ASTRONOMY: F. H. SEARES 
235 
mately the same color. Further, the result noted by earlier observers is at 
once evident, namely, that the giant G and K stars are redder than the dwarfs 
of the same spectrum. The difference becomes zero for Fs, and is again 
zero, or at least very small, for the M stars, although the difference in 
luminosity between the giants and dwarfs is here about 10 magnitudes with a 
corresponding range in intensity of 1 to 10,000. 
For further study of the relation of color to luminosity we consider groups 
of stars having the same spectral type. As they stand, the data are insuffi- 
cient; but provisionally we proceed by reducing all the stars within any spec- 
tral sub-division to the mean spectrum with the aid of the curves of fig. 1. 
Thus all the B stars are reduced to B5 by applying to the logarithms of the 
B A F G K Ma Mb Me 
9.5 
9.4 
9.3 
9.2 
9.1 
9.0 
8.9 
8.8 
FJG. 1. ORDINATES: COLOR (LOGARITHM OF EXPOSURE-RATIO) 
• ABSCISSA: SPECTRAL TYPE 
The open circles represent mean valves for stars of zero absolute magnitude. The dots 
refer to stars of fainter luminosity, the respective mean absolute magnitudes being 
indicated by the accompanying figures. 
exposure-ratio of each a small correction which makes the colors very nearly 
what they would have been had the types actually been B5 throughout. 
The six groups of results thus obtained have been plotted separately — loga- 
rithms of exposure-ratios as ordinates against absolute magnitudes as abscissae 
— and are shown in figs. 2 and 3. 
The results for the B stars are to be regarded as altogether provisional. 
They suggest that the color decreases, that is, becomes bluer, as the intrinsic 
brightness decreases. The A stars, all of which belong to the Taurus stream, 
increase in color with increasing absolute magnitude. This result seems defi- 
nite, and is confirmed by the Fo and Fl stars derived from the lists of Adams 
and Joy, which are shown by the crosses in fig. 2. When reduced to A5 they 
agree well with the A stars of the Taurus stream. A further confirmation is 
