Editing Extinction (astronomy)

{{short description|Interstellar absorption and scattering of light}}
{{other uses|Extinction (disambiguation)}}
[[File:The dark nebula LDN 483.jpg|thumb|upright=1.5|An extreme example of visible light extinction, caused by a [[dark nebula]]]]

In [[astronomy]], '''extinction''' is the [[absorption (electromagnetic radiation)|absorption]] and [[light scattering|scattering]] of [[electromagnetic radiation]] by dust and gas between an emitting [[astronomical object]] and the [[observation|observer]]. Interstellar extinction was first documented as such in 1930 by [[Robert Julius Trumpler]].<ref>
{{cite journal | first=R. J. | last=Trumpler 
| date=1930 
| title=Preliminary results on the distances, dimensions and space distribution of open star clusters 
| journal=Lick Observatory Bulletin 
| volume=14 
| issue=420 
| pages=154–188 
| bibcode=1930LicOB..14..154T
|doi = 10.5479/ADS/bib/1930LicOB.14.154T | doi-access=free 
}}</ref><ref name="Karttunen2003">
{{cite book | last=Karttunen | first=Hannu 
| title=Fundamental astronomy 
| work=Physics and Astronomy Online Library 
| publisher=Springer 
| date=2003 
| page=289 
| isbn=978-3-540-00179-9}}</ref> However, its effects had been noted in 1847 by [[Friedrich Georg Wilhelm von Struve]],<ref>Struve, F. G. W. 1847, St. Petersburg: Tip. Acad. Imper., 1847; IV, 165 p.; in 8.; DCCC.4.211 [http://adsabs.harvard.edu/abs/1847edas.book.....S]</ref> and its effect on the colors of [[star]]s had been observed by a number of individuals who did not connect it with the general presence of [[Cosmic dust|galactic dust]]. For stars lying near the plane of the [[Milky Way]] which are within a few thousand [[parsec]]s of the Earth, extinction in the [[visual band]] of frequencies ([[photometric system]]) is roughly 1.8&nbsp;[[Magnitude (astronomy)|magnitudes]] per kiloparsec.<ref>
{{cite book | first=Douglas C. B. | last=Whittet 
| title=Dust in the Galactic Environment 
| series=Series in Astronomy and Astrophysics 
| edition=2nd 
| publisher=CRC Press 
| date=2003 
| isbn=978-0750306249 
| page=10 
| url=https://books.google.com/books?id=k21lk4sORpEC&pg=PA10}}</ref>

For [[Observatory#Ground-based_observatories|Earth-bound observers]], extinction arises both from the [[interstellar medium]] and the [[Atmosphere of Earth|Earth's atmosphere]]; it may also arise from [[circumstellar dust]] around an observed object. Strong extinction in Earth's atmosphere of some [[wavelength]] regions (such as [[X-ray]], [[ultraviolet]], and [[infrared]]) is overcome by the use of [[space telescope|space-based observatories]]. Since [[blue|blue light]] is much more strongly [[Attenuation|attenuated]] than [[red]] light, extinction causes objects to appear redder than expected; this phenomenon is called '''interstellar reddening'''.<ref name=basicastronomy>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}}), and Kutner (2003, {{ISBN|978-0-521-52927-3}}) for applications in astronomy.</ref>

==Interstellar reddening==

Interstellar reddening is a phenomenon associated with interstellar extinction where the [[astronomical spectroscopy|spectrum]] of electromagnetic radiation from a [[astronomical object|radiation source]] changes characteristics from that which the object originally [[Emission spectrum|emitted]]. Reddening occurs due to the light scattering off [[Cosmic dust|dust]] and other [[matter]] in the [[interstellar medium]]. Interstellar reddening is a different phenomenon from [[redshift]], which is the proportional [[Doppler effect|frequency shifts]] of spectra without distortion. Reddening preferentially removes shorter wavelength [[Photon|photons]] from a radiated spectrum while leaving behind the longer wavelength photons, leaving the [[Spectroscopy#Atoms|spectroscopic lines]] unchanged.

