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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