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

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 ClassColor
-0.33   O5 Blue
-0.17   B5 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.


This page updated on June 27, 2015

About the Author

Marc Berthoud

Marc worked on the FORCAST camera for the SOFIA airborne observatory. He now is a staff astronomer at the Yerkes Observatory.

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