UBV photometric system
Encyclopedia
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. It is the first known standardized photoelectric photometric system
. The letters U, B, and V stand for ultraviolet
, blue, and visual magnitudes, which are measured for a star in order to classify it in the UBV system. The choice of colors on the blue end of the spectrum is because of the bias that photographic film has for those colors. It was introduced in the 1950s by American
astronomer
s Harold Lester Johnson
and William Wilson Morgan
.
The filters are selected in such manner, that the mean wavelengths of response functions are 364 nm for U, 442 nm for B, 540 nm for V.
The zero point of the B-V and U-B color indices
was defined to be zero for A0 V stars not affected by interstellar reddening.
The UBV system has some disadvantages. The short wavelength cutoff of the U filter is defined mainly by the terrestrial atmosphere rather than the filter itself. Thus, it (and observed magnitudes) can vary with altitude and atmospheric conditions. However, a large number of measurements have been made in this system, including many of the bright stars.
Harold Johnson (astronomer)
Harold Lester Johnson was an American astronomer.Johnson was awarded the Helen B. Warner Prize by the American Astronomical Society in 1956...
system (or Johnson-Morgan system), is a wide band 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...
for classifying stars
Stellar classification
In astronomy, stellar classification is a classification of stars based on their spectral characteristics. The spectral class of a star is a designated class of a star describing the ionization of its chromosphere, what atomic excitations are most prominent in the light, giving an objective measure...
according to their colors. It is the first known standardized photoelectric 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 letters U, B, and V stand for ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
, blue, and visual magnitudes, which are measured for a star in order to classify it in the UBV system. The choice of colors on the blue end of the spectrum is because of the bias that photographic film has for those colors. It was introduced in the 1950s by American
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
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 Harold Lester Johnson
Harold Johnson (astronomer)
Harold Lester Johnson was an American astronomer.Johnson was awarded the Helen B. Warner Prize by the American Astronomical Society in 1956...
and William Wilson Morgan
William Wilson Morgan
William Wilson Morgan was an American astronomer.The principal theme in Morgan's work was the study of stellar and galaxy classification. Along...
.
The filters are selected in such manner, that the mean wavelengths of response functions are 364 nm for U, 442 nm for B, 540 nm for V.
The zero point of the B-V and U-B color indices
Color index
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...
was defined to be zero for A0 V stars not affected by interstellar reddening.
The UBV system has some disadvantages. The short wavelength cutoff of the U filter is defined mainly by the terrestrial atmosphere rather than the filter itself. Thus, it (and observed magnitudes) can vary with altitude and atmospheric conditions. However, a large number of measurements have been made in this system, including many of the bright stars.