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How does the color index of a star relate to its actual color?

Articles in astronomy give the colors of stars as a number, called color index.

How do you turn this number into a real color?

The color index (CI) is usually

CI = mB - mV

where mB is the blue color magnitude of the star and mV the visible color magnitude. As the magnitude increases with decreasing brightness a star with a smaller index will be more blue and a star with a larger index more red. The following table should help you do the translation:
Color Index Spectral Class Color
-0.33O5 Blue
-0.17B5 Blue-white
0.15 A5 White with bluish tinge
0.44 F5 Yellow-White
0.68 G5 Yellow
1.15 K5 Orange
1.64 M5 Red

This table is only valid for the B-V (or Blue minus Visible) color index. Often astronomers use other color indexes such as U-B (Ultraviolet minus Blue) or H-K (H-band minus K-band) indexes.

January 2007, Marc Berthoud (more by Marc Berthoud) (Like this Answer)

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