An Etymological Dictionary of Astronomy and Astrophysics

The nearest known → classical T Tauri star,
situated some 56 → parsecs away in the constellation → Hydra, far from any → molecular cloud (abbreviated TW Hya). It is a variable star with large Hα-emission → equivalent width. TW Hya is similar in mass to the Sun, but is only about 8 million years old. The star appears to be → accreting from a remarkable → face-on→ protoplanetary disk of dust and gas, resolved in images from the → Hubble Space Telescope. Stellar light scattered from the optically thick dust disk is seen from 20 to 230 AU. The → scattering profile indicates that the disk is → flared, not geometrically flat. TW Hya is accompanied by a group of other low-mass stars with similar ages and space motions, comprising the → TW Hydrae association. An → exoplanet of mass nearly 10 → Jupiters has been detected around TW Hya. It orbits the star with a period of 3.56 days at 0.04 AU, inside the inner rim of the disk.

See also: → Hydra.

The nearest known → classical T Tauri star,
situated some 56 → parsecs away in the constellation → Hydra, far from any → molecular cloud (abbreviated TW Hya). It is a variable star with large Hα-emission → equivalent width. TW Hya is similar in mass to the Sun, but is only about 8 million years old. The star appears to be → accreting from a remarkable → face-on→ protoplanetary disk of dust and gas, resolved in images from the → Hubble Space Telescope. Stellar light scattered from the optically thick dust disk is seen from 20 to 230 AU. The → scattering profile indicates that the disk is → flared, not geometrically flat. TW Hya is accompanied by a group of other low-mass stars with similar ages and space motions, comprising the → TW Hydrae association. An → exoplanet of mass nearly 10 → Jupiters has been detected around TW Hya. It orbits the star with a period of 3.56 days at 0.04 AU, inside the inner rim of the disk.

See also: → Hydra.

A young (≤ 100 million years) association of stars (abbreviated TWA) with at least 27 members,
from → intermediate mass stars to planetary mass objects. Five of them, including → TW Hydrae, display signatures of → T Tauri stars. TWA is the first moving group of young nearby (≤ 100 → paesecs) stars to be identified.

See also: → TW Hydrae; → association.

A young (≤ 100 million years) association of stars (abbreviated TWA) with at least 27 members,
from → intermediate mass stars to planetary mass objects. Five of them, including → TW Hydrae, display signatures of → T Tauri stars. TWA is the first moving group of young nearby (≤ 100 → paesecs) stars to be identified.

See also: → TW Hydrae; → association.

A cardinal number, 10 times 2.

Etymology (EN): M.E.; O.E. twentig “group of twenty,” from twegen→ two; cf. O.Fris. twintich, Du. twintig, O.H.G. zweinzug, Ger. zwanzig.

Etymology (PE): Bist “twenty;” Mid.Pers. wist “twenty;” Av. vīsaiti “twenty;” cf. Skt. vimśati- “twenty;” Gk. eikosi “twenty;” L. uiginti “twenty.”

A cardinal number, 10 times 2.

Etymology (EN): M.E.; O.E. twentig “group of twenty,” from twegen→ two; cf. O.Fris. twintich, Du. twintig, O.H.G. zweinzug, Ger. zwanzig.

Etymology (PE): Bist “twenty;” Mid.Pers. wist “twenty;” Av. vīsaiti “twenty;” cf. Skt. vimśati- “twenty;” Gk. eikosi “twenty;” L. uiginti “twenty.”

Two times, as in succession; on two occasions.

Etymology (EN): M.E. twies, from O.E. twiga, twigea “two times,” from twi-, → two; cf. L. bis, Gk. dis, Skt. dvis, Av. biš.

Etymology (PE): Dobâr, from do, → two, + bâr “time, fold,” from Mid.Pers. bâr; Proto-Ir. *uara-; cf. Av. var- “to choose; to convince;” Skt. vāra- “time, turn.”

