Opacity

Opacity measures how strongly matter absorbs or scatters radiation as it passes through. It depends on the material, radiation wavelength, and physical conditions.

Absorption coefficient: A measure of the rate at which a medium absorbs radiation per unit path length. It quantifies the absorptive contribution to opacity along a radiation path.
Extinction coefficient: A quantity describing attenuation of radiation through absorption and scattering in a material. It combines the two interaction channels that contribute to opacity.
Stellar structure: The internal arrangement and physical state of a star, governed by gravity, pressure, energy generation, and transport. Stellar opacity regulates energy transport and helps set a star’s internal temperature profile.
Transmittance: The fraction of incident radiation that passes through a material without being reflected or absorbed. Transmittance describes the surviving radiation, whereas opacity describes its attenuation.
Scattering coefficient: A measure of the rate at which particles redirect radiation per unit path length. Scattering contributes to opacity by redirecting photons out of their original paths.
Mass attenuation coefficient: An attenuation coefficient divided by material density, expressing radiation attenuation per unit mass thickness. It is a common mass-specific way to express opacity in radiation physics.
Stellar atmosphere: The outer gaseous layers of a star from which most observed radiation emerges. Opacity determines which atmospheric layers contribute to the spectrum we observe.
Reflectance: The fraction of incident radiation reflected by a surface or material. Reflection can reduce transmitted intensity without being the same as absorption or opacity.
Optical depth: A dimensionless measure of the cumulative attenuation of radiation along a path through matter. Integrating opacity along a path gives the optical depth that determines transmission.
Rosseland mean opacity: A frequency-averaged opacity weighted for radiative diffusion in an optically thick medium. This average is suited to energy transport when radiation undergoes many interactions.

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