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

A → spectral line whose → energy levels have a → fine structure. Examples are [C II] 157.7 μm (→ singly ionized carbon), [O III] 88 μm, and [Ne II] 12.8 μm.

→ fine; → structure; → line.

Any of the terminal members of the hand, especially one other than the thumb (Dictionary.com).

M.E., from O.E. fingor, cognate with Ger. Finger, Du. vinger, O.N. fingr, Goth. figgrs.

Angošt, variants angol, angul (also angal "loop"); Mid.Pers. angust; Av. angušta- "toe," ank- "curved, crooked;" cf. Skt. angustha- "thumb," angula- "finger," ankah "hook, bent;" Gk. angkon "elbow," angkura "anchor;" L. angulum "corner;" Lith. anka "loop;" O.E. ancleo "ankle;" O.H.G. ango "hook;" PIE *ang-/*ank- "to bend".

A weak yet important kind of mixing that results from → fingering instability in stars within → radiative zones that have an unstable mean → molecular weight  → gradient. Also called → thermohaline convection.

→ finger; → -ing; → convection.

A type of instability that often occurs in fluids which are thermally stably stratified, but have an inhomogeneous composition. A well-known example, found in upper layers of the Earth's oceans, is → salt fingers. Similar fingering instabilities can occur in any other thermally stably stratified solution, provided the concentration of the slower-diffusing solute increases with height. The saturated state of this instability, → fingering convection, takes the form of tightly-packed, vertically-elongated plumes of sinking dense fluid and rising light fluid, and significantly enhances the vertical transport of both heat and chemical composition. The fingering instability occurs in stars within radiation zones that have an unstable mean → molecular weight  → gradient (μ gradient). This situation is often found as a result of material accretion onto a star by anything from a single or multiple planets, to material from a dust-enriched or debris accretion disk, or material from a more evolved companion. It also naturally arises in the vicinity of the → hydrogen shell burning in → red giant branch (RGB) stars, and in thin element-rich layers near the surface of intermediate-mass stars. The fingering instability initially takes the form of thin tubes, hence the name "finger," within which the fluid moves vertically. The tubes rapidly break down, however, as a result of parasitic shear instabilities that develop inbetween them, and the fingering instability eventually saturates into a state of homogeneous fingering convection where the typical aspect ratio of the eddies is closer to one (P. Garaud et al., 2015, arXiv:1505.07759).

→ finger; → -ing; → instability.

A → redshift space distortion which causes the dense central regions of → galaxy clusters to appear elongated along the → line of sight. This effect is attributed to random velocities in clusters of galaxies deviating from pure → Hubble flow. For an observer galaxies with peculiar velocity perpendicular to the line of sight do not change the redshift, which is given just by the normal Hubble expansion. On the contrary, galaxies with peculiar velocity along the line of sight appear with a different redshift, resulting from the Hubble expansion velocity plus the peculiar velocity. Since this affects only redshift and not position on the sky, the stretching occurs only radially, toward the observer. See also → Kaiser effect, → peculiar velocity.

→ finger; → God.

1) Math: The opposite of → infinite.
2) Physics: Either non-infinite or non-zero.

From L. finitus, p.p. of finire "to limit, set bounds, end."

Karânmand, from karân "boundary, side, end, coast" + -mand adjective suffix. Karân, variants karâné, kenâr, from Mid.Pers. karân, karânak, kenâr "edge, limit, boundary," Av. karana- "side, boundary, end."

A → statistical population consisting of individuals or items which are finite in number.

→ finite; → population.

A sum a₁ + a₂ + a₃ + · · · + a_N, where the a_i's are real numbers. In terms of Σ-notation, it is written as a₁ + a₂ + a₃ + · · · + a_N = Σ (n = 1 to N).  See also → infinite series.

→ finite; → series.

A → set whose elements can be numbered from 1 to n, for some positive integer n.

→ finite; → set.

A state, process, or instance of combustion in which a substance combines with oxygen producing heat, light, and flame.

O..E. fyr, from P.Gmc. *fuir (cf. O.N. fürr, M.Du. vuur, Ger. Feuer), from PIE *paewr-; cf. Mod.Pers. Lori porpor "blazing charcoal," Gilaki bur, biur "smokeless red fire" (Lori perisk, periska "spark," Kurd. biriske "spark," Lârestâni pelita "spark"); Tokharian por, puwar "fire;" Gk. pyr "fire;" Hitt. pahhur "fire;" Skt. pū- "to cleanse."

Âtaš, variants âzar, taš, from Mid.Pers. âtaxš, âtur "fire;" Av. ātar-, āθr- "fire," singular nominative ātarš-; O.Pers. ātar- "fire;" Av. āθaurvan- "fire priest;" Skt. átharvan- "fire priest;" cf. L. ater "black" ("blackened by fire"); Arm. airem "burns;" Serb. vatra "fire;" PIE base *āter- "fire."

