What does a B–V color index calculation resulting in a small or negative number signify about a star?
Answer
The star is hot and blue, being brighter in the B (blue) filter than the V filter.
The B–V color index is calculated by subtracting the magnitude measured through the V (visual/yellow-green) filter from the magnitude measured through the B (blue) filter. Since the magnitude scale is inverted where smaller numbers indicate brighter objects, a small or negative result (like -0.4) means the magnitude measured in the B filter was significantly lower (i.e., brighter) than the magnitude measured in the V filter. Consequently, the star is emitting much more energy in the blue wavelengths than the longer wavelengths, which is the definitive characteristic of a hot, blue star.
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