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CHAPTER I 
SURVEY OF THE PROBLEM 
1. At first sight it would seem that the deep interior of the sun and 
stars is less accessible to scientific investigation than any other region of 
the universe. Our telescopes may probe farther and farther into the 
depths of space; but how can we ever obtain certain knowledge of that 
which is hidden behind substantial barriers? What appliance can pierce 
through the outer layers of a star and test the conditions within ? 
The problem does not appear so hopeless when misleading metaphor 
is discarded. It is not our task actively to “probe”; we learn what we 
do learn by awaiting and interpreting the messages dispatched to us by 
the objects of nature. And the interior of a star is not wholly cut off from 
such communication. A gravitational field emanates from it, which sub 
stantial barriers cannot appreciably modify; further, radiant energy from 
the hot interior after many deflections and transformations manages to 
struggle to the surface and begin its journey across space. From these 
two clues alone a chain of deduction can start, which is perhaps the more 
trustwdrthy because it is only possible to employ in it the most universal 
rules of nature—the conservation of energy and momentum, the laws of 
chance and averages, the second law of thermodynamics, the fundamental 
properties of the atom, and so on. There is no more essential uncertainty 
in the knowledge so reached than there is in most scientific inferences. 
We should be unwise to trust scientific inference very far when it 
becomes divorced from opportunity for observational test. We do not, 
however, study the interior of a star merely out of curiosity as to the 
extraordinary conditions prevailing there. It appears that an understand 
ing of the mechanism of the interior throws light on the external manifesta 
tions of the star, and the whole theory is ultimately brought into contact 
with observation. At least that is the goal which we keep in view. 
2 . The gravitational field emanating from the interior and the radiant 
energy streaming out from the interior together control the conditions in 
the shallow layer or atmosphere examined with the telescope and spectro 
scope. We believe that they are by far the most important controlling 
factors. Spectrum analysis detects in the stellar atmospheres chemical 
substances which differ from one star to another; in some helium is 
prominent, in others oxygen, hydrogen, calcium, iron, titanium oxide, 
and so on. But it is not to be supposed that this is an indication of the