In most [[photometric system]]s, filters (passbands) are used from which readings of magnitude of light may take account of latitude and humidity among terrestrial factors. Interstellar reddening equates to the "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). The latter is the theoretical value which it would have if unaffected by extinction. In the first system, the [[UBV photometric system]] devised in the 1950s and its most closely related successors, the object's color excess <math>E_{B-V}</math> is related to the object's [[B−V color]] (calibrated blue minus calibrated visible) by:

<math display="block">E_{B-V} = (B-V)_{\textrm{observed}} - (B-V)_{\textrm{intrinsic}}\,</math>

For an A0-type main sequence star (these have median wavelength and heat among the main sequence) the color indices are calibrated at 0 based on an intrinsic reading of such a star (± exactly 0.02 depending on which spectral point, i.e. precise passband within the abbreviated color name is in question, see [[color index]]). At least two and up to five measured passbands in magnitude are then compared by subtraction: U, B, V, I, or R during which the color excess from extinction is calculated and deducted. The name of the four sub-indices (R minus I etc.) and order of the subtraction of recalibrated magnitudes is from right to immediate left within this sequence.

==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
| doi-access = free
  }}</ref><ref>{{Cite journal
  | last = Milne
  | 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
| doi-access = free
  }}</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
  | last = Valencic
  | first = Lynne A.
  | author-link = Lynne A. Valencic
  | 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
  | last1 = Predehl
  | first1 = P.
  | author-link = P. Predehl
  | last2 = Schmitt
  | first2 = J. H. M. M.
  | 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
  | last = Bohlin
  | first = Ralph C.
  | author-link = Ralph C. Bohlin
  | author2 = Blair D. Savage
  | author3-link = J. F. Drake
 | author3 = J. F. Drake
  | title = A survey of interstellar H I from L-alpha absorption measurements. II
  | journal = Astrophysical Journal
  | date = 1978
  | volume = 224
  | pages = 132–142
  | bibcode = 1978ApJ...224..132B
  | doi = 10.1086/156357
| author2-link = Blair D. Savage
 }}</ref><ref>{{Cite journal
  | last = Diplas
  | first = Athanassios
  | author-link = Athanassios Diplas
  | author2 = Blair D. Savage 
  | title = An IUE survey of interstellar H I LY alpha absorption. 2: Interpretations
  | journal = Astrophysical Journal
  | date = 1994
  | volume = 427
  | pages = 274–287
  | bibcode = 1994ApJ...427..274D
  | doi = 10.1086/174139
| author2-link = Blair D. Savage
 | doi-access = free
  }}</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
  | volume = 400
  | issue = 4
  | pages = 2050–2053
  | bibcode = 2009MNRAS.400.2050G
  | doi = 10.1111/j.1365-2966.2009.15598.x
  | doi-access = free
 | 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
  | first = Annie C.
  | author-link = Annie C. Robin
  | author2 = Reylé, C.
  | author3 = Derrière, S.
  | author4 = Picaud, S.
  | title = A synthetic view on structure and evolution of the Milky Way
  | journal = Astronomy and Astrophysics
  | date = Oct 2003
  | volume = 409
  | issue = 2
  | pages = 523–540
  | 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.

