Illustration of the CIE 1931 color space. This is a vectorized version of Image:CIExy1931.png by User:PAR. The spectral colours are embedded as a JPEG raster image with SVG blur applied to allow scaling to any size. From the description of Image:CIExy1931.png:

The colors for this diagram were generated using the RGB color space in en:Adobe photoshop. The transformation from xy chromaticity coordinates was done using the en:sRGB color space specification on the [X,Y,Z]=[x,y,1-x-y] tristimulus values, then multiplying by a constant so that one of the R, G, or B values was maximized. Assuming that one's monitor converts Adobe photoshop RGB according to the sRGB color space (probably a good assumption) then, within the sRGB gamut, the chromaticities are correct, but are incorrect outside the gamut. (See the sRGB article for a description of the sRGB gamut). The process of maximizing the value of R, G, or B results in a distinct 3-pointed star in the diagram, centered at the D65 white point. This is because, although the chromaticities are correct, the luminosities (brightness values) are not equal across the diagram. If the luminosities were all made equal, then the entire diagram would be rather dark, since pure blue has a low luminosity. Any attempt to equalize luminosity to remove the star will reduce the overall luminosity of the diagram, and the star will not completely disappear until the diagram is very dark. Alternatively, we could blur the colors to get rid of the star, which would give incorrect chromaticities. I have opted for correct chromaticities at maximum brightness, thus the presence of the star.