An Etymological Dictionary of Astronomy and Astrophysics

The second innermost satellite of → Neptune. It was discovered using NASA’s Voyager 2 mission in 1989. It orbits 50,070 km from the center of Neptune and is about 80 km in diameter.

See also: Named after a daughter of Aether and Hemera from Gk. mythology. Thalassa is also the Greek word for “sea”.

The second innermost satellite of → Neptune. It was discovered using NASA’s Voyager 2 mission in 1989. It orbits 50,070 km from the center of Neptune and is about 80 km in diameter.

See also: Named after a daughter of Aether and Hemera from Gk. mythology. Thalassa is also the Greek word for “sea”.

The fourth-closest of → Jupiter’s known → satellites, also known as Jupiter XIV. Thebe is 100 x 90 km in diameter and orbits its planet at 222,000 km in 0.6745 (Earth) day. It is in synchronous rotation, i.e. always keeps the same side facing Jupiter. Thebe was discovered by Stephen Synnott (Voyager 1) in 1979.

See also: In Gk. mythology, Thebe was a nymph, daughter of the river god Asopus.

The fourth-closest of → Jupiter’s known → satellites, also known as Jupiter XIV. Thebe is 100 x 90 km in diameter and orbits its planet at 222,000 km in 0.6745 (Earth) day. It is in synchronous rotation, i.e. always keeps the same side facing Jupiter. Thebe was discovered by Stephen Synnott (Voyager 1) in 1979.

See also: In Gk. mythology, Thebe was a nymph, daughter of the river god Asopus.

A hypothetical → protoplanet that collided with → earth some 4.5 billion years ago. Debris from the collision, a mixture of material from both bodies, spun out into Earth orbit and → coalesced into the → Moon. This scenario explains many aspects of → lunar geology including the size of the Moon’s → core and the → density and → isotopic  → composition of Moon rocks.

See also: Named for Theia, the mythical Greek Titan who was the mother of Selene, the goddess of the Moon.

A hypothetical → protoplanet that collided with → earth some 4.5 billion years ago. Debris from the collision, a mixture of material from both bodies, spun out into Earth orbit and → coalesced into the → Moon. This scenario explains many aspects of → lunar geology including the size of the Moon’s → core and the → density and → isotopic  → composition of Moon rocks.

See also: Named for Theia, the mythical Greek Titan who was the mother of Selene, the goddess of the Moon.

The belief in one God as the creator and ruler of the universe, without rejection of revelation (distinguished from → deism).

2. Belief in the existence of a god or gods (opposed to → atheism) (Dictionary.com).

Etymology (EN): From the- variant of theo- before a vowel, from Gk. theos “god,” from PIE root *dhes-, root of words applied to various religious concepts, such as L. feriae “holidays,” festus “festive,” fanum “temple.”

Etymology (PE): Yzadân-bâvari, from yazdân “god,” from Mid.Pers. yazetân “gods,” ultimately from Proto-Ir. *iaz- “to sacrifice, worship, venerate,” → deity.

The belief in one God as the creator and ruler of the universe, without rejection of revelation (distinguished from → deism).

2. Belief in the existence of a god or gods (opposed to → atheism) (Dictionary.com).

Etymology (EN): From the- variant of theo- before a vowel, from Gk. theos “god,” from PIE root *dhes-, root of words applied to various religious concepts, such as L. feriae “holidays,” festus “festive,” fanum “temple.”

Etymology (PE): Yzadân-bâvari, from yazdân “god,” from Mid.Pers. yazetân “gods,” ultimately from Proto-Ir. *iaz- “to sacrifice, worship, venerate,” → deity.

A person who believes in → theism.

See also: → theism; → -ist

A person who believes in → theism.

See also: → theism; → -ist

Of or relating to a → theme.

See also: → thematics.

Of or relating to a → theme.

See also: → thematics.

A body of topics for study or discussion.

See also: From L. themat-, from thema, → theme,

• → -ics.

A body of topics for study or discussion.

See also: From L. themat-, from thema, → theme,

• → -ics.

The act or process of thematizing.

See also: → thematize; → -tion.

The act or process of thematizing.

See also: → thematize; → -tion.

Present or select as a theme.

Etymology (EN): From L. themat-, from thema, → theme,

• E. suffix → -ize.

Etymology (PE): Verbal noun of nedanidan, → thematize.

Present or select as a theme.

Etymology (EN): From L. themat-, from thema, → theme,

• E. suffix → -ize.

Etymology (PE): Verbal noun of nedanidan, → thematize.

1. Generally, a topic or subject of discourse, discussion, meditation, or composition.
   

   2. A unifying or dominant idea, motif, etc., as in a work of art. More specifically, an implicit central concept or message found in a literary work.
      

   3. A recurrent melodic element in a musical composition.
      

   4. Linguistics: The stem of a noun or verb; the part to which inflections are added, especially one composed of the root and an added vowel.

Etymology (EN): M.E. teme, theme, from O.Fr. tesme, from L. thema “a subject, thesis,” from Gk. thema “a proposition, subject, deposit,” literally “something set down,” from PIE root *dhe- “to set, put.”

Etymology (PE): Nedan, literally “(something) set down,” from prefix ne- “down,” → ni- (PIE), + da-, variant of dâ-/dâdan “to give, set” (dah-/dahad “gives”); Mid.Pers. dâdan, dah- “to give, set; create;” O.Pers. dā- “to give;” Av. dā- “to place upon, bestow;” Proto-Ir. *da- “to give” (cf. Skt. dā- “to give, present, offer;” Gk. thema; PIE *dhe- “to set, put,” as above) + word forming suffix -an (as in rowzan, rasan, anjoman, barzan).

1. Generally, a topic or subject of discourse, discussion, meditation, or composition.
   

   2. A unifying or dominant idea, motif, etc., as in a work of art. More specifically, an implicit central concept or message found in a literary work.
      

   3. A recurrent melodic element in a musical composition.
      

   4. Linguistics: The stem of a noun or verb; the part to which inflections are added, especially one composed of the root and an added vowel.

Etymology (EN): M.E. teme, theme, from O.Fr. tesme, from L. thema “a subject, thesis,” from Gk. thema “a proposition, subject, deposit,” literally “something set down,” from PIE root *dhe- “to set, put.”

Etymology (PE): Nedan, literally “(something) set down,” from prefix ne- “down,” → ni- (PIE), + da-, variant of dâ-/dâdan “to give, set” (dah-/dahad “gives”); Mid.Pers. dâdan, dah- “to give, set; create;” O.Pers. dā- “to give;” Av. dā- “to place upon, bestow;” Proto-Ir. *da- “to give” (cf. Skt. dā- “to give, present, offer;” Gk. thema; PIE *dhe- “to set, put,” as above) + word forming suffix -an (as in rowzan, rasan, anjoman, barzan).

A small satellite of → Jupiter, ninth in order from the planet. It is about 8 km in diameter and orbits Jupiter at a mean distance of 7 500 000 km every 130 days. It was discovered in 1975, lost, and then rediscovered in 2000. Also known as Jupiter XVIII.

See also: Named after Themisto, daughter of the river god Inachus, who became the mother of Ister (the river Danube) by Zeus (Jupiter).

A small satellite of → Jupiter, ninth in order from the planet. It is about 8 km in diameter and orbits Jupiter at a mean distance of 7 500 000 km every 130 days. It was discovered in 1975, lost, and then rediscovered in 2000. Also known as Jupiter XVIII.

See also: Named after Themisto, daughter of the river god Inachus, who became the mother of Ister (the river Danube) by Zeus (Jupiter).

1. A form of government in which → God or a → deity is recognized as the supreme civil ruler, the God’s or deity’s laws being interpreted by the ecclesiastical authorities.
   

2. A system of government by priests claiming a divine commission (Dictionary.com)

See also: → theism; → -cracy.

1. A form of government in which → God or a → deity is recognized as the supreme civil ruler, the God’s or deity’s laws being interpreted by the ecclesiastical authorities.
   

2. A system of government by priests claiming a divine commission (Dictionary.com)

See also: → theism; → -cracy.

An instrument for the measurement of angles, used in surveying. It consists essentially of a telescope moving along a circular scale graduated in degrees.

See also: The first occurrence of the word theodolite is found in the surveying textbook A geometric practice named Pantometria (1571) by Leonard Digges, which was published posthumously by his son, Thomas Digges. The etymology of the word is unknown. The first part of the New Latin theo-delitus might stem from the Gk. theaomai “to behold, view attentively, contemplate,” but the second part is more puzzling and is often attributed to an un-scholarly variation of delos “evident, clear.”

An instrument for the measurement of angles, used in surveying. It consists essentially of a telescope moving along a circular scale graduated in degrees.

See also: The first occurrence of the word theodolite is found in the surveying textbook A geometric practice named Pantometria (1571) by Leonard Digges, which was published posthumously by his son, Thomas Digges. The etymology of the word is unknown. The first part of the New Latin theo-delitus might stem from the Gk. theaomai “to behold, view attentively, contemplate,” but the second part is more puzzling and is often attributed to an un-scholarly variation of delos “evident, clear.”

A person versed in theology, especially Christian theology (Dictionary.com)

See also: → theology.

A person versed in theology, especially Christian theology (Dictionary.com)

See also: → theology.

The field of study and analysis that treats of → God and of God’s attributes and relations to the universe; study of divine things or religious truth; divinity (Dictionary.com).

See also: → theism; → -logy.

The field of study and analysis that treats of → God and of God’s attributes and relations to the universe; study of divine things or religious truth; divinity (Dictionary.com).

See also: → theism; → -logy.

A → proposition, → statement, or → formula in → mathematics or → logic deduced from → axioms, other propositions, → assumptions, → premises, or formulas. Theorems are statements which can be proved. For example, → Fourier theorem; → Liouville’s theorem; → Woltjer’s theorem.

Etymology (EN): From M.Fr. théorème, from L.L. theorema, from Gk. theorema “spectacle, speculation,” in Euclid “proposition to be proved,” from theorein “to look at, speculate, consider.”

Etymology (PE): Farbin, from far- intensive prefix “much, abundant; elegantly; forward” (Mid.Pers. fra- “forward, before; much; around;”
O.Pers. fra- “forward, forth;” Av. frā, fərā-, fra- “forward, forth; excessive;” cf. Skt. prá- “before; forward, in front;” Gk. pro “before, in front of;” L. pro “on behalf of, in place of, before, for;” PIE *pro-)

• bin, present stem of didan “to see,” from
  Mid.Pers. wyn-; O.Pers. vain- “to see;” Av. vaēn- “to see;”
  cf. Skt. veda “I know;” Gk. oida “I know,” idein “to see;” L. videre “to see;” PIE base *weid- “to know, to see.”