==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).<ref>{{Cite journal
  | last = Cardelli
  | first = Jason A.
  | author-link = Jason A. Cardelli
  | author2 = Sembach, Kenneth R.
  | author3-link = John S. Mathis
 | author3 = Mathis, John S.
  | title = The quantitative assessment of UV extinction derived from IUE data of giants and supergiants
  | journal = Astronomical Journal
  | date = 1992
  | volume = 104
  | issue = 5
  | pages = 1916–1929
  | bibcode = 1992AJ....104.1916C
  | doi = 10.1086/116367
  | issn = 0004-6256
| author2-link = Kenneth R. Sembach
 }}</ref>  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 nebula]]e, it is common to look at the ratio of two [[Spectral line|emission lines]] which should not be affected by the [[temperature]] and [[density]] in the nebula. For example, the ratio of [[hydrogen-alpha]] to [[Balmer series|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 [[Angstrom|Å]], well into the [[ultraviolet]] region of the electromagnetic spectrum. This feature was first observed in the 1960s,<ref>{{Cite journal
  | author=Stecher, Theodore P.
  | title=Interstellar Extinction in the Ultraviolet
  | journal=Astrophysical Journal
  | date=1965
  | volume=142
  | page=1683
  | bibcode=1965ApJ...142.1683S
  | doi=10.1086/148462
}}</ref><ref>{{Cite journal
  | author=Stecher, Theodore P.
  | title=Interstellar Extinction in the Ultraviolet. II
  | journal=Astrophysical Journal
  | date=1969
  | volume=157
  | pages=L125
  | bibcode=1969ApJ...157L.125S
  | doi=10.1086/180400
| doi-access=free
  }}</ref> but its origin is still not well understood. Several models have been presented to account for this bump which include [[graphite|graphitic]] grains with a mixture of [[Polycyclic aromatic hydrocarbon|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.<ref>{{Cite journal
  | author=Bradley, John
  | title=An Astronomical 2175 Å Feature in Interplanetary Dust Particles
  | journal=Science
  | date=2005
  | volume=307
  | issue=5707
  | pages=244–247
  | bibcode=2005Sci...307..244B
  | doi=10.1126/science.1106717
  | pmid=15653501
  | display-authors=2
  | last2=Dai
  | first2=ZR
  | last3=Erni
  | first3=R
  | last4=Browning
  | first4=N
  | last5=Graham
  | first5=G
  | last6=Weber
  | first6=P
  | last7=Smith
  | first7=J
  | last8=Hutcheon
  | first8=I
  | last9=Ishii
  | first9=H
| s2cid=96858465
 }}</ref>

==Extinction curves of other galaxies==
[[File:Interstellar extinction ave curves local group.png|thumb|right|Plot showing the average extinction curves for the MW, LMC2, LMC, and SMC Bar.<ref name="mw_lmc_smc_comp" /> The curves are plotted versus 1/wavelength to emphasize the UV.]]

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 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.<ref>{{Cite journal
  | last = Fitzpatrick
  | first = Edward L.
  | author-link = Edward L. Fitzpatrick
  | title = An average interstellar extinction curve for the Large Magellanic Cloud
  | journal = Astronomical Journal
  | date = 1986
  | volume = 92
  | pages = 1068–1073
  | bibcode = 1986AJ.....92.1068F
  | doi = 10.1086/114237
| doi-access = free
  }}</ref><ref>{{Cite journal
  | last = Misselt
  | first = Karl A.
  | author-link = Karl A. Misselt
  | author2 = Geoffrey C. Clayton
  | author3-link = Karl D. Gordon
 | author3 = Karl D. Gordon
  | title = A Reanalysis of the Ultraviolet Extinction from Interstellar Dust in the Large Magellanic Cloud
  | journal = Astrophysical Journal
  | date = 1999
  | volume = 515
  | issue = 1
  | pages = 128–139
  | bibcode = 1999ApJ...515..128M
  | doi = 10.1086/307010
  | arxiv = astro-ph/9811036
| s2cid = 14175478
 | author2-link = Geoffrey C. Clayton
 }}</ref> In the SMC, more extreme variation is seen with no 2175 Å bump and very strong far-UV extinction in the star forming Bar and fairly normal ultraviolet extinction seen in the more quiescent Wing.<ref>{{Cite journal
  | last1 = Lequeux
  | first1 = J.
  | author-link = J. Lequeux
  | last2 = Maurice
  | first2 = E.
  | author-link2 = E. Maurice
  | last3 = Prevot-Burnichon
  | first3 = M. L.
  | author-link3 = M. L. Prevot-Burnichon
  | last4 = Prevot
  | first4 = L.
  | author-link4 = L. Prevot
  | last5 = Rocca-Volmerange
  | first5 = B.
  | author-link5 = B. Rocca-Volmerange
  | title = SK 143 - an SMC star with a galactic-type ultraviolet interstellar extinction
  | journal = Astronomy and Astrophysics
  | date = 1982
  | volume = 113
  | pages = L15–L17
  | bibcode = 1982A&A...113L..15L
}}</ref><ref>{{Cite journal
  | last1 = Prevot
  | first1 = M. L.
  | author-link = M. L. Prevot
  | last2 = Lequeux
  | first2 = J.
  | author-link2 = J. Lequex
  | last3 = Prevot
  | first3 = L.
  | author-link3 = L. Prevot
  | last4 = Maurice
  | first4 = E.
  | author-link4 = E. Maurice
  | last5 = Rocca-Volmerange
  | first5 = B.
  | author-link5 = B. Rocca-Volmerange
  | title = The typical interstellar extinction in the Small Magellanic Cloud
  | journal = Astronomy and Astrophysics
  | date = 1984
  | volume = 132
  | pages = 389–392
  | bibcode = 1984A&A...132..389P
}}</ref><ref>{{Cite journal
  | last = Gordon
  | first = Karl D.
  | author-link = Karl D. Gordon
  | author2 = Geoffrey C. Clayton 
  | title = Starburst-like Dust Extinction in the Small Magellanic Cloud
  | journal = Astrophysical Journal
  | date = 1998
  | volume = 500
  | issue = 2
  | pages = 816–824
  | bibcode = 1998ApJ...500..816G
  | doi = 10.1086/305774
  | arxiv = astro-ph/9802003
| s2cid = 18090417
 | author2-link = Geoffrey C. Clayton
 }}</ref>

