COSMOGONY—JEANS 153 
as their name implies, are closely packed groups of stars of approxi- 
mately globular shape; seen through a powertul telescope, they lool 
rather like a swarm of bees, and produce the impression of being 
nests or birthplaces of families of stars. Only 69 of these objects 
are known, and, as practically no new ones have been discovered 
since the time of the Herschels, it is likely that there are none left 
to discover. They are all rich in Cepheid variables. Doctor Shap- 
ley finds that the distances of these 69 clusters range from 21,000 to 
216,000 light-years. In this and similar measurements the light- 
year is taken as the unit of distance because it is futile to express 
astronomical distances in terms of miles or other ordinary terrestrial 
standards of measurement. Light takes some eight minutes to travel 
from the sun to the earth, so that in one year it travels about 64,000 
times the distance from the earth to the sun; this is the distance that 
the astronomer describes as one light-year and takes as his unit of 
length. We begin to realize what is meant by the distance of a star 
cluster being hundreds of thousands of light-years if we reflect that 
what our telescope shows us is not the star-cluster as it now is, but 
the cluster as it was when primeval man dwelt on earth. Through 
the long prehistoric ages, through the slow dawn of civilization, and 
through the rise and fall of empires and dynasties, the light which 
left the cluster in remote ages has been traveling toward us at the 
rate of 186,000 miles every second and has only just reached us. 
Quite recently Doctor Hubble, of Mount Wilson Observatory, has 
discovered Cepheid variables in certain of the spiral nebule, and 
so is able to estimate the distances of these nebule. The most 
remote of the nebule so far discussed proves to be the well-known 
“ Andromeda nebula” (M. 31), at a distance of 950,000 light-years; 
others are at comparable distances. Using a slightly different 
method, Doctor Shapley has estimated the distance of the star cloud 
N. G. C. 6822 as being about a million light-years. 
The two objects just mentioned are the most remote at present 
known. Are we to suppose that they fix the approximate limits of 
the universe, or must we look forward to a continual expansion of 
the observed size of the universe as the power of our telescopes 
continually increases? It is not possible to give a final answer to 
this question, but a considerable mass of evidence points to the 
former alternative as being probably the true one. Our sun is 
one of a group of some two or three thousand million stars which 
form a disk-shaped or biscuit-shaped structure girdled by the Milky 
Way. It has long been understood that this particular star group 
can not be of infinite extent. If it were, the sky would appear as a 
continuous blaze of light, and the gravitational force produced by 
this infinite mass of stars would be so intense that our sun and 
