An Etymological Dictionary of Astronomy and Astrophysics
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The earliest theoretical description of the inner structure of atoms whereby an atom consists of a sphere of positive electricity of uniform density, throughout which is distributed an equal and opposite charge in the form of electrons. The diameter of the sphere was supposed to be of the order of 10^-8 cm, the magnitude found for the size of the atom. → Rutherford atom.

→ Thomson; → atom.

The → cross section involved the → Thomson scattering of electromagnetic waves by a free electron. It is defined by: σ_T = 8πr_e²/3, where r_e is the classical → electron radius. Its value is 0.665 245 x 10^-28 m².

→ Thomson; → cross; → section.

The absorption or emission of heat when current is passed through a single conductor whose ends are kept at different temperatures. If current is passed from hotter end to colder end of a copper wire, then heat is evolved along the length of the wire. When current is passed from colder end to the hotter end, then heat is absorbed.

→ Thomson; → effect.

The classical, → non-relativistic scattering of photons by free charged particles. When an electromagnetic wave is incident on a charged particle, the electric and magnetic components of the wave exert a force on the particle, setting it into motion. As it accelerates, it in turn radiates in all directions. Such scattering is independent of wavelength and equal numbers of photons are scattered forward and backward. Thomson scattering occurs in stellar atmospheres and in any non-relativistic → plasma. Thomson scattering is normally taken as the minimum → opacity.

→ Thomson; → scattering.

A soft, ductile, lustrous, silver-white, → radioactive metal; symbol Th. → Atomic number 90; → atomic weight 232.0381; → melting point about 1,750 °C; → boiling point about 4,790 °C; → specific gravity 11.7 at 20 °C. It was discovered in the mineral thorite (ThSiO₄) by the Swedish chemist Jons Jacob Berzelius in 1828. It was first isolated by the chemists D. Lely Jr. and L. Hamburger in 1914.

Thorium-232 is a → fertile isotope, naturally occurring, from which the → fissile isotope uranium-233 can be bred.

From Thor, the "Scandinavian god of thunder."

A theoretical class of → stellar objects in which a → neutron star core is surrounded by a large and diffuse envelope. TŻOs are expected to form as a result of the evolution of two → massive stars in a → close binary, with the neutron star forming when the more massive star explodes as a → supernova. During subsequent evolution of the system, the expanding envelope of the companion may lead to a common envelope state and the spiral-in of the neutron star into the core of its companion. Alternately, a TŻO may be produced when a newly-formed neutron star receives a supernova "kick" velocity in the direction of its companion and becomes embedded. Supergiant TŻOs are predicted to be almost identical in appearance to → red supergiants (RSGs). The best features that can be used at present to distinguish TŻOs from the general RSG population are the unusually strong → heavy element and → lithium lines present in their spectra, products of the star's fully → convective envelope linking the → photosphere with the extraordinarily hot burning region in the vicinity of the neutron star core. These objects are thought to be extremely rare, with as few as 20-200 of them predicted to exist in the Galaxy at present, though some authors have doubted whether such an object could survive the merger with the envelope intact. A candidate is HV 2112 (Levesque et al., 2014, MNRAS, arXiv:1406.0001; Beasor et al., 2018, MNRAS, arXiv:1806.07399).

Thorne K. S., Żytkow A., 1975, ApJ 199, L19.

The product of mental activity; that which one thinks; the act or process of thinking.

M.E. thoght; O.E. (ge)thoht, from stem thencan "to think;" cf. O.Fris. thinka, O.S. thenkian, O.H.G. denchen, Ger. denken "to think."

Andišé, noun from andišidan, → think.

A demonstration which is carried out in the realm of the imagination, rather than in a laboratory. Thought experiments are designed to test ideas, theories, and hypotheses which cannot physically be tested, at least with current scientific equipment. Some examples: → Maxwell's demon; → Einstein's elevator; Heisenberg's gamma-ray microscope; → Schrodinger's cat. Also called Gedanken experiment.

→ thought; → experiment. Based on both the Ger./L. compound Gedankenexperiment and its Ger. equivalent Gedankenversuch.

1) A declaration of an intention or determination to inflict punishment, injury, etc., in retaliation for, or conditionally upon, some action or course; menace.
2) An indication or warning of probable trouble (Dictionary.com).

M.E. threte, O.E. threat "pressure, oppression;" cognate with O.N. thraut "hardship, bitter end," Du. verdreiten, Ger. verdrießen "to vex," L. trudere "to press, thrust."

Harš, from Kurd. haraša "threat," haraša kirdan "threaten," related to Mid/Mod.Pers. rašk "envy, jealousy;" Lori, Laki erešt "assault, attack;" Tabari ârâšt "curse, anathema;" Av. arš- "to be envious;" Skt. īrs- "to be envious, envy;" Arm. her "anger, quarrel;" O.E. eorsian "to be malicious;" Proto-Ir. Harš- "to be envious."