Two times, as in succession; on two occasions.

Etymology (EN): M.E. twies, from O.E. twiga, twigea “two times,” from twi-, → two; cf. L. bis, Gk. dis, Skt. dvis, Av. biš.

Etymology (PE): Dobâr, from do, → two, + bâr “time, fold,” from Mid.Pers. bâr; Proto-Ir. *uara-; cf. Av. var- “to choose; to convince;” Skt. vāra- “time, turn.”

The diffused light from the sky when the Sun is below the → horizon, either from daybreak to → sunrise or, more commonly, from → sunset to → dusk or nightfall.

There are three types of twilight: → astronomical twilight, → nautical twilight, and → civil twilight. They are divided on the basis of the → solar depression angle.

Etymology (EN): M.E., cognate with Du. tweelicht, Ger. zwielicht,
from twi- a combining form meaning “two, twice,” but it appears to refer to “half” light, rather than the fact that twilight occurs twice a day + → light.

Etymology (PE): Nimtâb, from nim, nimé, → half,

• tâb “light,” present stem of
  tâbidan “to shine,” → luminosity.

The diffused light from the sky when the Sun is below the → horizon, either from daybreak to → sunrise or, more commonly, from → sunset to → dusk or nightfall.

There are three types of twilight: → astronomical twilight, → nautical twilight, and → civil twilight. They are divided on the basis of the → solar depression angle.

Etymology (EN): M.E., cognate with Du. tweelicht, Ger. zwielicht,
from twi- a combining form meaning “two, twice,” but it appears to refer to “half” light, rather than the fact that twilight occurs twice a day + → light.

Etymology (PE): Nimtâb, from nim, nimé, → half,

• tâb “light,” present stem of
  tâbidan “to shine,” → luminosity.

A shining with intermittent light, as the twinkling of the stars. → scintillation.

Etymology (EN): M.E. twinklen (v.), O.E. twinclian, frequentative of twincan “to wink, blink;” related to M.H.G. zwinken, Ger. zwinkern.

Etymology (PE): Cešmak “twinke,” from cešm, → eye,

• -ak relation suffix.

A shining with intermittent light, as the twinkling of the stars. → scintillation.

Etymology (EN): M.E. twinklen (v.), O.E. twinclian, frequentative of twincan “to wink, blink;” related to M.H.G. zwinken, Ger. zwinkern.

Etymology (PE): Cešmak “twinke,” from cešm, → eye,

• -ak relation suffix.

A thought experiment in special relativity, according to which if one of a pair of twins (A) remains on Earth, and the other (B) travels at a speed near the speed of light, B will be younger than A upon returning to Earth.
In fact there is no paradox, because the two perspectives, A and B’s, are actually not completely symmetric. There is no fixed time difference between the events, and different observers experience different intervals of time between the same two events. In fact, B returns younger than A because only B travels in a non-inertial (accelerating) reference frame. From A’s point of view, B experiences time dilation, but from B’s point of view the distance traveled is shortened because of length contraction.

If B leaves in the year 2000 and returns in 2020, for A 20 years have elapsed. For B it depends on his travel speed. If has has moved as fast as 86% of the speed of light for him 10 years have passed. If his speed has been 99.5% of the speed of light the travel duration for him has been 2 years.
This effect has been verified experimentally by measurements with atomic clocks.

Etymology (EN): Twin M.E.; O.E. twinn; cf. O.N. tvinnr, O.Dan. tvinling, Du. tweeling, Ger. zwillung; → paradox.

Etymology (PE): Pârâdaxš, → paradox; hamzâdhâ, plural of hamzâd “twin,” literally “born together,” from ham- “together” → syn- + zâd “born,” from zâdan “to bring forth, give birth” (Mid.Pers. zâtan; Av. zan- “to bear, give birth to a child, be born,” infinitive zazāite, zāta- “born;” cf. Skt. janati “begets, bears,”
janitár “progenitor, father;” Gk. genetor “progenitor;” L. gignere “to beget,” nasci “to be born,” as above, PIE base *gen- “to give birth, beget”).

