Vol.. 6, 1920 
ASTRONOMY: H. SHAPLEY 
295 
Spectral types conceals the important differences for red and blue stars 
of the giant-dwarf phenomena; and the spectral (or color) curve that 
includes a great range of absolute magnitude is of limited value because 
FIG. 2 
Integrated color curve for 1145 giant stars of all 
absolute magnitudes brighter than zero. Coordinates 
as in Fig. 1. 
it integrates indiscriminately the various masses, densities, ages, and 
other factors that appear to change with luminosity. 
The study of magnitudes and colors in star clusters affords, however, 
for the brightest of giant stars, a fairly accurate determination of the 
frequencies of magnitudes and colors for small intervals of color and 
+0.4 +0.6 +0.8 +1.0 +1.2 
FIG. 3 
Change of mean color index with brightness for 1143 giant stars 
in clusters. Ordinates are absolute photo visual magnitudes; ab- 
scissae are color indices. (See last three columns of Table I.) 
magnitude, respectively. The relative apparent magnitudes and dis- 
tances, which in most cases are prerequisite to a knowledge of absolute 
brightness, are sufficiently known in nine systems to permit a combina- 
tion of all observations for a statistical investigation of the above-men- 
tioned astrophysical laws. 
The discussion of the observational data for 1152 stars brighter than 
absolute photovisual magnitude zero is summarized in the tabular results 
and illustrative curves of the present note. 
