the fourth magnitude. The temporary stars of 1572 and 1604 had directed the attention of astronomers to the variableness of the light of stars; and already, in 1050, Bouillaud had approximately determined the period of Mira Ceti, or the star o in the constellation of the Whale. Cassini, who observed sensible variations in the star of the Fox, supposed that its period could be fixed at ten months; but it was sought for in vain in February, 1672; it did not reappear till the end of March, being at that time of the sixth magnitude; then it disappeared once more, and has since never been seen.
Between the star of the Fox and the star discovered on April 28, 1848, by Hind in Ophiuchus (or Serpentarius), 177 years elapsed. The star of 1848, which was of a dark yellow or reddish color, did not exceed the fifth magnitude, but the variations of its light were carefully studied during the whole period of its visibility. In 1850 it was hardly of the eleventh magnitude, and this magnitude it has since kept.
Here we may refer to the researches made since Cassini's time into ancient writings, whether European or Chinese, which show that many similar apparitions of new stars have been noted in chronicles and afterward forgotten: for instance, the star of the year 125 b. c., observed by Hipparchus, as we learn from Pliny; another, which appeared in the Emperor Hadrian's time; the new star seen in the constellation of the Eagle in the year 389, and which possessed a brilliancy resembling that of Venus; that seen in the Scorpion in the ninth century; the new stars of the years 945 and 1264, both of which made their appearance in very nearly the same part of the heavens between Cepheus and Cassiopeia.
Before we come to the two latest temporary stars, which have succeeded each other in an interval often and a half years, and which are worthy of a detailed description, let us briefly state the questions to which these apparitions have given rise among astronomers, and the hypotheses which have been offered for their solution.
To what causes must we refer the nearly always sudden apparitions of these strange bodies, their variations of lustre, their intermittence, as also their changes of color? Why is it that, after alternations of great lustre and of paling, their light gradually faded away, and what is the cause of their ultimate disappearance?
A thousand conjectures have been made in the effort to answer this question. Among the weightiest of these, the one which compares temporary stars to variable periodic stars must be rejected at once—not on the ground that these two classes of stars are absolutely distinct, but because the periodicity being due, either to a movement of rotation or to occultation on the one hand, or to a phenomenon peculiar to the star itself on the other, the first hypothesis is clearly inadmissible for the explanation of new stars, and the second is precisely the question to be solved.
