Extinction (astronomy) - General Characteristics

General Characteristics

Interstellar reddening occurs because interstellar dust absorbs and scatters blue light waves more than red light waves, making stars appear redder than they are. This is similar to the effect seen when dust particles in the atmosphere of Earth contribute to red sunsets.

Broadly speaking, interstellar extinction is strongest at short wavelengths. This results in a change in the shape of an observed spectrum. Superimposed on this general shape are absorption features (wavelength bands where the intensity is lowered) that have a variety of origins and can give clues as to the chemical composition of the interstellar material, e.g. dust grains. Known absorption features include the 2175 Å bump, the diffuse interstellar bands, the 3.1 μm water ice feature, and the 10 and 18 μm silicate features.

In the solar neighborhood, the rate of interstellar extinction in the Johnson-Cousins V-band is usually taken to be 0.7-1.0 mag/kpc−simply an average due to the clumpiness of interstellar dust. In general, however, this means that a star will have its brightness reduced by about a factor of 2 in the V-band for every kiloparsec it is further away from us.

The amount of extinction can be significantly higher than this in specific directions. For example, some regions of the Galactic center have more than 30 magnitudes of extinction in the optical, meaning that less than 1 optical photon in 1012 passes through. This results in the so-called zone of avoidance, where our view of the extra-galactic sky is severely hampered, and background galaxies, such as Dwingeloo 1, were only discovered recently through observations in radio and infrared.

The general shape of the ultraviolet through near-infrared (0.125 to 3.5 μm) extinction curve in our own Galaxy, the Milky Way, is fairly well characterized by the single parameter R(V) (which is different along different lines of sight through the Galaxy), but there are known deviations from this single parameter characterization. Extending the extinction law into the mid-infrared wavelength range is difficult due to the lack of suitable targets and various contributions by absorption features. R(V) is defined to be A(V)/E(B-V), and measures the total, A(V), to selective, E(B-V) = A(B)-A(V), extinction in set bands. A(B) and A(V) are the total extinction at the B and V filter bands. Another measure used in the literature is the absolute extinction A(λ)/A(V) at wavelength λ, comparing the total extinction at that wavelength to that at the V band.

R(V) is known to be correlated with the average size of the dust grains causing the extinction. For our own Galaxy, the Milky Way, the typical value for R(V) is 3.1, but is found to be between 2.5 and 6 for different lines of sight. The relationship between the total extinction, A(V) (measured in magnitudes), and the column density of neutral hydrogen atoms column, N_H (usually measured in cm-2), shows how the gas and dust in the interstellar medium are related. From studies using ultraviolet spectroscopy of reddened stars and X-ray scattering halos in the Milky Way, Predehl and Schmitt (1995) found the relationship between N_H and A(V) to be approximately,

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Astronomers have determined the three-dimensional distribution of extinction in the solar circle of our Galaxy, using visible and near-infrared stellar observations and a model of the distribution of stars in the Galaxy. The dust giving rise to the extinction lies along the spiral arms, as observed in other spiral galaxies.