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

A combining form meaning "something drawn, written" (diagram, spectrogram, oscillogram).

From Gk. -gramma, combining form of gramma "something written or drawn," → graph.

-Negâšt, from negâšt "drawn, written;" p.p. of negâštan, negârdan "to paint, write," → graph.

A measure of the departure from a constant rate of the acceleration of the Universe, expressed by: q(t) = R(t)R ^..(t)/R ^.2(t), where R(t) represents the size of the Universe at time t. Traditionally, a negative sign is inserted in the above equation for the → deceleration parameter.

→ acceleration; → parameter.

The brightest star in → Cepheus and a → subgiant star of apparent visual magnitude 2.44. Its → spectral type is A7 and distance 49 → light-years.

Alderamin, from Ar. al dhirâ' al-yamin "right arm" (of Cepheus), from Ar. dhirâ' "arm" + yamin "right".

Zerâ'-e Yamin, from Ar. al dhira al-yamin.

The ratio J/M, where J is the → angular momentum of a → rotating black hole and M the mass of the black hole.

→ angular; → momentum; → parameter.

A geometrical representation of → complex numbers, which like the → Cartesian coordinates, uses two reference perpendicular axes. The horizontal axis represents the → real number part of the number and the perpendicular axis the → imaginary number part.

Named after Jean Robert Argand (1768-1822), a Swiss mathematician, who introduced this representation; → diagram.

An auxiliary observing program to be carried out at telescope in case the atmospheric conditions make the main program unfeasible.

Backup, from → back + up; → program.

Barnâmé, → program; yadaki "reserve, substitute," from yadak "a led horse."

A set of nebular → emission line diagrams used to distinguish the ionization mechanism of → nebular gas. The most famous version consists of [N II]λ6584/Hα versus [OIII] λ5007/Hβ. The next two more commonly used BPT diagnostics are [S II] λλ6717,6731/Hα versus [O III] λ5007/Hβ and [O I] λ6300/Hα versus [O III]λ5007/Hβ. These diagrams use strong, optical lines of close proximity in the ratios to limit → reddening and → spectrophotometric effects. They are able to clearly distinguish different classes of → ionization, for example → LINERs from normal → H II regions and → active galactic nuclei.

Baldwin, J. A., Phillips, M. M., Terlevich, R., 1981 PASP 93, 5; → diagram.

CCD frame with exposure time set to zero and giving the bias level.

→ bias; → frame.

Nurdâd, → exposure; varak→ bias

A graph on which the latitudes of → sunspots are plotted against time. It shows how sunspots migrate from high latitudes (30°- 40° north or south) to the solar equator (latitude of about 5°) during each → solar cycle, according to → Sporer's law. The shape of these distributions, when represented for both hemispheres, resembles the wings of a butterfly. The diagram was first created by Edward W. Maunder in 1904 to illustrate the solar cycle (M.S.: SDE).

Butterfly, from M.E. butterflye, from O.E. butorfleoge, from butor, butere "butter" floge "fly," but the etymology is not clear; → diagram.

Nemudâr, → diagram; parvânevâr "resembling a butterfly," from parvâné "butterfly" + -vâr similarity suffix.

One of a series of astronomical images obtained using a CCD detector in particular for calibration purposes.

→ CCD; → frame.

A diagram based on two photometric colors usually representing the same class of astronomical objects.

→ color; → diagram.

A form of → Hertzsprung-Russell diagram in which the luminosity is the vertical axis and the → color index the horizontal axis.

→ color; → luminosity, → diagram.

A form of → Hertzsprung-Russell diagram in which the visual absolute magnitude M_v is the vertical axis and the → color index the horizontal axis.

→ color; → magnitude, → diagram.

A → reference frame that is attached to a moving object. The object in this frame is therefore at rest.

→ comoving; → frame.

Same as → concept map.

→ conceptual; → diagram.

A plot showing a summary of correlation at different periods of time.

Correlo-, from → correlation; → -gram.

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.

One of the four terms that describe an arranged version of the → Friedmann equations. They are all time dependent.
1) For matter: Ω_m = 8πGρ_m/(3H²), where G is the → gravitational constant, ρ_m is the mean matter density, and H the → Hubble parameter. The matter density parameter is also expressed as Ω_m = ρ_m/ρ_crit, where ρ_crit is the → critical density.
2) For radiation: Ω_r = 8πGρ_r/(3H²), where ρ_r is the radiation equivalent of matter density. This parameter is also expressed as Ω_r = ρ_r/ρ_crit.
3) For the → cosmological constant: Ω_Λ = Λc²/(3H²). Similarly, Ω_Λ = ρ_Λ/ρ_crit, where &rho_Λ = Λc²/(8πG) is sometimes referred to as the density of → dark energy.
4) For the → curvature of space-timeΩ_k = -kc²/(R²H²), where k is the → curvature constant and R the → cosmic scale factor.
Note that: Ω_m + Ω_r + Ω_Λ + Ω_k = 1, and Ω_total = Ω_m + Ω_r + Ω_Λ = 1 - Ω_k.

→ density; → parameter.

A graphic representation of the behavior of one or several variables.

From Fr. diagramme, from L. diagramma, from Gk. diagramma "that which is marked out by lines," from diagraphein "to mark out by lines," from dia- "across, out" + graphein "to write, draw," → -graphy.

Nemudâr agent noun of nemudan "to show," → display, from the past stem nemud + -âr, such as xâstâr, foruxtâr, padidâr, parastâr (contraction of *parastidâr).

A → dimensionless parameter indicating the degree to which a star is close to the → Eddington limit. It is expressed as Γ = L / L_Edd = κ L / (4πGMc), where L and M are the star luminosity and mass respectively, κ is the opacity, c the speed of light, and G the → gravitational constant. At the Eddington limit, Γ = 1, the star would become unbound. Because stellar luminosity generally scales with a high power of the stellar mass (L∝ M^3-4), → massive stars with M larger than 10 Msun generally have electron Eddington parameters of order Γ ≅ 0.1-1.

After Arthur Stanley Eddington (1882-1944), prominent British astrophysicist; → parameter.