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

The quality or condition of being → false. Opposite of → truth.

→ false; → -ity.

Flux of radiation that falls on a detector per unit surface area of the detector per unit bandwidth of the radiation per unit time.

→ flux; → density.

A transition between two quantum mechanical → states that → violates the quantum mechanical → selection rules.

→ forbidden; → transition.

A coordinate positioning tool, using a combination of satellites that can rapidly and accurately determine the → latitude, → longitude, and the → altitude of a point on or above the Earth's surface. The GPS is based on a constellation of 24 Earth-orbiting satellites at an altitude of about 26,000 km. The system is a direct application of the thories of → special relativity and → general relativity.

→ global; → positioning; → system.

The total number of → Lyman continuum photons emitted by an → H II region. It is usually derived using → radio continuum observations which are less affected by → interstellar extinction. The measured value is often a lower limit because of photon leakage from the H II region and absorption. See also → density-bounded H II region.

→ H II; → region; → luminosity.

An → atomic transition involving a → hyperfine structure.

→ hyperfine; → transition.

1) The laying on of something as a burden or obligation.
2) Something imposed, as a burden or duty; an unusual or extraordinarily burdensome requirement or task.
3) The act of imposing by or as if by authority (Dictionary.com).

→ impose; → -tion.

Situated in the natural or original position or place.

From L. in, → in-; L. situ, → site.

Dar "in," → in-; jâ, → place.

General: The quality or condition of being intense.
Physics: Strength, as of energy or a force per unit of area, solid angle, time, etc.; e.g. → electric intensity; → magnetic intensity.

From → intense + → -ity.

Dartanuyi state, condition noun of dartanu, → intense.

The height of a spectral line above the continuum base.

→ intensity; → line.

The rate of emitted energy from unit surface area through unit solid angle. The radiation from a surface has different intensities in different directions.

→ intensity; → radiation.

The energy per second emitted by an astronomical object.

→ intrinsic; → luminosity.

The ratio of the → dynamic viscosity (η) to the density (ρ) of a fluid: ν = η/ρ. The unit of kinematic viscosity in the → SI system is m²s^-1. In the → cgs system, cm²s^-1, equal to 10^-4 m²s^-1, is called the → stokes (st).

→ kinematic; → viscosity.

A quantity, denoted L_d, describing a continuous system in the → Lagrangian formalism, and defined as the → Lagrangian per unit volume. It is related to the Lagrangian L by:
L = ∫∫∫L_d d³V.
Lagrangian density is often called Lagrangian when there is no ambiguity.

→ Lagrangian; → density.

A measure of the total effect of an absorption or emission line. The line intensity is equal to the integration of the absorption coefficient over the entire shape of the absorption line.

→ line; → intensity.

→ Einstein equivalence principle.

→ local; → position; → invariance.

The → total → brightness of a star or other astronomical object. It is expressed in watts and represents the total amount of → energy that the object radiates each → second over all wavelength regions of the → electromagnetic spectrum. Because this quantity is independent of distance, it is an → intrinsic brightness.
See also:
→ absolute luminosity, → anomalous luminosity effect, → bolometric luminosity, → color-luminosity diagram, → Eddington luminosity, → H II region luminosity, → intrinsic luminosity, → luminosity class, → luminosity distance, → luminosity function, → luminosity problem, → luminosity-size relation, → mass-luminosity ratio, → mass-luminosity relation, → peak luminosity, → period-luminosity relation, → solar luminosity, → stellar luminosity, → wind luminosity.

Verbal noun of → luminous.

A classification of stellar spectra according to luminosity for a given → spectral type. The luminosity class is an indication of a star's → surface gravity. It is shown by a Roman numeral as follows: I (→ supergiants), II (bright → giants), III (normal giants), IV (→ subgiants), and V (→ dwarf stars, or → main-sequence stars). Luminosity classes VI (→ subdwarfs) and VII (→ white dwarfs) are rarely used. Subclasses a, b, and c are especially used for supergiants, while the most luminous → hypergiants are assigned luminosity class Ia-0.

→ luminosity; → class.

1) Distance derived by comparison of → observed and → intrinsic luminosities. If an object has a known luminosity L, and the observed flux is S, the luminosity distance is defined by D_L = (L/4πS)^1/2.

2) In cosmology, the → expansion of the Universe results in a diminution of the photon flux and the above equation fails. The reason is that for a homogeneous and isotropic Universe (→ Robertson-Walker metric), the luminosity decreases by a factor (1 + z)⁴. Therefore, the luminosity distance is related to the → angular diameter distance (D_A) by: D_L = (1 + z)².D_A.

→ luminosity; → distance.

Number → distribution of → stars or galaxies (→ galaxy) with respect to their → absolute magnitudes. The luminosity function shows the → number of stars of a given intrinsic luminosity (or the number of galaxies per integrated magnitude band) in a given → volume of space.

→ luminosity; → function.