582 DISCUSSION ON THE 
opacity of degenerate matter. Degenerate matter obstructs the passage 
of radiation far less strongly than gaseous matter, and so experiences a 
smaller radiation pressure ; accordingly (at a certain luminosity) as soon 
as more degenerate matter is deposited on the core, radiation pressure 
weakens still further, further collapse occurs, more degenerate matter is 
deposited, and so on. The star has a canker at the core. Or, to put it 
another way, degeneracy is an internal disease which propagates itself 
with startling rapidity under favourable stellar conditions. The outward 
and visible symptom of the disease is the nova-phenomenon ; and, as is 
not unknown in medicine, a high surface temperature is developed. The 
existence of a discontinuity in core radius can be established mathe- 
matically, but I only provisionally identify this particular discontinuity 
with the observed discontinuity in Nature. We at least obtain a hint as 
to the character of one of the most important events in the evolutionary 
history of a star. 
But collapse alone is not the only possibility. If the collapsing star 
is in a state of rotation the increased angular velocity consequent on 
collapse may cause the star to divide into two, in the manner described 
by Jeans. Something of this kind has been observed in the case of Nova 
Pictoris. The two fragments so produced may remain collapsed (and 
thus very dense), or in certain circumstances, provided as they are with an 
internal source of energy, one or both may re-expand and re-form even 
giant stars. This may be the origin of the great frequency of occurrence 
of double stars in nature. In the fable the mountains gave birth to a 
mouse. In the heavens the dwarf may give birth to giants. 
The reduced opacity of degenerate matter, for which indirectly the 
nova phenomenon is evidence, has an important bearing on the source of 
stellar energy. It has usually been concluded that the white dwarf stars, 
which are amongst the least luminous of stars, have the hottest interiors, 
This has always been a difficulty, for if temperature has any influence 
whatever on generation of energy, we should have then that the hottest 
stars were the worst generators of energy. But this difficulty now dis- 
appears. For if the opacity of degenerate matter is very small, the 
interior of a white dwarf will be almost isothermal, and its temperature 
will depend on the thickness and opacity of the gaseous fringe or envelope 
which encases the degenerate interior. This will be modified by con- 
ductivity, but it is at least clear that the interiors of white dwarf stars 
must be amongst the coolest, not the hottest, of stellar interiors. In other 
words, we find what we should expect, that the coolest interiors are the 
worst generators of energy. For complete transparency of its interior 
Sirius B would have a temperature of at most 15 or 20 million degrees. 
Sir James Jeans recently drew my attention to certain aspects of hot- 
water engineering—or, as he put it, ‘ what every plumber knows.’ He 
pointed out that every plumber knows that a thin layer of asbestos is 
inadequate for keeping a hot-water container very hot. But the point 
is that if the hot-water container is surrounded by only a thin film of 
ashestos, we do not thereby infer that the asbestos is of no importance ; 
we infer that the water in the container is cold! The Celestial Plumber (if 
Dr. Barnes will permit me the phrase) knows this also; and we see the 
result in the internal state of the white dwarfs. 
poe 
gee Pete 
