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

The water current brought about by the → tides.

→ tidal; → current.

A luminosity enhancement in the → light curve of a galaxy observed in X-rays or ultraviolet surveys supposed to be associated with the → tidal disruption of a star that has passed close to a → supermassive black hole in the core of a → host galaxy. An → accretion disk forms after the tidal disruption. The flare event marks the beginning of the accretion process onto the black hole.

→ tidal; → disruption; → flare.

The stretching of an object under → tidal force. Tidal stretching results from a difference in the gravitational pull felt on two sides of a body. It is proportional to the inverse cube of the distance to the source of gravity (1/r³). As a consequence, nearby objects, even small ones like the Moon, raise high tides, whereas distant giants like Jupiter do not produce much of an effect.

→ tidal; → stretching.

Same → temporal resolution.

→ time; → resolution.

A transformation operating on time in the equations of motion of a dynamical system in which t is replaced by -t.

→ time; → reversal.

A technique for deriving extragalactic distances which uses the → luminosity of the brightest → red giant branch stars in old → stellar populations as a → standard candle. For old (> 2-3 Gyr), → metal-poor ([Fe/H] < -0.7) stellar populations, this luminosity is relatively well determined, and the → absolute magnitude of these stars in the I band is roughly constant (M_I = -4.1 ± 0.1).

→ tip; → red giant; → branch; → method.

Exhausted of strength and energy.

Past participle of tire "to weary; become weary," M.E. tyren, O.E. teorian, of unknown origin.

Xasté "tired; hurt, wounded;" Mid.Pers. xastan, xad- "to injure, wound;" Av. vīxaδ- "to crush;" Proto-Iranian *xad- "to wound, hurt."

The hypothesis that photons from distant objects lose energy during their intergalactic journey to us, thereby increasing in wavelength and becoming redshifted. This would provide an alternative to the → Big Bang model in accounting for the → redshifts of distant galaxies. However, there is no evidence for any such tired-light effect. First discussed by F. Zwicky (1929, Proceedings of the National Academy of Sciences, 15, 773).

→ tired; → light.

A criterion for the stability of the disk of a → spiral galaxy. It is expressed by the → Toomre parameter.

→ Toomre criterion; → criterion.

The scale beyond which for a thin, rotating disk, rotation stabilizes self-gravitational contraction. The Toomre length is given by: λ_T = 4π²GΣ / κ², where G is the → gravitational constant, Σ is the mass → surface density, and κ is the → epicyclic frequency (Toomre 1964, ApJ 139, 1217).

→ Toomre parameter; → length.

A quantity that measures the stability of a differentially rotating disk of matter against → gravitational collapse. It is expressed by the relation: Q = c_sκ / πGΣ, where c_s is the → sound speed, κ the → epicyclic frequency, G the → gravitational constant, and Σ the → surface density. The disk is linearly stable for Q > 1 and linearly unstable for Q < 1.

After Alar Toomre (1936-), an American astrophysicist of Estonian origin, professor of mathematics at the Massachusetts Institute of Technology; → parameter.

A cosmological model of → structure formation in which larger structures, such as galaxy → superclusters or perhaps even the vast → filaments and → voids, form earlier and then they fragment into smaller structures such as individual galaxies. Opposite of → bottom-up structure formation.

→ top; → down; → structure; → formation.

A stream of water flowing with great rapidity and violence. See also → rapids.

M.E., from M.Fr. torrent, from L. torrentem (nominative torrens) "a rushing (stream)," from torrere, "to parch, dry up, roast," → torrid.

Lâhez, from Tabari lahez "an overwhelming flow," Baxtiyâri lâhiz "flood." The first component lâh, lah maybe related to lur, Lori, Kordi laf, Tabari lé "flood," → cataclysm.

A phenomenon occurring when a light ray traveling cross an → interface from a higher → refractive index medium to a lower refractive index medium hits the interface at an angle larger than the → critical angle. In these conditions the light will not pass through to the second → medium at all. Instead, all of it will be reflected back into the first medium.

→ total; → internal; → reflection.

The sum of → static pressure, → dynamic pressure, and → hydrostatic pressure in the → Bernoulli equation.

→ total, → pressure.

Allowing → electromagnetic radiation  of → specific → wavelengths to pass through. See also → translucent.

From M.L. transparentem (nominative transparens), pr.p. of transparere "to show through," from L. → trans- "through" + parere "to come in sight, appear."

Tarânemâ, from tarâ-, → trans-, + nemâ present stem of nemudan "to show" (Mid.Pers. nimūdan, nimây- "to show," from O.Pers./Av. ni- "down; into," → ni- (PIE), + māy- "to measure;" cf. Skt. mati "measures," matra- "measure;" Gk. metron "measure;" L. metrum; PIE base *me- "to measure").

1) A tall, woody perennial plant usually with a single trunk.
2) Math.: → factor tree.
3) A → data structure that is used to represent hierarchical data.
4) In → graph theory, a → connected graph with no → cycles.
5) A mathematical structure that can be viewed as either a graph or as a data structure. Many powerful algorithms in computer science and software engineering are tree based algorithms.

M.E., from O.E. treo, treow "tree, wood," from P.Gmc. *trewan (cf. O.S. trio, O.N. tre, Goth. triu), from PIE *deru- "wood" (cf. Mod.Pers. dâr "tree;" O.Pers. dāruv- "wood;" Av. dāuru- "piece of wood, tree trunk;" Skt. dāru- "tree, wood;" Gk. drus "tree;" Serb. drvo "tree;" L. larix "larch tree;" Rus. drevo "tree, wood;" Pol. drwa "wood;" Lith. derva "pine wood;" O.Ir. daur, Welsh derwen "oak").

Deraxt "tree;" Mid.Pers. draxt "tree," two possible etymologies. A suffixed variant of dâr "tree," cognate with E. tree, as above. Alternatively, from Av. *draxta- "firmly stood, fixed" (as in handraxta-), from drang-, dranj- "to fix, fasten, strengthen, hold," dražaite "holds," infinitive drājnhe; cf. Khotanese drys- "to hold;" Sogdian drγ- "to hold;" Parthian Mid.Pers. drxs "to endure."

A type of → data structure in which each element is attached to one or more elements in a hierarchical manner. Trees are often called inverted trees because they are normally drawn with the root at the top.

→ tree; → structure.

A graphical representation that displays a hierarchical view of data.

→ tree; → view.

A general tendency, course, or direction.

From M.E. trenden "to roll about, turn, revolve," from O.E. trendan, (cf. O.E. trinde "round lump, ball," O.Fris. trind, M.L.G. trint "round," M.L.G. trent "ring, boundary," Du. trent "circumference," Dan. trind "round").

Ravâl, from row present stem of raftan "to go, walk, proceed" (Mid.Pers. raftan, raw-, Proto-Iranian *rab/f- "to go; to attack") + suffix -âl, → -al.