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

A large parabolic structure that collects radio waves and focuses them on a detector by means of a secondary reflector. A similar device used as antenna for radar transmitting.

O.E. disc "plate, bowl, platter," from L. discus "dish, platter, quoit," from Gk. diskos "disk, platter."

Jâm "cup, chalice, goblet, bowl," Mid.Pers. jâm "vessel, goblet; glass," Av. yama- "glass, glass vessel," yāmô.pacika- "baked glass;" related to Skt. camasa- "a vessel used at sacrifices for drinking Soma, kind of flat dish or cup?"

To remove infection the use of appropriate means.

→ dis-; → infect.

Any agent, such as a chemical, heat, or radiation, that destroys or inhibits the growth of disease carrying microorganisms.

→ disinfect; → -ant.

The breaking up of a body into fragments. For example, that of an unstable nuclei either spontaneously or as a result of bombardment by fast-moving particles, or the breaking up of a comet.

Disintegration, verbal noun of disintegrate, from → dis- + → integration.

Vâpâši, forupâši, from vâ-, frou-→ de- + pâši, noun of pâšidan→ disperse.

Any of the components in a logical → disjunction.

→ disjunction.

A → proposition of the form "A or B" (A ∨ B), where A and B are themselves propositions. A disjunction is → true when one or the other of its components (called → disjuncts) is true, and false otherwise.

→ dis-; → junction.

1) General: Any thin, flat, circular plate or object.
2) Math.: The domain bounded by a circle.
3) Astro.: A thin, flattened material structure encompassing a central object, such as a star or a galactic nuclear region. → accretion disk; → circumnuclear disk; → circumstellar disk; → galactic disk.

Computers: Any of several types of material support on which data can be encoded.

From L. discus "quoit, discus, disk," from Gk. diskos, from dikein "to throw." → dish.

Gerdé, from Mid.Pers. girdag "disk, round," from gird/girt "round, all around," Proto-Iranian *gart- "to twist, to wreathe," cf. Skt krt "to twist threads, spin; to wind; to surround;" kata- "a twist of straw," Pali kata- "ring, bracelet," Gk. kartalos "a kind of basket," kyrtos "curved;" disk loanword from Fr.

An accretion process involving an → accretion disk.

→ disk; → accretion.

A galaxy consisting of a thin disk of stars and → interstellar matter which may include → spiral arms and → bar.

→ disk; → galaxy.

1) General: The process by which an → accretion disk cools, causing it to fragment into self-gravitating → clumps.
2) A model of → planet formation in which a region of a → circumstellar disk becomes dense and cool enough to be unstable to → gravitational collapse, resulting in the formation of a gaseous → protoplanet. Disk instability has been put forward as a mechanism for forming massive → giant planets on short time-scales (1 Ky-1 My).
3) In models of galactic structure, a process that is thought to cause the formation of → pseudo-bulges.
4) → disk instability model (DIM).

→ disk; → instability.

A model describing → dwarf novae and → Soft X-ray Transient (SXT)s. Accordingly, these objects are triggered by an → accretion disk instability due to an abrupt change in opacities (→ opacity) at → temperatures at which hydrogen is partially ionized. All versions of the DIM have this ingredient. They differ in assumptions about → viscosity, and about what happens at the inner and outer disk radii. Basically, during → quiescence, material accumulates in the accretion disk until a critical point is reached. The disk then becomes unstable and is dumped onto the → compact object, releasing a burst of → X-rays. However, the greater duration of SXT bursts (months) and the time interval between bursts (decades) cannot be accounted for by the standard disk instability model used for dwarf novae, and additional factors such as X-ray illumination and irradiation of the accretion disk are required for the model to match the observed properties of SXTs (J-P Lasota and J-M Hameury, 1995).

→ disk; → instability; → model.

In star formation models involving magnetized accretion, a process whereby the stellar rotational → angular velocity becomes equal to the → Keplerian angular velocity of the → accretion disk. This happens at the → corotation radius. Disk locking is believed to be responsible for efficient loss of stellar → angular momentum during the → pre-main sequence contraction of → T Tauri stars. These stars are expected to spin up by a factor of about 3 due to contraction after being magnetically disconnected from the → circumstellar disk. However, observations show that a large fraction of pre-main sequence stars evolve at nearly constant angular velocity through the first 4 Myr. This process results from → magnetic braking. The idea of magnetic disk locking originated with the theory developed by Ghosh & Lamb (1979, ApJ 232, 259) for → neutron stars.

