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e-Journal

 

Diffuse Interstellar Bands: A Cosmic Mystery
(Released November 2009)

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  by Pam Graham  

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There is an unidentified light thief in the interstellar medium (ISM). Out there in the vast spaces between all the stars we can see, in every direction we can aim a telescope, there lurks a mystery material that has evaded positive identification for almost 90 years.

Back in 1922, Mary Lea Heger, a graduate student at Berkeley's Lick Observatory, photographed two peculiar features in the spectra of distant binary stars. These features appeared as absorption lines - dark lines resulting from the absence of photons at distinct frequencies in a continuous spectrum - but they were much wider and more diffuse than the atomic spectral lines ordinarily seen in stars. They also appeared to be stationary, even though spectral lines produced by stars in a binary system should shift back and forth in wavelength due to the Doppler effect.

observatory
Berkley's Lick Observatory where Diffuse Interstellar Bands were discovered in 1929

By 1938, astronomer Paul Merrill had confirmed the unidentified nature of these broad absorption features, called absorption bands, and established their origin in interstellar rather than circumstellar regions.

For the remainder of the twentieth century, the puzzle of the diffuse interstellar bands (DIB) continued to baffle scientists. A huge amount of observational, theoretical and laboratory effort has been applied to the attempt at identifying the carriers responsible for the roughly 300 DIBs currently verified in the wide optical spectrum between the ultraviolet and infrared.

diffuse interstellar bands
Diffuse Interstellar Bands

The difficulty in identifying the DIB carriers is surprising, given that spectra of other components of the ISM, such as H, Na, K and Ca+, are easily reproduced and matched in the laboratory. Yet not one of the bands has been positively assigned.1

Most researchers today believe that organic matter, probably in the form of large polycyclic aromatic hydrocarbon (PAH) structures, are responsible for the absorptions.

comparison chart
Similarity between the DIBs and spectral data on PAHs from Ames Research Center's astrochemistry lab and especially Dr. Farid Salama

To better understand the challenges involved and the motivation for solving this outstanding astrophysical problem, it's helpful to take a closer look at the ISM and the spectroscopic concepts employed in the quest.

Go To Interstellar Medium

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