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 [[metallicity|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<ref name="mw_lmc_smc_comp">{{Cite journal
  | last = Gordon
  | first = Karl D.
  | author-link = Karl D. Gordon
  | author2 = Geoffrey C. Clayton
  | author3-link = Karl A. Misselt
 | author3 = Karl A. Misselt
  | author4-link = Arlo U. Landolt
 | author4 = Arlo U. Landolt
  | author5-link = Michael J. Wolff
 | author5 = Michael J. Wolff
  | title = A Quantitative Comparison of the Small Magellanic Cloud, Large Magellanic Cloud, and Milky Way Ultraviolet to Near-Infrared Extinction Curves
  | journal = Astrophysical Journal
  | date = 2003
  | volume = 594
  | issue = 1
  | pages = 279–293
  | bibcode = 2003ApJ...594..279G
  | doi = 10.1086/376774
  | arxiv = astro-ph/0305257
| s2cid = 117180437
 | author2-link = Geoffrey C. Clayton
 }}</ref> and finding extinction curves in the Milky Way that look more like those found in the LMC2 supershell of the LMC<ref>{{Cite journal
  | last = Clayton
  | first = Geoffrey C.
  | author-link = Geoffrey C. Clayton
  | author2 = Karl D. Gordon
  | author3-link = Michael J. Wolff
 | author3 = Michael J. Wolff
  | title = Magellanic Cloud-Type Interstellar Dust along Low-Density Sight Lines in the Galaxy
  | journal = Astrophysical Journal Supplement Series
  | date = 2000
  | volume = 129
  | issue = 1
  | pages = 147–157
  | bibcode = 2000ApJS..129..147C
  | doi = 10.1086/313419
  | arxiv = astro-ph/0003285
| s2cid = 11205416
 | author2-link = Karl D. Gordon
 }}</ref> and in the SMC Bar<ref>{{Cite journal
  | last = Valencic
  | first = Lynne A.
  | author-link = Lynne A. Valencic
  | author2 = Geoffrey C. Clayton
  | author3-link = Karl D. Gordon
 | author3 = Karl D. Gordon
  | author4-link = Tracy L. Smith
 | author4 = Tracy L. Smith
  | title = Small Magellanic Cloud-Type Interstellar Dust in the Milky Way
  | journal = Astrophysical Journal
  | date = 2003
  | volume = 598
  | issue = 1
  | pages = 369–374
  | bibcode = 2003ApJ...598..369V
  | doi = 10.1086/378802
  | arxiv = astro-ph/0308060
| s2cid = 123435053
 | author2-link = Geoffrey C. Clayton
 }}</ref> 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) which shows that their dust lacks the 2175 Å bump.<ref>{{Cite journal
  | last = Calzetti
  | first = Daniela
  | author-link = Daniela Calzetti
  | author2 = Anne L. Kinney
  | author3-link = Thaisa Storchi-Bergmann
 | author3 = Thaisa Storchi-Bergmann
  | title = Dust extinction of the stellar continua in starburst galaxies: The ultraviolet and optical extinction law
  | journal = Astrophysical Journal
  | date = 1994
  | volume = 429
  | pages = 582–601
  | bibcode = 1994ApJ...429..582C
  | doi = 10.1086/174346
| author2-link = Anne L. Kinney
 | hdl = 10183/108843
 | hdl-access = free
  }}</ref><ref>{{Cite journal
  | last = Gordon
  | first = Karl D.
  | author-link = Karl D. Gordon
  | author2 = Daniela Calzetti
  | author3-link = Adolf N. Witt
 | author3 = Adolf N. Witt
  | title = Dust in Starburst Galaxies
  | journal = Astrophysical Journal
  | date = 1997
  | volume = 487
  | issue = 2
  | pages = 625–635
  | bibcode = 1997ApJ...487..625G
  | doi = 10.1086/304654
  | arxiv = astro-ph/9705043
| s2cid = 2055629
 | author2-link = Daniela Calzetti
 }}</ref>

