Vol. 7. 1921 ASTRONOMY: SHAPLE Y AND MA YBERR Y 
153 
but it is now possible with the aid of the variable stars to determine the dis- 
tance much more accurately. 
The following photographs of the cluster have been examined in the 
stereocomparator for the detection of variable stars. 
TABLE 1 
TABLE 2 
PLATE 
DATE 
TELESCOPE 
EXPOS- 
STAR 
X 
Y 
No. 
Kind 
URE TIME 
4168 
S27 
1917 Nov. 
10.387 
60-inch 
15 m 
1 
— 177". 6 
+ 113". 8 
5168a* 
S30 
1919 Dec. 
24.284 
60-inch 
30 
2 
—38.0 
—38.0 
5402 
S30 
1920 July 
16 . 622 
60-inch 
70 
3 
—27.6 
+32.8 
5500 
S30 
1920 Aug. 
10.515 
60-inch 
50 
4 
—24.2 
—42.2 
P 9 
S30 
1920 Sept. 
11.428 
100-inch 
30 
5 
—24.2 
+36.2 
P 23 
S30 
1920 Sept. 
12.381 
100-inch 
40 
6 
—15.5 
—44.8 
P 29 
S30 
1920 Sept. 
12.556 
100-inch 
30 
7 
0.0 
—38.0 
5601 
S30 
1920 Sept. 
14.312 
60-inch 
45 
8 
+31.0 
+ 13.8 
P 50 
S30 
1920 Sept. 
15.379 
60-inch 
60 
9 
+36.2 
+ 15.5 
10 
+38.0 
—13.8 
*Plate 5168a was made by Mr. Hoge, the others by Mr. 
Shapley . 
11 
+ 141.4 
+48.3 
Seven of the eleven variables found on the above plates show conspic- 
uous variation in the course of one night, and the other four appear also to 
be short period variables. In magnitude and range they are very much 
alike, and in this cluster, as in Messier 3, co Centauri and other systems, 
the variables are between one and two magnitudes fainter than the brightest 
stars. It is probably safe to assume, therefore, that these faint variable 
stars are typical short-period Cepheids, similar to those commonly found 
in globular clusters. 
The coordinates of the variable stars, referred to the center of the 
cluster, are given in seconds of arc in table II. Nos. 2 and 5 were found or 
suspected by Miss Ritchie, the others by Miss Mayberry. 
From fifty-three measures of the brightness of the eleven variable stars 
on the plates made with the 60-inch reflector, a median magnitude of 18.96 
is found. The median for the seven that appear most certainly to be short 
period Cepheids is also 18.96. The photographic magnitudes 4 of the vari- 
ables are based on the results for the bright stars of the cluster, published 
in Mt. Wilson Contr., No. 156, p. 4, 1918. 
The observed range in photographic magnitude for the eleven variables 
is 0.83; for the seven variables it is 0.99. 
If we adopt from Mt. Wilson Contr., No. 151, the value —0.23 as the me- 
dian photographic absolute magnitude of short period Cepheids, we have 
m — M = 19.2 = 5 (log d— 1), from which the distance, d, is found to be 
69,000 parsecs. Expressed as a parallax, this is 0*. 0000145 — a highly 
satisfactory confirmation of the former results. 
Although N. G. C. 7006 is the smallest known cluster in angular diameter, 
its linear dimensions are comparable with those of Messier 13. The di- 
