3.1.3 Observing through the interstellar medium
Material in the interstellar medium absorbs radiation. An extra term, A, the absorption in magnitudes, is required in Equation C:
Radiation is both scattered and absorbed by interstellar matter. The combined effect of scattering and absorption is called extinction.
Atoms (and molecules) can be excited by collisions as well as by absorption of photons.
Molecules have quantized vibrational and rotational energy states in addition to electron energy states. The energy gaps for vibrational and rotational states are generally much smaller than for electronic states so photoexcitation of (and photoemission from) vibrational states occurs at infrared wavelengths and of rotational states at microwave wavelengths.
Interstellar dust causes greater extinction at short wavelengths. Distant stars therefore appear fainter and redder due to interstellar extinction.
The properties of the interstellar medium itself can be inferred from its effects on starlight.