Cousin Ricky wrote:

> The coolest star in the BSC4 has a B-V of 5.74, and is type C6IIe.  (A carbon
> star would screw up the black body correlation, wouldn't it?)  I don't know if
> all the high B-V stars are carbon stars.  Also, I don't know what a carbon star
> looks like.

Ah, you are of course correct. For more exotic spectral classes,
where the color of the star is not dominated by a blackbody spectrum,
you can easily get B-V outside the common "temperature" range. In the
case of carbon stars the abundant carbon in the atmosphere of the star
absorbs much of the blue light that would otherwise be emitted.

To really be certain about this you need to have a spectrum of
the star so you know the spectral class. That information is much
less readily available. About 300K+ stars with known spectrum are
in http://en.wikipedia.org/wiki/Henry_Draper_Catalogue. On the
other hand, if you see a star with B-V > 2.0 it's probably a
good bet that you shouldn't interpret this as temperature.

Now, all of this may be a bit overkill for your purposes, since
you are not trying to understand the stellar physics but actually
want the color for rendering. The values of B, V and R already
represent direct measurement of emission strength for various
optical wavelength ranges. So you might be better of trying to
construct an RGB color from that directly (very crude first
approximation would be to simply convert the magnitudes into
some linear brightness scale and then use <R',V',B'> as RGB).

This is related to human color perception so astrophysical
formulae will not help you much here. Astrophotographers need
to consider similar issues for color balance (see, for example,
B-V calibration at http://astrophoton.com/tips.htm). But they
are taking images with human perception in mind so they don't
have to convert from catalog B-V, of course. Planetarium
software probably does something like this.

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