An Etymological Dictionary of Astronomy and Astrophysics

Having a constant entropy.

See also: → iso- + → entropy + → -ic.

A → reversible flow in which the value of → entropy remains → constant; i.e. no energy is added to the flow, and no energy losses occur due to friction or dissipative effects.

See also: → isentropic; → flow.

A → thermodynamic process for which there is no → heat transfer with the surroundings, and no change in → entropy.

See also: → isentropic; → process.

The astronomical activities that took place from the 8th to the 14th century in the Middle East, Central Asia, North Africa, and Moorish Spain.
The term Islamic should refer to a civilization rather than a religion, because much of the astronomy was secular. In fact more than 90% of “Islamic” astronomy deals with the Greek astronomy → Ptolemaic system, which has obviously nothing to do with religion.
Moreover, many non-Muslims within that civilization contributed to this science and must be acknowledged. Apart from these considerations, the term “Islamic astronomy” creates a conceptual disparity. In comparison, the works of European astronomers, such as Copernicus, Galileo, Newton, and others are not placed under “Christian astronomy,” and they are indeed not called “Christian scientists.” See also → Arabic astronomy, → Islamic calendar.

See also: From Islam, literally “submission” (to God); → astronomy.

A religious and strictly → lunar calendar which follows the visibility of the lunar crescent after → conjunction and ignores the seasons (see also → synodic month). The year, which consists of 12 months of 29 or 30 days, is approximately 354 days long (→ lunar year of 354.3672 days). Because the calendar follows a purely lunar cycle, each month begins 10 or 11 days earlier each year in relation to the 365-day → solar year. As a result, the cycle of 12 lunar months regresses through the seasons over a period of 33 years. For religious purposes, Muslims begin the months with the first visibility of the lunar crescent. The month length may be 30 or 29 days during four or three successive months respectively. However, astronomers consider a calendar with months of alternately 30 and 29 days. The 33-year period contains 11 → leap years of 355 days. The origin of the Islamic era is considered to be the migration (Hijra) of Prophet Muhammad from Mecca to Medina on 16 July, A.D. 622.
It was Caliph Umar (died 644) who, 17 years after the actual event, poised the migration as the beginning of the Muslim era.

See also: From Islam, literally “submission” (to God); → calendar.

A tract of land completely surrounded by water, and not large enough to be called a → continent (Dictionary.com).

Etymology (EN): M.E. iland, from O.E. igland “island,” from ieg “island;” PIE *akwa- “water,” cf. Pers. âb, → water,

• → land.

Etymology (PE): Âdâk, âdak, adak “island” (Dehxodâ), probably from Proto-Ir. *āpdaka- “placed in water,” from *âp-, → water, cf. Pers. âb,

• da- “to place, put,” cf. Pers. dâdan “to give,” → thesis, + suffix -ka.
  Âbxost, ultimately from āpxvasta-, literally “stricken/pounded by water,” from *āp-, → water, + *xvasta-, from Proto-Ir. *huah- “to strike, to thresh;” cf. Av. (paiti)xv^anh- “to thresh;” Mid.Pers. xwastan “to tread, trample, thresh;” Mod.Pers. xvast “trampled, beaten,” âbxun “island,” šabixun “night attack;” Kurd. xistin, xin-, xa- “to strike, to beat;” Hamedâni xostan/xus-, Esfahâni xosan “to throw.”
  Jaziré loan from Ar. jazirah.
  Tomb “island” in Iranian Hormozgâni dialects.

The hypothesis first put forward by Immanuel Kant (1724-1804) according to which the objects termed “spiral nebulae” were stellar systems comparable to our own → Milky Way galaxy. At the end of the 18th century, William Herschel (1738-1822) using his giant reflectors discovered thousands of such nebulae. However, in spite of advances in observations it was never possible to prove Kant’s idea until the second decade of the twentieth century. The observations using the Mount Wilson 2.50m (100 inch)
telescope allowed Edwin Hubble in 1924 to firmly establish that the “spiral nebulae” were unquestionably extragalactic.

See also: The term “island Universe” was first introduced by the German Alexander von Humboldt in 1850; → island; → Universe.

Prefix denoting “equal; homogeneous; uniform.” Also is- sometimes before a vowel.

Etymology (EN): From L.L., from Gk. isos “equal.”

Etymology (PE): Izo-, loan from Gk. isos, as above. The Pers. ham-→ com- is overused; therefore this dictionary adopts izo-.

1. Meteo.: A line connecting points having equal pressure.
   
2. Any of a group of nuclides which, although having different atomic numbers, have identical mass numbers; e.g. the tin isotope ¹¹⁵₅₀Sn and the indium isotope ¹¹⁵₄₉In.

