Color index: Difference between revisions

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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>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 body|black bodies]], using Ballesteros' formula<ref name=Ballesteros>Ballesteros, F. J. (2012). "New insights into black bodies". EPL 97 (2012) 34008. {{ArXiv|1201.1809}}.</ref> (also implemented in the PyAstronomy package for Python):<ref name=PyAstronomy>BallesterosBV_T API http://www.hs.uni-hamburg.de/DE/Ins/Per/Czesla/PyA/PyA/index.html.</ref>

:<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>

Color indices of distant objects are usually affected by [[extinction (astronomy)|interstellar extinction]], that is, 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.

For example, in the UBV photometric system we can write it for the B−V color:

:<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