168 © ANNUAL REPORT SMITHSONIAN INSTITUTION, 1944 
All these suns which can be recognized distinctly are grouped in a 
sort of flattened, disklike bunch which is whirling in empty space. 
Our own sun and planetary system is a member of this group, being 
located about 30,000 light-years? distant from the center, or hub, of 
this gigantic disk. When we look into space along the plane of the 
disk the stars seem to be distributed very densely. We see the milky 
way. This bunch of suns is called a spiral nebula. It is sometimes 
called a galaxy, or an island universe. The word “universe” in this 
sense has a restricted meaning because our island universe is not the 
only one in existence. There are millions of others distributed 
throughout space as far as our most powerful telescopes have been 
able to penetrate. 
The nebulae are by no means recent discoveries. Sir William 
Herschel, 150 years ago, suspected that they were distant groups of 
stars. The philosopher Kant believed that they were “systems of 
many stars, whose distance presents them in such a narrow space 
that the light which is individually imperceptible from each of them, 
reaches us, on account of their immense multitude, in a uniform pale 
glimmer.” They have been described as looking like “candlelight seen 
through horn.” A rough diagram, not drawn to scale, is given in 
figure 2 to indicate the total extent of the entire universe which has 
been observed, up to the present, with our most powerful telescopes. 
We might now indicate on the linear lay-out of figure 1 the approxi- 
mate size of the largest bunch of matter, the spiral nebula, as 100,000 
light-years. Also we might speculate as to the possibility of nebulae 
themselves forming still larger groups. Extensive surveys have been 
made by the astronomers at Harvard and Mount Wilson, of the dis- 
tribution in space of the nebulae, and there is, indeed, evidence of 
grouping of nebulae. It is legitimate to add another bunch of matter 
to the line lay-out—the supernebula, or supergalaxy. 
The supergalaxy is the largest known aggregation of matter in 
the universe. Its diameter may be of the order of a million light- 
years. At least that is the estimate made by Harlow Shapley of 
the diameter of the group of nebulae in which our own is located. 
Our local group contains perhaps 15 or 20 nebulae, but in some super- 
galaxies there are hundreds of members. 
So far, then, our picture of the universe reveals a granular, or 
atomic structure. We start near the zero point of size, with a particle 
of definite size. A fundamental law of attraction operates to cause 
the small particles to group together to form larger particles, these 
larger particles again group to form still larger particles, and so on 
until we reach the limit of observation, the enormous supergalaxy. 
2A light-year is the distance which light travels in one year. It is approximately 
6,000,000,000,000 miles. 
