[Illustration: Fig. 7.--Cassiopeia; showing where a new star appeared in the year 1572.]
In passing it may be remarked that there are reasons for expecting the return of Tycho Brahé's star in the course of a few years. For other new stars have been recorded as seen in the same part of the heavens in the years 945 and 1264, and though the interval from 945 to 1264 (or 319 years) exceeds by 11 years the interval from 1264 to 1572 (or 308 years), yet the difference is but small by comparison with either entire interval; and we may not unreasonably believe that the three new stars seen in Cassiopeia have been only three apparitions of one and the same star, which shines out, with superior lustre, for a few months, once in a period averaging about 313 years. It seems to me not at all unlikely that, some time during the next twenty years, astronomers will have an opportunity of examining, with the telescope and spectroscope, a star which last appeared before either instrument had been invented.
Already facts are known respecting the so-called new stars which will not permit us to accept the explanations of old so readily offered and admitted, simply because so little was certainly known.
In the year 1866 a star appeared suddenly in the constellation of the Northern Crown, where no star had before been visible to the naked eye. It was a little below the arc of stars forming the celestial coronet.[7]
It shone as a second magnitude star when first seen, but very rapidly diminished in lustre. It increased our knowledge in two important respects.
First, on examining Argelander's charts of the northern heavens, the new star was found to have been observed and charted as a tenth magnitude star, that is, four magnitudes below the lowest limit of naked eye vision. It was not, then, a new sun, though it might still truly be called a new star, in this sense, that it was a new member of the set of stars which adorn our skies as seen by ordinary vision.
In the second place, the star was subject to the searching scrutiny of spectroscopic analysis, with results of a most interesting character.
The reader is no doubt aware that when the light of a star is analysed into its component colours by the instrument called the spectroscope, it is found that all the colours of the rainbow are present, as in the case of solar light, but (also in the sun's case) not all the tints of these colours. Certain dark lines athwart the rainbow-tinted streak, called the _spectrum_ of the star, indicate the presence of absorbing vapours in the star's atmosphere. This general statement is true of every fixed star, though the dark lines of some stars differ in number and position from the dark lines of others, showing that other absorbing vapours are present. In the case of the new star in the Crown, the usual stellar spectrum was shewn,--a rainbow-tinted streak crossed by a number of dark lines. But besides these, there were seen four very bright lines,--lines so bright that the rainbow-tinted streak appeared as a dark background. The meaning of this is well understood by spectroscopists. It signifies that besides the vapours which, being cooler than the star, absorbed a portion of its light, and produced the dark lines, some vapours were present in the star's atmosphere which were a great deal hotter than the star, and so produced bright lines. Now two of the lines corresponded in position with two of the well known lines of the gas hydrogen, showing that this was one of the gases which had been raised to an unusual degree of heat.