A → proposition, → statement, or → formula in → mathematics or → logic deduced from → axioms, other propositions, → assumptions, → premises, or formulas. Theorems are statements which can be proved. For example, → Fourier theorem; → Liouville’s theorem; → Woltjer’s theorem.

Etymology (EN): From M.Fr. théorème, from L.L. theorema, from Gk. theorema “spectacle, speculation,” in Euclid “proposition to be proved,” from theorein “to look at, speculate, consider.”

Etymology (PE): Farbin, from far- intensive prefix “much, abundant; elegantly; forward” (Mid.Pers. fra- “forward, before; much; around;”
O.Pers. fra- “forward, forth;” Av. frā, fərā-, fra- “forward, forth; excessive;” cf. Skt. prá- “before; forward, in front;” Gk. pro “before, in front of;” L. pro “on behalf of, in place of, before, for;” PIE *pro-)

• bin, present stem of didan “to see,” from
  Mid.Pers. wyn-; O.Pers. vain- “to see;” Av. vaēn- “to see;”
  cf. Skt. veda “I know;” Gk. oida “I know,” idein “to see;” L. videre “to see;” PIE base *weid- “to know, to see.”

Of, pertaining to, or consisting in theory.

Etymology (EN): From L.L. theoreticus “of or pertaining to theory,” from Gk. theoretikos “contemplative, pertaining to theory;” → theory.

Etymology (PE): Negarik, contraction of negaréik, from negaré→ theory + -ik, → -ic.

Of, pertaining to, or consisting in theory.

Etymology (EN): From L.L. theoreticus “of or pertaining to theory,” from Gk. theoretikos “contemplative, pertaining to theory;” → theory.

Etymology (PE): Negarik, contraction of negaréik, from negaré→ theory + -ik, → -ic.

An astrophysical study or research group mainly concerned with theory rather than observation.

See also: → theoretical; → astrophysics.

An astrophysical study or research group mainly concerned with theory rather than observation.

See also: → theoretical; → astrophysics.

One who formulates or is expert in the theoretical side of a subject.

Etymology (EN): From theoretic, from theoretics, from → theory

• -ian.

Etymology (PE): Negare-pardâz, from negaré, → theory,

• pardâz, present stem of pardâxtan “to accomplish; bring to perfection; to attempt, to care; to clean; to free;” Mid.Pers. pardâxtan, pardâzidan “to accomplish; to be done with, freed of” ultimately from Proto-Iranian *para-tāxta-, *para-tāca- “to take away; to expel,”
  from *para- “along, forth,” → para-, + tāxta-, tāca- “to run, to flow;” cf. Av. tak- “to run, to flow;” Mod.Pers. tâxtan, tâz- “to flow, to cause to walk,” → flow.

One who formulates or is expert in the theoretical side of a subject.

Etymology (EN): From theoretic, from theoretics, from → theory

• -ian.

Etymology (PE): Negare-pardâz, from negaré, → theory,

• pardâz, present stem of pardâxtan “to accomplish; bring to perfection; to attempt, to care; to clean; to free;” Mid.Pers. pardâxtan, pardâzidan “to accomplish; to be done with, freed of” ultimately from Proto-Iranian *para-tāxta-, *para-tāca- “to take away; to expel,”
  from *para- “along, forth,” → para-, + tāxta-, tāca- “to run, to flow;” cf. Av. tak- “to run, to flow;” Mod.Pers. tâxtan, tâz- “to flow, to cause to walk,” → flow.

Same as → theoretician.

Etymology (EN): From theor-, from → theory + -ist a suffix of nouns, often corresponding to verbs ending in -ize or nouns ending in -ism.

Etymology (PE): → theoretician.

Same as → theoretician.

Etymology (EN): From theor-, from → theory + -ist a suffix of nouns, often corresponding to verbs ending in -ize or nouns ending in -ism.

Etymology (PE): → theoretician.

A coherent set of verified facts, propositions, or principles analyzed in their relation to one another and used to explain and predict phenomena, e.g. the → theory of relativity.
The criterion of the scientific status of a theory is its → falsifiability, → refutability, or → testability. See also → hypothesis, → model.

Etymology (EN): From L.L. theoria, from Gk. theoria “contemplation, speculation, a looking at, things looked at,” from theorein “to consider, view, look at,” from theoros “spectator,” from thea “a view” + horan “to see.”

Etymology (PE): Negaré, from negar present stem of negaridan, negaristan “to look, observe;” Mid.Pers. nigeridan, niger-, nikiritan, nikir- “to look, to watch, to notice, to consider;” ultimately from Proto-Iranian *ni-kar-, from *ni- “down, in, into,” → ni- (PIE), + *kar- “to observe, to consider;” cf. Av. kar- “to remember; to impress on memory;” Skt. kal- “to observe, consider,” kalayati “considers, observes;” Mid.Pers. kartan “to establish; to declare; to found,” (h)angârtan “to consider, to bear in mind, to regard as,” us-kâritan “to consider, deliberate, discuss,” sikâl, sigâl “thought;”
Mod.Pers. engâridan, engâštan “to suppose,”
segâl “thought,” segâlidan “to think, to resolve to injure, to deceive.”

A coherent set of verified facts, propositions, or principles analyzed in their relation to one another and used to explain and predict phenomena, e.g. the → theory of relativity.
The criterion of the scientific status of a theory is its → falsifiability, → refutability, or → testability. See also → hypothesis, → model.

Etymology (EN): From L.L. theoria, from Gk. theoria “contemplation, speculation, a looking at, things looked at,” from theorein “to consider, view, look at,” from theoros “spectator,” from thea “a view” + horan “to see.”

Etymology (PE): Negaré, from negar present stem of negaridan, negaristan “to look, observe;” Mid.Pers. nigeridan, niger-, nikiritan, nikir- “to look, to watch, to notice, to consider;” ultimately from Proto-Iranian *ni-kar-, from *ni- “down, in, into,” → ni- (PIE), + *kar- “to observe, to consider;” cf. Av. kar- “to remember; to impress on memory;” Skt. kal- “to observe, consider,” kalayati “considers, observes;” Mid.Pers. kartan “to establish; to declare; to found,” (h)angârtan “to consider, to bear in mind, to regard as,” us-kâritan “to consider, deliberate, discuss,” sikâl, sigâl “thought;”
Mod.Pers. engâridan, engâštan “to suppose,”
segâl “thought,” segâlidan “to think, to resolve to injure, to deceive.”

Any theory that attempts to describe all the forces of nature including gravity in a single mathematical formalism; e.g. → grand unified theory. → string theory.

Etymology (EN): → theory; every; M.E. every, everich; O.E. æfre ælc “ever each;” → thing.

Etymology (PE): Negaré, → theory; hamé, → all; ciz, → thing.

Any theory that attempts to describe all the forces of nature including gravity in a single mathematical formalism; e.g. → grand unified theory. → string theory.

Etymology (EN): → theory; every; M.E. every, everich; O.E. æfre ælc “ever each;” → thing.

Etymology (PE): Negaré, → theory; hamé, → all; ciz, → thing.

Any of the two theories put forward by Albert Einstein:
→ special relativity (1905) and → general relativity (1916).

See also: → theory; → relativity;

Any of the two theories put forward by Albert Einstein:
→ special relativity (1905) and → general relativity (1916).

See also: → theory; → relativity;

Any of several commercial units of heat energy, as one equivalent to 10⁶ calories.

See also: From Gk. therme “heat,” → thermal.

Any of several commercial units of heat energy, as one equivalent to 10⁶ calories.

See also: From Gk. therme “heat,” → thermal.

Of, pertaining to, or caused by heat or temperature.

Etymology (EN): From M.Fr. thermal, from Gk. therme “heat,” cognate with Pers. garm “warm,” as below.

Etymology (PE): Garmâyi, adj. of garmâ “heat, warmth,” from Mid.Pers. garmâg; O.Pers./Av. garəma- “hot, warm;” cf. Skt. gharmah “heat;” cognate with Gk. therme, thermos, as above; PIE *ghworm-/*ghwerm- “warm.”

Of, pertaining to, or caused by heat or temperature.

Etymology (EN): From M.Fr. thermal, from Gk. therme “heat,” cognate with Pers. garm “warm,” as below.

Etymology (PE): Garmâyi, adj. of garmâ “heat, warmth,” from Mid.Pers. garmâg; O.Pers./Av. garəma- “hot, warm;” cf. Skt. gharmah “heat;” cognate with Gk. therme, thermos, as above; PIE *ghworm-/*ghwerm- “warm.”

1. The random movement of the molecules of a substance, the energy of which is, by kinetic theory, synonymous with the heat content of the substance.
   

2. Solid state physics: The random motion of free electrons in a conductor due to heat energy.

See also: → thermal; → agitation.

1. The random movement of the molecules of a substance, the energy of which is, by kinetic theory, synonymous with the heat content of the substance.
   

2. Solid state physics: The random motion of free electrons in a conductor due to heat energy.

See also: → thermal; → agitation.

The emission of electromagnetic radiation from high temperature plasma, produced as electrons are deviated by positive ions. Same as → free-free emission

See also: → thermal; → bremsstrahlung.

The emission of electromagnetic radiation from high temperature plasma, produced as electrons are deviated by positive ions. Same as → free-free emission

See also: → thermal; → bremsstrahlung.

A process that occurs in a medium where a → temperature gradient exists:

dQ = -κ(dT/dx)dA.dt, where dQ is the amount of heat passing through the time dt across an area dA in the direction of the normal x to this area and toward the reduction in temperature, κ is the → thermal conductivity, and (dT/dx) the temperature gradient.

See also: → thermal; → conduction.

A process that occurs in a medium where a → temperature gradient exists:

dQ = -κ(dT/dx)dA.dt, where dQ is the amount of heat passing through the time dt across an area dA in the direction of the normal x to this area and toward the reduction in temperature, κ is the → thermal conductivity, and (dT/dx) the temperature gradient.

See also: → thermal; → conduction.

In → thermal conduction, the amount of heat passing across unit area per unit time and per unit → temperature gradient.

See also: → thermal; → conductivity.