See also: From Gk. isobares “of equal weight,” from → iso- +
bar, from baros “weight,” cognate with Pers. bâr “charge, weight” (Mid.Pers. bâr,
from O.Pers./Av. bar- “to bear, carry,” Mod.Pers. bordan “to carry;” L. brutus “heavy, dull, stupid, brutish;” Skt. bhara- “burden, load,” bharati “he carries;” Mod.Pers. gerân “heavy;” Skt. guru; L. gravis; PIE *gwere- “heavy;” *bher- “to carry, give birth”).

A process taking place at constant pressure. → polytropic process.

See also: Isobaric, adj. of → isobar; → process.

Same as → isospin.

See also: Isobaric, adj. of → isobar; → spin.

A process in which the volume remains unchanged.

Etymology (EN): From → iso- + chor-, from Gk. chora “place, land” + → ic.

Etymology (PE): Farâravand, → process; izogonj, from izo→ iso- + gonj “volume,” gonjdan “to be contained; to hold exactly; to be filled;” Mid.Pers. winj- “to be contained;” Proto-Iranian *uiac-/*uic-; cf. Skt. vyac- “to contain, encompass,” vyás- “extent, content, extension;” L. uincire “to bind.”

A curve on a → Hertzsprung-Russell diagram connecting all stars having the same age.

Etymology (EN): Isochrone, back formation from isochronal, from Gk. → iso- + khronos “time.”

Etymology (PE): Izozamân, from → iso- + zamân→ time.

Having similar angles.

See also: → iso- + -gonal, → diagonal

Math.: A curve which intersects every member of a given one-parameter family of curves at one and the same angle.

See also: → isogonal; → trajectory.

Line joining geographical points of equal insolation during a specific interval of time.

Etymology (EN): From Gk. → iso- + hel, from helios “sun,” → helio-.

Etymology (PE): Izohur, from izo-, → iso-,

• hur “sun,” cognate with → helio-.

Adaptive optics: The angle over which wavefront phases become de-correlated by 1 radian rms due to → tip-tilt alone. It is defined analogously to the → isoplanatic angle.

See also: → iso-; → kinetic; → angle.

To set or place apart; detach or separate so as to be alone.
Chemistry: To obtain a substance in an uncombined or pure state.
Physics: → isolated system.

Etymology (EN): Isolate, back-formation from isolated, from Fr. isolé “isolated,” from It. isolato, from L. insulatus “made into an island,” from insula “island;” maybe from
*en-salos “in the sea,” from salum “sea.”

Etymology (PE): Vâyutidan, from vâ- denoting “separation” (also “reversal, opposition; repetition; back, backward,” variant of bâz-, from Mid.Pers. abâz-, apâc-; O.Pers. apa- [pref.] “away, from;” Av. apa- [pref.] “away, from,”
apaš [adv.] “toward the back;” cf. Skt. ápāñc “situated behind”) + Mid.Pers. yut “separate, different,”
Mod.Pers. jodâ “separate, apart;” Av. yuta- “separate, apart” + -idan infinitive suffix.

Set apart or separated from others or other things.

See also: Past participle of → isolate.

A galaxy that is not a member of a dense aggregate. In other words, a galaxy that is formed in a low galactic density environment and has evolved without major interactions with other galaxies of similar mass.

See also: → isolated; → galaxy.

Massive star formation outside → OB associations. Recent observational findings suggest that → massive star formation is a collective process. In other words, massive stars form in → cluster environments
and the mass of the most massive star in a cluster is correlated with the mass of the cluster itself. Nevertheless, other observational results give grounds for supposing that massive stars do not necessarily form in clusters but that they can be formed as isolated stars or in very small groups. According to statistical studies
nearly 95% of Galactic → O star population is located in clusters or OB associations. This means that a small percentage, about 5%, of high mass stars may form in isolation. Isolation is meant not traceable to an origin in an OB association. This definition therefore excludes → runaway massive stars, which are thought to result from either dynamical interaction in massive dense clusters, or via a kick from a → supernova explosion in a → binary system. Alternatively, isolated massive star has been defined as follows: An O-type star belonging to a cluster whose total mass is < 100 Msun and moreover is devoid of → B stars (Selier et al. 2011, A&A 529, A40 and references therein).

See also: → isolated; → massive star; → formation.

A → neutron star which does not belong to a → binary system, does not have radio emission, and is not surrounded by a progenitor → supernova remnant. INSs appear to be thermally cooling with no emission outside the → soft X-ray band, except for faint optical/UV counterparts. Although these properties are similar to those of → compact central object (CCO)s, they are a distinct class because they lack any observable associated supernova remnant or nebula. There are presently seven confirmed INSs (sometimes referred to as The Magnificent Seven), six of which have measured weakly modulated X-ray pulsations with periods between 3 s and 11 s, much longer than those of CCOs (A. K. Harding, 2013, Front. Phys. 8, 679).

See also: → isolated; → neutron; → star.

