182 
The Evolution of the Stars [ch. vi 
smaller component will go further to the left of the diagram, and the smaller 
this component is, the further it will go. Thus if the mass-ratio is very 
uneven when fission takes place, there is a grave danger of the smaller com 
ponent becoming a white dwarf. The two systems of well-determined mass- 
ratio in which the ratio is furthest from equality are * Sirius (mf/m—0‘29) and 
Procyon ( m'/m — O'SS ). The less massive component of Sirius is certainly a 
white dwarf and that of Procyon is in all probability one. Even if fission does 
not cause the less massive component to fall into the white dwarf state a 
second fission may do so, since the secondary small components so formed 
tend to be thrown very forcibly against the barrier. Such consideration may 
explain why o 2 Eridani B (of mass only 022 times the sun) is a white dwarf, 
although again we cannot press these interpretations since it is very un 
certain (Chap, xn) whether these systems have originated by fission. In 
any case, a number of single stars of exceptionally small mass and originally 
of high luminosity may press so hard against the barrier as to fall through 
and end as white dwarfs. 
167. Let us next consider the evolution of a star in which no fission occurs. 
In any case this evolution must be in the direction of decreasing mass; if 
we neglect the probable inhibition of energy-generation caused by ionisation, 
it must also be in the direction of continually decreasing bolometric luminosity. 
The curve shewn in fig. 16a reproduces that already shewn in fig. 11 for the 
possible configurations of a star of given mass. As before, stable configurations 
are drawn thick and unstable configurations thin. 
When a contracting star reaches a configuration such as R, its stable track 
comes to an end, and it must drop on to the next stable branch ST. It cannot 
Aitken, Binary Stars, pp. 205 and 216.