An Etymological Dictionary of Astronomy and Astrophysics
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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.

Of or relating to → spectropolarimetry.

→ spectropolarimetry; → -ic.

A technique of observation in → astrophysics which combines → spectroscopy and → polarization measurements. Spectropolarimetry has a wide range of applications in astrophysics, including → stellar magnetic field studies. → ESPaDOnS, → HARPSpol.

→ spectro-; → polarimetry.

An optical instrument for forming and examining the spectrum of a light source. The instrument contains a narrow slit through which the light enters. The slit is placed at the focus of a positive lens called the collimator lens to form a beam of parallel rays. The beam of light falls on a dispersing element (prism, grating, or grism) which separates the light into its colors. This spectrum can be observed with an ocular (in the spectroscope) or recorded on a detector (in the spectrograph).

→ spectro-; → -scope.

Of or relating to → spectroscopy.

→ spectro-; → -scopy; → -ic.

A binary system that cannot be resolved by a telescope, but can be identified by means of the Doppler shift of the spectral lines. As stars revolve, they alternately approach and recede in the line of sight. This motion is shown up in their spectra as a periodic oscillation or doubling of spectral lines.

→ spectroscopic; → binary.

The situation in which spectroscopic features in a certain optical region are not sensitive enough to distinguish adjacent → luminosity classes, for instance → dwarf stars from → giant stars.

→ spectroscopic; → degeneracy.

A spacial → Hertzsprung-Russell diagram (HRD) which is independent of distance and extinction measurements. The sHRD is derived from the classical HRD by replacing the luminosity (L) to the quantity ℒ = T ⁴_eff/g which is the inverse of the flux-weighted gravity introduced by Kudritzki et al. (2003). The value of ℒ can be calculated from stellar atmosphere analyses without prior knowledge of the distance or the extinction. In contrast to the classical T_eff-log g diagram (→ Kiel diagram), the sHRD sorts stars according to their proximity to the → Eddington limit, because ℒ is proportional to the Eddington factor Γ = L/L_Edd according to the relation ℒ = (1/4πσG)(L/M) = (c/(σκ)Γ, where σ is the → Stefan-Boltzmann constant, κ is the electron → scattering  → opacity in the stellar envelope, and the other symbols have their usual meanings (Langer, N., Kudritzki, R. P., 2014, A&A 564, A52, arXive:1403.2212, Castro et al., 2014, A&A 570, L13.

→ spectroscopic; → H-R diagram.

The stellar mass derived from → gravity (g) and radius (R), expressed by M = gR²/G, where G is the → gravitational constant. Spectroscopic mass conveys the actual mass of the star, in contrast with its → initial mass.

→ spectroscopic; → mass.