488 
ASTRONOMY: SHAPLEY AND DAVIS Proc. N. A. S. 
sured. Outside this crowded central region the catalogue includes prac- 
tically all cluster stars to the apparent photovisual magnitude 16.8, and 
it is nearly complete to the seventeenth magnitude. Excluding question- 
able values, and also results for 64 stars fainter than pv. mag. 17, 
we have 750 magnitudes available for statistical discussion. 
4. By accepting the parallax of the cluster as 0 ".000072 {Mount Wilson 
Contribution No. 152), zero absolute magnitude is found to correspond to 
apparent magnitude 15.72. The frequency of stars may be determined 
throughout the interval of absolute photovisual magnitude from the 
brightest star in the cluster, —3.5, to a little fainter than +1.0. All the 
stars studied, therefore, are giants, the faintest ones catalogued being some 
twenty times as bright as the sun. 
5. The frequency curves of absolute magnitude, which are graphs of 
the so-called luminosity law, are shown in the accompanying figure for 
red, yellow, and blue stars, separately. The curves are comparable with 
the combined results for three clusters in figure 4 of the preceding commu- 
nication of the series,^ except that in the present case a fainter magni- 
tude is reached. 
The preliminary maximum in the curve for blue stars (and less distinctly 
for yellow stars) at absolute magnitude —0.3 shows the highly significant 
excess of stars at the median magnitude for cluster-type variables.^ The 
variety and lack of symmetry in these luminosity curves, and the certain 
existence of preliminary maxima, suggest the impracticability of using a 
small fragment of the general luminosity curve (all colors combined) as 
a means of estimating the distances of globular clusters^ — a procedure re- 
cently followed by Schouten and by Coeberg with surprizing results. 
6. No conspicuous dependence of mean brightness or color on distance 
from the centre is found in Messier 3. The characteristic relation of mean 
color to absolute brightness appears, much as in figure 3 of the communica- 
tion referred to above; and the maximum frequency of color index falls 
between -|-0.4 and +0.8, quite in agreement with the earlier results for 
Messier 13. 
7. A separate investigation of the color curves and the constancy of 
period is under way for the numerous Cepheid variables in Messier 3. 
It will include an examination of the small light variations that appear in 
a number of stars very similar in color and absolute brightness to the 
typical cluster-type Cepheid, but not heretofore recognized as variable. 
8. The apparent visual magnitude of the cluster as a whole is 6.6. 
The integrated absolute magnitude is, therefore, 6.6—15.7 = —9.1, 
which is equivalent to 360,000 times the light emission of the sun, or to a 
total energy of radiation of 1.4 X 10^^ ergs a second. 
1 Messier 3 = G. C. 3636 = N. G. C. 5272; R. A. = 13^^ 37"^ 35^ Decl. = +28° 52' 
56", 1900; galactic longitude = 8°, galactic latitude = +77°. 
