Glossary term: Spectrum

Redirected from Spectral Line

Description: A rainbow forms when water droplets split light into elementary colors, from violet, blue, and green, to yellow, orange, and red. Each color corresponds to a range of wavelengths, and the rainbow colors are arranged in order of increasing wavelength from violet to red. This kind of decomposed light, or electromagnetic radiation in general, into different wavelengths is called a spectrum.

Electromagnetic radiation is a mixture of light particles called "photons". Creating a spectrum amounts to sorting photons by energy and documenting how many photons there are in each given energy range. By a basic law of quantum mechanics, this is equivalent to sorting light by frequency – yet another way of documenting a spectrum.

If the amount of energy varies smoothly with wavelength (or photon energy, or frequency), the spectrum is called continuous. In contrast, sharp dips or peaks in a spectrum at certain wavelengths are called absorption and emission lines, respectively. Such lines arise due to transitions between different energy levels within atoms or molecules (or even atomic nuclei), either absorbing or emitting radiation at specific wavelengths. For example, in visible light, stars show continuous spectra with absorption lines. The lines carry information about a star's chemical composition. The analysis of spectra is known as spectroscopy; instruments that allow for the recording of spectra are called spectroscopes, spectrometers, or spectrographs.

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Term and definition status: This term and its definition have been approved by a research astronomer and a teacher

The OAE Multilingual Glossary is a project of the IAU Office of Astronomy for Education (OAE) in collaboration with the IAU Office of Astronomy Outreach (OAO). The terms and definitions were chosen, written and reviewed by a collective effort from the OAE, the OAE Centers and Nodes, the OAE National Astronomy Education Coordinators (NAECs) and other volunteers. You can find a full list of credits here. All glossary terms and their definitions are released under a Creative Commons CC BY-4.0 license and should be credited to "IAU OAE".

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Credit: Fabrizio Guasconi/IAU OAE (CC BY 4.0)

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Related Diagrams

A smooth line declining at longer wavelengths with a few sharp dips.

Spectrum of an O-type star

Caption: The spectrum of the O-type star HD 235673 with wavelength in nanometers on the x-axis and flux on the y-axis. The top part of the plot shows the same spectrum but with bright patches for wavelengths with high flux and dark patches for wavelengths with low flux. The colour of the line between 400 nm and 700 nm roughly corresponds to the colour the human eye would see light of that wavelength. Below 400 nm and above 700 nm, where the human eye can see little to no light, the lines are coloured blue and red respectively. The black lines show spectral absorption lines caused by atoms and ions of different elements in the star’s atmosphere. These atoms and ions absorb at specific wavelengths, causing sharp, dark lines in the spectra. How strong these lines are depends on the temperature of the star’s atmosphere. Two stars made from the same mix of elements could have spectra with vastly different sets of lines in their spectra if they have different temperatures in their atmospheres. For O-type stars the most important features are a small number of lines caused by ionized helium. These lines are stronger in O-type stars than in cooler stars. Lines from helium atoms and hydrogen atoms also appear in the spectrum. The spectrum has more flux at the blue end of the spectrum than at the red end of the spectrum.
Credit: IAU OAE/SDSS/Niall Deacon

License: CC-BY-4.0 Creative Commons Attribution 4.0 International (CC BY 4.0) icons

Spectrum of a B-type star

Caption: The spectrum of the B-type star HD 258982. The colour of the line between 400 nm and 700 nm roughly corresponds to the colour the human eye would see light of that wavelength. Below 400 nm and above 700 nm, where the human eye can see little to no light, the lines are coloured blue and red respectively. The black lines show spectral absorption lines caused by atoms and ions of different elements in the star’s atmosphere. These atoms and ions absorb at specific wavelengths, causing sharp, dark lines in the spectra. How strong these lines are depends on the temperature of the star’s atmosphere. Two stars made from the same mix of elements could have spectra with vastly different sets of lines in their spectra if they have different temperatures in their atmospheres. For B-type stars the most important lines are caused by helium atoms. These lines are strongest in B-type stars and weaker in hotter and cooler types. Lines from hydrogen atoms are also present but are not as strong as in cooler A-type stars.
Credit: IAU OAE/SDSS/Niall Deacon

License: CC-BY-4.0 Creative Commons Attribution 4.0 International (CC BY 4.0) icons

A smooth line peaking about 420 nm then declining at longer wavelengths with a few fairly broad dips.

Spectrum of an A-type star

Caption: The spectrum of the A-type star BD-11 1212. The colour of the line between 400 nm and 700 nm roughly corresponds to the colour the human eye would see light of that wavelength. Below 400 nm and above 700 nm, where the human eye can see little to no light, the lines are coloured blue and red respectively. The black lines show spectral absorption lines caused by atoms and ions of different elements in the star’s atmosphere. These atoms and ions absorb at specific wavelengths, causing sharp, dark lines in the spectra. How strong these lines are depends on the temperature of the star’s atmosphere. Two stars made from the same mix of elements could have spectra with vastly different sets of lines in their spectra if they have different temperatures in their atmospheres. Lines from hydrogen atoms dominate the spectra of A-type stars and are strongest at this spectral type.
Credit: IAU OAE/SDSS/Niall Deacon

License: CC-BY-4.0 Creative Commons Attribution 4.0 International (CC BY 4.0) icons

A relatively smooth line peaking about 430 nm then declining at longer wavelengths with a few fairly broad dips.

Spectrum of an F-type star

Caption: The spectrum of the F-type star 2MASS J22243289+4937443. The colour of the line between 400 nm and 700 nm roughly corresponds to the colour the human eye would see light of that wavelength. Below 400 nm and above 700 nm, where the human eye can see little to no light, the lines are coloured blue and red respectively. The black lines show spectral absorption lines caused by atoms and ions of different elements in the star’s atmosphere. These atoms and ions absorb at specific wavelengths, causing sharp, dark lines in the spectra. How strong these lines are depends on the temperature of the star’s atmosphere. Two stars made from the same mix of elements could have spectra with vastly different sets of lines in their spectra if they have different temperatures in their atmospheres. The lines from hydrogen atoms that are strongest in A-type stars are still relatively strong in F-type stars but lines from metals, particularly ionised calcium begin to become strong at this spectral type.
Credit: IAU OAE/SDSS/Niall Deacon

License: CC-BY-4.0 Creative Commons Attribution 4.0 International (CC BY 4.0) icons

A quite ragged line peaking about 470 nm then declining at longer wavelengths with a few deeper dips.

Spectrum of a G-type star

Caption: The spectrum of the G-type star UCAC4 700-069569. The colour of the line between 400 nm and 700 nm roughly corresponds to the colour the human eye would see light of that wavelength. Below 400 nm and above 700 nm, where the human eye can see little to no light, the lines are coloured blue and red respectively. The black lines show spectral absorption lines caused by atoms and ions of different elements in the star’s atmosphere. These atoms and ions absorb at specific wavelengths, causing sharp, dark lines in the spectra. How strong these lines are depends on the temperature of the star’s atmosphere. Two stars made from the same mix of elements could have spectra with vastly different sets of lines in their spectra if they have different temperatures in their atmospheres. In G-type stars lines from hydrogen atoms are weaker than in F-type stars and lines from ionised calcium stronger. Lines from metal atoms such as atoms of iron, sodium and calcium also begin to become prominent.
Credit: IAU OAE/SDSS/Niall Deacon

License: CC-BY-4.0 Creative Commons Attribution 4.0 International (CC BY 4.0) icons

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License: CC-BY-4.0 Creative Commons Attribution 4.0 International (CC BY 4.0) icons
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