In astronomy, the color index is a simple numericalexpression 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 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 0.656 ± 0.005,[2] 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).[3] Traditionally, the color index uses Vega as a zero point.
To measure the index, one observes the 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 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.[4] A good approximation can be obtained by considering stars as black bodies, using Ballesteros' formula[5] (also implemented in the PyAstronomy package for Python):[6]
Color indices of distant objects are usually affected by interstellar extinction, that is, they are redder than those of closer stars. The amount of reddening is characterized by 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:
The passbands most optical astronomers 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 system of filters is sometimes called 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. M. S. Bessell specified a set of filter transmissions for a flat response detector, thus quantifying the calculation of the color indices.[7] 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.
^David F. Gray (1992), The Inferred Color Index of the Sun, Publications of the Astronomical Society of the Pacific, vol. 104, no. 681, pp. 1035–1038 (November 1992).
color, index, colorant, reference, database, colour, index, international, term, geology, geology, sample, calibration, colors, class, teff, 000b0v, 000a0v, 790f0v, 300g0v, 940k0v, 150m0v, 840in, astronomy, color, index, simple, numerical, expression, that, de. For the colorant reference database see Colour Index International For the term in geology see Color index geology Sample calibration colors 1 Class B V U B V R R I Teff K O5V 0 33 1 19 0 15 0 32 42 000B0V 0 30 1 08 0 13 0 29 30 000A0V 0 02 0 02 0 02 0 02 9 790F0V 0 30 0 03 0 30 0 17 7 300G0V 0 58 0 06 0 50 0 31 5 940K0V 0 81 0 45 0 64 0 42 5 150M0V 1 40 1 22 1 28 0 91 3 840In 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 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 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 0 656 0 005 2 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 3 Traditionally the color index uses Vega as a zero point To measure the index one observes the 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 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 4 A good approximation can be obtained by considering stars as black bodies using Ballesteros formula 5 also implemented in the PyAstronomy package for Python 6 T 4600 K 1 0 92 B V 1 7 1 0 92 B V 0 62 displaystyle T 4600 mathrm K left frac 1 0 92 B text V 1 7 frac 1 0 92 B text V 0 62 right Color indices of distant objects are usually affected by interstellar extinction that is they are redder than those of closer stars The amount of reddening is characterized by 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 E B V B V observed B V intrinsic displaystyle E text B text V B text V text observed B text V text intrinsic The passbands most optical astronomers 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 system of filters is sometimes called 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 M S Bessell specified a set of filter transmissions for a flat response detector thus quantifying the calculation of the color indices 7 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 See also EditAsteroid color indices Color color diagram Distant object color indices UBV photometric system Zero pointReferences Edit Zombeck Martin V 1990 Calibration of MK spectral types Handbook of Space Astronomy and Astrophysics 2nd ed Cambridge University Press p 105 ISBN 0 521 34787 4 David F Gray 1992 The Inferred Color Index of the Sun Publications of the Astronomical Society of the Pacific vol 104 no 681 pp 1035 1038 November 1992 bet Ori SIMBAD Centre de donnees astronomiques de Strasbourg 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 Ballesteros F J 2012 New insights into black bodies EPL 97 2012 34008 arXiv 1201 1809 BallesterosBV T API http www hs uni hamburg de DE Ins Per Czesla PyA PyA index html Michael S Bessell 1990 UBVRI passbands Publications of the Astronomical Society of the Pacific vol 102 Oct 1990 p 1181 1199 Further reading EditQuery for Johnson H L and Morgan ApJ 117 313 1953 Query for Cousins A W J MNRAS 166 711 1974 Query for Cousins A W J MNASSA 33 149 1974 Query for Bessell M S PASP 102 1181 1990 Portals Physics Mathematics Astronomy Stars Outer space Solar System Science Retrieved from https en wikipedia org w index php title Color index amp oldid 1121449044, wikipedia, wiki, book, books, library,
