180 
[CH. VI 
The Evolution of the Stars 
left-hand edge of the main sequence, against the impenetrable barrier. 
They cannot cross, so that their evolutionary progress consists in a march 
down the main sequence. This explains why binary stars, and newly formed 
binaries in particular figure so prominently on the left-hand edge of the main 
sequence. So much is this the case that perhaps the best way of mapping out 
the edge of the main sequence is through a succession of binaries which 
appear to have recently broken up by fission—Pearce’s star, V Puppis, 
u Herculis, /3 Aurigae, etc. 
As a consequence the majority of newly formed binaries are of the early 
spectral types 0, B and A. The astronomer of only a decade ago explained this 
by saying that stars of types 0 and B were specially liable to fission; he 
thought of the spectral type as something so permanently attached to the con 
stitution of the star that even a cataclysm like fission could not alter it. Our 
explanation is rather that giant F, G, K and M stars shew a certain tendency 
to fission, but that as soon as fission has taken place the star ceases to be an 
F, G, K or M -type star; its components change their spectral type and 
become 0 or 5-type stars. In brief 0 and 5-type stars do not shew a tendency 
to fission, but to have fissioned. 
We can trace this tendency to its origin. Fission, as we shall see in a 
subsequent chapter, results from a star having more angular momentum than 
it can carry without bursting. As a star’s dimensions shrink, its angular 
momentum remains constant except for a slow loss in the form of radiation, 
but its capacity for carrying angular momentum diminishes. If a sudden 
shrinkage occurs, the star’s angular momentum remains constant while its 
capacity for carrying it undergoes a sudden decrease, and this may result in 
fission. Such a sudden shrinkage occurs when a star reaches the unstable edge 
of the giant branch and suddenly drops down to the main sequence. Even if 
fission does not occur, such a star becomes a main sequence, and so probably 
an early type, star—this explains the existence of 0 and 5-type stars which are 
not binaries—but if fission occurs the star not only becomes an early type 
star, but its components are thrown over to the extreme left-hand edge of the 
main sequence. 
The same process can of course occur in other parts of the diagram. For 
instance a giant star ionised only down to its M -ring may suddenly reach the 
limits of stability and undergo fission in falling down to the giant branch 
of L -ring stars. Such considerations probably explain the origin of giant 
binaries such as Capella and W Crucis. 
White Dwarfs. 
166. For the sake of simplicity in exposition, wre have spoken of the 
left-hand edge of the main sequence as forming an impenetrable barrier. 
Actually, as we have seen, the barrier is not absolutely impenetrable. The 
more a star presses against the barrier the greater (in general terms) become