In → thermal conduction, the amount of heat passing across unit area per unit time and per unit → temperature gradient.

See also: → thermal; → conductivity.

A detector that senses the change of temperature due to the absorption of photons.

See also: → thermal; → detector.

A detector that senses the change of temperature due to the absorption of photons.

See also: → thermal; → detector.

A physical process resulting from → temperature gradients in stellar interiors, whereby more highly charged and more massive chemical species are concentrated toward the hottest region of the star, its center. Therefore, thermal diffusion and → gravitational settling tend to make heavier species sink relative to the light ones.

See also: → thermal; → diffusion.

A physical process resulting from → temperature gradients in stellar interiors, whereby more highly charged and more massive chemical species are concentrated toward the hottest region of the star, its center. Therefore, thermal diffusion and → gravitational settling tend to make heavier species sink relative to the light ones.

See also: → thermal; → diffusion.

→ thermal radiation.

See also: → thermal; → emission.

→ thermal radiation.

See also: → thermal; → emission.

1. The energy in the form of heat emitted by an object by virtue of its temperature.
   

2. The total potential and kinetic energies associated with the random motions of the particles of a material. The quantity of thermal energy possessed by a body determines
   its temperature. The thermal energy which is absorbed, given up, or transferred from one material to another is heat.
   

3. The characteristic energy of → thermal neutrons at room temperature, about 0.025 eV.

See also: → thermal; → energy.

1. The energy in the form of heat emitted by an object by virtue of its temperature.
   

2. The total potential and kinetic energies associated with the random motions of the particles of a material. The quantity of thermal energy possessed by a body determines
   its temperature. The thermal energy which is absorbed, given up, or transferred from one material to another is heat.
   

3. The characteristic energy of → thermal neutrons at room temperature, about 0.025 eV.

See also: → thermal; → energy.

In thermodynamics, the state of a system all parts of which have attained a uniform temperature and no net heat exchange is taking place between it and its surroundings. If two bodies are in thermal equilibrium, they have the same temperature. Thermal equilibrium is the central criterion of the → zeroth law of thermodynamics. See also → local thermodynamic equilibrium (LTE).

See also: → thermal; → equilibrium.

In thermodynamics, the state of a system all parts of which have attained a uniform temperature and no net heat exchange is taking place between it and its surroundings. If two bodies are in thermal equilibrium, they have the same temperature. Thermal equilibrium is the central criterion of the → zeroth law of thermodynamics. See also → local thermodynamic equilibrium (LTE).

See also: → thermal; → equilibrium.

An → atmospheric escape that occurs when irradiation from a parent star (or a very high heat flux from a planet interior) heats a planetary atmosphere, causing its molecules to escape to space.

In basic models, the theory assumes neutral species with a → Maxwell-Boltzmann distribution of velocities, which occurs when collisions between molecules are frequent.

Thermal escape has two types: → Jeans’ escape and → hydrodynamic escape

(see, e.g., Catling, D. C. and Kasting, J. F., 2017, Escape of Atmospheres to

Space, pp. 129-167. Cambridge University Press).

See also: → thermal; → escape.

An → atmospheric escape that occurs when irradiation from a parent star (or a very high heat flux from a planet interior) heats a planetary atmosphere, causing its molecules to escape to space.

In basic models, the theory assumes neutral species with a → Maxwell-Boltzmann distribution of velocities, which occurs when collisions between molecules are frequent.

Thermal escape has two types: → Jeans’ escape and → hydrodynamic escape

(see, e.g., Catling, D. C. and Kasting, J. F., 2017, Escape of Atmospheres to

Space, pp. 129-167. Cambridge University Press).

See also: → thermal; → escape.

A process in which collisions that occur between particles cause atoms or molecules to obtain additional kinetic energy.

See also: → thermal; → excitation.

A process in which collisions that occur between particles cause atoms or molecules to obtain additional kinetic energy.

See also: → thermal; → excitation.

The change in dimensions of a material resulting from a change in temperature.

See also: → thermal; → expansion.

The change in dimensions of a material resulting from a change in temperature.

See also: → thermal; → expansion.

A vector quantity that depends on the distribution of temperature in three dimensions with respect to a given point. The magnitude and orientation of the maximum thermal gradient are given by:

∇T = (∂T/∂x)i + (∂T/∂y)j

• (∂T/∂z)k, where T is the temperature distribution function in three dimensions, and i, j, and k are the unit vectors along the x, y, and z axes defining the temperature field.
  Same as → temperature gradient.

See also: → thermal; → gradient.

A vector quantity that depends on the distribution of temperature in three dimensions with respect to a given point. The magnitude and orientation of the maximum thermal gradient are given by:

∇T = (∂T/∂x)i + (∂T/∂y)j

• (∂T/∂z)k, where T is the temperature distribution function in three dimensions, and i, j, and k are the unit vectors along the x, y, and z axes defining the temperature field.
  Same as → temperature gradient.

See also: → thermal; → gradient.

A mechanism for the → transport of → electrons which occurs when the → Fermi level lies
below a low but wide energy → barrier. The → tunneling probability across the barrier is considerably suppressed due to the width of the barrier. However, at higher temperatures, the electron can raise its energy with the assistance of a vibrational mode. The electron is said to hop from one side of the barrier to the other side via an intermediate state.

See also: → thermal; → hop; → -ing.

A mechanism for the → transport of → electrons which occurs when the → Fermi level lies
below a low but wide energy → barrier. The → tunneling probability across the barrier is considerably suppressed due to the width of the barrier. However, at higher temperatures, the electron can raise its energy with the assistance of a vibrational mode. The electron is said to hop from one side of the barrier to the other side via an intermediate state.

See also: → thermal; → hop; → -ing.

The tendency of a body to resist a change in temperature. A body with a low thermal inertia requires very few calories to change its surface temperature. A low thermal inertia material tends to be thermally insulating, so that the surface temperature changes readily, but those changes are not conducted to depth within the material (Ellis et al., 2007, Planetary Ring Systems, Springer).

See also: → thermal; → inertia.

The tendency of a body to resist a change in temperature. A body with a low thermal inertia requires very few calories to change its surface temperature. A low thermal inertia material tends to be thermally insulating, so that the surface temperature changes readily, but those changes are not conducted to depth within the material (Ellis et al., 2007, Planetary Ring Systems, Springer).

See also: → thermal; → inertia.

The → Jeans mass when → turbulence is insignificant.

See also: → thermal; → Jeans; → mass.

The → Jeans mass when → turbulence is insignificant.

See also: → thermal; → Jeans; → mass.

The random motions and collisions of molecules, atoms, electrons, or other subatomic particles constituting an object at all temperatures above → absolute zero. The thermal motion of particles rises with the temperature of those particles and is governed by the laws of → thermodynamics. The most convincing experimental proof of thermal motion → Brownian motion.

See also: → thermal; → motion.

The random motions and collisions of molecules, atoms, electrons, or other subatomic particles constituting an object at all temperatures above → absolute zero. The thermal motion of particles rises with the temperature of those particles and is governed by the laws of → thermodynamics. The most convincing experimental proof of thermal motion → Brownian motion.

See also: → thermal; → motion.

A neutron of very slow speed and consequently of low energy. The energy of thermal neutrons is of the same order as the → thermal energy of the atoms and molecules of the substance through which they are passing.

See also: → thermal; → neutron.

A neutron of very slow speed and consequently of low energy. The energy of thermal neutrons is of the same order as the → thermal energy of the atoms and molecules of the substance through which they are passing.

See also: → thermal; → neutron.

Electric noise signals that are produced by the random thermal motion of charges in circuit.

See also: → thermal; → noise.

Electric noise signals that are produced by the random thermal motion of charges in circuit.

See also: → thermal; → noise.

The ordinary pressure in a gas that is due to motions of particles and can be attributed to the object’s → temperature.

See also: → thermal; → pressure.

The ordinary pressure in a gas that is due to motions of particles and can be attributed to the object’s → temperature.

See also: → thermal; → pressure.

Repeated instabilities of the He burning shell which ignites in sudden burst during the final phases of the → AGB evolution.

See also: → thermal; → pulse.

Repeated instabilities of the He burning shell which ignites in sudden burst during the final phases of the → AGB evolution.

See also: → thermal; → pulse.

The energy radiated from an object in the form of → electromagnetic waves as a result of its → temperature. Thermal radiation ranges in → wavelength from the longest → infrared radiation
through the → visible light spectrum to the shortest → ultraviolet rays. In opposition, → non-thermal radiation is caused by energetic particles.

See also: → thermal; → radiation.

The energy radiated from an object in the form of → electromagnetic waves as a result of its → temperature. Thermal radiation ranges in → wavelength from the longest → infrared radiation
through the → visible light spectrum to the shortest → ultraviolet rays. In opposition, → non-thermal radiation is caused by energetic particles.

See also: → thermal; → radiation.

Stresses induced in a material because of rapid temperature change or a → thermal gradient .

See also: → thermal; → shock.

Stresses induced in a material because of rapid temperature change or a → thermal gradient .

See also: → thermal; → shock.

A → transient → rise in → temperature above the normal level in a medium.

See also: → thermal; → spike.

A → transient → rise in → temperature above the normal level in a medium.

See also: → thermal; → spike.

In star formation models, the gas pressure that counters the collapsing pull of gravity.

See also: → thermal; → support.

In star formation models, the gas pressure that counters the collapsing pull of gravity.

See also: → thermal; → support.

1. The process by which a system reaches → thermal equilibrium.
   Thermalization results from energy exchange between the components constituting the system and their exchange with the outside medium. In a gas at a given temperature, molecules move with different velocities. The gas temperature corresponds to the mean velocity of the molecules, but individual molecules may deviate largely from the mean velocity. Some move very fast others slowly and change velocity upon collisions. Collisions reduce the energy of fast moving molecules and increase that of slow ones.

In the process of thermalization → matter and → radiation are in constant interaction such that their → temperatures become identical.

The process goes on until energy distribution reaches → equilibrium. The system is said to be → thermalized.

2. The condition where in an → atomic or → molecular → transition the → Boltzmann factor for the two → levels of transition takes on the value it would have in → thermodynamic equilibrium.

See also: Verbal noun of → thermalize.

1. The process by which a system reaches → thermal equilibrium.
   Thermalization results from energy exchange between the components constituting the system and their exchange with the outside medium. In a gas at a given temperature, molecules move with different velocities. The gas temperature corresponds to the mean velocity of the molecules, but individual molecules may deviate largely from the mean velocity. Some move very fast others slowly and change velocity upon collisions. Collisions reduce the energy of fast moving molecules and increase that of slow ones.

