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

A type of → molecular cloud in which the → interstellar radiation field is sufficiently attenuated, so that the local fraction of → molecular hydrogen (H₂) becomes substantial (> 0.1). However, enough interstellar radiation is still present to → photoionize any atomic carbon, or to → photodissociate → carbon monoxide (CO) such that carbon is predominantly still in the form of C⁺ (> 0.5). In steady state, diffuse molecular clouds must necessarily be surrounded by diffuse atomic gas, in order to provide the → shielding of radiation. This means that most sightlines that cross a diffuse molecular cloud will also cross → diffuse atomic gas (Snow & McCall, 2006, ARA&A 44, 367).

→ diffuse; → molecular; → cloud.

An irregularly shaped and low density interstellar cloud visible in the optical wavelengths.

→ diffuse; → nebula.

Reflection of light from a rough or granular surface, which takes place in all directions due to the microscopic irregularities of the interface; opposed to → specular reflection.

→ diffuse; → reflection.

Transmission accompanied by diffusion or scatter to the extent that there is no regular or direct transmission.

→ diffuse; → transmission.

A device used to scatter or disperse light emitted from a source.

From → diffuse + -er.

From paxš, present stem of paxšidan, → diffuse, + -andé or -gar (→ detector).

1) Movement of a gas or liquid as a result of the random thermal motion of its atoms or molecules.
2) The random spreading out of a beam of radiation on reflection (→ diffuse reflection) or transmission through a translucent medium (such as amber, milk, or frosted glass).
3) In chemical thermodynamics, the process of attaining an equilibrium distribution of the → concentration within the phases. A result of diffusion at constant equilibrium is the equalization of the → chemical potentials.
Related terms: → diffraction; → dispersion; → distribution; → scatter; → scattering.

L. diffusionem, from stem of diffundere "scatter, pour out," from dif- "apart, in every direction," → dis-, + fundere "to melt, cast, pour out," from PIE *gheud-, from root *gheu- "to pour."

Paxš, verbal noun and stem of paxšidan→ diffuse.

A factor of proportionality involved in the → diffusion equation. It may be defined as the amount of the quantity diffusing across a unit area through a unit concentration gradient in unit time. → magnetic diffusivity.

→ diffusion; → coefficient.

An equation that expresses the time rate of change of a quantity in terms of the product of the diffusion coefficient and the → Laplacian operating on the quantity. For example the diffusion equation for temperature is: ∂T/∂t = D∇²T.

→ diffusion; → equation.

A narrow boundary layer above the solar → photosphere, between two magnetic field lines, where the plasma becomes demagnetized or unfrozen. The presence of a localized magnetic region is necessary for → magnetic reconnection.

→ local; → -ize; → diffusion; → region.

Tending to diffuse; characterized by → diffusion.

→ diffuse + → -ive.

1) The ability to permit or undergo diffusion.
2) → magnetic diffusivity.

→ diffusive; → -ity.

A symbol, numeral, or graphic character that represents an integer.

From L. digitus "finger, toe."

Raqam, from Ar.

1) Of or pertaining to a digit.
2) Representing data as a series of numerical values. Often opposed to → analogue.

→ digit + → -al.

A computer that accepts and operates with → discrete data in the form of combinations of digits, letters, or other characters. In modern terminology, generally called computer.

→ digital + → computer.

To convert an → analogue signal to a → digital signal that represents the same information as the analogue signal.

→ digit + → -ize.

1) Having formed by two half-planes which intersect.
2) Of or relating to a → dihedron.

→ dihedron; → -al.

An angle formed by two planes meeting along a common line. Compare with → face angle.

→ angle; → dihedral.

A figure formed by two intersecting planes.

→ di-; → -hedron.

1) (v.tr.) To make wider or larger; cause to expand.
2) (v.intr.) To spread out; expand.

M.E. dilaten, from O.Fr. dilater, from L. dilatare "make wider, enlarge," from → dis- "apart" + latus "wide," → latitude.

Farâxândan, farâxidan, infinitive from farâx "broad, wide, spacious;" Mid.Pers. frâxv, fraxv "broad, wide," frâxvitan, frâxvênitan "to spread."

The act of dilating; state of being dilated. Also dilatation.
Physics: The increase in volume per unit volume of a homogeneous substance. → time dilation.

Verbal noun of → dilate.