rate--inadequate. In accordance with the theory of
Helmholtz, the chief supply of solar energy is held to
be contraction of the solar mass itself; and plainly this
must have its limits. Therefore, unless some means as
yet unrecognized is restoring the lost energy to the
stellar bodies, each of them must gradually lose its lustre,
and come to a condition of solidification, seeming
sterility, and frigid darkness. In the case of our own
particular star, according to the estimate of Lord
Kelvin, such a culmination appears likely to occur
within a period of five or six million years.


The Astronomy of the Invisible

But by far the strongest support of such a forecast as
this is furnished by those stellar bodies which even now
appear to have cooled to the final stage of star development
and ceased to shine. Of this class examples in
miniature are furnished by the earth and the smaller of
its companion planets. But there are larger bodies of
the same type out in stellar space--veritable "dark
stars"--invisible, of course, yet nowadays clearly recognized.

The opening up of this "astronomy of the invisible"
is another of the great achievements of the nineteenth
century, and again it is Bessel to whom the honor of
discovery is due. While testing his stars for parallax;
that astute observer was led to infer, from certain
unexplained aberrations of motion, that various stars,