Extinction is a term used in astronomy to describe the absorption and scattering of electromagnetic radiation by matter (dust and gas) between an emitting astronomical object and the observer. Interstellar extinction—also called Galactic extinction, when it occurs in the Milky Way—was first recognized as such in 1930 by Robert Julius Trumpler. However, its effects had been noted in 1847 by Friedrich Georg Wilhelm von Struve, 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.
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. This is not to be confused with the quite separate phenomenon of red shift.
General characteristics 
||This article may be too technical for most readers to understand. (December 2009)|
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, NH (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 NH and A(V) to be approximately,
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.
Measuring extinction towards an object 
To measure the extinction curve for a star, the star's spectrum is compared to the observed spectrum of a similar star known not to be affected by extinction (unreddened). It is also possible to use a theoretical spectrum instead of the observed spectrum for the comparison, but this is less common. 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. A ratio other than 2.85 must therefore be due to extinction, and the amount of extinction can thus be calculated.
The 2175-angstrom feature 
One prominent feature in measured extinction curves of many objects within the Milky Way is a broad 'bump' at about 2175 Å, 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. Several models have been presented to account for this bump which include graphitic grains with a mixture of PAH molecules. Investigations of interstellar grains embedded in interplanetary dust particles (IDP) observed this feature and identified the carrier with organic carbon and amorphous silicates present in the grains.
Extinction curves of other galaxies 
The form of the standard extinction curve depends on the composition of the ISM, which varies from galaxy to galaxy. In the Local Group, the best-determined extinction curves are those of the Milky Way, the Small Magellanic Cloud (SMC) and the Large Magellanic Cloud (LMC). In the LMC, there is significant variation in the characertistics 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. 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. 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 and finding extinction curves in the Milky Way that look more like those found in the LMC2 supershell of the LMC and in the SMC Bar 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.
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. Nevertheless, the atmosphere is completely opaque to many wavelengths requiring the use of satellites to make observations.
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 
In astronomy, interstellar reddening is a phenomenon associated with interstellar extinction where the spectrum of electromagnetic radiation from a radiation source changes characteristics from that which the object originally emitted. Reddening occurs due to the light scattering off dust and other matter in the interstellar medium. Interstellar reddening should not be confused with the redshift, which is the proportional frequency shifts of spectra without distortion. Reddening preferentially removes shorter wavelength photons from a radiated spectrum while leaving behind the longer wavelength photons (in the optical, light that is redder), leaving the spectroscopic lines unchanged.
In any photometric system interstellar reddening can be described by color excess, defined as the difference between an objects observed color index and its intrinsic color index (sometimes referred to as its normal color index). An objects intrinsic color index is the theoretical color index which it would have if unaffected by extinction. In the UBV photometric system the color excess is related to the B-V colour by:
- Trumpler, R. J. (1930). "Preliminary results on the distances, dimensions and space distribution of open star clusters". Lick Observatory Bulletin 14 (420): 154–188. Bibcode:1930LicOB.420..154T.
- Karttunen, Hannu (2003). "Fundamental astronomy". Physics and Astronomy Online Library (Springer). p. 289. ISBN 978-3-540-00179-9.
- Struve, F. G. W. 1847, St. Petersburg: Tip. Acad. Imper., 1847; IV, 165 p.; in 8.; DCCC.4.211 
- Whittet, Doug C. B. (2003). Dust in the Galactic Environment. Series in Astronomy and Astrophysics (2nd ed.). CRC Press. p. 10. ISBN 0750306246.
- See Binney and Merrifeld, Section 3.7 (1998, ISBN 978-0-691-02565-0), Carroll and Ostlie, Section 12.1 (2007, ISBN 978-0-8053-0402-2), Kutner (2003, ISBN 978-0-521-52927-3) for applications in astronomy.
- "Interstellar Reddening and Extinction". Astro.virginia.edu. 2002-04-22. Retrieved 2011-11-08.
- Gottlieb, D. M.; Upson, W.L. (1969). "Local Interstellar Reddening". Astrophysical Journal 157: 611. Bibcode:1969ApJ...157..611G. doi:10.1086/150101.
- Milne, D. K.; Aller, L.H. (1980). "An average model for the galactic absorption". Astrophysical Journal 85: 17–21. Bibcode:1980AJ.....85...17M. doi:10.1086/112628.
- Lynga, G. (1982). "Open clusters in our Galaxy". Astronomy & Astrophysics 109: 213–222. Bibcode:1982A&A...109..213L.
