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The UBV photometric system (from ''Ultraviolet, Blue, Visual''), also called the 
The filters are selected so that the mean wavelengths of response functions (at which magnitudes are measured to mean precision) are for U, for B, for V. Zero-points were calibrated in the B−V (B minus V) and U−B (U minus B)
The UBV photometric system (from ''Ultraviolet, Blue, Visual''), also called the Johnson
Johnson may refer to:
People and fictional characters
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* Johnson (given name), a list of people
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*Johnson (composer) (1953–2011) ...
system (or Johnson-Morgan system), is a photometric system
In astronomy, a photometric system is a set of well-defined passbands (or optical filters), with a known sensitivity to incident radiation. The sensitivity usually depends on the optical system, detectors and filters used. For each photometric s ...
usually employed for classifying stars according to their colors.
It was the first standardized photometric system. The apparent magnitudes of stars in the system are often used to determine the color indices
In astronomy, the color index is a simple Numerical analysis, numerical Expression (mathematics), 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 ...
B−V and U−B, the difference between the B and V magnitudes and the U and B magnitudes respectively. The system is defined using a set of color optical filter
An optical filter is a device that selectively transmits light of different wavelengths, usually implemented as a glass plane or plastic device in the optical path, which are either dyed in the bulk or have interference coatings. The optic ...
s in combination with an RMA 1P21 photomultiplier tube
Photomultiplier tubes (photomultipliers or PMTs for short) are extremely sensitive detectors of light in the ultraviolet, visible light, visible, and near-infrared ranges of the electromagnetic spectrum. They are members of the class of vacuum t ...
.
The choice of colors on the blue end of the spectrum was assisted by the bias that photographic film has for those colors. It was introduced in the 1950s by American astronomer
An astronomer is a scientist in the field of astronomy who focuses on a specific question or field outside the scope of Earth. Astronomers observe astronomical objects, such as stars, planets, natural satellite, moons, comets and galaxy, galax ...
s Harold Lester Johnson and William Wilson Morgan. A telescope and the telescope at McDonald Observatory were used to define the system. The filters that Johnson and Morgan used were Corning 9 863 for U and 3 384 for V. The B filter used a combination of Corning 5 030 and Schott GG 13.
The filters are selected so that the mean wavelengths of response functions (at which magnitudes are measured to mean precision) are for U, for B, for V. Zero-points were calibrated in the B−V (B minus V) and U−B (U minus B) color indices
In astronomy, the color index is a simple Numerical analysis, numerical Expression (mathematics), 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 ...
selecting such A0 main sequence stars which are not affected by interstellar reddening
In astronomy, extinction is the absorption and scattering of electromagnetic radiation by dust and gas between an emitting astronomical object and the observer. Interstellar extinction was first documented as such in 1930 by Robert Julius Trum ...
. These stars correspond with a mean effective temperature
The effective temperature of a body such as a star or planet is the temperature of a black body that would emit the same total amount of electromagnetic radiation. Effective temperature is often used as an estimate of a body's surface temperature ...
(Teff (K)) of between 9727 and 9790 Kelvin, the latter being stars with class A0V (V meaning five).
The system has a key limit drawback. The short wavelength cutoff that is the shortest limit of the U filter is set by any given terrestrial atmosphere rather than the filter itself; thus, it (and observed magnitudes) varies chiefly with altitude and atmospheric water (humidity plus condensation into clouds). However, many measurements have been made in this system, including thousands of the bright stars.
Extensions
The Johnson-Kron- Cousins UBVRI photometric system is a common extension of Johnson's original system that provides redder passbands.See also
* Strömgren photometric system *Photometric System
In astronomy, a photometric system is a set of well-defined passbands (or optical filters), with a known sensitivity to incident radiation. The sensitivity usually depends on the optical system, detectors and filters used. For each photometric s ...
Notes
References
{{DEFAULTSORT:Ubv Photometric System Photometric systems American inventions Stellar astronomy