In the process of thermalization → matter and → radiation are in constant interaction such that their → temperatures become identical.

The process goes on until energy distribution reaches → equilibrium. The system is said to be → thermalized.

2. The condition where in an → atomic or → molecular → transition the → Boltzmann factor for the two → levels of transition takes on the value it would have in → thermodynamic equilibrium.

See also: Verbal noun of → thermalize.

To bring neutrons into → thermal equilibrium with their surroundings; to produce → thermal neutrons.

Etymology (EN): From → thermal + → -ize.

Etymology (PE): Yekgarmâyidan, literally “equal warming,” from yek-,
→ one, + garmâyidan, infinitive from garmâ, → thermo-.

To bring neutrons into → thermal equilibrium with their surroundings; to produce → thermal neutrons.

Etymology (EN): From → thermal + → -ize.

Etymology (PE): Yekgarmâyidan, literally “equal warming,” from yek-,
→ one, + garmâyidan, infinitive from garmâ, → thermo-.

A collisionally excited spectral line formed in high density condition well above the → critical density. At such densities the → excitation temperature is at (or very near) the → kinetic temperature
of the gas. At low densities, below the critical density, the excitation temperature will be only slightly above the radiation temperature and the emission line will be practically invisible.

See also: Thermalized, p.p. of → thermalize; → line.

A collisionally excited spectral line formed in high density condition well above the → critical density. At such densities the → excitation temperature is at (or very near) the → kinetic temperature
of the gas. At low densities, below the critical density, the excitation temperature will be only slightly above the radiation temperature and the emission line will be practically invisible.

See also: Thermalized, p.p. of → thermalize; → line.

An electron that has been emitted from a heated body such as the hot cathode of an electron tube.

See also: From therm- variant of → thermo- before a vowel + → ion.

An electron that has been emitted from a heated body such as the hot cathode of an electron tube.

See also: From therm- variant of → thermo- before a vowel + → ion.

Electrons gaining enough thermal energy to escape spontaneously from the cathode or dynodes and mimic photoelectrons.

See also: thermion; → emission.

Electrons gaining enough thermal energy to escape spontaneously from the cathode or dynodes and mimic photoelectrons.

See also: thermion; → emission.

A combining form meaning “heat, hot,” used in the formation of compound words. Also therm- before a vowel.

Etymology (EN): From Gk. therme “heat,” thermos “hot;” cf. L. fornax “oven, kiln,” related to fornus, furnus “oven,” and to formus “warm;” cognate with Pers. garm “warm,” as below; P.Gmc. *warmaz (O.E. wearm; E. warm; O.H.G., Ger. warm).

Etymology (PE): Garmâ “heat, warmth,” from Mid.Pers. garmâg; O.Pers./Av. garəma- “hot, warm;” cf. Skt. gharmah “heat;” cognate with Gk. therme, thermos, as above; PIE *ghworm-/*ghwerm- “warm.”

A combining form meaning “heat, hot,” used in the formation of compound words. Also therm- before a vowel.

Etymology (EN): From Gk. therme “heat,” thermos “hot;” cf. L. fornax “oven, kiln,” related to fornus, furnus “oven,” and to formus “warm;” cognate with Pers. garm “warm,” as below; P.Gmc. *warmaz (O.E. wearm; E. warm; O.H.G., Ger. warm).

Etymology (PE): Garmâ “heat, warmth,” from Mid.Pers. garmâg; O.Pers./Av. garəma- “hot, warm;” cf. Skt. gharmah “heat;” cognate with Gk. therme, thermos, as above; PIE *ghworm-/*ghwerm- “warm.”

A layer in a large body of water, such as a lake, in which temperature changes more rapidly with depth than it does in the layers above or below.

See also: → thermo- + → -cline.

A layer in a large body of water, such as a lake, in which temperature changes more rapidly with depth than it does in the layers above or below.

See also: → thermo- + → -cline.

Electrical circuit consisting of two dissimilar metals, in which an electromotive force is produced when the two junctions are at different temperatures.

See also: → thermo- + → couple.

Electrical circuit consisting of two dissimilar metals, in which an electromotive force is produced when the two junctions are at different temperatures.

See also: → thermo- + → couple.

Of or pertaining to → thermodynamics.

See also: → thermo-; → dynamic.

Of or pertaining to → thermodynamics.

See also: → thermo-; → dynamic.

The condition of a → thermodynamic system in which the available → energy is distributed uniformly among all the possible forms of energy. Furthermore,
all → thermodynamic process es must be exactly balanced by their reverse processes. For example, inside a star there will be as many → ionizations of helium per second as there are → recombinations of free electrons and helium ions. Se also
→ local thermodynamic equilibrium (LTE).

See also: → thermodynamic; → equilibrium.

The condition of a → thermodynamic system in which the available → energy is distributed uniformly among all the possible forms of energy. Furthermore,
all → thermodynamic process es must be exactly balanced by their reverse processes. For example, inside a star there will be as many → ionizations of helium per second as there are → recombinations of free electrons and helium ions. Se also
→ local thermodynamic equilibrium (LTE).

See also: → thermodynamic; → equilibrium.

The loci of various changes between two → states through which a → thermodynamic system passes during a → thermodynamic process.

See also: → thermodynamic; → path.

The loci of various changes between two → states through which a → thermodynamic system passes during a → thermodynamic process.

See also: → thermodynamic; → path.

A measure of the energy level of a → thermodynamic system. It represents the amount of → work obtainable when the system undergoes a → change. The main types of thermodynamic potential are: → internal energy,
→ enthalpy, the → Helmholtz free energy, and the
→ Gibbs free energy.

See also: → thermodynamic; → potential.

A measure of the energy level of a → thermodynamic system. It represents the amount of → work obtainable when the system undergoes a → change. The main types of thermodynamic potential are: → internal energy,
→ enthalpy, the → Helmholtz free energy, and the
→ Gibbs free energy.

See also: → thermodynamic; → potential.

An ordered set of → equilibrium states undergone by a
→ thermodynamic system. Thermodynamics processes have various types:
→ cyclic process, → reversible process, and → irreversible process, → isothermal process, → adiabatic process, → isentropic process.

See also: → thermodynamic; → process.

An ordered set of → equilibrium states undergone by a
→ thermodynamic system. Thermodynamics processes have various types:
→ cyclic process, → reversible process, and → irreversible process, → isothermal process, → adiabatic process, → isentropic process.

See also: → thermodynamic; → process.

A quantity of substance or a working machine which in a well-defined way is set apart from its → environment. The boundary between the system and its surroundings can be real or an imaginary mathematical envelope. A thermodynamic system is not necessarily bound to a predefined geometry. Thermodynamic systems can be divided into three types: → open systems, → closed systems, and → isomated systems.

See also: → thermodynamic; → system.

A quantity of substance or a working machine which in a well-defined way is set apart from its → environment. The boundary between the system and its surroundings can be real or an imaginary mathematical envelope. A thermodynamic system is not necessarily bound to a predefined geometry. Thermodynamic systems can be divided into three types: → open systems, → closed systems, and → isomated systems.

See also: → thermodynamic; → system.

A temperature scale, measured in → kelvin (K), that is related to the energy possessed by matter; it was formerly known as → absolute temperature. The zero point on the scale (0 K) is absolute zero. Thermodynamic temperature can be converted to temperature on the → Celsius scale by subtracting 273.15.

See also: → thermodynamic; → temperature.

A temperature scale, measured in → kelvin (K), that is related to the energy possessed by matter; it was formerly known as → absolute temperature. The zero point on the scale (0 K) is absolute zero. Thermodynamic temperature can be converted to temperature on the → Celsius scale by subtracting 273.15.

See also: → thermodynamic; → temperature.

A branch of physics concerned with the relations between heat and other forms of energy and how these affect temperature, pressure, volume, mechanical action, and work.

See also: → thermo-; → dynamics, coined by the Scottish physicist William Thomson (Lord Kelvin, 1824-1907), in 1849.

A branch of physics concerned with the relations between heat and other forms of energy and how these affect temperature, pressure, volume, mechanical action, and work.

See also: → thermo-; → dynamics, coined by the Scottish physicist William Thomson (Lord Kelvin, 1824-1907), in 1849.

Of, relating to, or produced by electric phenomena occurring in conjunction with a flow of heat.

See also: → thermo- + → electric.

Of, relating to, or produced by electric phenomena occurring in conjunction with a flow of heat.

See also: → thermo- + → electric.

A phenomenon occurring when temperature differences exist in an electrical circuit, such as
the → Peltier effect, the → Seebeck effect, and the → Thomson effect,

See also: → thermoelectric; → effect.

A phenomenon occurring when temperature differences exist in an electrical circuit, such as
the → Peltier effect, the → Seebeck effect, and the → Thomson effect,

See also: → thermoelectric; → effect.

The electricity produced by heat or temperature difference in a conductor.

See also: → thermo- + → electricity.

The electricity produced by heat or temperature difference in a conductor.

See also: → thermo- + → electricity.

An instability in the ocean water that occurs when
a layer of warm salt water is above a layer of fresh cold water of slightly higher density. In this process the hot salt water cools off and then, after having reached a higher density than the fresh water, sinks down even in the presence of stabilizing temperature gradients. This phenomenon explains the large-scale water movements in the oceans called themohaline circulation. First discussed by Melvin E. Stern (1960, Tellus 12, 172). → thermohaline mixing.

Etymology (EN): Thermohaline, from → thermo- + haline, from Gk. hals (genitive halos) “salt, sea;” cf. L. sal; O.Ir. salann; Welsh halen;
O.C.S. sali “salt;” O.E. sealt; cf. O.N., O.Fris., Goth. salt, Du. zout, Ger. Salz from PIE *sal- “salt.”

Etymology (PE): Garmâšur, from garmâ-→ thermo- + šur “salty” (Mid.Pers. šôr “salty,” šorag “salt land;” cf. Skt. ksurá- “razor, sharp knife;” Gk. ksuron “razor;” PIE base *kseu- “to rub, whet”).

An instability in the ocean water that occurs when
a layer of warm salt water is above a layer of fresh cold water of slightly higher density. In this process the hot salt water cools off and then, after having reached a higher density than the fresh water, sinks down even in the presence of stabilizing temperature gradients. This phenomenon explains the large-scale water movements in the oceans called themohaline circulation. First discussed by Melvin E. Stern (1960, Tellus 12, 172). → thermohaline mixing.

