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 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. 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 0.656±0.005^[1], while the blueish Rigel has B-V -0.03 (its B magnitude is 0.09 and its V magnitude is 0.12, B-V=-0.03).^[2] A giant Hubble mosaic of the Crab Nebula, a supernova remnant. ... The Pleiades, an open cluster of stars in the constellation of Taurus. ... Fig. ... In science, a magnitude is the numerical size of something: see orders of magnitude. ... UBV photometric system, also called the Johnson system (or Johnson-Morgan system), is a wide band photometric system for classifying stars according to their colors. ... In astronomy, a Photometric system is a set of discrete passbands (of filters), with a known sensitivity to incident radiation. ... The Sun is the star of our solar system. ... Rigel (pronounced ) (β Orionis) is the brightest star in the constellation Orion and the seventh brightest star in the sky, with visual magnitude 0. ...

Color indices of distant objects are usually affected by interstellar extinction —i.e. 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: Extinction is a term used in astronomy to describe the absorption of light from astronomical objects by matter between them and the observer. ... In astronomy, interstellar reddening is a phenomenon associated with interstellar extinction where the spectrum of electromagnetic radiation from a radiation source changes characteristics from that which was emitted. ...

E_{B − V} = (B − V)_Observed − (B − V)_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. Bessel specified a set of filter transmissions for a flat response detector, thus quantifying the calculation of the color indices. 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. An astronomer or astrophysicist is a person whose area of interest is astronomy or astrophysics. ... In astronomy, a Photometric system is a set of discrete passbands (of filters), with a known sensitivity to incident radiation. ...

References

• Johnson, H. L. and Morgan, ApJ 117, 313 (1953)
• Cousins, A. W. J., MNRAS 166, 711 (1974)
• Cousins, A. W. J., MNASSA 33, 149 (1974)
• Bessell, M. S., PASP 102, 1181 (1990)

William Wilson Morgan (January 3, 1906 – June 21, 1994) was an American astronomer. ...

Note

1. ↑  The Simbad Astronomical Database' Rigel page
2. ↑  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)