Editing Extinction (astronomy) (section)

==General characteristics==
Interstellar reddening occurs because [[Cosmic dust|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 [[Sunset#Colors|contribute to red sunsets]].<ref>{{cite web | url=http://faculty.virginia.edu/skrutskie/ASTR1210/notes/redden.html | title=Interstellar Reddening, Extinction, and Red Sunsets | publisher=Astro.virginia.edu | date=2002-04-22 | access-date=2017-07-14 | archive-date=2017-11-22 | archive-url=https://web.archive.org/web/20171122060346/http://faculty.virginia.edu/skrutskie/ASTR1210/notes/redden.html | url-status=dead }}</ref>

Broadly speaking, interstellar extinction is strongest at short wavelengths, generally observed by using techniques from spectroscopy. Extinction 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&nbsp;[[Angstrom|Å]] bump, the [[diffuse interstellar band]]s, the 3.1&nbsp;[[μm]] water ice feature, and the 10 and 18&nbsp;μm [[silicate]] features.

In the [[solar neighborhood]], the rate of interstellar extinction in the [[UBV photometric system|Johnson–Cousins V-band (visual filter)]] averaged at a wavelength of 540&nbsp;nm is usually taken to be 0.7–1.0 mag/kpc−simply an average due to the ''clumpiness'' of interstellar dust.<ref>{{Cite journal
  | last = Gottlieb
  | first = D. M.
  | author2 = Upson, W.L.
  | title = Local Interstellar Reddening
  | journal = Astrophysical Journal
  | date = 1969
  | volume = 157
  | page = 611
  | bibcode = 1969ApJ...157..611G
  | doi = 10.1086/150101
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  }}</ref><ref>{{Cite journal
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  | first = D. K.
  | author2 = Aller, L.H.
  | title = An average model for the galactic absorption
  | journal = Astrophysical Journal
  | date = 1980
  | volume = 85
  | pages = 17–21
  | bibcode = 1980AJ.....85...17M
  | doi = 10.1086/112628
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  }}</ref><ref>{{Cite journal
  | last = Lynga
  | first = G.
  | title = Open clusters in our Galaxy
  | journal = Astronomy & Astrophysics
  | date = 1982
  | volume = 109
  | pages = 213–222
  | bibcode = 1982A&A...109..213L
}}</ref> In general, however, this means that a star will have its brightness reduced by about a factor of 2 in the V-band viewed from a good night sky vantage point on earth for every [[Parsec|kiloparsec]] (3,260 light years) it is farther away from us.

The amount of extinction can be significantly higher than this in specific directions. For example, some regions of the [[Galactic Center]] are awash with obvious intervening dark dust from our spiral arm (and perhaps others) and themselves in a bulge of dense matter, causing as much as more than 30 magnitudes of extinction in the optical, meaning that less than 1 optical photon in 10<sup>12</sup> passes through.<ref>{{Cite journal
  | last = Schlegel
  | first = David J.
  | author-link = David J. Schlegel
  | author2 = Finkbeiner, Douglas P
  | author3-link = Marc Davis (astronomer)
 | author3 = Davis, Marc
  | title = Maps of Dust Infrared Emission for Use in Estimation of Reddening and Cosmic Microwave Background Radiation Foregrounds
  | journal = Astrophysical Journal
  | date = 1998
  | volume = 500
  | issue = 2
  | pages = 525–553
  | bibcode = 1998ApJ...500..525S
  | doi = 10.1086/305772
  | arxiv = astro-ph/9710327
| s2cid = 59512299
 | author2-link = Douglas P. Finkbeiner
 }}</ref> This results in the [[Zone of Avoidance|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 astronomy|radio]] and [[Infrared astronomy|infrared]].

