VARIABLE STARS
Reading edition
In addition to variations in the color of stars, they also vary greatly in brilliance, and certain stars are much brighter at times than at others. In some cases these changes in brilliance are regular; in others, irregular. “Omicron,” for example, which, Bayer recorded in his Atlas in 1603, is a regular variable; its period of change is 331 days, 8 hours; in other words, it reaches its greatest brightness about 12 times in 11 years, when it sometimes attains the brilliancy of a star of the 2nd magnitude, at which brilliancy it remains stationary for about a fortnight. It then diminishes during about three months, until it sinks down to a star of magnitude 9½, or even becomes totally invisible. It remains in this condition for about 5 months, and then gradually recovers—during the next following 3 months—its maximum brilliancy. In other words, its brilliancy is absolutely periodic. Other variables are by no means regular, however, but “come and go” at different intervals.
Various theories have been advanced by way of explanation—one of the simplest being that such stars are in reality double, one being luminous and the other not; and that, during their revolutions, the non-luminous star partially or totally eclipses the bright one, at stated intervals. The whole subject, however, is difficult, and much yet remains to be learned concerning these variable stars.