BEYOND THE MILKY WAY. 561 
small, central, fuzzy mass, surrounded by vast crowds of stars. At the extreme end of 
the sequence we have pure clouds of stars such as our own system” The comparison 
of the cart-wheel remains quite a good one, throughout the second half of the sequence, 
because the nebule here generally have a thick central projection, which we may 
describe as the hub of the wheel, while the rest of their structure is flat. In brief, our 
sequence is one of nebule arranged in order of flatness, and this suggests a simple 
theoretical interpretation of the seyuence. 
We know how increasing the speed of rotation of a body results in a flattening of 
its shape. The ordinary Watt governor provides an obvious instance—-as the engine 
runs faster, it flattens out. The sun rotating only once every 26 days, is an almost 
perfect sphere. The earth, rotating more rapidly, but still very slowly, is slightly 
flattened, so that we usually describe it as orange-shaped. Jupiter rotates more 
rapidly—once every ten hours—and is much flatter in shape. Finally, astronomical 
bodies which are rotating very rapidly may be almost completely flat. It is natural, 
then, to try to interpret our sequence of nebule as one of bodies which are rotating at 
different speeds. And as we know that the speed of rotation of a body increases as 
it shrinks, it seems likely that we may interpret this sequence of nebule as one of 
different stages of development or evolution. If this conjecture is sound, a nebula 
starting with little rotation at first and shrinking in size, would gradually increase 
its speed of rotation as it shrank, and would move steadily along the sequence as it did so. 
The way to test this conjecture is to calculate for ourselves how a mass of rotating 
gas would change in shape as it condensed and shrank. Although the mathematical 
analysis is not simple, and cannot be absolutely precise, it is, I think, fairly conclusive ; 
we find that the evolution of a mass of rotating and shrinking gas would be represented 
exactly by passage along the sequence. 
It is not worth enumerating all the detailed changes which, as theory predicts, 
would accompany this evolution, hut one is worthy of attention. The more highly 
developed nebule do not show a uniform distribution of gas in their outer fringes, 
but an uneven distribution which first forms condensations or knots, and finally 
develops into separate stars, such as we have already seen in the Great Andromeda 
Nebula. Mathematical theory not only predicts this, hut enables us to calculate the 
amounts of gas which would go into each of these condensations ; in other words, we 
can calculate theoretically what the masses of the stars ought to be if our theory is 
sound. It is gratifying to find that these theoretical masses agree pretty well with 
the masses of actual stars. We may, then, be fairly confident that this is the way the 
stars come into being; our sequence of nebular configurations is, in effect, a sort of 
cinematograph film of the birth of the stars. 
We may, then, feel on fairly safe ground in tracing the evolution of the universe 
back from stars to nebule, but how did the nebule themselves come into being? The 
conjecture which at once jumps to the mind is that the nebule may have been formed 
by the same process as the stars; just as the stars came into being as condensations 
in a tenuous, uniformly spread gas—the outer fringes of the nebula—so the nebula may 
themselves have previously come into being as condensations in an earlier mass of 
uniformly spread, tenuous gas. This can never be more than a conjecture, but there 
are strong arguments in its favour, as we shall now see. 
We have already seen how differences in size and brightness between nebulz of 
the same shape are almost entirely due to a distance effect. In other words, the 
faintness of a nebula gives us a measure of its distance. This makes it possible to 
estimate the distances of all nebulz, even the very faintest, with fair accuracy. The 
faintest which can be observed photographically in the 100-inch telescope prove to 
be at the amazing distance of about 140,000,000 light-years. Dr. Hubble finds that 
the two million nebulz which lie within this distance are fairly uniformly spaced at 
about 1,800,000 light-years apart. We can construct a model, by taking apples and 
spacing them at about 10 yards apart, until we have filled a sphere a mile in diameter. 
This will use about 300 tons of apples. This sphere is the part of space we can see 
in the 100-inch telescope ; each apple is a nebula containing matter enough for the 
creation of several thousand million stars like our sun; each atom in each apple is 
as big as Betelgeux with a diameter equal to, or slightly larger than, that of the 
earth’s orbit. 
The circumstance that the nebule are fairly uniformly distributed through space 
certainly supports the conjecture that they may have originated out of a primeval 
gas spread uniformly through space. Moreover, it can be proved that such a gas 
would not stay uniformly spread through space but would break up into condensa- 
1931 00 
