CHAPTER XIV 
THE GALACTIC SYSTEM OF STARS 
330. As we have seen (§ 13), our sun is a member of a huge system of 
stars whose number must be counted in thousands of millions. In general 
shape this system may be compared to an oblate spheroid with very unequal 
axes, or, less mathematically, to a coin or round biscuit. The stars are not uni 
formly distributed throughout this system, being much more thickly scattered 
in its central parts than in its outer regions. Probably there is no clearly 
defined boundary, the star-density diminishing indefinitely as we recede from 
the centre, but never becoming quite zero. The sun lies almost exactly in the 
central plane of the system, although not precisely at the centre. Those stars 
which lie near the edge of the coin or biscuit are so remote as to appear very 
faint to us and constitute the Milky Way. The system of stars bounded by 
the Milky Way is commonly called the Galactic System. 
The stars shew so little motion that for a long time astronomers failed to 
detect any motion at all, and they became known as “ fixed stars ” to dis 
tinguish them from the planets or “wandering stars” whose motion was 
obvious to everyone. But modern astronomy finds it possible to measure the 
motions of a great number of stars. By measuring a stars apparent displace 
ment in the sky over a number of years, it is possible to determine its angular 
motion round the sun, and if the star’s distance is known this can immediately 
be translated into an actual velocity of kilometres per second. By measuring 
the displacement of lines in the star’s spectrum, it is often possible to determine 
the speed with which the star is approaching, or receding from, the sun. The 
combination of both methods makes it possible to determine the star’s motion 
completely, and to announce that it is moving at so many kilometres a 
second in such or such a direction. 
There are two ways of studying the motion of a large collection of in 
dependently moving bodies, such as stars, the molecules of a gas, or the 
asteroids in the solar system. It may be possible to measure the motion of 
each individual and so acquire a complete knowledge of the motion in question. 
If this is impossible it may still be possible, by the use of statistical methods, 
to discover certain general characteristics of the motion. In the kinetic theory 
of gases, for instance, no attempt is made to follow the motions of individual 
molecules, but a general statistical treatment shews that they must neces 
sarily move equally in all directions, no preference being shewn for any one 
direction over others, and Maxwell’s well-known law gives the statistical 
distribution between different velocities. By contrast, the motion of the 
asteroids shews very marked preferential motion, being confined very closely