Etymology (EN): Thermohaline, from → thermo- + haline, from Gk. hals (genitive halos) “salt, sea;” cf. L. sal; O.Ir. salann; Welsh halen;
O.C.S. sali “salt;” O.E. sealt; cf. O.N., O.Fris., Goth. salt, Du. zout, Ger. Salz from PIE *sal- “salt.”

Etymology (PE): Garmâšur, from garmâ-→ thermo- + šur “salty” (Mid.Pers. šôr “salty,” šorag “salt land;” cf. Skt. ksurá- “razor, sharp knife;” Gk. ksuron “razor;” PIE base *kseu- “to rub, whet”).

In stars, an instability phenomenon, reminiscent of the
→ thermohaline convection in the oceans, that takes place
when layers of higher molecular weight occur above a region of lower molecular weight. A situation of heavier material being above lighter gas in a star can occur during the → helium flash when → helium burning does not start in the center but in the shell. Similarly, in → close binary systems it may happen that helium-rich material is transferred to a → main sequence star. Then a helium-rich outer layer is formed and the instability occurs at the interface between that layer and the original stellar material. This process can explain several surface abundance variations in stars. First discussed by S. Kato (1966, Publ. Astron. Soc. Japan 18, 374).

See also: → thermohaline; → mixing.

In stars, an instability phenomenon, reminiscent of the
→ thermohaline convection in the oceans, that takes place
when layers of higher molecular weight occur above a region of lower molecular weight. A situation of heavier material being above lighter gas in a star can occur during the → helium flash when → helium burning does not start in the center but in the shell. Similarly, in → close binary systems it may happen that helium-rich material is transferred to a → main sequence star. Then a helium-rich outer layer is formed and the instability occurs at the interface between that layer and the original stellar material. This process can explain several surface abundance variations in stars. First discussed by S. Kato (1966, Publ. Astron. Soc. Japan 18, 374).

See also: → thermohaline; → mixing.

A theoretical interpretation for the → X-ray bursts observed toward → low-mass X-ray binary (LMXB) stars. According to models, X-ray bursts are produced on the surface of → neutron stars as a result of violent thermonuclear processes in a → hydrogen or → helium rich → layer. It is the → nuclear energy released in the → fusion of hydrogen and helium to heavier elements (e.g., Ni, Zn, and Se) in the → accreted matter which heats the upper layers of the neutron star so that X-rays are emitted from the surface (see, e.g., Taam, R.E., 1984, AIP Conf. Proc. 115, 263).

See also: → thermonuclear; → flash.

A theoretical interpretation for the → X-ray bursts observed toward → low-mass X-ray binary (LMXB) stars. According to models, X-ray bursts are produced on the surface of → neutron stars as a result of violent thermonuclear processes in a → hydrogen or → helium rich → layer. It is the → nuclear energy released in the → fusion of hydrogen and helium to heavier elements (e.g., Ni, Zn, and Se) in the → accreted matter which heats the upper layers of the neutron star so that X-rays are emitted from the surface (see, e.g., Taam, R.E., 1984, AIP Conf. Proc. 115, 263).

See also: → thermonuclear; → flash.

A nuclear reaction in which two or more atomic nuclei fuse into a single heavier nucleus by a collision of the interacting particles at extremely high temperatures.
Chains of thermonuclear reactions, such as the → proton-proton chain and the → CNO cycle, account for the energy radiated from the Sun and more massive stars.

See also: → thermo- + → nuclear; → reaction.

A nuclear reaction in which two or more atomic nuclei fuse into a single heavier nucleus by a collision of the interacting particles at extremely high temperatures.
Chains of thermonuclear reactions, such as the → proton-proton chain and the → CNO cycle, account for the energy radiated from the Sun and more massive stars.

See also: → thermo- + → nuclear; → reaction.

1. The uncontrolled → fusion of hydrogen into helium.
   

2. A → thermonuclear reaction process occurring at electron → degenerate conditions in stellar material, such as in → Type Ia supernovae.

Etymology (EN): → thermonuclear; → runaway.

Etymology (PE): Vâžireš, → reaction; garmâhaste-yi, → thermonuclear; legâm gosixté literally “rampant, unrestrained,” from legâm “bridle, rein” + gosixté “broken off, torn away,” p.p. of gosixtan “to tear away, to break off.”

1. The uncontrolled → fusion of hydrogen into helium.
   

2. A → thermonuclear reaction process occurring at electron → degenerate conditions in stellar material, such as in → Type Ia supernovae.

Etymology (EN): → thermonuclear; → runaway.

Etymology (PE): Vâžireš, → reaction; garmâhaste-yi, → thermonuclear; legâm gosixté literally “rampant, unrestrained,” from legâm “bridle, rein” + gosixté “broken off, torn away,” p.p. of gosixtan “to tear away, to break off.”

Same as → type Ia supernova

See also: → thermonuclear; → supernova.

Same as → type Ia supernova

See also: → thermonuclear; → supernova.

The region of the upper atmosphere in which temperature increases continuously with height, starting at roughly 100 km. The thermosphere includes the exosphere and most of the ionosphere.

See also: → thermo-; → sphere.

The region of the upper atmosphere in which temperature increases continuously with height, starting at roughly 100 km. The thermosphere includes the exosphere and most of the ionosphere.

See also: → thermo-; → sphere.

A device for maintaining a system at constant temperature by automatically terminating or restoring the heating or cooling source. It consists of a temperature sensing instrument connected to a switching device. The sensing
device is often a bimetallic strip which triggers a simple electric switch.

See also: → thermo- + → -stat.

A device for maintaining a system at constant temperature by automatically terminating or restoring the heating or cooling source. It consists of a temperature sensing instrument connected to a switching device. The sensing
device is often a bimetallic strip which triggers a simple electric switch.

See also: → thermo- + → -stat.

1. A controlled and structured list of terms or descriptors usually with a cross-reference system used in subject analysis and information retrieval in a particular field.
   

2. More generally, a work that lists words arranged and grouped according to their semantic similarities, including synonyms and sometimes antonyms. This is different from the dictionary, which
   contains definitions and pronunciations. The first major work of this kind in English is Peter Mark Roget’s Thesaurus of English words and phrases, Classified and Arranged so as to Facilitate the Expression of Ideas (1852).

Etymology (EN): From L. thesaurus “treasury, treasure,” from Gk.  thesauros  “treasure, treasury, storehouse,” from root of  tithenai “to put, to place,” → thesis.

Etymology (PE): Vâžganj, from vâž, → word, + ganj “treasure,” from Mid.Pers. ganj “treasure.”

1. A controlled and structured list of terms or descriptors usually with a cross-reference system used in subject analysis and information retrieval in a particular field.
   

2. More generally, a work that lists words arranged and grouped according to their semantic similarities, including synonyms and sometimes antonyms. This is different from the dictionary, which
   contains definitions and pronunciations. The first major work of this kind in English is Peter Mark Roget’s Thesaurus of English words and phrases, Classified and Arranged so as to Facilitate the Expression of Ideas (1852).

Etymology (EN): From L. thesaurus “treasury, treasure,” from Gk.  thesauros  “treasure, treasury, storehouse,” from root of  tithenai “to put, to place,” → thesis.

Etymology (PE): Vâžganj, from vâž, → word, + ganj “treasure,” from Mid.Pers. ganj “treasure.”

1. A proposition put forward for consideration, especially one to be discussed and proved or to be maintained against objections.
   Philo.: The first of three stages in Hegelian dialectic; the inevitable transition of thought, by contradiction and reconciliation, from an initial conviction to its opposite and then to a new, higher conception that involves but transcends both of them. → antithesis; → synthesis.
   

2. A dissertation based on original research, especially as work toward an academic degree.

Etymology (EN): M.E., from L., from Gk. thesis “a proposition; a setting down, something set down,” from root of tithenai “to place, put, set,” cognate with Pers. dâdan “to give,” as below.

Etymology (PE): 1) Dâyan “giving, setting down,” from O.Pers./Av. dā- “to give, grant, put,” dadāiti “he gives;” Mid.Pers./Mod.Pers. dâdan “to give; to put”
(cf. Skt. dadáti “he gives;” Gk. tithenai “to place, put, set,” didomi “I give;”
L. dare “to give, offer;” Rus. delat’ “to do;” O.H.G. tuon, Ger. tun, O.E. don “to do;” PIE base *dhe- “to put, to do”)

• -y- epenthetic vowel + -an noun/adjective suffix appearing in many words (such as rowzan, mihan, barzan, rasan, barzan, rowšan).
  

2. Dâyan-nâme, from dâyan “thesis” + nâme “diploma, letter” (Mid.Pers. nâmag “book, letter, inscription,” from O.Pers./Av. nāman- “name;” cf. Skt. nama-;
   Gk. onoma, onuma; L. nomen; PIE *nomen-).
   Pâyân-nâme, literally “ending, final diploma,” from pâyân “end,” → terminal + nâme.

1. A proposition put forward for consideration, especially one to be discussed and proved or to be maintained against objections.
   Philo.: The first of three stages in Hegelian dialectic; the inevitable transition of thought, by contradiction and reconciliation, from an initial conviction to its opposite and then to a new, higher conception that involves but transcends both of them. → antithesis; → synthesis.
   

2. A dissertation based on original research, especially as work toward an academic degree.

Etymology (EN): M.E., from L., from Gk. thesis “a proposition; a setting down, something set down,” from root of tithenai “to place, put, set,” cognate with Pers. dâdan “to give,” as below.

Etymology (PE): 1) Dâyan “giving, setting down,” from O.Pers./Av. dā- “to give, grant, put,” dadāiti “he gives;” Mid.Pers./Mod.Pers. dâdan “to give; to put”
(cf. Skt. dadáti “he gives;” Gk. tithenai “to place, put, set,” didomi “I give;”
L. dare “to give, offer;” Rus. delat’ “to do;” O.H.G. tuon, Ger. tun, O.E. don “to do;” PIE base *dhe- “to put, to do”)

• -y- epenthetic vowel + -an noun/adjective suffix appearing in many words (such as rowzan, mihan, barzan, rasan, barzan, rowšan).
  

2. Dâyan-nâme, from dâyan “thesis” + nâme “diploma, letter” (Mid.Pers. nâmag “book, letter, inscription,” from O.Pers./Av. nāman- “name;” cf. Skt. nama-;
   Gk. onoma, onuma; L. nomen; PIE *nomen-).
   Pâyân-nâme, literally “ending, final diploma,” from pâyân “end,” → terminal + nâme.

