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

In medieval astronomy, a spurious oscillatory motion of the equinoxes thought to have a period of 7,000 years. This concept, attributed to Thâbit ibn Qurra (c. 830-901), had a profound influence on medieval astronomy. In order to explain trepidation, Thâbit was said to have added a new sphere to the eight Ptolemaic spheres beyond the sphere of fixed stars.

From L. trepidationem "agitation, trembling," from trepidare "to tremble, hurry," from trepidus "anxious, scared;" cf. Skt. trprá- "hasty;" PIE base *trep- "to shake, tremble."

Larzeš, verbal noun of larzidan "to tremble, shiver;" Mid.Pers. larzidan "to shake, tremble;" Manichean Mid.Pers. rarz- "to shiver with fever;" Proto-Iranian *rarz- "to shake, tremble."

1) Law: An unlawful act causing injury to the person, property, or rights of another, committed with force or violence, actual or implied. → light trespass.
2) To commit a transgression or offense; transgress; offend; sin (Dictionary.com).

M.E. trespas, from O.Fr. trespasser "pass beyond or across, cross, traverse; infringe, violate," from tres- "beyond," from L. → trans-, + passer "go by, pass," → pass, → passage.

Tarâraft, tarâraftan, from tarâ-, → trans-, + raft, raftan "to go," → process.

The formation of second-generation stars in a → molecular cloud, as set off by the action of → massive stars. → sequential star formation; → radiation-driven implosion.

→ trigger; → star formation.

The lower part of the Earth's atmosphere in which temperature decreases with height, except for local areas of → temperature inversion.

→ tropo- + → sphere.

The → sidereal time with respect to the → true equinox.

→ true; → sidereal; → time.

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.

A star formation scenario whereby → massive stars form from gravitationally bound → pre-stellar cores, which are supersonically → turbulent and in approximate pressure equilibrium with the surrounding protocluster medium. The high → accretion rates that characterize such media allow accretion to overcome the radiation pressure due to the luminosity of the star. The core is assumed to → collapse via an → accretion disk to form a single star or binary. The core density structure adopted is ρ ∝ r^-k, with k = 1.5 set from observations. This choice affects the evolution of the accretion rate, which increases linearly with time. The high densities in regions of massive-star formation lead to typical time scales for the formation of a massive star of about 10⁵ years (McKee & Tan 2003, ApJ 585, 850).

→ turbulent; → core.

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.

A galaxy that emits more than 90% of its energy in the infrared (8-1000 µm) and whose infrared luminosity exceeds 10¹² solar luminosities. → luminous infrared galaxy (LIRG). Quasars can also have such high or even higher bolometric luminosities. However LIRGs and ULIRGs emit the bulk of their energy in the infrared. Most of ULIRGs are found in merging and interacting galaxy systems. It is thought that their luminosity results from galactic collisions, which increase the rate of star formation.

→ ultraluminous; → infrared; → galaxy.

Describing a system or situation for which the → Lorentz factor, γ, is much larger than 1. See also → subrelativistic.

→ ultra- + → relativistic

A gas composed of ultrarelativistic particles.

→ ultrarelativistic; → gas.

1) To estimate at too low a value, rate, or the like.
2) An estimate that is too low. → overestimate.

→ under-; → estimate

A type of meteorite in which the constituting materials (stone, glass, metal) are mixed together in a disorderly mass, in contrast to → differentiated meteorites.

→ un-; → differentiated meteorite.

A no longer in general use name for → Aromatic Infrared Band.

→ unidentified; → infrared; → band.

1) Physics: A → potential that satisfies both → Poisson's equation and the → boundary conditions pertinent to a particular field is the only possible potential.
2) Math.: If two → continuous functions φ(t) and ψ(t) have one and the same → Laplace transform F(p), then these functions are identically equal.
3) Astro.: A → black hole can only be characterized by its → mass, → electric charge, and → angular momentum. See also → no hair theorem.

→ uniqueness; → theorem.

Same as → weak equivalence principle.

→ universality; → free; → fall.

A star whose light is not affected by→ interstellar absorption. → reddening

→ un-; → reddened star.

Describing an image whose constituent or elementary parts are not resolved. → unresolved source.

→ un- + → resolved.

A source of radiation whose angular size is too small for details of its structure to be revealed.

→ unresolved; → source.

The general term applied to the atmosphere above the → troposphere.

→ upper; → atmosphere.