An Etymological Dictionary of Astronomy and Astrophysics
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A large circle of the → celestial sphere whose plane is perpendicular to the radius of the Earth through the point. Same as → astronomical horizon. The → visible horizon usually lies lower than the true horizon. See also → dip of the horizon.

→ true; → horizon.

The coordinates of an object for a given date, with respect to the true equator and the true equinoxes for the instant of time in question.

→ true; → position.

The act or process of truncating, as → disk truncation.

Verbal noun from → truncate.

A → total function from → truth values to truth values (a sequence of truth values).

→ truth; → function.

An observed correlation between the luminosity of a spiral galaxy and its rate of rotation (measured from its 21 cm hydrogen line). This means that more luminous galaxies have stars that are moving faster. Knowing the rotational velocity of a spiral galaxy, this relation provides its absolute magnitude and then its distance. → Faber-Jackson relation.

Named after R. B. Tully and J. R. Fisher who first derived this relationship (1977, A&A 54, 661); → relation.

The motion of a solid body whose rotation axis is not fixed in space. For example, that of an asteroid that does not rotate about one of their principal axes. → tumbling asteroid.

→ tumble; → motion.

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.

→ TW Hydrae; → association.

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.

→ two; → micron; → all-sky survey.

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).

→ 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.

→ two; → photon; → emission.

A hybrid of → geocentric and → heliocentric systems in which the Earth remains at the center, with the → Sun orbiting the Earth, but with all the other planets revolving about the Sun. Moreover, the fixed stars were located in a crystalline shell centered on the Earth.

Related to the Danish astronomer Tycho Brahe (1546-1601), who imagined the model; → model.

An → orbital migration of low-mass → planets in which no gap is created in the → protoplanetary disk. According to planetary models, beyond a critical core mass for the forming planet, a gap in the protoplanetary disk is created. The critical mass depends on the mass and → metallicity of the disk and therefore it does not have a singular value, but has been shown to be between about 10-30 Earth masses. Compare with → Type II migration.

→ type; → migration.

The → orbital migration of forming → planets that → accrete enough disk material to exceed the critical mass. This type migrates in a gap created in the → protoplanetary disk. Compare with → Type I migration.

→ type; → migration.

A very young → H II region fully embedded in its natal molecular cloud. Ultracompact H II regions are distinguished from classical H II regions by their small sizes (diameter ≥ 0.1 pc), high densities (Ne ≥ 10⁵ cm^-3), and high emission measures (EM ≥ 10⁷ pc cm^-6). Their typical ionized gas content is about 10^-2  → solar masses, in contrast to classical H II regions with a mass of about 10⁵ solar masses. Due to very important extinction, ultracompact H II regions are not accessible to visible wavelengths.

→ ultra-; → compact; → H II region.

The branch of physics dealing with elastic waves of frequencies above 20 kHz to 10¹⁰ kHz propagated in solids, liquids, and gases.

→ ultra-; sonic from L. sonus→ sound + → -ics.

The study of astronomical objects in the ultraviolet portions of the electromagnetic spectrum, in the waveband 3000 Å to about 10 Å. At these wavelengths, the atmosphere prevents ultraviolet radiation from reaching the Earth surface. Therefore ground-based observatories cannot observe in the ultraviolet. Only with the advent of space-based telescopes has this area of astronomy become available for research.

→ ultraviolet; → astronomy.

The function y = f(x) in a given range of the argument x if there is no number M such that for all values of x in the range under consideration the inequality | f(x) | ≤ M will be fulfilled. → bounded function.

→ unbound; → function.

Not limited by conditions; absolute.

→ un-; → conditional.

Not awake and aware; not responding to one's environment.

→ non-; → conscious.

The → state of being → unconscious.

→ unconscious; → -ness.