The proper name of the → extrasolar planet  → Pollux b.

See also: In Greek and Roman mythology, Thestias was the patronym of Leda, → Pollux’s mother.

The proper name of the → extrasolar planet  → Pollux b.

See also: In Greek and Roman mythology, Thestias was the patronym of Leda, → Pollux’s mother.

Relatively great in extent from one surface to the opposite; deep or profound. &rarroptically thick

Etymology (EN): M.E. thikke, O.E. thicce “not thin, dense,” from P.Gmc. *theku-, *thekwia- (cf. O.S. thikki, O.H.G. dicchi, Ger. dick), from PIE *tegu- “thick.”

Etymology (PE): Setabr, from Mid.Pers. stabr “strong, big,” stambag “pugnacious, opposing;” O.Pers. (mā) stabava [2sg.inj.] “to revolt;” Av. stabra- “strong, firm;” cf. Skt. stabh- “support,” stambh- “to support, fix firmly,” stabhnāti “supports;” Gk. astemphes “steadfast,” stephein “to tie around, encircle,” astemphes “firm, rigid;” Lith. stebas “staff, pillar,” stembti “to oppose.”

Relatively great in extent from one surface to the opposite; deep or profound. &rarroptically thick

Etymology (EN): M.E. thikke, O.E. thicce “not thin, dense,” from P.Gmc. *theku-, *thekwia- (cf. O.S. thikki, O.H.G. dicchi, Ger. dick), from PIE *tegu- “thick.”

Etymology (PE): Setabr, from Mid.Pers. stabr “strong, big,” stambag “pugnacious, opposing;” O.Pers. (mā) stabava [2sg.inj.] “to revolt;” Av. stabra- “strong, firm;” cf. Skt. stabh- “support,” stambh- “to support, fix firmly,” stabhnāti “supports;” Gk. astemphes “steadfast,” stephein “to tie around, encircle,” astemphes “firm, rigid;” Lith. stebas “staff, pillar,” stembti “to oppose.”

A disk component of a → spiral galaxy that lies above the → thin disk and mainly consists of stars. The thick disk of our → Galaxy makes up about 10-50% of the stellar mass of the → Milky Way and has a scale height of ~ 1,000-3,000 → light-years.
Thick disk stars are, on average, moving faster in a vertical direction with respect to the → galactic plane than thin disk stars. In contrast to thin disk, the stars within the thick disk are almost all older than 10 billion years and typically have a smaller → metallicity than the average values for the thin disk stars. These facts suggest that the formation scenarios for the thin and thick disks were different. In particular, it is thought that the thick disk is much older than the thin disk.

See also: → thick; → disk;

A disk component of a → spiral galaxy that lies above the → thin disk and mainly consists of stars. The thick disk of our → Galaxy makes up about 10-50% of the stellar mass of the → Milky Way and has a scale height of ~ 1,000-3,000 → light-years.
Thick disk stars are, on average, moving faster in a vertical direction with respect to the → galactic plane than thin disk stars. In contrast to thin disk, the stars within the thick disk are almost all older than 10 billion years and typically have a smaller → metallicity than the average values for the thin disk stars. These facts suggest that the formation scenarios for the thin and thick disks were different. In particular, it is thought that the thick disk is much older than the thin disk.

See also: → thick; → disk;

A lens whose thickness is not small compared with its focal length. The thick lens may include several components, which may or may not be in contact. → thin lens.

See also: → thick; → lens.

A lens whose thickness is not small compared with its focal length. The thick lens may include several components, which may or may not be in contact. → thin lens.

See also: → thick; → lens.

The state or quality of being thick. → optical thickness.

Etymology (EN): M.E. thiknesse, O.E. thicnes, from → thick + -nes(s) suffix of action, quality or state, cf. M.Du. -nisse, O.H.G. -nissa, Ger. -nis, Goth. -inassus.

Etymology (PE): Setabrâ, from setabr→ thick + -â a suffix forming nouns from adjectives.

The state or quality of being thick. → optical thickness.

Etymology (EN): M.E. thiknesse, O.E. thicnes, from → thick + -nes(s) suffix of action, quality or state, cf. M.Du. -nisse, O.H.G. -nissa, Ger. -nis, Goth. -inassus.

Etymology (PE): Setabrâ, from setabr→ thick + -â a suffix forming nouns from adjectives.

The part of the human → leg between the → hip and the → knee.

Etymology (EN): M.E. thi, thigh(e), the(h), O.E. thioh, theoh; cognate with Du. dij, O.H.G. dioh, O.Norse thjo.

Etymology (PE): Rân, from Mid.Pers. rân “thigh;” Av. rāna- “the outer part of the thigh.”

The part of the human → leg between the → hip and the → knee.

Etymology (EN): M.E. thi, thigh(e), the(h), O.E. thioh, theoh; cognate with Du. dij, O.H.G. dioh, O.Norse thjo.

Etymology (PE): Rân, from Mid.Pers. rân “thigh;” Av. rāna- “the outer part of the thigh.”

Having relatively little extent from one surface or side to the opposite; not thick. → optically thin; → superthin galaxy.

Etymology (EN): M.E. thyn(ne), O.E. thynne, from P.Gmc. *thunnuz, *thunw- (cf. W.Fris. ten, M.L.G. dunne, Du. dun, O.H.G. dunni, Ger. dünn), from PIE *tnus-, *tnwi-, from base *ten- “stretch;” cf. Pers. tonok “thin, slender,” → attenuate.

Etymology (PE): Nâzok, from Mid.Pers. nâzuk, nâzik “tender, gentle.”

Having relatively little extent from one surface or side to the opposite; not thick. → optically thin; → superthin galaxy.

Etymology (EN): M.E. thyn(ne), O.E. thynne, from P.Gmc. *thunnuz, *thunw- (cf. W.Fris. ten, M.L.G. dunne, Du. dun, O.H.G. dunni, Ger. dünn), from PIE *tnus-, *tnwi-, from base *ten- “stretch;” cf. Pers. tonok “thin, slender,” → attenuate.

Etymology (PE): Nâzok, from Mid.Pers. nâzuk, nâzik “tender, gentle.”

A disk component of a → spiral galaxy containing → stars, → gas, and → dust
which are confined to the galaxy’s → plane of rotation. In contrast to → thick disks, thin disks contain the bulk of the → baryonic matter in spiral galaxies. For example, on the order of 60-90% of the baryonic matter in the → Milky Way is located in the thin disk. The scale height of the thin disk in the Milky Way is about 400 → light-years,
whereas its scale length is about 10,000 light-years. Moreover, the outer regions of
thin disks appear to be bent by the → warp phenomenon.
The thin disks of spiral galaxies are active sites of
→ star formation, especially in the spiral arms. For this reason, stars in the thin disk tend to be relatively young. Thin disk stars also tend to be → metal-rich compared with thick disk and → halo stars, and typically have a → metallicity similar to that of the Sun.

See also: → thin; → disk;

A disk component of a → spiral galaxy containing → stars, → gas, and → dust
which are confined to the galaxy’s → plane of rotation. In contrast to → thick disks, thin disks contain the bulk of the → baryonic matter in spiral galaxies. For example, on the order of 60-90% of the baryonic matter in the → Milky Way is located in the thin disk. The scale height of the thin disk in the Milky Way is about 400 → light-years,
whereas its scale length is about 10,000 light-years. Moreover, the outer regions of
thin disks appear to be bent by the → warp phenomenon.
The thin disks of spiral galaxies are active sites of
→ star formation, especially in the spiral arms. For this reason, stars in the thin disk tend to be relatively young. Thin disk stars also tend to be → metal-rich compared with thick disk and → halo stars, and typically have a → metallicity similar to that of the Sun.

See also: → thin; → disk;

A lens whose thickness is considered small in comparison with the distances generally associated with its optical properties. Such distances are, for example, radii of curvature of the two spherical surfaces, primary and secondary focal lengths, and object and image distances. → thick lens.

See also: → thin; → lens.

A lens whose thickness is considered small in comparison with the distances generally associated with its optical properties. Such distances are, for example, radii of curvature of the two spherical surfaces, primary and secondary focal lengths, and object and image distances. → thick lens.

See also: → thin; → lens.

1. A material object without life or consciousness; an inanimate object.
   

   2. Some entity, object, or creature that is not or cannot be specifically designated or precisely described.
      

   3. Anything that is or may become an object of thought (Dictionary.com).

Etymology (EN): M.E. thing; O.E. þing “meeting, assembly, discussion;” cf. O.Norse þing “assembly, meeting, council;” O.Frisian thing “assembly, action, matter, thing;” O.Saxon thing; O.Du. dinc “law suit, matter, thing;” M.Du. ding; Du. ding; O.Ger. ding, dinc “assembly;” M.H.G. dinc “assembly;” Ger. ding “matter, affairs, thing.” Hence, the word originally meant “assembly, meeting,” then came to mean a specific issue discussed at such an assembly, and finally came to indicate “an object.”

Etymology (PE): Ciz, from Mid.Pers. ciš, tis “thing, affair;” O.Pers. cišciy “anything.”

1. A material object without life or consciousness; an inanimate object.
   

   2. Some entity, object, or creature that is not or cannot be specifically designated or precisely described.
      

   3. Anything that is or may become an object of thought (Dictionary.com).

Etymology (EN): M.E. thing; O.E. þing “meeting, assembly, discussion;” cf. O.Norse þing “assembly, meeting, council;” O.Frisian thing “assembly, action, matter, thing;” O.Saxon thing; O.Du. dinc “law suit, matter, thing;” M.Du. ding; Du. ding; O.Ger. ding, dinc “assembly;” M.H.G. dinc “assembly;” Ger. ding “matter, affairs, thing.” Hence, the word originally meant “assembly, meeting,” then came to mean a specific issue discussed at such an assembly, and finally came to indicate “an object.”

Etymology (PE): Ciz, from Mid.Pers. ciš, tis “thing, affair;” O.Pers. cišciy “anything.”

To employ one’s mind rationally and objectively in evaluating or dealing with a given situation.

Etymology (EN): M.E. thinken, variant of thenken, O.E. thencan;
P.Gmc. *thankjan (cf. O.Fris. thinka, O.S. thenkian, O.H.G. denchen, Ger. denken, Goth. thagkjan).