A thought experiment in special relativity, according to which if one of a pair of twins (A) remains on Earth, and the other (B) travels at a speed near the speed of light, B will be younger than A upon returning to Earth.
In fact there is no paradox, because the two perspectives, A and B’s, are actually not completely symmetric. There is no fixed time difference between the events, and different observers experience different intervals of time between the same two events. In fact, B returns younger than A because only B travels in a non-inertial (accelerating) reference frame. From A’s point of view, B experiences time dilation, but from B’s point of view the distance traveled is shortened because of length contraction.

If B leaves in the year 2000 and returns in 2020, for A 20 years have elapsed. For B it depends on his travel speed. If has has moved as fast as 86% of the speed of light for him 10 years have passed. If his speed has been 99.5% of the speed of light the travel duration for him has been 2 years.
This effect has been verified experimentally by measurements with atomic clocks.

Etymology (EN): Twin M.E.; O.E. twinn; cf. O.N. tvinnr, O.Dan. tvinling, Du. tweeling, Ger. zwillung; → paradox.

Etymology (PE): Pârâdaxš, → paradox; hamzâdhâ, plural of hamzâd “twin,” literally “born together,” from ham- “together” → syn- + zâd “born,” from zâdan “to bring forth, give birth” (Mid.Pers. zâtan; Av. zan- “to bear, give birth to a child, be born,” infinitive zazāite, zāta- “born;” cf. Skt. janati “begets, bears,”
janitár “progenitor, father;” Gk. genetor “progenitor;” L. gignere “to beget,” nasci “to be born,” as above, PIE base *gen- “to give birth, beget”).

A cardinal number, 1 plus 1. → two-color diagram; → two-photon emission.

Etymology (EN): M.E., from O.E. twa, feminine and neuter form of twegen “two,” from P.Gmc. *twai (cf. O.S., O.Fris. twene, twa, O.N. tveir tvau, Du. twee, O.H.G. zwene, zwo, Ger. zwei, Goth. twai), cognate with Pers. do, as below.

Etymology (PE): Do “two,” from Mid.Pers. do; Av. dva-; cf.
Skt. dvi-; Gk. duo; L. duo (Fr. deux), di;
Lith. dvi; O.C.S. duva; PIE base *duwo.

A cardinal number, 1 plus 1. → two-color diagram; → two-photon emission.

Etymology (EN): M.E., from O.E. twa, feminine and neuter form of twegen “two,” from P.Gmc. *twai (cf. O.S., O.Fris. twene, twa, O.N. tveir tvau, Du. twee, O.H.G. zwene, zwo, Ger. zwei, Goth. twai), cognate with Pers. do, as below.

Etymology (PE): Do “two,” from Mid.Pers. do; Av. dva-; cf.
Skt. dvi-; Gk. duo; L. duo (Fr. deux), di;
Lith. dvi; O.C.S. duva; PIE base *duwo.

An astronomical → survey conducted from 1997 to 2001 of the entire sky in near-infrared J, H, and K bands (wavelengths 1.25, 1.65, and 2.17 microns respectively).
The aim was to detect and characterize point sources brighter than about 1 → mJy in each band, with → signal-to-noise ratio
greater than 10, using a pixel size of 2".0. Two automated 1.3-m telescopes were used, one at Mt. Hopkins, AZ, and one at CTIO, Chile. 2MASS is currently producing the following data products:

1. A digital atlas of the sky comprising approximately 4 million 8’ × 16’ images, having about 4" spatial resolution in each of the wavelength bands.

2. A point source catalog containing accurate positions and fluxes for 300 million stars and other unresolved objects.

3. An extended source catalog containing positions and total magnitudes for more than 1,000,000 galaxies and nebulae.

See also: → two; → micron; → all-sky survey.

