UBV photometric system
First standardized photometric system for stellar classification.
The UBV photometric system—short for Ultraviolet, Blue, Visual—is also known as the Johnson system or Johnson-Morgan system. It is primarily used to classify stars by their color and was the first standardized photometric system, developed in the 1950s by American astronomers Harold Lester Johnson and William Wilson Morgan. The system relies on a set of color optical filters paired with an RMA 1P21 photomultiplier tube. The choice of blue-end colors was influenced by the sensitivity of photographic film to those wavelengths. The system was defined using a 13-inch (330 mm) telescope and the 82-inch (2,100 mm) telescope at McDonald Observatory. Johnson and Morgan used Corning 9 863 for the U filter and Corning 3 384 for the V filter; the B filter combined Corning 5 030 and Schott GG 13.
A key limitation is that the shortest wavelength cutoff of the U filter is determined by the terrestrial atmosphere rather than the filter itself, so the cutoff—and the observed magnitudes—varies mainly with altitude and atmospheric water content (humidity and cloud condensation). Despite this, thousands of bright stars have been measured using this system.
The filters are selected so that their mean response wavelengths are 364 nm for U, 442 nm for B, and 540 nm for V. Zero-points for the B−V and U−B color indices were calibrated using A0 main sequence stars unaffected by interstellar reddening, which have mean effective temperatures between 9727 and 9790 Kelvin (class A0V). The following table shows the approximate characteristics of each filter (colors are only approximate, based on wavelength-to-sRGB representation where possible):
| Filter | Mean Wavelength (nm) | Approximate Color | |--------|----------------------|-------------------| | U | 364 | Violet | | B | 442 | Blue | | V | 540 | Green-yellow |
A common extension, the Johnson-Kron-Cousins UBVRI photometric system, adds redder passbands.
- field
- Astronomy
- known_for
- First standardized photometric system for classifying stars by color
- introduced_by
- Harold Lester Johnson and William Wilson Morgan
- era
- 1950s
Lore & Background
The UBV photometric system, also known as the Johnson or Johnson-Morgan system, was the first standardized system for classifying stars by color. It was introduced in the 1950s by American astronomers Harold Lester Johnson and William Wilson Morgan. The system uses three color optical filters—Ultraviolet (U), Blue (B), and Visual (V)—in combination with an RMA 1P21 photomultiplier tube. The choice of blue-end colors was influenced by the sensitivity of photographic film. The defining filters were Corning 9 863 for U, Corning 3 384 for V, and a combination of Corning 5 030 with Schott GG 13 for B. The system was calibrated using telescopes at McDonald Observatory. The mean wavelengths of the response functions are approximately 364 nm for U, 442 nm for B, and 540 nm for V. Zero-points for the B−V and U−B color indices were set using A0 main sequence stars unaffected by interstellar reddening, corresponding to effective temperatures between 9727 and 9790 Kelvin. A key drawback is that the short wavelength cutoff of the U filter is determined by the terrestrial atmosphere, not the filter itself, causing observed magnitudes to vary with altitude and atmospheric water content. Despite this limitation, thousands of bright stars have been measured in this system. An extension, the Johnson-Kron-Cousins UBVRI system, adds redder passbands.
Reader's Guide
The UBV photometric system's significance lies in being the first standardized method for measuring stellar colors, enabling consistent classification and the calculation of color indices B−V and U−B. These indices are fundamental for determining stellar temperatures and interstellar reddening. The system's legacy includes its extension into the Johnson-Kron-Cousins UBVRI photometric system, which added redder passbands. Although limited by atmospheric effects on the U filter, the system provided a foundational framework for photometric astronomy, with thousands of bright stars cataloged using its filters. Its introduction by Johnson and Morgan in the 1950s marked a shift toward reproducible, quantitative stellar photometry.
Did You Know?
- The UBV system was the first standardized photometric system.
- The U filter's short wavelength cutoff is set by the terrestrial atmosphere, not the filter itself.
Frequently Asked Questions
What is the UBV photometric system?
The UBV system (also called the Johnson or Johnson-Morgan system) is a standardized method for measuring a star's brightness through ultraviolet, blue, and visual wavelength bands to determine its color and temperature. It became the foundational color-classification tool in observational astronomy.
Who developed the UBV photometric system?
American astronomers Harold Lester Johnson and William Wilson Morgan introduced it in the 1950s, establishing the first formally standardized photometric system for classifying stars by their colors.
Why is the UBV system important in astronomy?
Before UBV, researchers at different observatories used inconsistent filter definitions, making cross-site comparisons of stellar colors unreliable. By fixing a single set of bandpass specifications, it gave the entire field a common language for describing how hot or cool a star's surface is.
What do the letters U, B, and V stand for in the UBV system?
They designate the three photometric bands: Ultraviolet, Blue, and Visual (the visible-light region). Comparing a star's measured magnitudes across these three bands reveals its surface temperature and helps assign a spectral type.
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