reasons for suspecting that the processes of evolution in our
Sun, and in other stars as well, may be enormously prolonged
through the influence of energy within the atoms or molecules
of matter composing them. The subatomic forces residing in the
radioactive elements represent the most condensed form of
energy of which we have any conception. It is believed that the
subatomic energy in a mass of radium is at least a million-fold
greater than the energy represented in the combustion or other
chemical transformation of any ordinary substance having the
same mass. These radioactive forces are released with extreme
slowness, in the form of heat or the equivalent; and if these
substances exist moderately in the Sun and stars, as they do in
the Earth, they may well be important factors in prolonging the
lives of these bodies.

Speaking somewhat loosely, I think we may say that the
processes of evolution from an extended nebula to a condensed
nebula and from the latter to a spherical star, are
comparatively rapid, perhaps normally confined to a few tens of
millions of years; but that the further we proceed in the
development process, from the blue star to the yellow, and
possibly but not certainly on to the red star, the slower is
the progress made, for the radiating surface through which all
the energy from the interior must pass becomes smaller and
smaller in proportion to the mass, and the convection currents
which carry heat from the interior to the surface must slow
down in speed.