ht, indeed, collide together, or a collision might
take place between a dark star and a star too faint to be seen even with
the most powerful telescope. The conflagration produced by the impact
would thus appear where nothing had been seen previously. Again, a
similar effect might be produced by a dark body, or a star too faint to
be seen, being heated to incandescence by plunging in its course through
a nebulous mass of matter, of which there are many examples lying about
in space.

The last explanation, which is strongly reminiscent of what takes place
in shooting stars, appears more probable than the collision theory. The
flare-up of new stars continues, indeed, only for a comparatively short
time; whereas a collision between two bodies would, on the other hand,
produce an enormous nebula which might take even millions of years to
cool down. We have, indeed, no record of any such sudden appearance of a
lasting nebula.

The other temporary star, known as Nova Persei, or the new star in the
constellation of Perseus, was discovered early in the morning of
February 22, 1901, also by Dr. Anderson. A day later it had grown to be
brighter than Capella. Photographs which had been taken, some three days
previous to its discovery, of the very region of the sky in which it had
burst forth, were carefully examined, and it was not found in these. At
the end of two days after its discovery Nova Persei had lost one-third
of its light. During the ensuing six months it passed through a series
of remarkable fluctuations, varying in brightness between the third and
fifth magnitudes. In the month of August it was seen to be surrounded by
luminous matter in the form of a nebula, which appeared to be gradually
spreading to some distance around. Taking into consideration the great
way off at which all this was taking place, it looked as if the new star
had ejected matter which was travelling outward with a velocity
equivalent to that of light. The remarkable theory was, however, put
forward by Professor Kapteyn and the late Dr. W.E. Wilson that there
might be after all no actual transmission of matter; but that perhaps
the real explanation was the gradual _illumination_ of hitherto
invisible nebulous matter, as a consequence of the flare-up which had
taken place about six months before. It was, therefore, imagined that
some dark body moving through space at a very rapid rate had plunged
through a mass of invisible nebulous matter, and had