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

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  by Pam Graham  

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Interstellar Medium

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Even though most of outer space has a lower density of atoms than the strongest vacuums we can create in laboratories on Earth, it is far from empty. To get an idea of scale, compare the density of the air we breathe, which is about 109 molecules per cubic centimeter, with the lowest density regions of interstellar space at approximately 0.1 atoms per cubic centimeter.2 Over astronomical distances, even that extremely sparse distribution adds up to an estimated 5-10 billion solar masses, which means that interstellar matter in the Milky Way comprises several percent of the total mass of visible stars in our galaxy.

pie chart
Composition of interstellar matter

Because only a few large molecules (like the PAHs that top the list of likely DIB carriers) can survive the high radiation fields in the interstellar environment, most of this material is in the form of gas (99%) and dust (1%).

polycyclic aromatic hydrocarbons
Typical polycyclic aromatic hydrocarbons
Far from being evenly distributed among the stars, the ISM exhibits a wide range of temperatures and densities. Deep-sky images show regions where the gas and dust collects to form diffuse clouds that scatter and absorb photons coming from remote stars. Due to the typical size of interstellar dust (which corresponds to the wavelength of blue light), when starlight passes through a region rich in interstellar matter, the blue photons are more easily and efficiently scattered than the red photons. Not only is the intensity of the light decreased (interstellar (IS) extinction), it is also reddened (IS reddening).3 Because of their general correlation with dust extinction, DIBs were originally attributed to dust particles.4

Unlocking the secret of the diffuse interstellar bands is not purely a matter of astronomical observation. The field of spectral analysis offers equally valuable tools for straightening out the conundrum.

Go To Spectral Characteristics

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