early spectral class star
setâré bâ rade-ye binâbi-ye âqâzin
Fr.: étoile de type spectral précoce
power spectral density
cagâli-ye binâbi-ye tavân
Fr.: densité spectrale de puissance
Same as → spectral density.
Of or pertaining to a → spectrum.
radebandi-ye binâbi (#)
Fr.: classification spectrale
A system that assigns a → spectral type to a star according to characteristics of its spectrum. The earliest attempt to divide stars on the basis of their spectra was the → Secchi classification in the 1860s. This scheme paved the way for the → Harvard classification that led to the current → Morgan-Keenan classification of spectral types. In the Harvard system stars were originally thought to follow an evolutionary sequence from the "early" O and B types to the "late" K and M types. Although this is now known to be wrong, the terms → early-type star and → late-type star are still in use. In the Morgan-Keenan system stars are classified as type O, B, A, F, G, K, or M in order of decreasing → effective temperature, and each type further subdivided into subclasses from 0 (hottest, except for → O-type stars) to 9 (coolest). They are also accompanied by a → luminosity class. In the late 1990s, spectral types L and T were added to the sequence to accommodate the coolest stars and → brown dwarfs (with class Y reserved for the coolest brown dwarfs of all, as yet unobserved).
Fr.: couverture spectrale
Fr.: densité spectrale
For a specified → bandwidth of radiation consisting of a continuous → frequency spectrum, the total → power in the bandwidth divided by the bandwidth. Spectral density describes how the power (or variance) of a time series is distributed with frequency. Also called power spectral density.
spectral energy distribution (SED)
vâbâžeš-e kâruž-e binâbi
Fr.: distribution de l'énergie spectrale
A plot showing the energy emitted by a source as a function of the radiation
wavelength or frequency. It is used in many branches of astronomy to characterize
astronomical sources, in particular mainly in → near infrared
and → middle infrared to study
→ protostars or
→ young stellar objects. The SED of these objects is
divided in four classes.
ârang-e binâb, ~ binâbi
Fr.: motif spectral
An emission or absorption mark in the spectrum of an astronomical object, of known or unknown origin, usually with complex structure.
Fr.: indice spectral
1) The → exponent
of the → frequency on which depends
the intensity of the → continuum emission, that is:
The exponent (α) typically
takes positive values from 0 to 2 for → thermal emission,
while → non-thermal emission, such as
→ synchrotron radiation,
leads to negative values of the spectral index ranging from about -0.5 to -1.5.
Fr.: raie spectrale
A dark or bright line in an otherwise uniform and continuous spectrum, resulting from an excess or deficiency of photons in a narrow wavelength range, compared with the nearby wavelengths.
Fr.: domain spectral
The observable spectral range provided by a spectroscope, as determined by the grating dispersion, camera focal length, and detector size.
Fr.: région spectrale
An extent of wavelengths into which the electromagnetic spectrum is divided; e.g. infrared or ultraviolet region.
vâgošud-e binâb, ~ binâbi
Fr.: résolution spectrale
The capacity of a spectrograph to separate two adjacent spectral lines. The theoretical spectral resolution depends on the grating dispersion, grating position, pixel size, collimator and camera focal length, and the entrance slit-width.
Fr.: réponse spectrale
Domain of the electromagnetic spectrum over which a detector is sensitive. Same as spectral sensitivity.
Fr.: série spectrale
Spectral lines or group of lines occurring in sequence.
Fr.: synthèse spectrale
The process of computing line strengths in stellar spectra using an appropriate stellar atmosphere model, atomic and molecular data, and the numerical solution of the → radiative transfer equation at each point in the spectrum.
Fr.: type spectral
A group into which stars may be classified according to the characteristics of their spectra. Spectral type correlates with the star's → effective temperature and → color. There are seven main spectral types. From hot and blue to cool and red, they are O, B, A, F, G, K, and M. Each spectral type is divided into several subtypes. For example, from warmest to coolest, spectral type G is G0, G1, G2, G3, and so on to G9. A precise → spectral classification requires determining the → luminosity class. The Sun is spectral type G2 V.
Fr.: variabilité spectrale
The state of a spectrum from an astronomical object in which the lines change with time as far as their intensity, profile, and wavelength are concerned.