Etymology (PE): Andišidan, infinitive from andiš-; Mid.Pers. handeš-,
handešidan “to think, consider, reflect,” ultimately from Proto-Iranian *ham-dis-, from *ham- “together, with, same,” → syn- + *dis- “form, appearance,” cf. Av. daēs- “to show,” daēsa- “sign, omen;” Mod.Pers. dis, disé “form, appearance,” variants -diz, -diš; Mid.Pers. dêsag “form, appearance,” dêsidan “to form, build;” Sogd. andiš “to seem,” andêš “to show,” andêšik “appearing;” cf. Skt. deś- “to show, point out;” PIE *deik- “to show” (cf. Gk. deiknumi “to show,” dike “manner, custom;” L. dicere “to utter, say;” O.H.G. zeigon, Ger. zeigen “to show;” O.E. teon “to accuse,” tæcan “to teach”).

To employ one’s mind rationally and objectively in evaluating or dealing with a given situation.

Etymology (EN): M.E. thinken, variant of thenken, O.E. thencan;
P.Gmc. *thankjan (cf. O.Fris. thinka, O.S. thenkian, O.H.G. denchen, Ger. denken, Goth. thagkjan).

Etymology (PE): Andišidan, infinitive from andiš-; Mid.Pers. handeš-,
handešidan “to think, consider, reflect,” ultimately from Proto-Iranian *ham-dis-, from *ham- “together, with, same,” → syn- + *dis- “form, appearance,” cf. Av. daēs- “to show,” daēsa- “sign, omen;” Mod.Pers. dis, disé “form, appearance,” variants -diz, -diš; Mid.Pers. dêsag “form, appearance,” dêsidan “to form, build;” Sogd. andiš “to seem,” andêš “to show,” andêšik “appearing;” cf. Skt. deś- “to show, point out;” PIE *deik- “to show” (cf. Gk. deiknumi “to show,” dike “manner, custom;” L. dicere “to utter, say;” O.H.G. zeigon, Ger. zeigen “to show;” O.E. teon “to accuse,” tæcan “to teach”).

Next after the second; the ordinal number for three. → Newton’s third law of motion; → third contact; → third dredge-up; → third law of thermodynamics.

Etymology (EN): M.E. thirde, O.E. (north) thirda, variant of ridda, from P.Gmc. *thridjas (cf. O.Fris. thredda, O.S. thriddio, M.L.G. drudde, Du. derde, O.H.G. dritto, Ger. dritte, Goth. thridja).

Etymology (PE): Sevom, ordinal number for sé, → three.

Next after the second; the ordinal number for three. → Newton’s third law of motion; → third contact; → third dredge-up; → third law of thermodynamics.

Etymology (EN): M.E. thirde, O.E. (north) thirda, variant of ridda, from P.Gmc. *thridjas (cf. O.Fris. thredda, O.S. thriddio, M.L.G. drudde, Du. derde, O.H.G. dritto, Ger. dritte, Goth. thridja).

Etymology (PE): Sevom, ordinal number for sé, → three.

The end of the total phase of a solar eclipse marked by the trailing edge of the Moon first revealing the Sun.

See also: → third; → contact.

The end of the total phase of a solar eclipse marked by the trailing edge of the Moon first revealing the Sun.

See also: → third; → contact.

A → dredge-up process that occurs in the stellar interior during He shell burning, as in → asymptotic giant branch (AGB) stars.
These stars consist of a degenerate carbon-oxygen core, surrounded by a helium-rich region, above which lies a hydrogen-rich convective envelope. Following thermal pulses of the helium-burning shell, the convective envelope moves inward in mass, penetrating the hydrogen-exhausted regions. This is known as third dredge-up. As convection moves inward, nuclear processed materials are carried to the surface.

See also: → third; → dredge-up.

A → dredge-up process that occurs in the stellar interior during He shell burning, as in → asymptotic giant branch (AGB) stars.
These stars consist of a degenerate carbon-oxygen core, surrounded by a helium-rich region, above which lies a hydrogen-rich convective envelope. Following thermal pulses of the helium-burning shell, the convective envelope moves inward in mass, penetrating the hydrogen-exhausted regions. This is known as third dredge-up. As convection moves inward, nuclear processed materials are carried to the surface.

See also: → third; → dredge-up.

The → entropy of an idealized state of maximum order is zero at the temperature of → absolute zero. Another version of this law: As a system approaches absolute zero, all processes cease and the entropy of the system approaches a minimum value.

See also: → third; → law; → thermodynamics.

The → entropy of an idealized state of maximum order is zero at the temperature of → absolute zero. Another version of this law: As a system approaches absolute zero, all processes cease and the entropy of the system approaches a minimum value.

See also: → third; → law; → thermodynamics.

The phase of the Moon when half the visible disk of the waning Moon is illuminated. This quarter occurs when the celestial longitude of the Moon is 270° greater than the Sun’s.

See also: → third; → quarter.

The phase of the Moon when half the visible disk of the waning Moon is illuminated. This quarter occurs when the celestial longitude of the Moon is 270° greater than the Sun’s.

See also: → third; → quarter.

A cardinal number, 10 times 3.

Etymology (EN): M.E. thritty, O.E. thritig, from thri, threo “three” + -tig “group of ten, -ty.”

Etymology (PE): Si, from Mid.Pers. sih; O.Pers. *ciθas nominative singular of *ciθant-; Av. θrisant- “thirty;” cf. Skt. trimśát- “thirty.”

A cardinal number, 10 times 3.

Etymology (EN): M.E. thritty, O.E. thritig, from thri, threo “three” + -tig “group of ten, -ty.”

Etymology (PE): Si, from Mid.Pers. sih; O.Pers. *ciθas nominative singular of *ciθant-; Av. θrisant- “thirty;” cf. Skt. trimśát- “thirty.”

A fundamental system for the classification of → asteroids based on → albedo and → spectral characteristics. The Tholen scheme includes 14 types with the majority of asteroids falling into one of three broad categories, and several smaller types.
→ C-type asteroid, → M-type asteroid, → S-type asteroid.

See also: David J. Tholen (1984) Ph.D. thesis, University of Arizona; → classification.

A fundamental system for the classification of → asteroids based on → albedo and → spectral characteristics. The Tholen scheme includes 14 types with the majority of asteroids falling into one of three broad categories, and several smaller types.
→ C-type asteroid, → M-type asteroid, → S-type asteroid.

See also: David J. Tholen (1984) Ph.D. thesis, University of Arizona; → classification.

The British physicist Sir Joseph John Thomson (1856-1940), discoverer of the electron (1897), Nobel Prize in Physics (1906). → Thomson atom, → Thomson cross section, → Thomson effect, → Thomson scattering, → Joule-Thomson effect.

The British physicist Sir Joseph John Thomson (1856-1940), discoverer of the electron (1897), Nobel Prize in Physics (1906). → Thomson atom, → Thomson cross section, → Thomson effect, → Thomson scattering, → Joule-Thomson effect.

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.

See also: → Thomson; → atom.

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.

See also: → 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.

See also: → Thomson; → cross; → section.

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.

See also: → 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.

See also: → Thomson; → effect.

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.

See also: → 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.

See also: → Thomson; → scattering.

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.

See also: → 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.

See also: From Thor, the “Scandinavian god of thunder.”

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.

See also: 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).

See also: Thorne K. S., Żytkow A., 1975, ApJ 199, L19.

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).

See also: Thorne K. S., Żytkow A., 1975, ApJ 199, L19.

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

Etymology (EN): 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.”

Etymology (PE): Andišé, noun from andišidan, → think.

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

Etymology (EN): 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.”

Etymology (PE): 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.

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

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.

See also: → 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).

Etymology (EN): 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.”

Etymology (PE): 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. 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).

Etymology (EN): 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.”

Etymology (PE): 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).

Etymology (EN): From M.E. thretnen, from O.E. thrêatnian, → threat

Etymology (PE): Infinitive from harš, → threat.

1. To utter a threat against; menace.
   

2. To be a menace or source of danger to (Dictionary.com).

Etymology (EN): From M.E. thretnen, from O.E. thrêatnian, → threat

Etymology (PE): Infinitive from harš, → threat.

A cardinal number, 2 plus 1.

Etymology (EN): 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.

Etymology (PE): 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-.

A cardinal number, 2 plus 1.

Etymology (EN): 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.

Etymology (PE): 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.

See also: → three; → body; → problem.

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.

See also: → 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).

See also: → three; → dimensional; → flow.

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).

See also: → three; → dimensional; → flow.

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

Etymology (EN): M.E. threschold, O.E. threscold, threscwald “doorsill, point of entering.”

Etymology (PE): 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 level that must be reached for a physical effect to begin or be noticeable.

Etymology (EN): M.E. threschold, O.E. threscold, threscwald “doorsill, point of entering.”

Etymology (PE): 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.

See also: → threshold; → energy.

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

See also: → threshold; → energy.

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

See also: → threshold; → reaction.

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

See also: → threshold; → reaction.

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

See also: → threshold; → signal.

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

See also: → threshold; → signal.

The front part of the neck. → nozzle throat.

Etymology (EN): M.E. throte, O.E. throte, throta, throtu; cognate with O.H.G. drozza “throat,” O.N. throti “swelling.”

Etymology (PE): 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,”

The front part of the neck. → nozzle throat.

Etymology (EN): M.E. throte, O.E. throte, throta, throtu; cognate with O.H.G. drozza “throat,” O.N. throti “swelling.”

Etymology (PE): 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).

Etymology (EN): 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.

Etymology (PE): Korsi “throne, chair, seat,” from Ar. kursī.

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).

Etymology (EN): 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.

Etymology (PE): 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).

Etymology (EN): M.E. throtelen, from throten “to cut the throat of (someone), strangle,”
from → throat.

Etymology (PE): 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).

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).

Etymology (EN): M.E. throtelen, from throten “to cut the throat of (someone), strangle,”
from → throat.

Etymology (PE): 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).

1. Thermodynamics: A process in which a gas, originally at a constant high pressure, passes → adiabatically
   through a porous wall or a narrow opening into a region of constant lower pressure. The throttling process is → irreversible and is accompanied by an increase of → entropy. The → enthalpy of the gas is the same in the → initial and → final → states. Also called → Joule-Thomson expansion. The change in the temperature of the gas in throttling is known as the → Joule-Thomson effect.
   

