An Etymological Dictionary of Astronomy and Astrophysics
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1) The → exponent of the → frequency on which depends the intensity of the → continuum emission, that is: F_ν∝ ν^α. 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.
2) The ratio α_IR = dlog(λF_λ)/dlogλ, where F represents the flux and λ the wavelength, in the range 2.2 μm ≤ λ ≤ 25 μm, particularly used in the classification of → protostars (→ Class I, → Class II, and → Class III).

→ spectral; → index.

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.

→ spectral; → line.

The observable spectral range provided by a spectroscope, as determined by the grating dispersion, camera focal length, and detector size.

→ spectral; → range.

An extent of wavelengths into which the electromagnetic spectrum is divided; e.g. infrared or ultraviolet region.

→ spectral; → region.

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.

→ spectral; → resolution.

Domain of the electromagnetic spectrum over which a detector is sensitive. Same as spectral sensitivity.

→ spectral; → response.

Spectral lines or group of lines occurring in sequence.

→ spectral; → series.

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.

→ spectral; → synthesis.

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.

→ spectral; → type.

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.

→ spectral; → variability.

A combining form representing → spectrum in compound words.
→ spectrogram, → spectrograph, → spectroheliogram, → spectroheliograph, → spectrometer, → spectrophotometer, → spectropolarimeric, → spectropolarimetry, → spectroscope, → spectroscopy, → spectroscopic.

→ spectrum

The → extreme adaptive optics system and → coronagraphic facility at the → European Southern Observatory (ESO) → Very Large Telescope (VLT) (UT3) available from May 2014. Its primary science goal is imaging, low-resolution spectroscopic, and polarimetric characterization of → exoplanetary system at → visible and → near-infrared wavelengths (0.5-2.32 μm). SPHERE is capable of obtaining → diffraction-limited images at 0''.02 to 0''.08 resolution depending on the wavelength. Its → spectral resolution is 30 to 350, depending on the mode.

→ spectro-; → polarimetric; → high; → contrast; → exoplanet.

A plot of the intensity of light at different wavelengths obtained using a spectrograph.

→ spectro-; → -gram.

An instrument that disperses the light into spectral lines and records them.

→ spectro-; → -graph.

An image of the Sun taken in the light of one particular wavelength.

→ spectro-; → heliogram.

An instrument for recording monochromatic images of the Sun.

→ spectro-; → heliograph.

1) A spectrograph in which the spectrum is recorded by electronic means so that wavelength, intensity, etc. can be measured.
2) An instrument for determining the distribution of energies in a beam of particles.

→ spectro-; → -meter.

An instrument designed to measure the intensity of a particular spectral line or a series of spectral lines.

→ spectro-; → photometer.

Of or relating to → spectrophotometry.

→ spectrum; → photometry.

In astronomy, measurement of the absolute fluxes of the components of different frequencies in the spectrum of a light source.

→ spectrum; → photometry.