nae eel 
BEYOND THE MILKY WAY. 
BY 
SIR JAMES H. JEANS, F-.R.S. 
Ov earth is one of a system of nine planets which, together with millions of smaller 
bodies, asteroids, comets and meteors, circle round the sun; our sun is one of a 
system of millions of stars which circle about one another: this star-system is one of 
millions of star-systems—-and here so far as we know, the sequence ends abruptly. 
These star-systems are the biggest objects known to science ; there is nothing beyond 
them except the great universe itself. They form the largest subdivision of the 
universe, and it is from this circumstance that they derive their special importance 
to science. 
The star-system which is best known to us is, of course, our own; we call it the 
Galactic system, because it is hounded by the Milky Way. We get the best picture 
which modern science can give us of this system if we think of it as being shaped 
like a cart-wheel, with the sun perhaps a third or a half-way along one of the spokes, 
and rotating like a cart-wheel. We still do not know the size of the wheel with any 
approach to accuracy, but its diameter is probably of the order of two hundred 
thousand light-years. Still less do we know how many stars there are in the whole 
wheel. It is almost certainly greater than a hundred thousand million, and may 
quite well be two, three, four or even five times this number. The hand of light we © 
call the Milky Way is formed hy the combined light of all the stars at great distances 
from the sun, including those which form the rim of the wheel. The wheel is held 
together by the gravitational attractions of the diflerent stars of which it is composed. © 
“As a consequence, the outermost stars move most slowly and take the most time to 
perform a complete revolution, just a3 in the Solar System the outermost planets 
move most slowly and take longest to deseribe their orbits round the sun. The sun 
probably moves round the hub at about 200 miles per second, and takes something 
over 20) million years to perform a complete revolution. ; 
In the early days of astronomy this was assumed to be the only system of stars in 
the sky. Then it began to be conjectured by Kant and Herschel that it was only 
one oi innumerable systems. Recent research has confirmed this conjecture very 
fully. Something like two million such systems can now be observed ; they are the 
objects we descrihe as extra-galactic nebula. 
A random collection of extra-galactic nebule seems at first to show a bewildering — 
variety of size, shape, brightness and constitution, but a scientific study soon reduces | 
them to law and order. It soon transpires that size and brightness go together, and — 
that variations in both originate in differences of distance. The nebula which 
appears small also appears faint ; and does both, merely because it is far away. Thus, 
the faintness of a nebula gives us a measure of its distance ; and we can easily get rid 
of effects which arise from mere differences of distance. 
Also we can avoid all complications caused by the different orientations of the 
nebule Ly a very simple artifice ; we reject all those which are not seen edge on, and 
confine our attention to those that are. We can do this quite recklessly, as some two 
million nebule can be seen in all. : 
We have now eliminated all the purely geometrical effects arising from differences _ 
of distance and of orientation, and are left with real physical differences of shape and — 
constitution. We find that by far the greater number of our edge-on nebule can he 
arranged in a single continuous sequence ; it is, in brief, a sequence which begins wit! 
spheres and ends with flat discs, although other features besides shape change as we 
move along it. The nebula at one end of the sequence consist solely of round, fuzz 
masses—-even the most powerful telescope shows no stars in these. About half wa: 
along, stars begin to appear in the outer edges of the nebule. Then, further along, 
come nebule such as the great Nebula in Andromeda, which consist of a comparativel 
—a re. 
