Extinction (astronomy)

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For other uses, see Extinction (disambiguation).

Extinction is a term used in astronomy to describe the absorption and scattering of electromagnetic radiation by dust and gas between an emitting astronomical object and the observer. Interstellar extinction was first recognized as such in 1930 by Robert Julius Trumpler.[1][2] However, its effects had been noted in 1847 by Friedrich Georg Wilhelm von Struve,[3] and its effect on the colors of stars had been observed by a number of individuals who did not connect it with the general presence of galactic dust. For stars that lie near the plane of the Milky Way and are within a few thousand parsecs of the Earth, extinction in the V band is on the order of 1.8 magnitudes per kiloparsec.[4]

For Earth-bound observers, extinction arises both from the interstellar medium (ISM) and the Earth's atmosphere; it may also arise from circumstellar dust around an observed object. The strong atmospheric extinction in some wavelength regions (such as X-ray, ultraviolet, and infrared) requires the use of space-based observatories. Since blue light is much more strongly attenuated than red light, extinction causes objects to appear redder than expected, a phenomenon referred to as interstellar reddening.[5]

General characteristics[edit]



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.[7][8][9] 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 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),[11][12] but there are known deviations from this single parameter characterization.[13] 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.[14]

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,[15] but is found to be between 2.5 and 6 for different lines of sight.


\frac{N_H}{A(V)} \approx 1.8 \times 10^{21}~\mbox{atoms}~\mbox{cm}^{-2}~\mbox{mag}^{-1}

(see also:[17][18][19]).

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.[20][21] The dust giving rise to the extinction lies along the spiral arms, as observed in other spiral galaxies.

Measuring extinction towards an object[edit]


The 2175-angstrom feature[edit]


Extinction curves of other galaxies[edit]

Plot showing the average extinction curves for the MW, LMC2, LMC, and SMC Bar.[26] The curves are plotted versus 1/wavelength to emphasize the UV.


In the LMC, there is significant variation in the characteristics of the ultraviolet extinction with a weaker 2175 Å bump and stronger far-UV extinction in the region associated with the LMC2 supershell (near the 30 Doradus starbursting region) than seen elsewhere in the LMC and in the Milky Way.[27][28] In the SMC, more extreme variation is seen with no 2175 Å and very strong far-UV extinction in the star forming Bar and fairly normal ultraviolet extinction seen in the more quiescent Wing.[29][30][31]

This gives clues as to the composition of the ISM in the various galaxies. Previously, the different average extinction curves in the Milky Way, LMC, and SMC were thought to be the result of the different metallicities of the three galaxies: the LMC's metallicity is about 40% of that of the Milky Way, while the SMC's is about 10%. Finding extinction curves in both the LMC and SMC which are similar to those found in the Milky Way[26] and finding extinction curves in the Milky Way that look more like those found in the LMC2 supershell of the LMC[32] and in the SMC Bar[33] has given rise to a new interpretation. The variations in the curves seen in the Magellanic Clouds and Milky Way may instead be caused by processing of the dust grains by nearby star formation. This interpretation is supported by work in starburst galaxies (which are undergoing intense star formation episodes) that their dust lacks the 2175 Å bump.[34][35]

Atmospheric extinction[edit]


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 sources of telluric absorption are molecular oxygen and ozone, which absorb strongly in the near-ultraviolet, and water, which absorbs strongly in the 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.

Interstellar reddening[edit]


In any photometric system interstellar reddening can be described by color excess, defined as the difference between an object's observed color index and its intrinsic color index (sometimes referred to as its normal color index). An object's intrinsic color index is the theoretical color index which it would have if unaffected by extinction. In the UBV photometric system the color excess E_{B-V} is related to the B-V colour by:

E_{B-V} = (B-V)_{\textrm{observed}} - (B-V)_{\textrm{intrinsic}}\,


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Further reading[edit]

  • Howarth, I. D. (1983). "LMC and galactic extinction". Royal Astronomical Society, Monthly Notices 203: 301–304. Bibcode:1983MNRAS.203..301H. 
  • Rouleau, F.; Henning, T.; Stognienko, R. (1997). "Constraints on the properties of the 2175Å interstellar feature carrier". Astronomy and Astrophysics 322: 633–645. arXiv:astro-ph/9611203. Bibcode:1997A&A...322..633R.