EVOLUTION OF THE UNIVERSE. 581 
maximum brightness, getting rid of an enormous amount of energy very 
quickly indeed. 
Since the rate of brightening is very rapid, we infer that the process 
of shrinkage is very rapid—in fact cataclysmic. The process of shrinkage 
is a veritable collapse. In a nova outburst the star is seen to be collapsing 
on itself ; and the suddenness of the collapse, and the resulting enormous 
amount of gravitational energy that must be got rid of in the short time 
available, conspire to produce the huge brightening of the star as observed. 
This sudden liberation of energy produces enormously increased radiation, 
which in turn expels the outer layers of gas. Such is the probable explana- 
tion of the origin of novae or ‘ new stars.’ 
These outer layers, thus expelled, constitute an exceedingly small 
proportion of the whole mass of the star. Thus, the mass of the star, 
after the outburst, is practically the same as before, yet it occupies a much 
smaller volume. Hence, its mean density must be much larger than before. 
The increase in density as observed varies from one hundredfold to ten- 
thousandfold. The gases expelled from the star during the outburst are 
chips from the old block, but the star itself does not remain an old block, 
it becomes very much of a new block—a very dense block. 
We can now link up this widely spread phenomenon with our general 
views on stellar evolution. A star may be supposed to fade, very slowly 
but quite definitely, as it ages. At some particular luminosity it collapses, 
giving rise to the appearance of a nova. This must be due to some 
structural weakness in the star, some insecurity in its foundations. It 
falls on itself like a house of cards. 
Samson, in the Old Testament story, lost his strength when he was 
shorn of his locks. The star also loses its strength when it is shorn of its 
locks. But the shearing of the stellar locks—the liberation of wisps of 
atmospheric gases—is a consequence, not the cause, of the internal 
weakening. 
We know other stars which are very dense—the stars which are known 
as ‘ white dwarfs’ and the nuclei of the planetary nebulae. Planetary 
nebulae are probably exceptionally large ex-novae in which the expelled 
gases linger round the star as a permanently accompanying nebula. It is 
reasonable to assume that the white dwarfs have also undergone the 
process of collapse—that every white dwarf has been at one time a nova. 
Though few white dwarfs are known, all those with which we are 
acquainted are close to the sun, whence it is concluded that they are 
very common in nature. This is in general agreement with the ideas [ 
am describing, since if novae are common the heavens must be littered up 
with ex-novae, and these remain in the form of white dwarfs. We must 
now endeavour to explain the origin of the weakening which causes the 
cataclysm. 
It is probable that all stars possess a degenerate core, a central 
region of matter in the degenerate state. As the luminosity of such 
a star decreases the core will vary im size. The behaviour of the 
core is very complicated, and I have not yet been able to trace 
it in full detail. But it appears that ‘at a certain critical value of 
the luminosity the core increases suddenly in radius, due to collapse 
of the gaseous envelope. This is a consequence of the very low 
