this, such objects invariably fade away after a few months, and
are changed into something very like an ordinary nebula. A
question of transcendent interest is that of the cause of these
outbursts. It cannot be said that science has, up to the present
time, been able to offer any suggestion not open to question. The
most definite one is the collision theory, according to which the
outburst is due to the clashing together of two stars, one or both
of which might previously have been dark, like a planet. The stars
which may be actually photographed probably exceed one hundred
millions in number, and those which give too little light to
affect the photographic plate may be vastly more numerous than
those which do. Dark stars revolve around bright ones in an
infinite variety of ways, and complex systems of bodies, the
members of which powerfully attract each other, are the rule
throughout the universe. Moreover, we can set no limit to the
possible number of dark or invisible stars that may be flying
through the celestial spaces. While, therefore, we cannot regard
the theory of collision as established, it seems to be the only
one yet put forth which can lay any claim to a scientific basis.
What gives most color to it is the extreme suddenness with which
the new stars, so far as has yet been observed, invariably blaze
forth. In almost every case it has been only two or three days
from the time that the existence of such an object became known
until it had attained nearly its full brightness. In fact, it
would seem that in the case of the star in Perseus, as in most
other cases, the greater part of the outburst took place within
the space of twenty-four hours. This suddenness and rapidity is
exactly what would be the result of a collision.

The most inexplicable feature of all is the rapid formation of a