But the theories about Nova Persei have been almost as numerous as the
astronomers who have speculated about it. One of the most startling of
them assumed that the outburst was caused by the running amuck of a
dark star which had encountered another star surrounded with planets,
the renewed outbreaks of light after the principal one had faded being
due to the successive running down of the unfortunate planets! Yet
another hypothesis is based on what we have already said of the tidal
influence that two close approaching suns would have upon each other.
Supposing two such bodies which had become encrusted, but remained
incandescent and fluid within, to approach within almost striking
distance; they would whirl each other about their common center of
gravity, and at the same time their shells would burst under the tidal
strain, and their glowing nuclei being disclosed would produce a great
outburst of light. Applying this theory to a ``nova,'' like that of
1866 in the ``Northern Crown,'' which had been visible as a small star
before the outbreak, and which afterward resumed its former aspect, we
should have to assume that a yet shining sun had been approached by a
dark body whose attraction temporarily burst open its photosphere. It
might be supposed that in this case the dark body was too far advanced
in cooling to suffer the same fate from the tidal pull of its victim.
But a close approach of that kind would be expected to result in the
formation of a binary system, with orbits of great eccentricity,
perhaps, and after the lapse of a certain time the outburst should be
renewed by another approximation of the two bodies. A temporary star
of that kind would rather be ranked as a variable.

The celebrated French astronomer, Janssen, had a different theory of
Nova Persei, and of temporary stars in general. According to his idea,
such phenomena might be the result of chemical changes taking place in
a sun without interference by, or collision with, another body.