The resemblance to actual clouds is often startling. Some
are close-packed and dense, like cumuli; some are wispy or mottled,
like cirri. The rifts and modulations, as well as the general
outlines, are the same as those of clouds of vapor or dust, and one
notices also the characteristic thinning out at the edges. But we must
beware of supposing that the component suns are thickly crowded as the
particles forming an ordinary cloud. They look, indeed, as if they
were matted together, because of the irradiation of light, but in
reality millions and billions of miles separate each star from its
neighbors. Nevertheless they form real assemblages, whose members are
far more closely related to one another than is our sun to the stars
around him, and if we were in the Milky Way the aspect of the
nocturnal sky would be marvelously different from its present
appearance.
Stellar clouds are characteristic of the Galaxy and are not found
beyond its borders, except in the ``Magellanic Clouds'' of the
southern hemisphere, which resemble detached portions of the Milky
Way. These singular objects form as striking a peculiarity of the
austral heavens as does the great ``Coal-sack'' described in Chapter
1. But it is their isolation that makes them so remarkable, for their
composition is essentially galactic, and if they were included within
its boundaries they would not appear more wonderful than many other
parts of the Milky Way.
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