Thermodynamics: A system which has no exchange of energy or matter with surroundings. The internal energy of such a system remains constant. → closed system; → open system.

See also: → isolated; → system.

An act or instance of isolating; the state of being isolated.

See also: Verbal noun of → isolate.

Physics: Any of two or more nuclei having the same atomic number A and mass number Z but different half-lives.
Chemistry: One of two or more substances that have the same chemical composition but differ in structural form.

Etymology (EN): Back formation from isomeric, from → iso- + mer a combining form meaning “part,” from Gk. meros “part, portion, share.”

Etymology (PE): Izomer, loan from Fr. isomère.

A one-to-one → mapping between two → sets, which preserves the relations existing between elements in its → domain. An isomorphism is a → bijective morphism.

See also: → iso-; → morphism.

A line drawn through all points on a weather map having the same amount of → cloud cover.

Etymology (EN): From Gk. → iso- + nephos “cloud,” cognate with Pers. nam “humidity, moisture;” Av. napta- “moist,” nabās-câ- “cloud,” nabah- “sky;” Skt. nábhas- “moisture, cloud, mist;” L. nebula “mist,” nimbus “rainstorm, rain cloud;” O.H.G. nebul; Ger. Nebel “fog;” O.E. nifol “dark;” from PIE *nebh- “cloud, vapor, fog, moist, sky.”

Etymology (PE): Izoabr, from izo-, → iso-, + abr → cloud.

Of or relating to → isophotes.

See also: → isophote; → -al.

The size attributed to a galaxy corresponding to a particular level of → surface brightness. The reason is that galaxies do not have sharp edges.

See also: → isophotal; → radius.

A line joining points with the same surface brightness on a plot or in image of a celestial object such as a nebula or galaxy.

Etymology (EN): Isophote, from → iso- + a combining form of Gk. phos (gen. photos) “light.”

Etymology (PE): Izošid, from izo-, → iso-, + šid
“light, sunlight,” from Mid.Pers. šêt “shining, radiant, bright;” Av. xšaēta- “shining, brilliant, splendid, excellent.”

The quality of an imaging system which is characterized by → isoplanatism.

Etymology (EN): From → iso- “equal, uniform”

• Gk. plane “wandering,” from planasthai “to wander” + -tic adjective-forming suffix, → -ic.

Etymology (PE): Izobirâh, from izo-, → iso-, + birâh “a devious path; a wanderer, who deviates, errs.”

The angle in which the → point spread function of the atmosphere/telescope system is space invariant. Because of the presence of → turbulence in high layers of the atmosphere, this angle is extremely small, often only a few seconds of arc at visible wavelengths. See also → aplanatism.

See also: → isoplanatic; → angle.

The spatial region where the variation of the → point spread function of an imaging system is considered negligible.

See also: → isoplanatic; → patch.

In an imaging system, the unvarying of the → point spread function over an extended field of view.

See also: From isoplanat(ic), → isoplanatic, + → -ism.

The condition in which the wavefronts arriving from different parts of a region of sky undergo almost identical phase perturbations. See also → isoplanatic patch.

See also: → isoplanatic; → patch.

A line on a map connecting points of equal value. For example the contour lines joining points of equal altitude on a topographic map. Some other examples of isopleths are: → isobar, → isoneph, and → isohel.

Etymology (EN): From Gk. isoplethes, from → iso- + pleth(os) “great number, multitude;” cognate with Pers. por, → full,

• -es adj. suffix.

Etymology (PE): Izocand, from izo-, → iso-, + cand replacing candi (after prefix) “quantity,” from Mid.Pers. candih “amount, quantity,” from cand “how many, how much; so many, much;” O.Pers. yāvā “as long as;” Av. yauuant- [adj.] “how great?, how much?, how many?,” yauuat [adv.] “as much as, as far as;” cf. Skt. yāvant- “how big, how much;” Gk. heos “as long as, until.”

A triangle having two sides equal.

Etymology (EN): From L.L. isosceles, from Gk. isoskeles “with equal legs; that can be divided into two equal parts,” from isos “equal, identical,” → iso-, + skelos “leg.”

Etymology (PE): Sebar, → triangle, do-pahlu-baraabar, from do, → two, pahlu, → side, barâbar, → equal.

A quantum number based on the assumption that the nucleon (proton and neutron) is a single entity having two states, like → spin. It is conserved by the strong interaction. Same as isotopic spin, isobaric spin.

See also: From → iso- + → spin

Meteo.: A line on a map or graph joining points of equal temperature.

Etymology (EN): Isotherm, back formation from → isothermal.

Etymology (PE): Izodamâ, from → iso- + damâ,
→ temperature.

