200-INCH HALE TELESCOPE—HUBBLE 183 
as a swarm of bees drifts through the air. From our position some- 
where within the system, we look out through the swarm of stars, 
past the boundaries, into the universe beyond. 
Those outer regions are empty for the most part—vast stretches 
of empty space. But here and there, scattered at immense intervals, 
we find other stellar systems comparable with our own. They are so 
remote that individual stars can be seen only in a few of the nearest 
systems. In general they appear as faint patches of light, resembling 
tiny clouds, and have long been called by the Latin word for clouds— 
that is, ‘‘nebulae.”’ 
We now know that these nebulae are huge stellar systems averaging 
about a hundred million times as bright as the sun. They are the 
true inhabitants of space—vast beacons that serve as landmarks for 
the exploration of the universe. We see a few that appear large and 
bright. These are the nearer nebulae. Then we find them smaller 
and fainter in constantly increasing numbers, and we know we are 
reaching out into space farther and ever farther, until, with the faintest 
nebulae that can be detected with the largest telescope, we have 
reached the frontiers of the observable region. 
This region has been explored with the 100-inch out to distances so 
remote that light, speeding at 186,000 miles per second, requires 500 
million years to make the journey. Thus the observable region at 
present is a sphere, centered on the observer, with a radius of about 
500 million light-years. Throughout this sphere about a hundred 
million nebulae are scattered, each a stellar system comparable to 
our own system of the Milky Way. 
The study of this observable region as a sample of the universe has 
led to the recognition of two large-scale features. The first feature 
is homogeneity. The nebulae are scattered singly, in groups, and 
even in great clusters, but when very large volumes of space are com- 
pared, their contents are found to be quite similar. On the grand 
scale, the observable region appears to be very much the same, in all 
directions and at all distances. 
The second characteristic is the fact that light waves from distant 
nebulae seem to grow longer in proportion to the distance they have 
traveled. It is as though the stations on your radio dial were all 
shifted toward the longer wave lengths in proportion to the distances 
of the stations. In the nebular spectra the stations (or lines) are 
shifted toward the red, and these red-shifts vary directly with dis- 
tance—an approximately linear relation. 
The red-shifts are most easily interpreted as evidence of motion in 
the line of sight away from the earth—as evidence that the nebulae 
in all directions are rushing away from us, and that the farther away 
they are, the faster they are receding. This interpretation lends itself 
directly to theories of an expanding universe. The interpretation is 
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