ASTRONOMY: F. H. SHARES 
481 
A NOTATION FOR USE IN THE DISCUSSION OF STAR COLORS 
By Frederick H. Seares 
MOUNT WILSON SOLAR OBSERVATORY. CARNEGIE INSTITUTION OF WASHINGTON 
Presented to the Academy, August 4, 1915 
The extension of absolute scales of photographic and photovisual 
magnitudes to the fainter stars provides a method of determining the 
colors of objects at present beyond the reach of spectroscopic investi- 
gation. For the statistical discussion of such color results it is con- 
venient to adopt, tentatively at least, in place of the conventional color 
index, a notation similar to that used for spectral classification. Where 
for the latter we employ the letters B, A, F, G, etc., to represent various 
spectral types, we choose h, a^f, g, etc., to indicate different classes of 
color. When a more exact specification is required, a decimal subdivision 
of the classes may be employed. 
Spectral type is determined by the number, character, and distribu- 
tion of the lines in the spectrum. Color class depends upon the relative 
intensity of the continuous spectrum background in two different re- 
gions whose location varies somewhat with the instrumental equipment 
and the method of observing. Since both spectral type and color class 
stand in an intimate relation to the temperature, they are necessarily 
connected with each other, and when properly defined the color class 
indicates at once the general character of the spectrum. To secure this 
advantage, the classes b, a, f, etc., are assumed to represent the colors 
corresponding to the mean for typical spectra of the classes B, A, F, etc., 
respectively. 
We cannot assume that there is an invariable correspondence between 
b and B, a and A,f and F, etc., for the observed color C, which specifically 
is represented by some one of the letters b, a, f, etc., may consist of at 
least three elements, namely, 
C = Cs^ Cm,s + (1) 
The terms in the right member are functions of the quantities repre- 
sented by the subscript letters. Thus Cs depends on the spectrum 5; 
it is the average color index corresponding to S, a mean result derived 
from a large number of stars. Three or four determinations are available, 
for example, those of King, Parkhurst, and Schwarzschild. For the 
Mount Wilson photometric system we may adopt provisionally 
Cs = 0.40 5, (2) 
in which S has the values —1, 0, etc., according as the spectrum 
is 50, ^0, FO, etc. Cs is thus a part of the observed color index C and 
is expressed in magnitudes. 
