Page:Popular Science Monthly Volume 69.djvu/185

Rh Reference will be made in what follows only to classes II., IV. and V. New stars may well be considered as a class apart. There is, possibly, no sufficient reason for including them among variable stars, technically so-called. The stars of class III. are few, doubtful, not well understood, and relatively unimportant.

II. Variable stars of long period and large variation in light are perhaps the easiest to observe and the most difficult to interpret of all. Many of them are bright enough to be observed, near maximum at least, by the naked eye. and the variations are so great that observations of the highest precision are not essential for the determination of the light-curves. The length of period ranges in general from 100

days to 400 days. Some of these stars have been observed by different astronomers during the last two centuries, and elaborate investigations concerning them have been made by Argelander, Schönfeld, Chandler, Pickering, and others. Omicron Ceti, or Mira, The Wonderful, has been studied more carefully than any other. Even here, however, much remains to be learned. The light-curve of Mira is shown in Fig. 1, and is fairly typical of the group. The variations in brightness are irregular and a single light-curve can only represent mean results. Irregularity characterizes all the phases; the exact time of any return of maximum is uncertain, and the brightness at different maxima, and at different minima, varies greatly.

The spectrum of stars of this class is in general of Seechi's third type, with heavy banded lines and flutings. A short time before maximum the bright lines of hydrogen appear, and persist till the star has grown somewhat faint again. At least, this is true of Mira, and of