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

A → photometric system defined by reference to → monochromatic magnitudes in such a way that, when monochromatic → flux f_ν is measured in ergs sec^-1 cm^-2 Hz^-1, the magnitude will be: AB = -2.5 logf_ν - 48.60. The constant is set so that AB is equal to the V magnitude for a source with a flat → spectral energy distribution. The → zero point is defined by the flux of the star → Vega at 5546 Å. In this system, an object with constant flux per unit frequency interval has zero color.

→ magnitude; → system.

1) The → magnitude a star would have if it were at a distance of 10 → parsecs in a void space, without → interstellar absorption. The absolute magnitude is usually deduced from the → visual magnitude, measured through a V filter (→ UBV system), when it is written as M_V. If it is defined for another wavelength, it gets another index (U, B, etc). If the radiation on all wavelengths is included, it becomes absolute → bolometric magnitude, M_bol. The Sun has the absolute magnitude + 4.8. Most of the stars have absolute magnitudes ranging between -9 (→ supergiants) and + 19 (→ red dwarfs) (M.S.: SDE).
2) The brightness a → comet or → asteroid would have if it were at a distance of 1 → astronomical unit both from the Sun and the Earth and were completely illuminated by the Sun (M.S.: SDE).

→ absolute; → magnitude.

A measure of a star's observed brightness (opposed to → absolute magnitude); symbol m. It depends on the star's → intrinsic brightness, its distance from the observer, and the amount of → interstellar absorption. The brightest star → Sirius has an apparent magnitude of -1.46, while the weakest stars visible with the naked eye in the most favorable observation conditions have magnitudes of about +6.5. The stars of magnitudes less than +23 are measured by professional observatories, whereas those of magnitudes less than +30 by a telescope such as the → Hubble Space Telescope (M.S.: SDE).

→ apparent; → magnitude.

→ Apparent magnitude in the visual wavelengths, around 5600 Å. → visual magnitude.

→ apparent; → visual; → magnitude.

The magnitude of an astronomical object for the entire range of its electromagnetic spectrum.

→ bolometric; → magnitude.

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.

In photometric studies of a → population of astronomical objects (usually stars or galaxies), the magnitude that represents the faintest members of the population.

→ completeness; → magnitude.

A magnitude which has been corrected for the interstellar reddening.

De-reddened, p.p. of → de-redden; → magnitude.

Borz, → magnitude; vâsorxidé p.p. of vâsorxidan, → de-redden.

The fraction of the Sun's diameter occulted by the Moon. It is strictly a ratio of diameters and should not be confused with → eclipse obscuration, which is a measure of the Sun's surface area occulted by the Moon. Eclipse magnitude may be expressed as either a percentage or a decimal fraction (e.g., 50% or 0.50). By convention, its value is given at the instant of → greatest eclipse (F. Espenak, NASA).

→ eclipse; → magnitude.

The magnitude derived directly using → Pogson's relation. The instrumental magnitude depends on → detector→ sensitivity, telescope → aperture, exact filter → bandpass, etc. It must be → calibrated to some standard → photometric system.

→ instrumental; → magnitude.

The magnitude an extended object (nebula, galaxy,etc.) would have if all of its light were concentrated into a point source

Integrated, p.p. of → integrate; → magnitude.

The faintest magnitude reachable by an instrument.

→ limit; → magnitude.

A measure of brightness in astronomy on a → logarithmic scale in which a difference of five magnitudes represents a difference of 100 times in brightness. In this scale the lower a magnitude, the brighter the object. The faintest magnitude reached by → unaided eye is 6.

From L. magnitudo "greatness, bulk, size," from magnus "great," cognate with Pers. meh "great, large" (Mid.Pers. meh, mas; Av. maz-, masan-, mazant- "great, important," mazan- "greatness, majesty," mazišta- "greatest;" cf. Skt. mah-, mahant-; Gk. megas; PIE *meg- "great") + -tudo, suffix forming abstract nouns from adjectives and participles.

Borz "height, magnitude" (it occurs also in the name of the mountain chain Alborz), related to boland "high," bâlâ "up, above, high, elevated, height," berg "mountain, hill" (Mid.Pers. buland "high;" O.Pers. baršan- "height;" Av. barəz- "high, mount," barezan- "height;" cf. Skt. bhrant- "high;" L. fortis "strong" (Fr. & E. force); 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"). Qadr, from Ar.

A scale for measuring and comparing the brightness of astronomical objects.

→ magnitude; → scale.

A survey in which the observed objects are bighter than a given → apparent magnitude.

→ magnitude; → limited; → volume.

Value of a number or of a physical quantity given roughly, usually expressed as a power of 10. Thus, 2.5 x 10⁵ and 6.4 x 10⁵ are of the same order of magnitude, and 2 x 10⁷ is 2 orders of magnitude greater than either.

→ order; → magnitude.

The magnitude of an object as measured with a photoelectric photometer.

→ photoelectric; → magnitude.

The apparent magnitude of a star as determined by measuring its brightness on a photographic plate. The photographic magnitude scale is now considered obsolete.

Adj. of → photography; → magnitude.

Magnitude defined for the combination of a photographic plate and a yellow filter, approximating the spectral sensitivity of the eye.

→ photo- + → visual; → magnitude.

The → apparent magnitude of a celestial body in the color sensitivity region of the human eye at a wavelength of 5600 Å. Visual magnitude is now essentially synonymous with V magnitude, which is determined photometrically.

→ visual; → magnitude.