Photoelectric absorption in the interstellar medium

The photoelectric effect is an important mode of photon absorption in the Galaxy. It effectively prevents photons with energies hv from 13.6 eV to about 2 keV (ultraviolet to "soft" x-ray) from traveling galactic distances in the interstellar medium. At higher photon energies, the cross sections for this process have decreased sufficiently for the interstellar medium to become transparent for distances comparable to the radius of the Galaxy. This allows one to carry out x-ray observations (>2 keV) of sources near the galactic center.

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