- Schlegel, David J.; Finkbeiner, Douglas P and Davis, Marc (1998). "Maps of Dust Infrared Emission for Use in Estimation of Reddening and Cosmic Microwave Background Radiation Foregrounds". Astrophysical Journal 500 (2): 525–553. arXiv:astro-ph/9710327. Bibcode:1998ApJ...500..525S. doi:10.1086/305772.
- Cardelli, Jason A.; Clayton, Geoffrey C. and Mathis, John S. (1989). "The relationship between infrared, optical, and ultraviolet extinction". Astrophysical Journal 345: 245–256. Bibcode:1989ApJ...345..245C. doi:10.1086/167900.
- Valencic, Lynne A.; Clayton, Geoffrey C. and Gordon, Karl D. (2004). "Ultraviolet Extinction Properties in the Milky Way". Astrophysical Journal 616 (2): 912–924. arXiv:astro-ph/0408409. Bibcode:2004ApJ...616..912V. doi:10.1086/424922.
- Mathis, John S.; Cardelli, Jason A. (1992). "Deviations of interstellar extinctions from the mean R-dependent extinction law". Astrophysical Journal 398: 610–620. Bibcode:1992ApJ...398..610M. doi:10.1086/171886.
- Fritz, T. K.; S. Gillessen; K. Dodds-Eden; D. Lutz; R. Genzel; W. Raab; T. Ott; O. Pfuhl; F. Eisenhauer and F. Yusuf-Zadeh (2011). "Line Derived Infrared Extinction toward the Galactic Center". The Astrophysical Journal 737: 73. arXiv:1105.2822. Bibcode:2011ApJ...737...73F. doi:10.1088/0004-637X/737/2/73.
- Schultz, G. V.; Wiemer; Wiemer, W. (1975). "Interstellar reddening and IR-excess of O and B stars". Astronomy and Astrophysics 43: 133–139. Bibcode:1975A&A....43..133S.
- Predehl, P.; Schmitt; Schmitt, J. H. M. M. (1995). "X-raying the interstellar medium: ROSAT observations of dust scattering halos". Astronomy and Astrophysics 293: 889–905. Bibcode:1995A&A...293..889P.
- Bohlin, Ralph C.; Blair D. Savage; J. F. Drake (1978). "A survey of interstellar H I from L-alpha absorption measurements. II". Astrophysical Journal 224: 132–142. Bibcode:1978ApJ...224..132B. doi:10.1086/156357.
- Diplas, Athanassios; Blair D. Savage (1994). "An IUE survey of interstellar H I LY alpha absorption. 2: Interpretations". Astrophysical Journal 427: 274–287. Bibcode:1994ApJ...427..274D. doi:10.1086/174139.
- Güver, Tolga; Özel,Feryal (2009). "The relation between optical extinction and hydrogen column density in the Galaxy". Monthly Notices of the Royal Astronomical Society 400: 2050–2053. arXiv:0903.2057. Bibcode:2009MNRAS.400.2050G. doi:10.1111/j.1365-2966.2009.15598.x.
- Marshall, Douglas J.; Robin, A.C., Reylé, C., Schultheis, M. and Picaud, S. (jul 2006). "Modelling the Galactic interstellar extinction distribution in three dimensions". Astronomy and Astrophysics 453 (2): 635–651. arXiv:astro-ph/0604427. Bibcode:2006A&A...453..635M. doi:10.1051/0004-6361:20053842.
- Robin, Annie C.; Reylé, C. and Derrière, S. and Picaud, S. (oct 2003). "A synthetic view on structure and evolution of the Milky Way". Astronomy and Astrophysics 409 (2): 523–540. Bibcode:2003A&A...409..523R. doi:10.1051/0004-6361:20031117.
- Cardelli, Jason A.; Sembach, Kenneth R. and Mathis, John S. (1992). "The quantitative assessment of UV extinction derived from IUE data of giants and supergiants". Astronomical Journal 104 (5): 1916–1929. Bibcode:1992AJ....104.1916C. doi:10.1086/116367. ISSN 0004-6256.
- Stecher, Theodore P. (1965). "Interstellar Extinction in the Ultraviolet". Astrophysical Journal 142: 1683. Bibcode:1965ApJ...142.1683S. doi:10.1086/148462.
