; → rift.

The length of time required for half of a given quantity of → radioactive material to → decay.

→ half; → life.

A classification of stellar spectra published in the Henry Draper catalogue, which was prepared in the early twentieth century by E. C. Pickering and Miss Annie Canon. It is based on the characteristic lines and bands of the chemical elements. The most important classes in order of decreasing temperatures are as follows: O, B, A, F, G, K, M.

Harvard, named for John Harvard (1607-1638), the English colonist, principal benefactor of Harvard College, now Harvard University. → classification

A galaxy or quasar having a → redshift larger than about 0.8, corresponding to a → look-back time half the present age of the Universe. The qualifier "high" is, however, relative and depends on context and authors' assessment.

→ high; → redshift; → object.

A → linear differential equation if the right-hand member is zero, Q(x) = 0, on interval I.

→ homogeneous; → linear; → differential; → equation.

The classification of galaxies according to their visual appearance into four basic types suggested by E. Hubble: → ellipticals (E), → spirals (S), → barred spirals (SB), and → irregulars (Ir). Later on a separate class of → lenticulars (S0) was appended as an intermediate type between ellipticals and spirals. The sequence starts with round elliptical galaxies (E0). Flatter galaxies are arranged following a number which is calculated from the ratio (a - b)/a, where a and b are the major and minor axes as measured on the sky. Ellipticals are divided into eight categories (E0, E1, ..., E7). Beyond E7 a clear disk is apparent in the lenticular (S0) galaxies. The sequence then splits into two parallel branches of disk galaxies showing spiral structure: ordinary spirals, S, and barred spirals, SB. The spiral and barred types are subdivided into Sa, Sb, Sc, and SBa, SBb, SBc, respectively. Along the sequence from Sa to Sc, the central bulge becomes smaller, while the spiral arms become more and more paramount. The original, erroneous idea that such arrangement of the galaxies might represent an evolutionary sequence led to the ellipticals being referred to as early-type galaxies, and the spirals and Irr I irregulars as late-type galaxies. See also → dwarf galaxy, → dwarf elliptical galaxy, → dwarf spheroidal galaxy.

→ Hubble; → classification.

Same as the → Hubble classification.

→ Hubble; → Hubble-Sandage variable; → sequence.

1) An act or instance of identifying; the state of being identified.
2) Psychology: The process of associating the self closely with other persons and assuming their characteristics or views unconsciously or partially so.
3) In psychological theories, a phase of recognition, the way in which the child's personality is formed by modeling it on a chosen parent.

Verbal noun of → identify.

Idâneš, verbal noun of idânidan→ identify.

Recognizing the lines in the spectrum of a star, nebula, galaxy, etc. using a calibration template.

→ identification; → line.

Computers: 1) One or more characters used to identify, name, or characterize the nature, properties, or contents of a set of data elements.
2) A string of bits or characters that names an entity, such as a program, device, or system, in order that other entities can call that entity.
3) In programming languages, a lexical unit that names a language object, such as a variable, array, record, label, or procedure.

Agent noun of → identify.

To recognize or establish as being a particular person or thing; e.g. → identification of lines.

Identify, from Fr. identifier, from identité, → identity.

Idânidan, infinitive from idâni, → identity.

(Conjunction) in case that; granting or supposing that; on condition that. → if and only if.

M.E., from O.E. gif; akin to O.H.G. ibu "if."

Agar "if," from Mid.Pers. agar, hakar "if;" O.Pers. ha-karam "once;" from ha- "one," variant ham-, → com- (cf. Av. ha-; Skt. sa-; Gk. ha-; L. sem-; PIE *sem- "one"), + karam- "a time," maybe from kar- "to do; doing," → work; cf. Av. hakərət "once;" Skt. sakrt "once;" Gk. hapaks "once;" L. semel "once."

Logic, Math.: An → expression indicating that two → statements so connected are → necessary and sufficient conditions for one another. The corresponding logical symbols usually used are: ↔, ⇔, ≡, and iff.

→ if; → and; → only.