UBV photometric system
First standardized photometric system for stellar classification.
The UBV photometric system, also called the Johnson system or Johnson-Morgan system, is a photometric system usually employed for classifying stars according to their colors. It was the first standardized photometric system, introduced in the 1950s by American astronomers Harold Lester Johnson and William Wilson Morgan.
- 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
A key limitation is that the short wavelength cutoff of the U filter is set by the terrestrial atmosphere rather than the filter itself, causing observed magnitudes to vary with altitude and atmospheric water content. Despite this, thousands of bright stars have been measured in this system.
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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