52 Recent Advances in Stellar Astronomy
It is now generally accepted that the atoms of the vari-
ous chemical elements are complicated structures, consist-
ing of a very minute and heavy nucleus, carrying a posi-
tive electrical charge, surrounded (at a great distance in
comparison with its own size) by electrons, in numbers
just sufficient to neutralize the charge on the nucleus (so
far as its action on things outside the atom is concerned),
and arranged in some sort of successive rings, layers, or
shells—each much larger than the last—though even the
outermost is perhaps a hundred-millionth of an inch in
diameter. These electrons are probably not at rest, but
in motion; and there appear to be certain possible steady
states of motion, in which they can continue to move for
long intervals, if not indefinitely. While this happens, the
atom neither emits nor absorbs radiation; but if one of
the electrons is shifted from one of these steady states
to another, radiation takes place. If the electron has less
energy in the new position than in the old, the balance
is liberated in the form of monochromatic radiation—
a train of waves of a definite rate, or frequency, of vibra-
tion. If the change is in the other direction, the energy
necessary to effect it is absorbed. In either case, the fre-
quency of the radiation is exactly proportional to the
amount of energy involved in the change of state on the
part of the electron—the factor of proportionality being
the “quantum constant” h, which keeps cropping up in
all sorts of physical problems, but whose real nature still
evades our analysis.
When the energy change is large, the vibrations are
very rapid. For example, if one of the innermost elec-
trons, close to the nucleus, is shifted to an outer level,
and falls back again, we get radiations of very short
wave length—X-rays, in fact. But if one of the outer-