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Opacity

Definition

Opacity measures the property of a medium to attenuate light. The opacity depends on the composition of the medium, its density, and temperature, but also on the wavelength considered. The absorption coefficient  aν (with units [cm−1]) enters into the definition of the optical depth. More often in astronomy, it is the mass absorption coefficient (with units [cm2 g−1]) which is used.

The main sources of opacity in an astronomical