Stellar Magnitudes

In the 140 years since Pogson created the modern magnitudes, astronomers have developed many different brightness systems, and the most popular are less than 50 years old.

For example, in 1953, H. L. Johnson created the UBV system of brightness measurements. We've already met V, which is measured over that portion of the electromagnetic spectrum to which our eyes are most sensitive. B is the star's brightness in magnitudes measured in the blue part of the spectrum, while U is the brightness in the ultraviolet—a spectral region our eyes cannot detect. There are many other brightness measurement systems in use, so many that astronomers often disagree on how measurements in one system should be converted to another.

Accurate measurement of stellar brightness is important because subtracting the brightness in one part of the spectrum from the brightness in another part reveals important information about the star. For many stars the quantity B-V gives a good approximation of the star's temperature. And it was established in 1978 that the quantity V-R, where R is the brightness in the red part of the spectrum, can be used to estimate a star's radius. This is important, because advances in our understanding of the stars require knowledge of basic parameters like temperature and radius, and careful measurements of brightness can provide some of that information.