184 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1926 
Let us admit that the island universe theory is true, and that our 
galaxy of stars is one among the many galaxies represented by the 
spiral nebulae. What is found is this: 
The various galaxies represented as spiral nebulae rush hither and 
yonder in all directions at speeds ranging up to several hundred 
miles a second. Our own galaxy is rushing from the general center 
of them along a way represented in direction by a line drawn toward 
the constellation Sagittarius, at the rate of about 200 miles per 
second. Its older and smaller stars are more scattered from the com- 
mon direction than the younger and more massive ones, so that, 
for example, the diffusion of yellow G type stars much exceeds that 
of the blue B type stars. Our sun happens to be moving in a direc- 
tion not very far from along the general direction of our whole 
galaxy, but at a somewhat faster rate than the average of its stars, 
and at the present time is nearer to the center than to the circum- 
ference of our lens-shaped system of stars. 
19. Finally, we may inquire how numerous are the heavenly bodies, 
and whether there are limits to the extent of the entire universe. 
In our own galaxy it is believed that the number of the stars reaches 
or exceeds 30 billions. Its greater and lesser diameters may be set 
at perhaps 100,000 and 20,000 light-years. Of spiral nebulae which 
could be photographed by the greatest telescopes, there are little 
less than 1,000,000. Those whose distances have been measured prove 
to be about 1,000,000 light-years distant. Smaller ones may very 
likely be much farther away. Indeed, one can not set bounds to the 
whole universe. There may be other galaxies far beyond the faint- 
est which our photographs reveal, and others still beyond these, 
indefinitely. Possibly their individual star populations may 
approach or exceed that of our own. We can not know that they do 
not. Mathematics, indeed, as Professor Moulton has shown, informs 
us that if space were infinite and populated with galaxies, they 
could not be distributed with approximate uniformity in this infinite 
extension as the dust particles are in a room. It would, however, 
be possible that if galaxies in great numbers constitute supergalaxies 
of enormous dimensions compared with individual galaxies, and if 
these supergalaxies are the units of which supergalaxies of the second 
order are composed, and so on in an unending sequence, each cosmic 
unit being made up of smaller units which are very far apart com- 
pared with their dimensions, then there will be no contradictions 
with observational evidence or dynamical requirements. 
In contemplation of these things, one has more reason than the 
psalmist had to exclaim: “ When I consider the heavens, the work 
of Thy fingers, the moon and the stars, which Thou has ordained; 
what is man that Thou art mindful of him and the Son of Man that 
Thou visitest him ?” 
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