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 a _ν (with units [cm⁻¹]) enters into the definition of the optical depth. More often in astronomy, it is the mass absorption coefficient ${\kappa}_{\nu }={a}_{\nu }/\rho$ (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 one 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

• Radiative Transfer