So why use B-V index to find temperature of a star when you can use Wien's law? What is the point of B-V index besides finding Temp?
For one, from an observational standpoint, one can measure B-V index with just two magnitude measurements with a B and a V filter, as opposed to needing a full spectrometer to use Wien's law. Otherwise, it is just another way to measure temperature/color of a star to my knowledge.
Isn't Wien's law more common to use (at least on Scioly tests)? On my state test a few weeks ago I remember seeing two maximum wavelength to temperature problems but zero B-V color index questions. That being said, you'll regularly see B-V color on the X-axis of H-R diagrams.
Wien's Law is more common on Scio tests as it is a simple relation that is easy to understand and use. B-V and temperature relations are much more complicated, depending on terms of B-V to different powers with differing coefficients that are affected by things such as metallicity and the life stage of a star.
Wien's Law is for blackbodies, and stars are only approximated as black bodies. Wien's Law gives the temperature of a blackbody that would emit the same total amount of light as the star, called the effective temperature.
Also, as said above, in observational astronomy you will be working with magnitudes of stars in some sort of filter system, commonly UBVRI. A common goal, especially in studies of star clusters, is to generate an H-R Diagram to determine things like the age and distance of the cluster. It is easiest to use the B-V value as a temperature indicator. This is why most H-R Diagrams of star clusters will be Color-Magnitude Diagrams (ie. V magnitude versus B-V Index).
2007-2012. Paul J Gelinas Jr High and Ward Melville High School
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