When light passes through a cloud of gas, the atoms in the gas may absorb some of the photons that are trying
to pass through. As the electrons in the atoms can have only a limited set of possible energies, they can only absorb certain specific
wavelengths. The light that emerges from the cloud of gas will be "diminished" at the wavelengths that are absorbed by the gas atoms.
The spectrum shown above is for one of the stars seen through the dark cloud in the photo. There are some dark
absorption lines in the emergent radiation (the background "rainbow") caused by absorptions by the gas atoms in the cloud. In addition, the
stars seen through the dark gas cloud appear distinctly "reddish" in color because the dust in the cloud has preferentially scattered the small
wavelength blue photons (like the "blue sky" effect in our sky and in reflection nebulae).
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