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

An → atomic transition involving a → hyperfine structure.

→ hyperfine; → transition.

A system consisting of a dominant → spiral galaxy associated with → dwarf satellite galaxies and intergalactic matter. Examples in the → Local Group are our Galaxy and the → Andromeda galaxy.

→ hyper- + → galaxy.

A high luminosity star with absolute visual magnitude around -10, about 10⁶ times as luminous as the Sun. Hypergiant stars are evolved → massive stars belonging to the luminosity class Ia+ or Ia0. Their spectra show very broadened emission and absorption lines resulting from the high luminosity and low surface gravity which favor strong → stellar wind. See also → Humphreys-Davidson limit; → yellow hypergiant.

→ hyper-; → giant.

The sixteenth of → Saturn's known → natural satellites. It is shaped like a potato with dimensions of 410 x 260 x 220 km and has a bizarre porous, sponge-like appearance. Many of the sponge holes or craters have bright walls, which suggests an abundance of → water  → ice. The crater floors are mostly the areas of the lowest → albedo and greatest red coloration. This may be because the average temperature of roughly -180 °C might be close enough to a temperature that would cause → volatiles to → sublimate, leaving the darker materials accumulated on the crater floors. Hyperion is one of the largest bodies in the → Solar System known to be so irregular. Its density is so low that it might house a vast system of caverns inside. Hyperion rotates chaotically and revolves around Saturn at a mean distance of 1,481,100 km. It was discovered by two astronomers independently in 1848, the American William C. Bond (1789-1859) and the British William Lassell (1799-1880).

Hyperion, in Gk. mythology was the Titan god of light, one of the sons of Ouranos (Heaven) and Gaia (Earth), and the father of the lights of heaven, Eos the Dawn, Helios the Sun, and Selene the Moon.

A condition of the eye that occurs when light rays entering the eye are focused behind the retina; also called farsightedness, hyperopia, long sight (opposed to → myopia).

From Gk. hupermetros "beyond measure," from → hyper- + metron "measure;" → meter + -opia a combining form denoting a condition of sight or of the visual organs hemeralopia; myopia.

Durbini "farsightedness," from dur "far" (Mid.Pers. dūr "far, distant, remote;" O.Pers. dūra- "far (in time or space)," dūraiy "afar, far away, far and wide;" Av. dūra-, dūirē "far," from dav- "to move away;" cf. Skt. dūrá- "far; distance (in space and time);" PIE base *deu- "to move forward, pass;" cf. Gk. den "for a long time," deros "lasting long") + bin- "to see" (present stem of didan; Mid.Pers. wyn-; O.Pers. vain- "to see;" Av. vaēn- "to see;" Skt. veda "I know;" Gk. oida "I know," idein "to see;" L. videre "to see;" PIE base *weid- "to know, to see") + -i noun suffix.

A highly energetic → supernova explosion. This phenomenon, which is more violent than a typical → supernova event, is accompanied by a → gamma-ray burst.

→ hyper-; → nova.

An unstable elementary particles, belonging to the class called → baryons, which have greater mass than the neutron but very short lives (10^-8 to 10^-10 seconds).

From → hyper- + → -on a suffix used in the names of elementary particles (gluon; meson; neutron; graviton, and so on).

In aerodynamics, adjective used to describe a → sound speed in excess of Mach 5. See also → supersonic.

Hypersonic, from → hyper- + → sonic.

A star whose velocity is so great that it will escape the → gravitational potential of our → Galaxy. Depending on the location and direction of motion, this criterion typically corresponds to a stellar velocity in the Galactic → rest frame larger than 400 km s^-1, and up to about 1200 km s^-1. The nature of the HVSs spans a wide range of types from → OB stars, to metal-poor → F-type stars and G/K dwarfs. While there is evidence from many late-type B HVSs in the → halo to originate from the Galactic → supermassive black hole (SMBH), other HVSs seem to originate from the → galactic disk. HVSs can obtain their large velocities from a number of different processes:
1) → Tidal disruption of → close binary stars by the central SMBH of the Milky Way. In this process one star is captured by the SMBH while the other is ejected at high speed via the → gravitational slingshot mechanism.
2) Exchange encounters in other dense stellar environments between hard binaries (→ hard binary) and → massive stars may cause stars to be ejected and escape our Galaxy.
3) Disruption of close binaries via → supernova explosions. The → runaway velocities of both ejected stars can reach large values when asymmetric supernovae are considered, i.e. when the newborn → neutron star receives a momentum kick at birth.
(see, e.g., T. M. Tauris, 2014, and references therein, arXiv:1412.0657).

→ hyper-; → velocity; → star.

An operator which takes a real number to the same real number.

→ identity; → operator.

The minimum temperature to which a fuel must be heated in order to initiate self sustained combustion independent of another heat source.

→ ignition; → temperature.

Latin verb meaning "to command, rule, reign."

L. imperare "to command, give orders, exercise authority," from → in- "into, in" + parare "to make ready, supply, order," related to parire "produce, give birth to," from PIE root *pere- "to produce, procure;" cf. Skt. prthukah "child, calf, young of an animal;" Gk. poris "calf, bull;" Czech spratek "brat, premature calf;" Lith. periu, pereti "to brood;" O.H.G. farro, Ger. Farre, Du. varre "bull," O.E. fearr "bull;" see below for possible Iranian cognates.

Parmâtidan, from BMP plm'(d)y "to command, order," Sogd. framat- "to command," variants of farmudan, farmâyidan "to command, to order," ultimately from prefixed Proto-Ir. *fra-maH-, from *maH- "to measure," → experiment.

1a) A command or order.
1b) → imperative case.
2a) Absolutely necessary or required; extremely important.
2b) Of the nature of or expressing a command; commanding.

From L.L. imperativus "pertaining to a command," from imperat-, p.p. stem of → imperare "to command."

Parmât, noun from present stem of parmâtidan, → imperare; parmâtii, adj. from parmât.

The grammatical mood of a verb that expresses a command or a request, as in close the door!.

→ imperative; → case.

In an imperative manner.

Adverb from → imperative; → -ly.

Not perfect; faulty or incomplete.

→ in-; → perfect.

A fault, flaw, or undesirable feature; the state or condition of being imperfect.

→ in-; → perfection.

Of, relating to, or suggestive of an empire or a sovereign, especially an emperor or empress (TheFreeDictionary).

Adjective of → empire.

1) The extension of a nation's authority by territorial acquisition or by the establishment of economic and political dominance over other nations.
2) A political doctrine or system promoting such extension of authority (TheFreeDictionary.com).

→ imperial; → -ism.

Math.: An operator whose inverse is a differential operator.

→ integral; → operator.