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{{Short description|
{{About||the colorant reference database|Colour Index International|the term in geology|Color index (geology)}}
{| class="wikitable sortable" style="float: right; text-align: center; border: 1px; margin-left: 0.5em;"
|+ Sample calibration colors<ref name=zombeck/> {{Failed verification|date=December 2023}}
! [[Stellar classification|Class]] || B−V || U−B || V−R || R−I
! [[Effective temperature|''T''<sub>eff</sub>]] ([[Kelvin|K]])
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In [[astronomy]], the '''color index''' is a simple [[Numerical analysis|numerical]] [[Expression (mathematics)|expression]] that determines the [[color]] of an object, which in the case of a [[star]] gives its [[temperature]]. The lower 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 inverse [[Logarithmic scale|logarithmic magnitude scale]], in which brighter objects have smaller (more negative) magnitudes than dimmer ones. For comparison, the whitish [[Sun]] has a B−V index of {{nowrap|0.656 ± 0.005}},<ref name=sun/> whereas 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/> Traditionally, the color index uses [[Vega]] as a [[Zero Point (photometry)|zero point]]. The [[blue supergiant]] [[Theta Muscae]] has one of the lowest B−V indices at −0.41,<ref>{{Cite book
To measure the index, one observes the [[Magnitude (astronomy)|magnitude]] of an object successively through two different [[Astronomical filter|filter]]s, 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 index respectively.
In principle, the temperature of a star can be calculated directly from the B−V index, and there are several formulae to make this connection.<ref name=Sekiguchi>
:<math>T = 4600\,\mathrm{K} \left( \frac{1}{0.92\;(B\text{-}\!V) + 1.7} + \frac{1}{0.92\;(B\text{-}\!V) + 0.62} \right). </math>
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:<math>E_{\text{B-}\!\text{V}} = {B\text{-}\!V}_\text{observed} - {B\text{-}\!V}_\text{intrinsic}.</math>
The [[passband]]s 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 the Johnson–Kron–Cousins filter system, named after the originators of the system (see references).<ref>{{Cite journal |last=Landolt |first=Arlo U. |date=1992-07-01 |title=UBVRI Photometric Standard Stars in the Magnitude Range 11.5 < V < 16.0 Around the Celestial Equator |url=https://ui.adsabs.harvard.edu/abs/1992AJ....104..340L |journal=The Astronomical Journal |volume=104 |pages=340 |doi=10.1086/116242 |bibcode=1992AJ....104..340L |issn=0004-6256}}</ref> These filters were specified as particular combinations of glass filters and [[photomultiplier|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.
Color indices can also be determined for other celestial bodies, such as planets and moons:
{| class="wikitable sortable"
|+Color indices of Solar System bodies<ref>{{Citation|last = Pace |first = G. |title = UBV: Subroutine to Compute Photometric Magnitudes of the Planets and Their Satellites |type = Technical report |publisher = [[Jet Propulsion Laboratory]] |pages = |date = February 15, 1971 |url = https://ntrs.nasa.gov/api/citations/19710009758/downloads/19710009758.pdf}}</ref><ref name=":0" />
!Celestial body
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== Quantitative color index terms ==
{| class="wikitable sortable"<!-- Do not change; these are not literal and just quantify ranges of certain B-V values. Stick to what the source says.-->
|+Quantitative color index terms<ref name=":0">{{Cite journal |
!Color<br />(Vega reference)
!Color
!Spectral class<br />([[
!Spectral class<br />([[
▲!Spectral class ([[supergiant stars]])
!Examples
|-
|Red
|≥1.40
|M
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|-
|Orange
|0.80-1.40
|K
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|-
|Yellow
|0.60-0.80
|G
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|[[Sun]], [[Rigil Kent]]
|-
|Green
|0.30-0.60
|F
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|-
|White
|0.00-0.30
|A
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|[[Sirius]], [[Vega]]
|-
|Blue
| -0.33-0.00
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|}
The common color labels (
==See also==
{{div col|colwidth=20em}}
* [[Asteroid color indices]]
* [[Color–color diagram]]
* [[Distant object color indices]]
* [[Spectral index]]
* [[UBV photometric system]]
* [[Zero point (photometry)|Zero point]]
{{div col end}}
== Notes ==
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* [http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1990PASP..102.1181B&db_key=AST&high=3c321cbf8304649 Query] for [[Michael S. Bessell|Bessell, M. S.]], PASP 102, 1181 (1990)
{{Portal bar|Physics|Mathematics|Astronomy|Stars|Outer space|Solar System|Science}}
[[Category:Photometric systems]]
[[Category:Index numbers]]
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