M.E., from O.E. lucan "to lock, to close," from loc "bolt, fastening, enclosure;" cf. M.L.G. lok, O.H.G. loh, O.N. lok "a cover, lid," Goth. -luk in usluk "opening," Ger. Loch "opening, hole," Du. luck "shutter."

Girkard, from gir "hold, block," from gereftan "to seize, hold, take," → eclipse, + kard noun from kardan "to do," → work.

A logical division of a hard disk that is treated as a separate unit by operating systems and file systems.

→ disk; → partition.

Of a spiral galaxy, those stars that lie in a flattened disk and move in nearly circular orbits around its centre. They are Population I stars of all ages up to the age of the disk, but in general are younger than stars in → halo population.

→ disk; population, from L.L. populationem "a people, multitude," from populatio, from populare "to inhabit," from populus "people," related to plebes "the common people," cf. Gk. plethos "people, multitude, great number," from PIE base *pel- "to be full;" Mod.Pers. por "full," O.Pers. paru- "much, many," Av. pouru- "much, many," pərəna- "full," par- "to fill," Skt. puru-, Gk. polus, O.E. full "completely, full," from P.Gmc. *fullaz, O.H.G. fol, Ger. voll, Goth. full.

Like Gk., Pers. uses the concepts of "multitude, many, full" to denote "people, group, herd, flock". The following examples are all terms derived from O.Pers. paru- "much, many," Av. par- "to fill," pouru- "much, many," pərəna- "full" (Mod.Pers. por "full"): literary Pers. bâré "herd, flock," parré "a rank or file of soldiers, a circular disposition of troops," Lori, Qâyeni bor "group, tribe, herd," Torbat-Heydariyeyi, Qomi borr "heap, bundle, group," Qomi borreh "group, assemblage of people," Pashtu parrak "flock, herd," Urdu para "flock, herd," Lârestâni baila "group, tribe," Tabari balik "herd, flock." With this introduction, porineš "population," verbal noun of porinidan "to populate," infinitive of porin "populous," from por "mutitude, many, full" + -in attribution suffix.

Computers: The specific amount of disk space that a user or service is allowed to use.

→ disk; → quota.

The process whereby a → galaxy evolves from a disturbed to an ordered system, as it develops into a → rotation dominated → settled disk.

→ disk; → settling.

A star that lies within the → galactic disk of a → spiral galaxy. Stars belonging to the → thin disk, such as the Sun or Alpha Centauri, lie at a typical distance of about 1,000 → light-years from the galactic midplane. There are also → thick disk stars, such as Lalande 21185, that lie at an average distance of about 3,500 light-years from the midplane.

→ disk; → star.

In models of magnetized → accretion disks, the process whereby the disk is disrupted at a radius where the → magnetic pressure overcomes the → ram pressure of the accreted material. This occurs at a distance typically 3-7 stellar radii, below the → corotation radius.

→ disk; → truncation.

In → magnetocentrifugal models of → protostars, the wind arising from a significant range of radii in the → accretion disk. The contribution from innermost parts of the disk is dealt with by the → X-wind model. (Königl A. and Pudritz R. E., 2000, In Protostars and Planets IV, V. Mannings, et al. (eds.), Tucson: Univ. Arizona Press, p. 759).

→ disk; → wind.

A → galaxy bulge that is flatter than a → classical bulge. Such bulges might be difficult to see in very inclined galaxies. They may contain sub-structures such as nuclear → bars, → spiral arms, or → rings. They usually show signs of → dust obscuration, younger → stellar populations, or ongoing → star formation. These systems seem to form mostly through disk instabilities (→ disk instability), such as bars, in a relatively slow, continuous and smooth process. Essentially, such instabilities induce a redistribution of → angular momentum along the galaxy, and, as a result, mostly gas but also stars are driven to the disk center. Also called → pseudo-bulge (Kormendy & Kennicutt, 2004, ARA&A 42, 603; Fisher & Drory, 2010, ApJ 716, 942).

→ -disk; → like; → bulge.