==Atmospheric extinction==
Atmospheric extinction gives the [[sunrise|rising]] or [[sunset|setting]] Sun an orange hue and varies with location and [[altitude]]. Astronomical [[observatory|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 [[satellite]]s to make observations.

This extinction has three main components: [[Rayleigh scattering]] by air molecules, [[light scattering by particles|scattering by particulates]], and molecular [[absorption (electromagnetic radiation)|absorption]]. Molecular absorption is often referred to as [[telluric contamination|telluric absorption]], as it is caused by the [[Earth]] (''telluric'' is a synonym for ''terrestrial''). The most important sources of telluric absorption are [[oxygen|molecular oxygen]] and [[ozone]], which strongly absorb radiation near [[ultraviolet]], and [[water]], which strongly absorbs [[infrared]].

The amount of such extinction is lowest at the observer's [[zenith]] and highest near the [[horizon]]. A given star, preferably at solar opposition, reaches its greatest [[horizontal coordinate system|celestial altitude]] and optimal time for observation when the star is near the local [[meridian (astronomy)|meridian]] around solar [[midnight]] and if the star has a favorable [[declination]] (''i.e.'', similar to the observer's [[latitude]]); thus, the seasonal time due to [[axial tilt]] is key. Extinction is approximated by multiplying the standard atmospheric extinction curve (plotted against each wavelength) by the mean [[air mass (astronomy)|air mass]] calculated over the duration of the observation. A dry atmosphere reduces infrared extinction significantly.

==References==
{{Reflist}}

==Further reading==
*{{Cite book|last1=Binney |first1=J. |name-list-style=amp |last2=Merrifield |first2=M. |date=1998 |title=Galactic Astronomy |publisher=Princeton University Press |location=Princeton |isbn=978-0-691-00402-0 }}
*{{Cite journal|last=Howarth |first=I. D. |date=1983 |title=LMC and galactic extinction |journal=Monthly Notices of the Royal Astronomical Society |volume=203 |issue= 2|pages=301–304 |doi=  10.1093/mnras/203.2.301|bibcode=1983MNRAS.203..301H |doi-access=free }}
*{{Cite journal|last=King |first=D. L. |date=1985 |title=Atmospheric Extinction at the Roque de los Muchachos Observatory, La Palma |journal=RGO/La Palma Technical Note |volume=31 }}
*McCall, M. L. (2004). "On Determining Extinction from Reddening". ''The Astronomical Journal''. '''128''': 2144–2169. http://adsabs.harvard.edu/abs/2004AJ....128.2144M
*{{Cite journal|last1=Rouleau |first1=F. |last2=Henning |first2=T. |last3=Stognienko |first3=R. |date=1997 |title=Constraints on the properties of the 2175Å interstellar feature carrier |journal=Astronomy and Astrophysics |volume=322 |pages=633–645 |arxiv=astro-ph/9611203 |bibcode=1997A&A...322..633R }}

[[Category:Observational astronomy]]
[[Category:Galactic astronomy]]
[[Category:Extragalactic astronomy]]
[[Category:Concepts in astronomy]]