!_ Its visual appearance in the great telescope
was now also that of a planetary nebula. Its spectrum during the first
period of its visibility had been carefully studied, so that the means
existed for making a spectroscopic comparison of the phenomenon in its
two phases. During the first period, when only a stellar spectrum was
noticed, remarkable shiftings of the spectral lines occurred, indicating
that two and perhaps three bodies were concerned in the production of
the light of the new star, one of which was approaching the earth, while
the other or the others receded with velocities of several hundred miles
per second! On the revival in the form of a planetary nebula, while the
character of the spectrum had entirely changed, evidences of rapid
motion in the line of sight remained.

But what was the meaning of all this? Evidently a catastrophe of some
kind had occurred out there in space. The idea of a collision involving
the transformation of the energy of motion into that of light and heat
suggests itself at once. But what were the circumstances of the
collision? Did an extinguished sun, flying blindly through space, plunge
into a vast cloud of meteoric particles, and, under the lashing impact
of so many myriads of missiles, break into superficial incandescence,
while the cosmical wrack through which it had driven remained glowing
with nebulous luminosity? Such an explanation has been offered by
Seeliger. Or was Vogel right when he suggested that Nova Aurigae could
be accounted for by supposing that a wandering dark body had run into
collision with a system of planets surrounding a decrepit sun (and
therefore it is to be hoped uninhabited), and that those planets had
been reduced to vapor and sent spinning by the encounter, the second
outburst of light being caused by an outlying planet of the system
falling a prey to the vagabond destroyer? Or some may prefer the
explanation, based on a theory of Wilsing's, that _two_ great bodies,
partially or wholly opaque and non-luminous at their surfaces, but
liquid hot within, approached one another so closely that the tremendous
strain of their tidal attraction burst their shells asunder so that
their bowels of fire gushed briefly visible, amid a blaze of spouting
vapors. And yet Lockyer thinks that there was no solid or semisolid mass
concerned in the phenomenon at all, but that what occurred was simply
the clash of two immense swarms of meteors that had crossed one
anoth