It has already been observed that a number of stars are arranged in
clusters of groups, while others, like our own sun, are at vast
distances from their nearest neighbors. Some of these clusters, of which
the Pleiades afford the best example to the naked eye, can be resolved
by a keen eye into separate stars; some, like Præsepe in Cancer, which
only show to the naked eye as a hazy spot of light, break up in a good
field-glass into clusters of stars; but the majority of stellar clusters
require a powerful telescope for their resolution.
It was long ago noticed that, the more powerful a telescope was, the
greater was the number of these hazy spots of light which it would
resolve into clusters of stars. Consequently the opinion was formed that
all the hazy little clouds or nebulæ which are so prevalent throughout a
large part of the sky were simply clusters of stars, so far away that
their light merged into a single impression on the eye. A great number
of these nebulæ were only resolved by large telescopes; many were found
to be irresolvable by any telescope. It was simply concluded from this
that they were still more distant than the clusters which had yielded to
the resolving powers of the telescope; and it was further supposed that
each of these clusters of stars might be a separate universe or galaxy,
comparable in extent and importance with our own universe, bounded by
the vast girdle of the Milky Way.
THE NEBULAR HYPOTHESIS. This grand conception of innumerable universes
scattered throughout space was speedily destroyed by the spectroscope,
which distinguishes with entire certainty between the light sent to us
from a solid star and that emitted by a gas. When it was turned upon the
nebulæ which had been supposed in reality to be star-clusters so distant
that no telescope could resolve them, it showed unmistakably that these
nebulæ were not star-groups, but simply masses of incandescent gas.
Besides, nebulæ vary greatly in form and appearance; some are clearly
clusters of stars, others are perfectly hazy. A round or oval form is
sometimes exhibited, with a gradual condensation towards the center, and
a number of stars standing in the center of a nebulous haze can be
observed. Such observations on nebulæ caused Kant and Laplace to suggest
a theory--now known as the nebular theory--as to the formation of
worlds. They considered that the solar system, for example, originally
existed as uncondensed nebulous matter. This gradually condensed towards
the center, forming the nucleus of the sun, and later the outer parts
separated into distinct parts, each part condensing into a planet. The
different forms of nebulæ observed in the heavens are then supposed to
be systems in different stages of development.