The general shape of the ultraviolet through near-infrared (0.125 to 3.5 μm) extinction curve (plotting extinction in magnitude against wavelength, often inverted) looking from our vantage point at other objects in the [[Milky Way]], is fairly well characterized by the stand-alone parameter of relative visibility (of such visible light) R(V) (which is different along different lines of sight),<ref name="ca89">{{Cite journal
  | last = Cardelli
  | first = Jason A.
  | author-link = Jason A. Cardelli
  | author2 = Clayton, Geoffrey C.
  | author3-link = John S. Mathis
 | author3 = Mathis, John S.
  | title = The relationship between infrared, optical, and ultraviolet extinction
  | journal = Astrophysical Journal
  | date = 1989
  | volume = 345
  | pages = 245–256
  | bibcode = 1989ApJ...345..245C
  | doi = 10.1086/167900
| author2-link = Geoffrey C. Clayton
 }}</ref><ref>{{Cite journal
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  | author2 = Clayton, Geoffrey C.
  | author3-link = Karl D. Gordon
 | author3 = Gordon, Karl D.
  | title = Ultraviolet Extinction Properties in the Milky Way
  | journal = Astrophysical Journal
  | date = 2004
  | volume = 616
  | issue = 2
  | pages = 912–924
  | bibcode = 2004ApJ...616..912V
  | doi = 10.1086/424922
  | arxiv = astro-ph/0408409
| s2cid = 119330502
 | author2-link = Geoffrey C. Clayton
 }}</ref> but there are known deviations from this characterization.<ref>{{Cite journal
  | last = Mathis
  | first = John S.
  | author-link = John S. Mathis
  | author2 = Cardelli, Jason A. |author-link2=Jason A. Cardelli 
  | title = Deviations of interstellar extinctions from the mean R-dependent extinction law
  | journal = Astrophysical Journal
  | date = 1992
  | volume = 398
  | pages = 610–620
  | bibcode = 1992ApJ...398..610M
  | doi = 10.1086/171886
}}</ref> 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.<ref>{{Cite journal
  |author1=[[T. K. Fritz]] |author2=[[S. Gillessen]] |author3=[[K. Dodds-Eden]] |author4=[[D. Lutz]] |author5=[[Reinhard Genzel|R. Genzel]] |author6=[[W. Raab]] |author7=[[T. Ott]] |author8=[[O. Pfuhl]] |author9=[[F. Eisenhauer]] |author10=[[F. Yusuf-Zadeh]] | title = Line Derived Infrared Extinction toward the Galactic Center
  | journal = The Astrophysical Journal
  | date = 2011
  | volume = 737
  | issue = 2
  | page = 73
  | bibcode = 2011ApJ...737...73F
  | doi = 10.1088/0004-637X/737/2/73
  | arxiv = 1105.2822
| s2cid = 118919927
 }}</ref>

R(V) compares aggregate and particular extinctions. It is 
<math display="block">A_{V} / E(B-V)\,</math>

Restated, it is the total extinction, A(V) divided by the selective total extinction (A(B)−A(V)) of those two wavelengths (bands). A(B) and A(V) are the ''total extinction'' at the [[UBV photometric system|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 the Milky Way Galaxy, the typical value for R(V) is 3.1,<ref>{{Cite journal
  | last1 = Schultz
  | first1 = G. V.
  | author-link = G. V. Schultz
  | first2 = W.
  | last2 = Wiemer
  | author-link2 = W. Wiemer
  | title = Interstellar reddening and IR-excess of O and B stars
  | journal = Astronomy and Astrophysics
  | date = 1975
  | volume = 43
  | pages = 133–139
  | bibcode = 1975A&A....43..133S
}}</ref> but is found to vary considerably across different lines of sight.<ref name="ma16">{{Cite journal
  | last = Majaess
  | first = Daniel
  | author-link = Daniel Majaess
  | author2 = David Turner
  | author3-link = Istvan Dekany
 | author3 = Istvan Dekany
  | author4-link = Dante Minniti
 | author4 = Dante Minniti
  | author5-link = Wolfgang Gieren
 | author5 = Wolfgang Gieren
  | title = Constraining dust extinction properties via the VVV survey
  | journal = Astronomy and Astrophysics
  | date = 2016
  | volume = 593
  | pages = A124
  | bibcode = 2016A&A...593A.124M
  | doi = 10.1051/0004-6361/201628763
| arxiv = 1607.08623
  | s2cid = 54218060
 | author2-link = David G. Turner
 }}</ref> As a result, when computing cosmic distances it can be advantageous to move to star data from the near-infrared (of which the filter or passband Ks is quite standard) where the variations and amount of extinction are significantly less, and similar ratios as to R(Ks):<ref>R(Ks) is, mathematically likewise, A(Ks)/E(J−Ks)</ref> 0.49±0.02 and 0.528±0.015 were found respectively by independent groups.<ref name="ma16" /><ref name="ni09">{{Cite journal
  | last = Nishyiama
  | first = Shogo
  | author-link = Shogo Nishiyama
  | author2 = Motohide Tamura
  | author3-link = Hirofumi Hatano
 | author3 = Hirofumi Hatano
  | author4-link = Daisuke Kato
 | author4 = Daisuke Kato
  | author5-link = Toshihiko Tanabe
 | author5 = Toshihiko Tanabe
  | author6-link = Koji Sugitani
 | author6 = Koji Sugitani
  | author7-link = Tetsuya Nagata
 | author7 = Tetsuya Nagata
  | title = Interstellar Extinction Law Toward the Galactic Center III: J, H, KS Bands in the 2MASS and the MKO Systems, and 3.6, 4.5, 5.8, 8.0&nbsp;μm in the Spitzer/IRAC System
  | journal = The Astrophysical Journal
  | date = 2009
  | volume = 696
  | issue = 2
  | pages = 1407–1417
  | bibcode = 2009ApJ...696.1407N
  | doi = 10.1088/0004-637X/696/2/1407
| arxiv = 0902.3095
  | s2cid = 119205751
 | author2-link = Motohide Tamura
 }}</ref>  Those two more modern findings differ substantially relative to the commonly referenced historical value ≈0.7.<ref name="ca89" />