A → meteor that is brighter than the brightest planets, i.e. with an apparent magnitude of -5 or greater. Fireballs are often followed by → meteorite falls. Also called → bolide.

From → fire + ball, from O.E., from O.N. bollr "ball," from P.Gmc. *balluz (cf. O.H.G. ballo, Ger. Ball), from PIE base *bhel- "to swell."

Tašguy, from taš "fire," variant of âtaš→ fire + guy "ball, sphere," variants golulé, gullé, goruk, gulu, gudé (cf. Skt. guda- "ball, mouthful, lump, tumour," Pali gula- "ball," Gk. gloutos "rump," L. glomus "ball," globus "globe," Ger. Kugel, E. clot; PIE *gel- "to make into a ball").

1) Having a solid structure that resists pressure.
2) Solidly or securely fixed in place.

M.E. ferm, from O.Fr. ferm "strong, vigorous; healthy, sound; steadfast," from L. firmus "strong, steadfast, stable," from PIE root *dher- "to hold firmly, support."

Deš (Hamadâni) "firm, steady," (Ilâmi) dež "intact, entire," (Hamadâni) daj "intact, full," (Šuštari) dec "full," may be related to Proto-Ir. *dar- "to hold, keep;" Av. dār- "to hold, keep;" Pers./ Mid.Pers. dâštan/dâr- "to have, hold, keep;" cf. Skt. dhar- "to hold, keep, preserve;" L. firmus "firm, steady, as above."

Being before all others with respect to time, order, importance, etc., used as the ordinal number of one.

O.E. fyrst "foremost," superlative of fore, from P.Gmc. *furisto (cf. O.H.G. furist, O.N. fyrstr, Dan. første, M.Du. vorste "first," Ger. Fürst "prince"), superlative of *fur-/*for-, from PIE *pro- (cf. Av. pouruua- "first," fra- "forward, forth;" Skt. pūrva- "first," pra- "before, formerly," Gk. pro; L. pro; E. fore).

Naxost, from Mid.Pers. naxust "the first," Parthian Mid.Pers. nxwšt, from naxu, Manichean Parthian nwx "beginning" + -ist superlative suffix, Av. -išta-, cf. Skt. -istha-, Gk. -istos, O.H.G. -isto, -osto, O.E. -st, -est, -ost; naxostin, from naxost + suffix -in.
Yekom, from yek, → one, + -om suffix of ordinal numbers.
Âqâz "beginning," from Proto-Iranian *āgāza-, from prefix ā- + *gāz- "to take, receive," cf. Sogdian āγāz "beginning, start," pcγz "reception, taking."

1) Generally, an expression to indicate that a comment or result is only approximate.
2) Math.: In calculus, limiting a differential equation to its → first derivative, for example: e^x≅ 1 + x. Also called → linear approximation.

→ first; → approximation.

An early phase in the process of star formation which begins when the mass of a → molecular cloud → clump exceeds the → Jeans mass. The collapse is initially → optically thin to the thermal emission from → dust grains, and the compressional heating rate is much smaller than the cooling rate by the → thermal radiation. The collapse proceeds → isothermally. The isothermal condition is broken when the central density reaches about 10^-13 g cm^-3 and a small region at the center of the cloud starts to become → opaque. The heat generated by the collapse in this region is no longer freely radiated away, and the compression becomes approximately → adiabatic. The central temperature and pressure then begin to rise rapidly, soon becoming sufficient to decelerate and stop the collapse at the center. There then arises a small central core, called the → first core, in which the material has stopped collapsing and is approaching → hydrostatic equilibrium. Outside this core, the material is still nearly isothermal and continues to fall inward almost in → free fall. Consequently a shock front arises at the boundary of the core, where the infalling material is suddenly stopped. The initial mass and radius of the core are about 10³¹ g and 6 x 10¹³ cm, respectively, and the central density and temperature are about 2 x 10^-10 g cm^-3 and 170 K, respectively. As the collapse proceeds, the core grows in mass due to the infall of the surrounding material; at the same time, however, the core radius decreases because of radiative energy losses from the outer layers of the core. The process leads to the → second collapse (R. B. Larson, 1969, MNRAS 145, 271).

→ first; → collapse.

1) The beginning of a → solar eclipse when the eastern part of the lunar limb touches the western limb of the Sun, marking the beginning of an eclipse.
2) For a → lunar eclipse, the moment when the eastern limb of the Moon is the first to enter the Earth's shadow.

→ first; → contact.

A first object in → hydrostatic equilibrium predicted to form during early dynamical contraction of a → molecular cloud → clump in the course of the → first collapse.

→ first; → core.

A equation in which the highest → exponent of the → variable is 1. Same as → linear equation.

→ first; → degree; → equation.

The simplest mathematical → differentiation of one quantity relative to another; df(x)/dx. It is the same as → derivative, contrasted with the → second derivative or the nth derivative.

→ first; → derivative.

In → calculus, a method for determining whether an → inflection point is a → local minimum, → local maximum, or neither.

→ first; → derivative; → test.