There are many other stars
scattered over its expanse which manifestly owe their origin to the
same source. But compare the general appearance of this nebula with
the others that we have studied, and remark the difference. If the
unmistakably spiral nebul? resemble bursting fly-wheels or grindstones
from whose perimeters torrents of sparks are flying, the Orion Nebula
rather recalls the aspect of a cloud of smoke and fragments produced
by the explosion of a shell. This idea is enforced by the look of the
outer portion farthest from the bright half of the nebula, where
sharply edged clouds with dark spaces behind seem to be billowing away
as if driven by a wind blowing from the center.
Next let us consider what scientific speculation has done in the
effort to explain these mysteries. Laplace's hypothesis can certainly
find no standing ground either in the Orion Nebula or in those of a
spiral configuration, whatever may be its situation with respect to
the grand Nebula of Andromeda, or the ``ring'' and ``planetary''
nebul?. Some other hypothesis more consonant with the appearances must
be found. Among the many that have been proposed the most elaborate is
the ``Planetesimal Hypothesis'' of Professors Chamberlin and Moulton.
It is to be remarked that it applies to the spiral nebul?
distinctively, and not to an apparently chaotic mass of gas like the
vast luminous cloud in Orion.
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