Star Colors

For doing visualizations of stars, it would be really helpful to know what colors they are. This page details some of the steps required to get reasonably accurate star colors.

NOTE: Write better intro later.

Blackbody Colors

Stars are, to a reasonable approximation, blackbody radiators. Hence, one can start by calculating the proper colors for a blackbody.

To calculate a black body color:

Choose a blackbody temperature
Choose a version of the CIE color matching functions
Integrate over the blackbody spectrum to get CIE xy values
Convert CIE xy to normalized RGB and sRGB

NOTE: Add some comments/references about CIE xy and such.

This procedure is straightforward, but a bit complicated by the choice of color matching functions and white point. A little web searching on the topic, and one finds some web pages by Mitchell Charity. On his pages, he discusses details of the calculation and provides a table of computed values using good choices. He also provides references and comparisons to other web pages that have made other choices.

The table of RGB and sRGB values for the 2 degee color matching functions (which is most appropriate for astronomy) is here:

blackbody_color_mncharity_2deg.txt

When plotted, the values look like this in RGB:

and like this in sRGB:

Here are links to larger versions of these plots:

RGB Linear Temperature Scale: JPEG, TIFF, and Postscript
RGB Logarithmic Temperature Scale: JPEG, TIFF, and Postscript
sRGB Linear Temperature Scale: JPEG, TIFF, and Postscript
sRGB Logarithmic Temperature Scale: JPEG, TIFF, and Postscript

The first thing one should note about this plot is that the RGB colors are normalized - one of the values is always 1. This is because the calculation only provides color, and not brightness. In the HSB (hue-saturation-brightness) color space, only hue and saturation are specified, with brightness a constant at maximum.

The second noteworthy point is the smoothness of the curves. The symbols are plotted at every tenth data point, while the curves are drawn through all the points. The smoothness of the curves indicates that linear interpolation from these tables is good enough for most purposes.

The RGB and sRGB colors associated with these numbers look like this using a linear temperature scale:

and this using a logarithmic temperature scale:

The above images should make it obvious to those who know star colors that a logarithmic temperature scale is more approriate to astronomy.

This images were created with a Renderman shader using BMRT (Blue Moon Rendering Tools):

Renderman Shader: fjs_blackbody_spectrum.sl
RGB Linear temperature scale RIB file: bb_spectrum_rgb_lin.rib
RGB Logarithmic temperature scale RIB file: bb_spectrum_rgb_log.rib
sRGB Linear temperature scale RIB file: bb_spectrum_srgb_lin.rib
sRGB Logarithmic temperature scale RIB file: bb_spectrum_srgb_log.rib

Page last updated on: January 14, 2005