1) To utter a threat against; menace.
2) To be a menace or source of danger to (Dictionary.com).

From M.E. thretnen, from O.E. thrêatnian, → threat

Infinitive from harš, → threat.

A cardinal number, 2 plus 1.

M.E.; O.E. threo, thrib, feminin and neuter of thri(e); cf. O.Fris. thre, M.Du., Du. drie, O.H.G. dri, Ger. drei, Dan. tre), cognate with Pers. sé, as below.

Sé, from Mid.Pers. sé; Av. θrayô, θrayas, tisrô, θri; cf. Skt. tráya, tri, trini; Gk. treis, L. tres, Lith. trys, O.C.S. trye, Ir., Welsh tri, O.E. threo, as above; PIE base *trei-.

The mathematical problem of studying the positions and velocities of three mutually attracting bodies (such as the Sun, Earth and Moon) and the stability of their motion. This problem is surprisingly difficult to solve, even in the simple case, called → restricted three-body problem, where one of the masses is taken to be negligibly small so that the problem simplifies to finding the behavior of the mass-less body in the combined gravitational field of the other two. See also → two-body problem, → n-body problem.

→ three; → body; → problem.

A flow whose parameters (velocity, pressure, and so on) vary in all three coordinate directions. Considerable simplification in analysis may often be achieved, however, by selecting the coordinate directions so that appreciable variation of the parameters occurs in only two directions, or even only one (B. Massey, Mechanics of Fluids, Taylor & Francis, 2006).

→ three; → dimensional; → flow.

The level that must be reached for a physical effect to begin or be noticeable.

M.E. threschold, O.E. threscold, threscwald "doorsill, point of entering."

Âstâné "threshold; a place of rest or sleeping," variant âstân; Mid.Pers. âstânak; ultimately from Proto-Iranian *ā-stānaka-, from *stā- "to stand;" cf. O.Pers./Av. sta- "to stand, stand still; set;" Av. hištaiti; Mid.Pers. êstâtan "to stand;" Mod.Pers. istâdan "to stand;" cf. Skt. sthâ- "to stand;" Gk. histemi "put, place, weigh," stasis "a standing still;" L. stare "to stand;" Lith. statau "place;" Goth. standan; PIE base *sta- "to stand."

The minimum energy necessary for the occurrence of some chemical/physical effect.

→ threshold; → energy.

The minimum energy, for an incident particle or photon, below which a particular reaction does not occur.

→ threshold; → reaction.

The minimum intensity of a signal that can be detected and recognized.

→ threshold; → signal.

The front part of the neck. → nozzle throat.

M.E. throte, O.E. throte, throta, throtu; cognate with O.H.G. drozza "throat," O.N. throti "swelling."

Galu "throat," related to geri, geribân "collar," gerivé "low hill," gardan "neck;" Mid.Pers. galôg, griv "throat," gartan "neck;" Av. grīvā- "neck;" cf. Skt. gala- "throat, neck;" Gk. bora "food;" L. gula "throat" (Fr. gueule "(animal) mouth"), gluttire "to gulp down," vorare "to devour;" PIE base *g^wer- "to swallow, devour." L. gula; cf. Mod.Pers. galu "throat,"

A jointed ring placed at the upper end of a → planispheric astrolabe astrolabe. By slipping one's thumb into the ring, one raises the instrument so that its weight and symmetrical design keeps it perpendicular to the ground (online museo galileo, VirtualMuseum).

From L. thronus, from Gk. thronos "elevated seat, chair, throne," from PIE root *dher- "to hold firmly, support;" cf. L. firmus "firm, steadfast, strong, stable," Skt. dharma- "statute, law;" Pers. dâr-, dâštan "to have, to possess," → property.

Korsi "throne, chair, seat," from Ar. kursī.

1a) To stop the breath of by compressing the throat; strangle.
1b) To compress by fastening something tightly around.
2a) To obstruct or check the flow of (a fluid), as to control the speed of an engine.
2b) To reduce the pressure of (a fluid) by passing it from a smaller area to a larger one (Dictionary.com).

M.E. throtelen, from throten "to cut the throat of (someone), strangle," from → throat.

Tâsenidan, from Dezfuli tâsenidan "to choke, compress the throat," cf. Laki, Šuštari tâsenan "to strangle," Ilâmi tâsânen "to strangle," Baxtiyâri tâsest "suffocated," Fini Bandar-Abbâs tâsaki "exhausted," (Dehxodâ) tâsidan "to be afflicated or sad," tâsidé "tired, emaciated," Baluci ta(n)sit "to be out of breath, pant;" Skt. tam- "to become suffocated, exhausted;" L. temulentus "drunken;" PIE *temH- "to faint, be exhausted" (Cheung 2007).