- Stecher, Theodore P. (1969). "Interstellar Extinction in the Ultraviolet. II". Astrophysical Journal 157: L125. Bibcode:1969ApJ...157L.125S. doi:10.1086/180400.
- Bradley, John; Dai, ZR et al. (2005). "An Astronomical 2175 Å Feature in Interplanetary Dust Particles". Science 307 (5707): 244–247. Bibcode:2005Sci...307..244B. doi:10.1126/science.1106717. PMID 15653501.
- Gordon, Karl D.; Geoffrey C. Clayton; Karl A. Misselt; Arlo U. Landolt; Michael J. Wolff (2003). "A Quantitative Comparison of the Small Magellanic Cloud, Large Magellanic Cloud, and Milky Way Ultraviolet to Near-Infrared Extinction Curves". Astrophysical Journal 594 (1): 279–293. arXiv:astro-ph/0305257. Bibcode:2003ApJ...594..279G. doi:10.1086/376774.
- Fitzpatrick, Edward L. (1986). "An average interstellar extinction curve for the Large Magellanic Cloud". Astronomical Journal 92: 1068–1073. Bibcode:1986AJ.....92.1068F. doi:10.1086/114237.
- Misselt, Karl A.; Geoffrey C. Clayton; Karl D. Gordon (1999). "A Reanalysis of the Ultraviolet Extinction from Interstellar Dust in the Large Magellanic Cloud". Astrophysical Journal 515 (1): 128–139. arXiv:astro-ph/9811036. Bibcode:1999ApJ...515..128M. doi:10.1086/307010.
- Lequeux, J.; Maurice; Prevot-Burnichon; Prevot; Rocca-Volmerange; E. Maurice; M. L. Prevot-Burnichon; L. Prevot; B. Rocca-Volmerange (1982). "SK 143 - an SMC star with a galactic-type ultraviolet interstellar extinction". Astronomy and Astrophysics 113: L15–L17. Bibcode:1982A&A...113L..15L.
- Prevot, M. L.; Lequeux; Prevot; Maurice; Rocca-Volmerange; J. Lequex; L. Prevot; E. Maurice; B. Rocca-Volmerange (1984). "The typical interstellar extinction in the Small Magellanic Cloud". Astronomy and Astrophysics 132: 389–392. Bibcode:1984A&A...132..389P.
- Gordon, Karl D.; Geoffrey C. Clayton (1998). "Starburst-like Dust Extinction in the Small Magellanic Cloud". Astrophysical Journal 500 (2): 816–824. arXiv:astro-ph/9802003. Bibcode:1998ApJ...500..816G. doi:10.1086/305774.
- Clayton, Geoffrey C.; Karl D. Gordon; Michael J. Wolff (2000). "Magellanic Cloud-Type Interstellar Dust along Low-Density Sight Lines in the Galaxy". Astrophysical Journal Supplements Series 129 (1): 147–157. arXiv:astro-ph/0003285. Bibcode:2000ApJS..129..147C. doi:10.1086/313419.
- Valencic, Lynne A.; Geoffrey C. Clayton; Karl D. Gordon; Tracy L. Smith (2003). "Small Magellanic Cloud-Type Interstellar Dust in the Milky Way". Astrophysical Journal 598 (1): 369–374. arXiv:astro-ph/0308060. Bibcode:2003ApJ...598..369V. doi:10.1086/378802.
- Calzetti, Daniela; Anne L. Kinney; Thaisa Storchi-Bergmann (1994). "Dust extinction of the stellar continua in starburst galaxies: The ultraviolet and optical extinction law". Astrophysical Journal 429: 582–601. Bibcode:1994ApJ...429..582C. doi:10.1086/174346.
- Gordon, Karl D.; Daniela Calzetti; Adolf N. Witt (1997). "Dust in Starburst Galaxies". Astrophysical Journal 487 (2): 625–635. arXiv:astro-ph/9705043. Bibcode:1997ApJ...487..625G. doi:10.1086/304654.
Further reading 
- Binney, J. & Merrifield, M. (1998). Galactic Astronomy. Princeton: Princeton University Press. ISBN 0-691-00402-1.
- Howarth, I. D. (1983). "LMC and galactic extinction". Royal Astronomical Society, Monthly Notices 203: 301–304. Bibcode:1983MNRAS.203..301H.
- King, D. L. (1985). "Atmospheric Extinction at the Roque de los Muchachos Observatory, La Palma". RGO/La Palma technical note 31.
- 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.