dustextinction

Dust extinction refers to the dimming of light from astronomical sources caused by absorption and scattering by dust grains along the line of sight. It affects stars, star clusters, galaxies, and quasars, and is strongly wavelength dependent: blue and ultraviolet light is attenuated more than red light, producing a reddening of the observed object.

Quantitatively, extinction at a given wavelength is expressed as A_lambda, the decrease in magnitudes. A common summary is the color excess E(B-V) = (B-V)observed − (B-V)intrinsic, with the total V-band extinction given by A_V = R_V × E(B-V). The parameter R_V, the ratio of total to selective extinction, reflects dust properties and environment; in the Milky Way it averages about 3.1 but can vary from roughly 2 to 5.5.

The wavelength dependence of extinction is described by extinction curves. Prominent parametrizations include the Cardelli–Clayton–Mathis (CCM) law and the Fitzpatrick law. Extinction curves differ among the Milky Way, Large Magellanic Cloud, and Small Magellanic Cloud, notably in the presence and strength of the 2175 Å ultraviolet bump, which is strong in the Milky Way and often weaker or absent in the Magellanic Clouds.

Dust grains are typically composed of silicates, carbonaceous material, and ices, with sizes on the order of 0.01–0.5 micrometres. Both absorption and scattering contribute to extinction, and in extended or complex geometries, attenuation effects can dominate over simple extinction along a single sightline.

Observationally, extinction biases luminosity measurements and colors, influencing distance estimates and inferred stellar populations. Corrections use measured reddening (from colors or spectral energy distributions), extinction laws, Balmer decrements for nebular lines, and dust maps or 3D reddening models to estimate line-of-sight extinction.