ASTRONOMY: H. SHAPLEY 
349 
Sagittarius, and Scorpio, is symmetrical about the point that is indi- 
cated by the distribution of globular clusters as the center of the galactic 
system. It is mainly the absence of globular clusters from low galactic 
latitudes throughout this interval of 70° in longitude that gives rise to 
the phenomenon of a region of avoidance. The diagram shows that the 
distribution of stellar material is probably fairly continuous along the 
galactic plane; from the local cluster the Cepheid variables (and various 
other types of highly luminous galactic stars) extend to the nearer star 
clouds and open clusters, and the latter are recorded among the more 
distant star clouds along the plane nearly as far as the center of the 
system of globular clusters. 
Parsecs 0 10,000 20,000 30,000 40,000 50,000 
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* 
^ 3k 
i-^ 
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* 
• 
— 
* * 
* 
* 
** 

♦ * * 
* 
* 
— ^ 
FIG. 2. DISTRIBUTION OF THE GLOBULAR CLUSTERS (ASTERISKS), OPEN CLUSTERS (OPEN 
CIRCLES), AND CEPHEID VARIABLES (DOTS) THAT FALL BETWEEN 
GALACTIC LONGITUDE 290° AND 360" 
Ordinates are distances from the galactic plane; abscissae are distances along the plane 
If, as appears very probable, the system of globular clusters outlines 
the galactic system, why do we not find large numbers of open clusters 
in the vicinity of and beyond the center, between the two halves of the 
assemblage of globular clusters? Nearer the sun there are some 70 
open groups within the mid-galactic segment — the segment which ap- 
pears to be their natural and only domain. Is the distant central 
region that is devoid of globular clusters also in part avoided either 
actually or apparently by open clusters? The observed scarcity of 
open clusters in this direction leads us to question the reality of the 
avoidance; it may be that patches of obscuring material conceal both 
open and globular clusters, as well as many of the more distant stars, 