1. Relating to or having a constant temperature.
   

2. Pertaining to an → isotherm.

Etymology (EN): Isothermal, from → iso- + therm,
from Gk. therme “heat” thermos “warm;” cognate with Pers. garm “warm” (garmâ “heat, warmth,” from Mid.Pers. garmâg; O.Pers./Av. garəma- “hot, warm;” cf. Skt. gharmah “heat;” L. formus “warm,” fornax “oven;” P.Gmc. *warmaz; O.E. wearm; E. warm; O.H.G., Ger. warm; PIE *ghworm-/*ghwerm- “warm”)

• → -al; → process.

Etymology (PE): Izodamâ, from → iso- + damâ,
→ temperature.

A → thermodynamic process that takes place at → constant  → temperature. For the temperature of a system to remain strictly constant, the changes in other coordinates (pressure and volume) must be carried out slowly, and → heat must enter or leave the system to maintain a constant
temperature. → polytropic process. Astrophysical examples include the → collapse of a → protostar down the → Hayashi track, and the collapse of a star at the end of its life to become a → white dwarf.

See also: → isothermal; → process.

A → stellar wind in which the gas is subject to only two forces: the inward directed gravity and the outward directed gradient of the gas pressure.

See also: → isothermal; → wind.

One of several nuclides having the same number of neutrons in their nuclei but differing in the number of protons.

See also: Isotone, from → iso- + tone, from
Gk tonos “strain, tone, mode,” literally, “a stretching,” akin to teinein “to stretch,” cognate with Pers. tanidan “to spin, twist, weave” (Mid.Pers. tanitan; Av. tan- “to stretch, extend;” cf. Skt. tan- to spin, stretch;" tanoti “stretches,” tantram “loom;” Gk. teinein “to stretch, pull tight;” L. tendere “to stretch;
PIE base *ten- “to stretch;” Pers. târ “string,” tân “thread,” tur “fishing net, net, snare,” and tâl “thread” (Borujerdi dialect) belong to this family; variants tanta “cobweb,” tanadu, tafen, kartané, kârtané, kâtené,
Pashtu tanistah “cobweb;” cf. Skt. tantu- “cobweb, thread, string”).

One of two or more atoms having the same number of protons in its nucleus, but a different number of neutrons and, therefore, a different mass.

Etymology (EN): Isotope, from → iso- + -tope, from Gk. topos “place.”

Etymology (PE): Izotop, loan from Fr., as above. hamjâ, from ham- “together” → com- + jâ “place” (from Mid.Pers. giyag “place;” O.Pers. ā-vahana- “place, village;” Av. vah- “to dwell, stay,” vanhaiti “he dwells, stays;” Skt. vásati “he dwells;” Gk. aesa (nukta) “to pass (the night);”
Ossetic wat “room; bed; place;” Tokharian B wäs- “to stay, wait;” PIE base ues- “to stay, live, spend the night”).

A slight difference between the → abundances of → isotopes of the same → chemical element owing to → physical or → chemical  → processes. It results in the → enrichment or → depletion of an isotope. Same as → isotopic fractionation.

See also: → isotope; → fractionation

A displacement in the spectral lines due to the different isotopes of an element.

See also: → isotope; → shift.

Of or relating to an → isotope.

See also: → isotope; → -ic.

Same as → isotope fractionation.

See also: → isotopic; → fractionation

The difference between the number of neutrons in an isotope and the number of protons. Neutron excess.

See also: → isotopic; → number.

The relative abundances of two isotopes of a given chemical element, such as D/H (deuterium/hydrogen), (carbon) ¹²C/¹³C, and (uranium) ²³⁵U/²³⁸U.

See also: → isotopic; → ratio.

Same as → isospin.

See also: → isotopic; → spin.

Any of molecular entities which differ in their isotopic composition but retain the same → chemical elements, e.g. H₂O and HDO.

See also: Abbreviation of → isotopic → analogue.

Any of → isomers having the same number of each isotopic atom but differing in their positions. For example, CH₃CHDCH₃ and CH₃CH₂CH₂D are a pair of isotopomers.

Etymology (EN): Short for isotopic isomers.

Having physical properties that do not vary with direction.

See also: → iso- + → -tropic.

A Universe having observed properties that appear identical in every direction.

See also: → isotropic; → universe.

The property by which physical properties are equal along all directions. → anisotropy.

See also: Isotropy, noun of → isotropic.

1a) A result or outcome of something.

1b) The action of flowing or coming out.

1c) Each of a regular series of publications.

1d) An important topic or problem for debate or discussion.

2. Formally send out or make known (OxfordDictionaries.com).

Etymology (EN): M.E., from O.Fr. issue “a way out, a going out, exit; final event,” from L. exire “to go out, go forth; become public; flow, pour forth,” from → ex- “out,” + ire “to go,”

Etymology (PE): Borunây, literally “what comes out, exits,” from borun, → out, + ây- present stem of âmadan “to come, arrive, become,” → precession.