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| {{For|the colorant reference database|Colour Index International}}
| | Hello! <br>I'm Dutch male :). <br>I really love The Vampire Diaries!<br><br>my web blog; [http://quittingelectricity.com quittingelectricity.com] |
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| {| class="wikitable" style="float: right; text-align: center; border: 1px; margin-left: 0.5em;"
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| |+ Sample calibration colors<ref name=zombeck/>
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| ! [[Stellar classification|Class]] || B–V || U–B || V–R || R–I
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| ! [[Effective temperature|T<sub>eff</sub>]] (K) | |
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| | O5V || –0.33 || –1.19 || –0.15 || –0.32 || 42,000
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| |-
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| | B0V || –0.30 || –1.08 || –0.13 || –0.29 || 30,000
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| | A0V || –0.02 || –0.02 || 0.02 || –0.02 || 9,790
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| | F0V || 0.30 || 0.03 || 0.30 || 0.17 || 7,300
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| |-
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| | G0V || 0.58 || 0.06 || 0.50 || 0.31 || 5,940
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| | K0V || 0.81 || 0.45 || 0.64 || 0.42 || 5,150
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| | M0V || 1.40 || 1.22 || 1.28 || 0.91 || 3,840
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| |}
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| In [[astronomy]], the '''color index''' is a simple numerical expression that determines the color of an object, which in the case of a [[star]] gives its [[temperature]]. To measure the index, one observes the [[Magnitude (astronomy)|magnitude]] of an object successively through two different filters, such as U and B, or B and V, where U is sensitive to ultraviolet rays, B is sensitive to blue light, and V is sensitive to visible (green-yellow) light (see also: [[UBV system]]). The set of passbands or filters is called a [[photometric system]]. The difference in magnitudes found with these filters is called the U-B or [[B-V color|B–V]] color index, respectively. The smaller the color index, the more blue (or hotter) the object is. Conversely, the larger the color index, the more red (or cooler) the object is. This is a consequence of the logarithmic magnitude scale, in which brighter objects have smaller (more negative) magnitudes than dimmer ones. For comparison, the yellowish [[Sun]] has a B–V index of {{nowrap|0.656 ± 0.005}},<ref name=sun/> while the bluish [[Rigel]] has a B–V of –0.03 (its B magnitude is 0.09 and its V magnitude is 0.12, B–V = –0.03).<ref name=rigel/>
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| In principle, the temperature of a star can be calculated directly from the B-V index, and there are in the literature several formulae to make this connection.<ref name=Sekiguchi>Sekiguchi M. and Fukugita (2000). "A STUDY OF THE B-V COLOR-TEMPERATURE RELATION". AJ (Astrophysical Journal) 120 (2000) 1072. http://iopscience.iop.org/1538-3881/120/2/1072.</ref> A good approximation can be obtained by considering stars as black bodies, using Ballesteros' formula:<ref name=Ballesteros>Ballesteros, F.J. (2012). "New insights into black bodies ". EPL (Europhysics Letters) 97 (2012) 34008. http://arxiv.org/pdf/1201.1809.pdf.</ref>
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| :<math>T = 4600\left( {\frac{1}{{0.92(B - V) +1.7}} + \frac{1}{{ 0.92(B - V)+0.62}}} \right) </math>
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| Color indices of distant objects are usually affected by [[extinction (astronomy)|interstellar extinction]] —i.e. they are [[interstellar reddening|redder]] than those of closer stars. The amount of reddening is characterized by [[Interstellar reddening|color excess]], defined as the difference between the '''Observed color index''' and the '''Normal color index''' (or '''Intrinsic color index'''), the hypothetical true color index of the star, unaffected by extinction.
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| For example, in the UBV photometric system we can write it for the [[B-V color]]:
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| :<math>E_{B-V} = (B-V)_{\textrm{Observed}} - (B-V)_{\textrm{Intrinsic}}</math>
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| The passbands most optical [[astronomer]]s use are the [[UBVRI]] filters, where the U, B, and V filters are as mentioned above, the R filter passes red light, and the I filter passes infrared light. This [[photometric system|system of filters]] is sometimes called
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| the [[Johnson-Cousins filter system]], named after the originators of the system (see references). These filters were specified as particular combinations of glass filters and photomultiplier tubes. [[Michael S. Bessell|M. S. Bessell]] specified a set of filter transmissions for a flat response detector, thus quantifying the calculation of the color indices.<ref name=filters/> For precision, appropriate pairs of filters are chosen depending on the object's color temperature: B-V are for mid-range objects, U-V for hotter objects, and R-I for cool ones.
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| ==See also==
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| * [[UBV photometric system]]
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| * [[Color–color diagram]]
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| == References ==
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| {{reflist|refs=
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| <ref name=zombeck>{{cite book | first=Martin V. | last=Zombeck | year=1990 | page=105 | contribution=Calibration of MK spectral types | title=Handbook of Space Astronomy and Astrophysics | publisher=Cambridge University Press | edition=2nd | isbn=0-521-34787-4 }}</ref>
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| <ref name=rigel>The [http://simbad.u-strasbg.fr Simbad Astronomical Database]' [http://simbad.u-strasbg.fr/sim-id.pl?protocol=html&Ident=Rigel&NbIdent=1&Radius=10&Radius.unit=arcmin&CooFrame=FK5&CooEpoch=2000&CooEqui=2000&output.max=all&o.catall=on&output.mesdisp=N&Bibyear1=1983&Bibyear2=2006&Frame1=FK5&Frame2=FK4&Frame3=G&Equi1=2000.0&Equi2=1950.0&Equi3=2000.0&Epoch1=2000.0&Epoch2=1950.0&Epoch3=2000.0 Rigel page]</ref>
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| <ref name=sun>David F. Gray (1992), ''[http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1992PASP..104.1035G&db_key=AST&data_type=HTML&format=&high=44b52c369023103 The Inferred Color Index of the Sun]'', Publications of the Astronomical Society of the Pacific, vol. 104, no. 681, pp. 1035-1038 (November 1992)</ref> | |
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| <ref name=filters>Michael S. Bessell (1990), ''[http://adsabs.harvard.edu/abs/1990PASP..102.1181B UBVRI passbands]'', Astronomical Society of the Pacific, Publications (ISSN 0004-6280), vol. 102, Oct. 1990, p. 1181-1199</ref>
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| }}
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| ==Further reading==
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| * [http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1953ApJ...117..313J&db_key=AST&high=3c321cbf8303780 [[Harold Johnson (astronomer)|Johnson, H. L.]] and [[William Wilson Morgan|Morgan]], ApJ 117, 313 (1953)]
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| * [http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1974MNRAS.166..711C&db_key=AST&high=3c321cbf8304063 [[Alan William James Cousins|Cousins, A. W. J.]], MNRAS 166, 711 (1974)]
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| * [http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1974MNSSA..33..149C&db_key=AST&high=3c321cbf8304063 [[Alan William James Cousins|Cousins, A. W. J.]], MNASSA 33, 149 (1974)]
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| * [http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1990PASP..102.1181B&db_key=AST&high=3c321cbf8304649 [[Michael S. Bessell|Bessell, M. S.]], PASP 102, 1181 (1990)]
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| [[Category:Photometric systems]]
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| [[Category:Index numbers]]
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Hello!
I'm Dutch male :).
I really love The Vampire Diaries!
my web blog; quittingelectricity.com