Color-Color Diagrams
If astronomers don't have a spectrum for a star, they use another tool to find
its temperature: a "color-color diagram." If you
plot the wavelengths of SDSS's five filters on a thermal radiation curve,
you get a graph like this:
To find one of the star's colors from this diagram, subtract the magnitude of the star seen through
one filter from the magnitude seen through another filter. The thermal source
graphed above has a curve that peaks in the green. Therefore, its g-r color will be
negative (remember, brighter objects have lower magnitudes). Its u-g color will be
positive. What about its r-i and i-z colors?
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