2. A technique implemented in the architecture of computers whereby the frequency produced by the microprocessor is automatically adjusted. Throttling process allows the computer to conserve
   power and also reduce the heat generated by its chip.

See also: → throttle; → process.

1. Thermodynamics: A process in which a gas, originally at a constant high pressure, passes → adiabatically
   through a porous wall or a narrow opening into a region of constant lower pressure. The throttling process is → irreversible and is accompanied by an increase of → entropy. The → enthalpy of the gas is the same in the → initial and → final → states. Also called → Joule-Thomson expansion. The change in the temperature of the gas in throttling is known as the → Joule-Thomson effect.
   

2. A technique implemented in the architecture of computers whereby the frequency produced by the microprocessor is automatically adjusted. Throttling process allows the computer to conserve
   power and also reduce the heat generated by its chip.

See also: → throttle; → process.

1. In at one end, side, or surface and out at the other.
   

2. From one to the other of; between or among the individual members or parts of.
   

3. Over the surface of, by way of, or within the limits or medium of.
   

4. During the whole period of (Dictionary.com).

Etymology (EN): M.E. (preposition and adv.), metathetic variant of thourgh, O.E. thurh, (cognates O.S. thuru, OFris. thruch, Du. door, O.H.G. thuruh, Ger. durch), cognate with Av. tarô, tarə “over, across, beyond,” L. trans-, → trans-.

Etymology (PE): Târu, related to tarâ-, → trans-, and from Av. tarô, tarə, as above.

1. In at one end, side, or surface and out at the other.
   

2. From one to the other of; between or among the individual members or parts of.
   

3. Over the surface of, by way of, or within the limits or medium of.
   

4. During the whole period of (Dictionary.com).

Etymology (EN): M.E. (preposition and adv.), metathetic variant of thourgh, O.E. thurh, (cognates O.S. thuru, OFris. thruch, Du. door, O.H.G. thuruh, Ger. durch), cognate with Av. tarô, tarə “over, across, beyond,” L. trans-, → trans-.

Etymology (PE): Târu, related to tarâ-, → trans-, and from Av. tarô, tarə, as above.

To propel something through the air in any way, especially by
swinging the arm and releasing the object from the hand; → ejecter.

Etymology (EN): M.E. throwen, thrawen, “to twist, turn writhe” (cf. O.S. thraian, M.Du. dræyen, Du. draaien, O.H.G. draen, Ger. drehen “to turn, twist”).

Etymology (PE): Andâxtan, andâz-, from Mid.Pers. handâxtan, handâz-; ultimately from Proto-Iranian *ham-tak-, from *ham- “together, with, same,” → syn- + *tak- “to run, to flow;” cf. Av. tak- “to run, to flow,” taciāp- “flowing water,” tacinti (3pl.pers.act.) “to flow,”
tacar- “course,” tacan “current, streaming;” Mod.Pers. tacidan, tâxtan, tâzidan “to run; to hasten; to assault,” tajan name of a river (initially “flowing, streaming, stream”), tâzi “swift (greyhound),” tak “running, rush,” from
Mid.Pers. tâz-, tâxtan “to flow, to cause to walk,” tc- “to flow, to walk,” tag “running, attack,” tâzig “swift, fast;”
Khotanese ttajs- “to flow, to walk;” Skt. tak- to rush, to hurry," takti “runs;” O.Ir. tech- “to flow;” Lith. teketi “to walk, to flow;” O.C.S. tešti “to walk, to hurry;” Tokharian B cake “river;” PIE base *tek^w- “to run; to flow.

To propel something through the air in any way, especially by
swinging the arm and releasing the object from the hand; → ejecter.

Etymology (EN): M.E. throwen, thrawen, “to twist, turn writhe” (cf. O.S. thraian, M.Du. dræyen, Du. draaien, O.H.G. draen, Ger. drehen “to turn, twist”).

Etymology (PE): Andâxtan, andâz-, from Mid.Pers. handâxtan, handâz-; ultimately from Proto-Iranian *ham-tak-, from *ham- “together, with, same,” → syn- + *tak- “to run, to flow;” cf. Av. tak- “to run, to flow,” taciāp- “flowing water,” tacinti (3pl.pers.act.) “to flow,”
tacar- “course,” tacan “current, streaming;” Mod.Pers. tacidan, tâxtan, tâzidan “to run; to hasten; to assault,” tajan name of a river (initially “flowing, streaming, stream”), tâzi “swift (greyhound),” tak “running, rush,” from
Mid.Pers. tâz-, tâxtan “to flow, to cause to walk,” tc- “to flow, to walk,” tag “running, attack,” tâzig “swift, fast;”
Khotanese ttajs- “to flow, to walk;” Skt. tak- to rush, to hurry," takti “runs;” O.Ir. tech- “to flow;” Lith. teketi “to walk, to flow;” O.C.S. tešti “to walk, to hurry;” Tokharian B cake “river;” PIE base *tek^w- “to run; to flow.

The force that is exerted by a rocket, propeller, or jet engine to propel an aircraft. It is directed forward along the axis of the engine. → drag; → lift.

Etymology (EN): M.E. thrusten, thrysten (v.); O.N. thrysta “to thrust, force.”

Etymology (PE): Pišrâné, from
piš “before; in front” (Mid.Pers. pêš “before, earlier,” O.Pers. paišiya “before; in the presence of”) + râné, from rândan “to push, drive, cause to go”
(causative of raftan “to go, walk, proceed,”
present tense stem row-, Mid.Pers. raftan, raw-, Proto-Iranian *rab/f- “to go; to attack”).

The force that is exerted by a rocket, propeller, or jet engine to propel an aircraft. It is directed forward along the axis of the engine. → drag; → lift.

Etymology (EN): M.E. thrusten, thrysten (v.); O.N. thrysta “to thrust, force.”

Etymology (PE): Pišrâné, from
piš “before; in front” (Mid.Pers. pêš “before, earlier,” O.Pers. paišiya “before; in the presence of”) + râné, from rândan “to push, drive, cause to go”
(causative of raftan “to go, walk, proceed,”
present tense stem row-, Mid.Pers. raftan, raw-, Proto-Iranian *rab/f- “to go; to attack”).

A fourth magnitude star (V = 3.65), called also α Draconis,
in the constellation → Draco. Despite its designation as Alpha (α), it is the seventh brightest star of the constellation. Thuban is a → giant star of → spectral type A0 III lying 310 → light-years away. It has an faint → companion in an orbit with a 51 day period. Thuban was the → pole star at about 2700 BC. Other designations: HR 5291, HD 123299, and SAO 16273.

See also: Thuban, from Ar. Ath-thu’bân (الثعبان) “a large male snake; basilisk.”

A fourth magnitude star (V = 3.65), called also α Draconis,
in the constellation → Draco. Despite its designation as Alpha (α), it is the seventh brightest star of the constellation. Thuban is a → giant star of → spectral type A0 III lying 310 → light-years away. It has an faint → companion in an orbit with a 51 day period. Thuban was the → pole star at about 2700 BC. Other designations: HR 5291, HD 123299, and SAO 16273.

See also: Thuban, from Ar. Ath-thu’bân (الثعبان) “a large male snake; basilisk.”

A soft, malleable, ductile, lustrous silver-white metal; symbol Tm. Atomic number 69; atomic weight 168.9342; melting point about 1,545°C; boiling point 1,947°C; specific gravity 9.3.
Thulium was discovered in 1879 by the Swedish chemist Per Theodor Cleve in a sample of erbium mineral. It was first isolated by the American chemist Charles James in 1911.

See also: From Thule, the earliest name for Scandinavia.

A soft, malleable, ductile, lustrous silver-white metal; symbol Tm. Atomic number 69; atomic weight 168.9342; melting point about 1,545°C; boiling point 1,947°C; specific gravity 9.3.
Thulium was discovered in 1879 by the Swedish chemist Per Theodor Cleve in a sample of erbium mineral. It was first isolated by the American chemist Charles James in 1911.

See also: From Thule, the earliest name for Scandinavia.

A loud rumbling sound emitted by rapidly expanding air along the path of the electrical discharge of lightning.

Etymology (EN): M.E. thonder, thunder, O.E. thunor, from P.Gmc. *thunraz (cf. O.Fris. thuner, M.Du. donre, Du. donder, O.H.G. donar, Ger. Donner “thunder”), cognate with Pers. tondar, as below.

Etymology (PE): Tondar; Mid.Pers. tndwr, Sogdian twntr; cf. Skt. stan- “to thunder, resound,” tanyati “thunders, roars,” tanyu- “thundering,” stanatha- “thunder;” L. tonare “to thunder,” tonitrus “thunder” (Fr. tonnerre); PIE base *(s)tene- “to resound, thunder.”

A loud rumbling sound emitted by rapidly expanding air along the path of the electrical discharge of lightning.

Etymology (EN): M.E. thonder, thunder, O.E. thunor, from P.Gmc. *thunraz (cf. O.Fris. thuner, M.Du. donre, Du. donder, O.H.G. donar, Ger. Donner “thunder”), cognate with Pers. tondar, as below.

Etymology (PE): Tondar; Mid.Pers. tndwr, Sogdian twntr; cf. Skt. stan- “to thunder, resound,” tanyati “thunders, roars,” tanyu- “thundering,” stanatha- “thunder;” L. tonare “to thunder,” tonitrus “thunder” (Fr. tonnerre); PIE base *(s)tene- “to resound, thunder.”

A → storm of → thunder and → lightning. Thunderstorms are associated with → convective clouds (Cumulonimbus) and are often accompanied
by → precipitation. They are usually short-lived and hit on only a small area.

Etymology (EN): → thunder; → storm.

Etymology (PE): Tondar-tuf, tufân-e tondari, from tondar, → thunder + tuf stem of tufidan “to roar, to raise a tumult,” tufân “storm, the roaring of the sea, the confused hum of men or animals.” This Persian word may be related to Gk. typhon “whirlwind, mythical monster associated with tempests.”

A → storm of → thunder and → lightning. Thunderstorms are associated with → convective clouds (Cumulonimbus) and are often accompanied
by → precipitation. They are usually short-lived and hit on only a small area.

Etymology (EN): → thunder; → storm.

Etymology (PE): Tondar-tuf, tufân-e tondari, from tondar, → thunder + tuf stem of tufidan “to roar, to raise a tumult,” tufân “storm, the roaring of the sea, the confused hum of men or animals.” This Persian word may be related to Gk. typhon “whirlwind, mythical monster associated with tempests.”