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

Dispersion of a light beam by using two dispersing elements (grating, grism), one for separating spectral orders, the other for resolving spectral features within an order.

→ cross; → dispersion.

The identification of an object in a data base or catalog and matching it with the same object identified in another catalog.

→ cross; → identification.

1) Math: The intersection of a plane with a geometric figure, usually at right angles to an axis of symmetry.
2) Physics: A quantity that expresses the effective area that a given particle presents as a target to another incident particle, giving a measure of the probability that the incident particle will induce a particular atomic or nuclear reaction. Also called collision cross section. The cross section has the dimensions of a surface. In nuclear and particle physics, the commonly used units for the cross section are the barn and cm².

In classical mechanics, the cross section for the collision of a point particle with a hard sphere is just be the surface of a section through the middle of the sphere. This explains the name "cross section." → cross; → section.

A process by which a homogeneous solution becomes crystal.

Noun from crystallize, → crystal.

Noun from bolur, from verb boluridan "to crystallize" + verbal noun suffix -eš.

An equation containing unknowns of the third power; the general form: ax³ + bx² + cx + d = 0.

Cubic, of or pertaining to → cube; → equation.

A function defined by a → polynomial of → degree three. Its generalized form is: f(x) = ax³ + bx² + cx + d, where a, b, c and d are constants, and a≠ 0.

→ cubic; → function.

The act of culminating or the instant at which a celestial object reaches its highest altitude above the horizon by crossing the observer's meridian. → upper culmination; → lower culmination.

Verbal noun of → culminate.

Bâlest, from Mid.Pers. bâlist "culmination point of a star; highest, summit," from bâlây "high" + -ist superlative suffix, Av. barəzišta- "highest," from barəz- "high, mount" (Skt. bhrant- "high," O.E. burg, burh "castle, fortified place," from P.Gmc. *burgs "fortress," Ger. Burg "castle," Goth. baurgs "city," E. burg, borough, Fr. bourgeois, bourgeoisie, faubourg; PIE base *bhergh- "high") + -išta- superlative suffix (Skt. -istha-, Gk. -istos, O.H.G. -isto, -osto, O.E. -st, -est, -ost).

A function that gives the probability that a → random variable X is less than or equal to x, at each possible outcome: F(x) = P(X ≤ x), for -∞ < x < ∞. Same as → distribution function.

→ cumulative; → distribution; → function.

One of nine → OB associations located in the Cygnus → constellation. The central association, → Cygnus OB2, is the most famous and the youngest of the Cygnus region. (L. Mahy et al. 2013, astro-ph/1301.0500 and references therein).

→ Cygnus; → OB star; → association.

An → ionization front of → H II regions whose expansion speed is comparable to the → sound speed in the gas (~ 10 km/sec for hydrogen at 10⁴ K). A D-type ionization front results from → R-type ionization front when its propagation speed decreases as the volume of gas ahead of the ionization front grows. If front velocity is equal to a lower limit (C₁² / 2C₂, where C₁ and C₂ are the sound speed ahead and behind the front respectively), the front is called D critical.

D referring to a dense gas; → type; → ionization; → front.

The automatic adjustment of the iris and retina of the eye to allow maximum vision in the dark, following exposure of the eye to a relatively brighter illumination.

→ dark; → adaptation.

A hypothetical process whereby hypothetical → non-baryonic dark matter particles undergo → annihilation interactions with themselves. The process results in observable by-products such as high-energy photons, neutrinos, and other detectable particles. See also → dark matter decay.

→ dark; → matter; → annihilation.

The process of controlling telescope operations during observation and obtaining data.

→ data; acquisition, from L. acquisitionem, from acquirere "get in addition," from → ad- "extra" + quærere "to search for, obtain."

Alfanješ, verbal noun of alfanjidan (variant alfaqdan) "to acquire, get," Bactrian αλφανζ "to acquire," Sogdian δβ'yz "to acquire, gain, get" (Cheung 2007); → data.

The process of converting crude observational data into usable information for scientific interpretation, by correcting, rearranging, ordering, and simplifying.

→ data; → reduction.

According to the → de Broglie hypothesis, which has been verified by experiments, every → particle of matter, whatever its nature, has a characteristic → wavelength associated with its wavelike quantum aspect. The de Broglie equation gives the equivalent wavelength of a moving particle: λ = h/mv, where h is → Planck's constant, m the mass of the particle, and v its velocity.
See also: → de Broglie wavelength, → Davisson-Germer experiment.

Named after Louis Victor de Broglie (1892-1987), French physicist, creator of a new field in physics, wave mechanics, who won the Nobel prize in physics in 1929. → equation

Transition from an excited energy state to a lower energy level, as in spectral line formation or particle emission from an atomic nucleus.

From → de- + → excitation.

A method for estimating the real orientation of a field and/or related velocities/separations from two-dimensional images.

→ de-; → projection.

The act or process of moving, or of causing to move, with decreasing speed. Sometimes called negative acceleration.

Verbal noun of decelerate.

A parameter designating the rate at which the expansion of the Universe would slow down owing to the braking gravitational effect of the matter content of the Universe. It is expressed by: q(t) = -R(t)R ^..(t)/R ^.2(t), where R(t) represents the size of the Universe at time t. See also → expansion parameter; compare with → acceleration parameter.

→ deceleration; → parameter.

A fraction expressed by using → decimal representation, as opposed to a vulgar fraction. For example, 2/5 is a vulgar fraction; 0.40 is a decimal fraction.

→ decimal; → fraction.