The relationship between the total extinction, A(V) (measured in [[magnitude (astronomy)|magnitude]]s), and the [[column density]] of neutral [[hydrogen]] atoms column, N<sub>H</sub> (usually measured in cm<sup>−2</sup>), 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<ref>{{Cite journal
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  | last2 = Schmitt
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  | author-link2 = J. H. M. M. Schmitt
  | title = X-raying the interstellar medium: ROSAT observations of dust scattering halos
  | journal = Astronomy and Astrophysics
  | date = 1995
  | volume = 293
  | pages = 889–905
  | bibcode = 1995A&A...293..889P
}}</ref> found the relationship between N<sub>H</sub> and A(V) to be approximately:

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

(see also:<ref>{{Cite journal
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 }}</ref><ref>{{Cite journal
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  | title = An IUE survey of interstellar H I LY alpha absorption. 2: Interpretations
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  }}</ref><ref>{{Cite journal
  | last = Güver
  | first = Tolga
  | author-link = Tolga Güver
  | author2 = Özel, Feryal
  | title = The relation between optical extinction and hydrogen column density in the Galaxy
  | journal = [[Monthly Notices of the Royal Astronomical Society]]
  | date = 2009
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  | bibcode = 2009MNRAS.400.2050G
  | doi = 10.1111/j.1365-2966.2009.15598.x
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 | arxiv = 0903.2057
}}</ref>).

Astronomers have determined the three-dimensional distribution of extinction in the "solar circle" (our region of our galaxy), using visible and near-infrared stellar observations and a model of distribution of stars.<ref>{{Cite journal
  | last = Marshall
  | first = Douglas J.
  | author-link = D.J. Marshall
  | author2 = Robin, A.C.
  | author3 = Reylé, C.
  | author4 = Schultheis, M.
  | author5 = Picaud, S.
  | title = Modelling the Galactic interstellar extinction distribution in three dimensions
  | journal = Astronomy and Astrophysics
  | date = Jul 2006
  | volume = 453
  | issue = 2
  | pages = 635–651
  | doi = 10.1051/0004-6361:20053842
  | bibcode = 2006A&A...453..635M
  | arxiv = astro-ph/0604427
| s2cid = 16845046
 }}</ref><ref>{{Cite journal
  | last = Robin
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  | author2 = Reylé, C.
  | author3 = Derrière, S.
  | author4 = Picaud, S.
  | title = A synthetic view on structure and evolution of the Milky Way
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  | doi = 10.1051/0004-6361:20031117
  | bibcode = 2003A&A...409..523R
| arxiv = astro-ph/0401052
  }}</ref> The dust causing extinction mainly lies along the [[Spiral galaxy#Spiral arms|spiral arms]], as observed in other spiral galaxies.