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

One possible explanation for → spiral arms, first put forward by B. Lindblad in about 1925 and developed later by C.C. Lin and F. H. Shu. According to this theory, spiral arms are not material structures, but regions of somewhat enhanced density, created by → density waves. Density waves are perturbations amplified by the self-gravity of the → galactic disk. The perturbation results from natural non-asymmetry in the disk and enhanced by environmental processes, such as galaxy encounters. Density waves rotate around the → galactic center and periodically compress the disk material upon their passage. If the spiral arms were rigid structures rotating like a pinwheel, the → differential rotation of the galaxy would wind up the arms completely in a relatively short time (with respect to the age of the galaxy), → winding problem. Inside the region defined by the → corotation radius, density waves rotate more slowly than the galaxy's stars and gas; outside that region they rotate faster.
As the density waves rotate, they are overtaken by the individual stars and nebulae/molecular clouds that are rotating around the galaxy at a higher rate. The molecular clouds passing through the density wave are subjected to compression because it is a region of higher density. This triggers the formation of clusters of new stars, which continue to move through the density wave.
The short-lived stars die, most likely as supernovae, before they can leave the spiral density wave. But the longer-lived stars that are formed pass through the density wave and eventually emerge on its front side and continue on their way as a slowly dissipating cluster of stars. Density wave theory explains much of the spiral structure that we see, but there are some problems. First, computer simulations with density waves tend to produce very orderly "grand design" spirals with a well-defined, wrapped 2-arm structure. But there are many spiral galaxies that have a more complex structure than this (→ flocculent spiral galaxy). Second, density wave theory assumes the existence of spiral density waves and then explores the consequences.
See also: → stochastic self-propagating star formation.

→ density; → wave; → theory.

An → H II region which lacks enough matter to absorb all → Lyman continuum photons of the → exciting star(s). In such an H II region a part of the ionizing photons escape into the → interstellar medium. See also → ionization-bounded H II region.

→ density; → bounded; → region.

1a) Something precipitated, delivered and left, thrown down, or accumulated, as by a natural process.
1b) Substance which settles down from a solution or a suspension, such as the natural sediment of wine in a bottle. See also → sediment.
2) To leave or form a layer of some substance (sand, sediment, etc.) as a gradual process in one place, or be left in this way.

From L. depositus, p.p. of deponere "to lay aside, put down," from → de- "away" + ponere "to put," → position.

Lerd "sediment, tartar of wine," probably a variant of dord "dregs, lees, sediment, tartar of wine."

The process by which water vapor changes directly to ice without first becoming a liquid. This is how snow forms in clouds, as well as frost and hoar frost on the ground. The opposite of deposition is → sublimation. → condensation.

M.E., from O.Fr. deposition, from L. deposition- "putting aside, testimony, burial," from deposit(us) "laid down," p.p. of deponere "to put down," from → de- + ponere "to put, place."

Vâneheš, from vâ-→ de- + neheš verbal noun of nehâdan "to put, place," Mid.Pers. nihâtan, O.Pers./Av. ni- "down; into," → ni-, + dā- "to put; to establish; to give," dadāiti "he gives," cf. Skt. dadâti "he gives," Gk. didomi "I give," L. do "I give;" PIE base *do- "to give."

A transition between two quantum-mechanical energy levels. See also → discrete spectrum.

→ discrete; → transition.

The state or fact of being diverse; difference; unlikeness.

→ diverse; → -ity.

Same as → viscosity and → absolute viscosity.

→ dynamic; → viscosity.

Same as → Eddington limit.

→ Eddington limit; → luminosity.

Same as → geodetic precession.

→ Einstein-de Sitter Universe; → effect.

The → Friedmann-Lemaitre model of → expanding Universe that only contains matter and in which space is → Euclidean (Ω_M > 0, Ω_R = 0, Ω_Λ = 0, k = 0). The Universe will expand at a decreasing rate for ever.

→ Einstein; de Sitter, after the Dutch mathematician and physicist Willem de Sitter (1872-1934) who worked out the model in 1917; → Universe.

The strength of an electric field at any point as measured by the force exerted upon a unit positive charge placed at that point.

→ electric; → intensity.

The number of electrons per unit volume in an ionized medium, like an → H II region, as determined from → emission lines.

→ electron; → density.

The simultaneous formation of an → electron and a → positron in the → pair production process.

→ electron; → positron; → pair.

The → transfer of an → electron from one → energy level to another.

→ electronic; → transition.

The amount of energy in the form of radiation per unit volume, expressed in ergs cm^-3. In particular, the energy density of blackbody radiation at temperature T is aT⁴, where the radiation constant a = 7.56 × 10^-15 erg cm^-3 (K)^-4.

→ energy; → density.

The passage of a celestial body or point across the → ephemeris meridian.

→ ephemeris; → transit.

The position of an oscillating body at which no net force acts on it.

→ equilibrium; → position.

Complete set of points in any given space group which are obtained by performing the symmetry operations of the space group on a single point (x, y, z).

→ equivalent; → position.

The passage of an → exoplanet across the face its star.

→ exoplanetary; → transit.

1) The act of setting forth information or a viewpoint.
2) Public exhibition or show.

Verbal noun of → expose.