But the identity in substance of earth and sun and
stars was not more clearly shown than the diversity of
their existing physical conditions. It was seen that sun
and stars, far from being the cool, earthlike, habitable
bodies that Herschel thought 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