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 | |
|---|---|---|---|
| -0.33 | 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.
Related questions:
- What do stellar classifications mean?
- What can we learn from the color of a star?
- What is apparent magnitude?
More questions about Stars: Previous | Next
More questions about Stargazing: Previous | Next
How to ask a question:
If you have a follow-up question concerning the above subject, submit it here. If you have a question about another area of astronomy, find the topic you're interested in from the archive on our site menu, or go here for help.
Main Page | About Us | For Teachers | Astronomy Links | Ask a Question | View a Random Question
URL: http://curious.astro.cornell.edu/question.php?number=715
This page has been accessed 9082 times since January 23, 2007.
Last modified: January 23, 2007 5:07:31 PM
Comments? curious@astro.cornell.edu
Legal questions? See our copyright, disclaimer and privacy policy.
Ask an Astronomer is hosted by the Astronomy Department at Cornell University and is produced with PHP and MySQL.
Warning: Your browser is misbehaving! This page might look ugly. (Details)