Colour of the stars. The radiation of a star is different for different wave-lengths (λ). As regarding other mass phenomena we may therefore mention:—(1) the total radiation or intensity (I), (2) the mean wave-length (λ0), (3) the dispersion of the wave-length (σ). In the preceding paragraphs we have treated of the total radiation of the stars as this is expressed through their magnitudes. The mean wave-length is pretty closely defined by the colour, whereas the dispersion of the wave-length is found from the spectrum of the stars.
There are blue (B), white (W), yellow (Y) and red (R) stars, and intermediate colours. The exact method is to define the colour through the mean wave-length (and not conversely) or the effective wave-length as it is most usually called, or from the colour-index. We shall revert later to this question. There are, however, a great many direct eye-estimates of the colour of the stars.
Colour corresponding to a given spectrum.
| Sp. | Colour | Number |
| B3 | YW- | 161 |
| A0 | YW- | 788 |
| A5 | YW | 115 |
| F5 | YW, WY- | 295 |
| G5 | WY | 216 |
| K5 | WY+, Y- | 552 |
| M | Y, Y+ | 95 |
| Sum ... | 2222 | |
Spectrum corresponding to a given colour.
| Colour | Sp. | Number |
| W, W+ | A0 | 281 |
| YW- | A0 | 356 |
| YW | A5 | 482 |
| YW+, YW- | F3 | 211 |
| WY | G4 | 264 |
| WY+, Y- | K1 | 289 |
| Y, Y+ | K4 | 254 |
| RY-, RY | K5 | 85 |
| Sum ... | 2222 | |
The signs + and - indicate intermediate shades of colour.
The preceding table drawn up by Dr. Malmquist from the colour observations of Müller and Kempf in Potsdam, shows the connection between the colours of the stars and their spectra.
The Potsdam observations contain all stars north of the celestial equator having an apparent magnitude brighter than 7m.5.
We find from these tables that there is a well-pronounced regression in the correlation between the spectra and the colours of the stars. Taking together all white stars we find the corresponding mean spectral type to be A0, but to A0 corresponds, upon an average, the colour yellow-white. The yellow stars belong in the mean to the K-type, but the K-stars have upon an average a shade of white in the yellow colour. The coefficient of correlation (r) is not easy to compute in this case, because one of the attributes, the colour, is not strictly graduated (i.e. it is not expressed in numbers defining the colour).[5] Using the coefficient of contingency of Pearson, it is, however, possible to find a fairly reliable value of the coefficient of correlation, and Malmquist has in this way found r = +0.85, a rather high value.