astronomy Astronomy () is a natural science that studies astronomical object, celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and chronology of the Universe, evolution. Objects of interest ...

, a spectral atlas is a collection of spectra of one or more objects, intended as a reference work for comparison with spectra of other objects. Several different types of collections are titled spectral atlases: those intended for spectral classification, for key reference, or as a collection of spectra of a general type of object. In any spectral atlas, generally all the spectra have been taken with the same equipment, or with very similar instruments at different locations, to provide data as uniform as possible in its

spectral resolution The spectral resolution of a spectrograph, or, more generally, of a frequency spectrum, is a measure of its ability to resolve features in the electromagnetic spectrum. It is usually denoted by \Delta\lambda, and is closely related to the resolvi ...

wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tro ...

coverage,

noise Noise is unwanted sound considered unpleasant, loud or disruptive to hearing. From a physics standpoint, there is no distinction between noise and desired sound, as both are vibrations through a medium, such as air or water. The difference arise ...

characteristics, etc.

Types

For spectral classification

When assigning a

spectral classification In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the ...

, a spectral atlas is a collection of standard spectra of stars with known spectral types, against which a spectrum of an unknown star is compared. It is analogous to an

identification key In biology, an identification key, taxonomic key, or biological key is a printed or computer-aided device that aids the identification of biological entities, such as plants, animals, fossils, microorganisms, and pollen grains. Identification k ...

in biology. Originally, such atlases included reproductions of the

monochrome A monochrome or monochromatic image, object or palette is composed of one color (or values of one color). Images using only shades of grey are called grayscale (typically digital) or black-and-white (typically analog). In physics, monochrom ...

spectra as recorded on

photographic plate Photographic plates preceded photographic film as a capture medium in photography, and were still used in some communities up until the late 20th century. The light-sensitive emulsion of silver salts was coated on a glass plate, typically thinn ...

s, as in the original Morgan-Keenan-Kellman atlas and other atlases. These atlases include identifications and notations for use of those spectral features to be used as discriminators between close spectral types. With very large surveys of the sky which include automated assignment of spectral classification from the digital spectra data, graphical atlases have been supplanted by libraries of spectra of standard stars which often can be downloaded from

VizieR A vizier (; ar, وزير, wazīr; fa, وزیر, vazīr), or wazir, is a high-ranking political advisor or minister in the near east. The Abbasid caliphs gave the title ''wazir'' to a minister formerly called ''katib'' (secretary), who was a ...

and other sources.

For key reference

A spectral atlas can be a very high-quality spectrum of a key reference object, often made with very high spectral resolution, generally presented in large-format graphical form as a

line chart A line chart or line graph or curve chart is a type of chart which displays information as a series of data points called 'markers' connected by straight line segments. It is a basic type of chart common in many fields. It is similar to a s ...

(but normally strictly without markers at specific data points) of intensity or relative intensity (which for a star whose spectrum is dominated by

absorption lines A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from emission (electromagnetic radiation), emission or absorption (electromagnetic radiation), absorption of light in a narrow frequency range, ...

runs from zero to a normalized continuum) as a function of wavelength. Such spectral atlases have been made several times for the

Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...

(''e.g.,''),

Arcturus , - bgcolor="#FFFAFA" , Note (category: variability): , , H and K emission vary. Arcturus is the brightest star in the northern constellation of Boötes. With an apparent visual magnitude of −0.05, it is the third-brightest of th ...

, other bright stars, planets, quasars, etc. Unlike classification atlases, such atlases made from data taken with photographic media were digitized with a

densitometer A densitometer is a device that measures the degree of darkness (the optical density) of a photographic or semitransparent material or of a reflecting surface. The densitometer is basically a light source aimed at a photoelectric cell. It determ ...

and the digitized result is presented. Sometimes these atlases include identifications of spectral features, but often such identifications are presented in tabular form in separate publications (''e.g.,'').

For objects of a general type

A spectral atlas can be a collection of spectra of different individual objects of a general type, showing the variety of spectra inherent to the type, or a collection of spectra from a single object that varies over time, showing the variations in the spectra.

Notes

External links

Morgan-Keenan-Kellman spectral atlas, at University College London

BASS2000: queryable solar spectral atlas, at Observatoire de Paris/Meudon

of

Hα H-alpha (Hα) is a specific deep-red visible spectral line in the Balmer series with a wavelength of 656.28 nm in air and 656.46 nm in vacuum; it occurs when a hydrogen electron falls from its third to second lowest energy level. H-alpha ...

profiles of

Be star Be stars are a heterogeneous set of stars with B spectral types and emission lines. A narrower definition, sometimes referred to as ''classical Be stars'', is a non-supergiant B star whose spectrum has, or had at some time, one or more Balmer e ...

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