An astronomical → survey conducted from 1997 to 2001 of the entire sky in near-infrared J, H, and K bands (wavelengths 1.25, 1.65, and 2.17 microns respectively).
The aim was to detect and characterize point sources brighter than about 1 → mJy in each band, with → signal-to-noise ratio
greater than 10, using a pixel size of 2".0. Two automated 1.3-m telescopes were used, one at Mt. Hopkins, AZ, and one at CTIO, Chile. 2MASS is currently producing the following data products:

1. A digital atlas of the sky comprising approximately 4 million 8’ × 16’ images, having about 4" spatial resolution in each of the wavelength bands.

2. A point source catalog containing accurate positions and fluxes for 300 million stars and other unresolved objects.

3. An extended source catalog containing positions and total magnitudes for more than 1,000,000 galaxies and nebulae.

See also: → two; → micron; → all-sky survey.

In classical mechanics, the study concerned with the dynamics of an isolated system of two particles subject only to the Newtonian gravitational
force between them. The problem can be separated into two single-particle problems with the following solutions. The equation of the → center of mass is governed by the equation of the same form as that for a single particle. Moreover, the motion of either particle, with respect to the other as origin, is the same as the motion with respect to a fixed origin of a single particle of → reduced mass acted on by the same internal force. See also → three-body problem, → n-body problem.

See also: → two; → body; → problem.

In classical mechanics, the study concerned with the dynamics of an isolated system of two particles subject only to the Newtonian gravitational
force between them. The problem can be separated into two single-particle problems with the following solutions. The equation of the → center of mass is governed by the equation of the same form as that for a single particle. Moreover, the motion of either particle, with respect to the other as origin, is the same as the motion with respect to a fixed origin of a single particle of → reduced mass acted on by the same internal force. See also → three-body problem, → n-body problem.

See also: → two; → body; → problem.

A → dynamical system consisting of two masses that interact via → central forces.

See also: → two; → body; → system.

A → dynamical system consisting of two masses that interact via → central forces.

See also: → two; → body; → system.

A graph on which two color indices such as B-V and U-B are plotted, one along each axis, for a sample of stars or other objects, such as stars.

See also: → two; → color; → diagram.

A graph on which two color indices such as B-V and U-B are plotted, one along each axis, for a sample of stars or other objects, such as stars.

See also: → two; → color; → diagram.

A flow whose parameters are functions of time and two space coordinates (x and y) only. There is no variation in the z direction and therefore the same → streamline pattern could at any instant be found in all planes in the fluid perpendicular to the z direction (B. Massey, Mechanics of Fluids, Taylor & Francis, 2006).

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

A flow whose parameters are functions of time and two space coordinates (x and y) only. There is no variation in the z direction and therefore the same → streamline pattern could at any instant be found in all planes in the fluid perpendicular to the z direction (B. Massey, Mechanics of Fluids, Taylor & Francis, 2006).

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

The simultaneous emission of two photons whose sum of energies is equal to that of a single electron transition. The energy of each individual photon of the pair is not fixed, so that the spectrum of two-photon emission is continuous from the wavelength of that transition to infinity. In practice, there is a peak in wavelength distribution of the emitted photons. Two-photon emission is studied atomic physics with application in astrophysics, as it contributes to the continuum radiation from → planetary nebulae. It was recently observed in condensed matter and specifically in → semiconductors.

See also: → two; → photon; → emission.

The simultaneous emission of two photons whose sum of energies is equal to that of a single electron transition. The energy of each individual photon of the pair is not fixed, so that the spectrum of two-photon emission is continuous from the wavelength of that transition to infinity. In practice, there is a peak in wavelength distribution of the emitted photons. Two-photon emission is studied atomic physics with application in astrophysics, as it contributes to the continuum radiation from → planetary nebulae. It was recently observed in condensed matter and specifically in → semiconductors.

See also: → two; → photon; → emission.