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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\(\ {a_\nu } \) (with units [cm−1]) enters into the definition of the optical depth. More often in astronomy, it is the mass absorption coefficient \( {\kappa_\nu } = {{{{a_\nu }}} \left/ {\rho } \right.} \) (with units [cm2 g−1]) which is used.

The main sources of opacity in an astronomical context are:

  • Electron scattering (at temperatures of the order 1 billion K)

  • Electronic transitions (free-free, bound-free, or bound-bound at temperature of the order 10,000 K)

  • Molecular or dust absorption (around or below 3,000 K)

The opacity plays an essential role in radiative transfer, and thus in the energetic equilibrium of stars.

See also

Mean Free Path

Optical Depth