Extinction and reddening
The interstellar medium in nowhere completely transparent and nowhere completely opaque; a dusty region of space dims rather than blocks the- light passing through it. Thin dimming process is called INTERSTELLAR EXTINCTION. Interstellar extinction in front of a star can be expressed quantitatively, and is usually given the symbol Av whore Av is the number of magnitudes by which the star’s light in dimmed by the dust at visual wavelengths. For example, a star behind a dust cloud with Av = 2 would appear two magnitudes fainter than if the dust cloud had not intervened. The extinction due to the interstellar dust varies enormously from place to place. In the plane of the Galaxy, Av is typically two magnitudes for every kiloparsec the light travels through it, but there are some clouds that are so dusty that Av reaches values of several hundred magnitudes per parsec.

The amount by which light is dimmed by the interstellar medium depends on its wavelength. Red light suffers less extinction than blue light with the result that the apparent colour of a star is altered if it is observed through a patch of dust. This phenomenon is called INTERSTELLAR REDDENING, and it is a consequence of the fact that the particles causing the extinction are smaller than the wavelength of light. A similar kind of reddening occurs in the Earth’s atmosphere, and is the cause of the Sun’s apparent daily colour changes as it rises and sets. The reddening effect of the Earth’s atmosphere is small when the Sun is high in the sky; in the evening, however, when sunlight has to travel a long distance through the atmosphere at a shallow angle, the effect be-becomes more pronounced and the setting Sun appears orange rather than white.

Interstellar extinction and reddening are important in the infrared and ultraviolet parts of the spectrum as well as the visible. In this context the term ‘reddening’ is used, with some chromatic licence, to indicate the relative enhancement of long-wavelength over short-wavelength radiation. The extinction (A ?) varies with wavelength from 0.1 to l0 ? m. The curve shows that at short ultraviolet wavelengths the extinction is several times larger than for visible light, while longward of 2 ? m it is reduced by a factor of at least ten. This dramatic improvement in the transparency of the interstellar medium at long wavelengths is of great importance as it allows astronomers to see through layers of dust with Av = 50 magnitudes or more, which would be impenetrable at shorter wavelengths. Several ‘bumps’ have been found in the interstellar extinction curve, and others will probably be discovered as techniques improve. The three features shown in figure 13.4 are at wavelengths where graphite, ice and silicate particles preferentially absorb radiation, but these chemical identifications are not yet certain.

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