h in no way distinguishable from others by any
apparent change of place, nor by any difference of appearance in
telescopes, yet undergo a more or less regular periodical increase and
diminution of lustre, involving in one or two cases a complete
extinction and revival. These are called periodic stars. The longest
known, and one of the most remarkable, is the star _Omicron_ in the
constellation Cetus (sometimes called Mira Ceti), which was first
noticed as variable by Fabricius in 1596. It appears about twelve times
in eleven years, remains at its greatest brightness about a fortnight,
being then on some occasions equal to a large star of the second
magnitude, decreases during about three months, till it becomes
completely invisible to the naked eye, in which state it remains about
five months, and continues increasing during the remainder of its
period. Such is the general course of its phases. But the mean period
above assigned would appear to be subject to a cyclical fluctuation
embracing eighty-eight such periods, and having the effect of gradually
lengthening and shortening alternately those intervals to the extent of
twenty-five days one way and the other. The irregularities in the degree
of brightness attained at the maximum are also periodical.

Such irregularities prepare us for other phenomena of stellar variation
which have hitherto been reduced to no law of periodicity--the phenomena
of temporary stars which have appeared from time to time in different
parts of the heavens blazing forth with extraordinary lustre, and after
remaining awhile, apparently immovable, have died away and left no
trace. In the years 945, 1264, and 1572 brilliant stars appeared in the
region of the heavens between Cepheus and Cassiopeia; and we may suspect
them, with Goodricke, to be one and the same star with a period of 312,
or perhaps 156 years. The appearance of the star of 1572 was so sudden
that Tycho Brahe, a celebrated Dutch astronomer, returning one evening
from his laboratory to his dwellinghouse, was surprised to find a group
of country people gazing at a star which he was sure did not exist half
an hour before. This was the star in question. It was then as bright as
Sirius, and continued to increase till it surpassed Jupiter when
brightest, and was visible at midday. It began to diminish in December
of the same year, and in March 1574 had entirely disappeared.

In 1803 it was announced by Sir William Herschel that there ex