Color index
Encyclopedia
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, while the bluish Rigel
has B-V -0.03 (its B magnitude is 0.09 and its V magnitude is 0.12, B-V=-0.03).
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:
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 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. 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.
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...
, the color index is a simple numerical expression that determines the color of an object, which in the case of a star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
gives its temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
. To measure the index, one observes the magnitude
Magnitude (astronomy)
Magnitude is the logarithmic measure of the brightness of an object, in astronomy, measured in a specific wavelength or passband, usually in optical or near-infrared wavelengths.-Background:...
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
Photometric system
In astronomy, a Photometric system is a set of well-defined passbands , with a known sensitivity to incident radiation. The sensitivity usually depends on the optical system, detectors and filters used. For each photometric system a set of primary standard stars is provided.The first known...
. 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
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
has a B-V index of 0.656±0.005, while the bluish Rigel
Rigel
Rigel is the brightest star in the constellation Orion and the sixth brightest star in the sky, with visual magnitude 0.18...
has B-V -0.03 (its B magnitude is 0.09 and its V magnitude is 0.12, B-V=-0.03).
Color indices of distant objects are usually affected by interstellar extinction
Extinction (astronomy)
Extinction is a term used in astronomy to describe the absorption and scattering of electromagnetic radiation by matter between an emitting astronomical object and the observer. Interstellar extinction—also called Galactic extinction, when it occurs in the Milky Way—was first...
—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:
The passbands most optical astronomer
Astronomer
An astronomer is a scientist who studies celestial bodies such as planets, stars and galaxies.Historically, astronomy was more concerned with the classification and description of phenomena in the sky, while astrophysics attempted to explain these phenomena and the differences between them using...
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 system of filters
Photometric system
In astronomy, a Photometric system is a set of well-defined passbands , with a known sensitivity to incident radiation. The sensitivity usually depends on the optical system, detectors and filters used. For each photometric system a set of primary standard stars is provided.The first known...
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. 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.
Note
- The Simbad Astronomical Database' Rigel page
- 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)
- Michael S. Bessell (1990), UBVRI passbands, Astronomical Society of the Pacific, Publications (ISSN 0004-6280), vol. 102, Oct. 1990, p. 1181-1199