ought them (surrounded by glowing
clouds, and protected from undue heat by other clouds), are in truth
seething caldrons of fiery liquid, or gas made viscid by condensation,
with lurid envelopes of belching flames. It was soon made clear, also,
particularly by the studies of Rutherfurd and of Secchi, that stars
differ among themselves in exact constitution or condition. There are
white or Sirian stars, whose spectrum revels in the lines of hydrogen;
yellow or solar stars (our sun being the type), showing various metallic
vapors; and sundry red stars, with banded spectra indicative of carbon
compounds; besides the purely gaseous stars of more recent discovery,
which Professor Pickering had specially studied. Zollner's famous
interpretation of these diversities, as indicative of varying stages
of cooling, has been called in question as to the exact sequence it
postulates, but the general proposition that stars exist under widely
varying conditions of temperature is hardly in dispute.

The assumption that different star types mark varying stages of cooling
has the further support of modern physics, which has been unable to
demonstrate any way in which the sun's radiated energy may be restored,
or otherwise made perpetual, since meteoric impact has been shown to
be--under existing conditions, at any 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
grea