163-165] Liquid Stars 179 
a Russell diagram in fig. 14. We may think of the stars as a vast army 
marching through the Russell diagram, each individual taking his marching 
orders from the rate of generation of energy in his interior. If all individuals of 
the same age were precisely similar, the army would of course march in Indian 
file through the diagram, the position of any individual being entirely fixed 
by his age. As individuals of the same age do not appear to differ very widely 
from one another, we may suppose that the line of march of the army forms a 
fairly clearly marked and well-trodden track. The main army marches down 
the giant branch in the upper half of the diagram to the point where this joins 
the main sequence and then wheeling left, marches down the main sequence. 
At intervals, when a member of the army breaks up by fission and forms 
a binary system, this routine is departed from. Both components of the new 
system are transported over to the left of the diagram and start independent 
evolutionary marches from there. 
The Evolution of Liquid Stars. 
Giant and Main-sequence Stars. 
165. The scheme just described undergoes substantial modification when 
deviation from the gas-laws are taken into account, and the diagram is 
divided up into stable and unstable regions. 
The evolution of the stars must no longer be compared to the steady 
march of an army through a perfectly flat featureless plain, but rather to the 
movements of an army scrambling down, and possibly at times up, a succes 
sion of terraces. The different terraces are the bands of stable configurations 
which correspond to the jamming of atoms ionised down to different rings of 
electrons. A star stands for a time on the terrace corresponding to one ring, 
and then, stepping on to the slippery unstable region between this terrace 
and the next, drops down to the next lower terrace—from the giant branch 
to the main sequence, let us say. 
Apart from the considerations advanced in § 143, which we shall disregard 
for the moment and return to later, the lowest terrace of all, the main 
sequence, does not lead to a further drop down. It is not bounded on its 
further side by a slippery unstable slope but by an impenetrable barrier 
formed by configurations in which the atoms lie as close together as they 
can be packed. In course of time most stars reach this barrier but cannot 
cross it, and sidle along it indefinitely. This explains the great concentration 
of stars along the left-hand edge of the main sequence, against which the 
stars seem to press like flies against a window-pane. 
When a star breaks up by fission into a binary system we have seen that 
both components of the binary are instantaneously transferred a long way 
over to the left of the diagram. If the effective temperature of the parent 
star was fairly high, both components will be thrown right over to the