352. Variable Stars.—There are many stars which undergo changes of brilliancy, sometimes slight, but occasionally very marked. These changes are in some cases apparently irregular, and in others periodic. All such stars are said to be variable, though the term is applied especially to those stars whose variability is periodic.
Fig. 416.
353. Algol.—Algol, a star of Perseus, whose position is shown in Fig. 416, is a remarkable variable star of a short period. Usually it shines as a faint second-magnitude star; but at intervals of a little less than three days it fades to the fourth magnitude for a few hours, and then regains its former brightness. These changes were first noticed some two centuries ago, but it was not till 1782 that they were accurately observed. The period is now known to be two days, twenty hours, forty-nine minutes. It takes about four hours and a half to fade away, and four hours more to recover its brilliancy. Near the beginning and end of the variations, the change is very slow, so that there are not more than five or six hours during which an ordinary observer would see that the star was less bright than usual.
This variation of light was at first explained by supposing that a large dark planet was revolving round Algol, and passed over its face at every revolution, thus cutting off a portion of its light; but there are small irregularities in the variation, which this theory does not account for.
354. Mira.—Another remarkable variable star is Omicron Ceti, or Mira (that is, the wonderful star). It is generally invisible to the naked eye; but at intervals of about eleven months it shines forth as a star of the second or third magnitude. It is about forty days from the time it becomes visible until it attains its greatest brightness, and is then about two months in fading to invisibility; so that its increase of brilliancy is more rapid than its waning. Its period is quite irregular, ranging from ten to twelve months; so that the times of its appearance cannot be predicted with certainty. Its maximum brightness is also variable, being sometimes of the second magnitude, and at others only of the third or fourth.
Fig. 417.
355. Eta Argus.—Perhaps the most extraordinary variable star in the heavens is Eta Argus, in the constellation Argo, or the Ship, in the southern hemisphere (Fig. 417). The first careful observations of its variability were made by Sir John Herschel while at the Cape of Good Hope. He says, "It was on the 16th of December, 1837, that, resuming the photometrical comparisons, my astonishment was excited by the appearance of a new candidate for distinction among the very brightest stars of the first magnitude in a part of the heavens where, being perfectly familiar with it, I was certain that no such brilliant object had before been seen. After a momentary hesitation, the natural consequence of a phenomenon so utterly unexpected, and referring to a map for its configuration with other conspicuous stars in the neighborhood, I became satisfied of its identity with my old acquaintance, Eta Argus. Its light was, however, nearly tripled. While yet low, it equalled Rigel, and, when it attained some altitude, was decidedly greater." It continued to increase until Jan. 2, 1838, then faded a little till April following, though it was still as bright as Aldebaran. In 1842 and 1843 it blazed up brighter than ever, and in March of the latter year was second only to Sirius. During the twenty-five years following it slowly but steadily diminished. In 1867 it was barely visible to the naked eye; and the next year it vanished entirely from the unassisted view, and has not yet begun to recover its brightness. The curve in Fig. 418 shows the change in brightness of this remarkable star. The numbers at the bottom show the years of the century, and those at the side the brightness of the star.
