Extinction is a term used in astronomy to describe the absorption of light from astronomical objects by matter between them and the observer. Extinction arises both from the interstellar medium and the atmosphere. In both cases, blue light is much more strongly absorbed than red light.

Interstellar extinction

Broadly speaking, interstellar extinction varies with wavelength in a way which is generally surprisingly uniform along different lines of sight within our galaxy, and can be characterised by a standard extinction curve. However, the amount of extinction varies greatly, from almost no absorption at all to many cases where objects are entirely invisible at optical wavelengths and can only be seen in infrared or radio wavelengths.

Superimposed on the standard extinction curve are many small absorption features, which have various origins and can give clues as to the composition of the interstellar medium. One of the most important types of absorption features are the diffuse interstellar bands, about 100 of which are seen in typical stellar spectra. The origin of these bands has been a hotly disputed topic for many years, but current ideas suggest that polycyclic aromatic hydrocarbons may be responsible for most or all of them.

Calculating a standard extinction curve

Several methods can be used to calculate a standard extinction curve. One way is by comparing the spectra of stars thought to be very similar, but at different distances. The difference between the shape of the spectra will then be due only to extinction. Another way is by calculating a theoretical unreddened spectrum, and comparing it to the observed spectrum.

The 2175Å feature

One prominent feature in derived standard extinction curves in our galaxy is a broad 'bump' at about 2175Å, well into the ultraviolet region of the electromagnetic spectrum. This feature was first observed in the 1960s, but its origin is still not well understood.

Measuring extinction towards an object

Once a standard extinction curve has been obtained, the amount of extinction towards an individual object can be determined. With stars, theoretical spectra can be compared to the observed spectra to determine the amount of redenning. In the case of emission nebulae, it is common to look at the ratio of two emission lines which should not be affected by the temperature and density in the nebula. For example, the ratio of hydrogen alpha to hydrogen beta emission is always around 2.85 under a wide range of conditions prevailing in nebulae. Therefore, a ratio other than 2.85 must be due to extinction, and the amount of extinction can thus be calculated.

Extinction curves of other galaxies

The form of the standard extinction curve depends on the composition of the interstellar medium, which varies between galaxies. In the Local Group, the best-determined extinction curves are those of our Galaxy, the Small Magellanic Cloud (SMC) and the Large Magellanic Cloud (LMC). In the LMC, the 2175Å feature is less prominent than it is in our Galaxy, while ultraviolet extinction is stronger. In the SMC, ultraviolet extinction is intermediate but the 2175Å feature is absent. This gives clues as to the composition of the ISM in the various galaxies, and implies that whatever is responsible for the 2175Å feature is related to the metallicity of the galaxy: the LMC's metallicity is about 40% of that of the Milky Way, while the SMC's is about 10%.

Atmospheric extinction

Atmospheric extinction varies with location and altitude. Astronomical observatories generally are able to characterise the local extinction curve very accurately, to allow observations to be corrected for the effect.

Atmospheric extinction has three main components: Rayleigh scattering by air molecules, scattering by aerosols, and molecular absorption. Molecular absorption is often referred to as 'telluric absorption', as it is caused by the Earth (telluric is a synonym of terrestrial). The most important source of telluric absorption is ozone, which absorbs strongly in the near-infrared.

The amount of atmospheric extinction depends on the altitude of an object, being lowest at the zenith and at a maximum near the horizon. It is calculated by multiplying the standard atmospheric extinction curve by the mean airmass calculated over the duration of the observation.

References

1. Howarth I.D. (1983), LMC and galactic extinction, Royal Astronomical Society, Monthly Notices, vol. 203, Apr. 1983, p. 301-304.
2. King D.L. (1985), Atmospheric Extinction at the Roque de los Muchachos Observatory, La Palma, RGO/La Palma technical note 31
3. Rouleau F., Henning T., Stognienko R. (1997), Constraints on the properties of the 2175Å interstellar feature carrier, Astronomy and Astrophysics, v.322, p.633-645