Page:The fairy tales of science.djvu/229

Rh like that of Sirius, supposing the two bodies to be placed at the same distance from us.

Several among the stars exhibit the most remarkable phenomenon of a regular periodical increase and diminution of lustre, involving, in some rare instances, an alternate total extinction and revival. These are called periodical, or variable stars. One of the most remarkable is the star Omicron, in the constellation Cetus, which has a period of 334 days. It remains about a fortnight at its greatest brightness, equal to a large star of the second magnitude; it then decreases during about three months until it disappears altogether; after remaining invisible during about five months, it reappears again, and continues increasing in brilliancy during the remaining three months of its period. It shows, however, occasionally considerable irregularity in its phases, and has actually been known on one occasion to remain altogether invisible during more than four years (between October, 1672, and December, 1676). Another remarkable specimen of a variable star is Beta, in the constellation of Perseus. The whole period of change of this star is rather less than 2 days 20 hours and 49 minutes, during which time it varies in brightness from the second