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Earth and Space Sciences · Glossary

What is Colour index?

Definition 2.17 University Earth and Space Sciences — Year 1 · Chapter 2 — Telescopes, Detectors and Photometry

The colour index of a star is the difference of its magnitudes in two bands, the bluer first; the most used is B−VB - V. A small or negative B−VB - V marks a blue star, a large one a red star.

Black-body spectra at three temperatures, each divided by its own maximum, with the pivot wavelengths of the U, B, V, R and I filters (dashed). The peak moves to shorter wavelengths as the temperature rises (Wien’s law): 828\, nm, 502\, nm and 290\, nm.
Black-body spectra at three temperatures, each divided by its own maximum, with the pivot wavelengths of the U, B, V, R and I filters (dashed). The peak moves to shorter wavelengths as the temperature rises (Wien’s law): 828 nm828\,\mathrm{nm}, 502 nm502\,\mathrm{nm} and 290 nm290\,\mathrm{nm}.
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