STAR 311 distance as to present a milky luminosity. Many stars are variable in brilliancy. These may be divided into periodic variables, irreg- ular variables, and temporary stars. Periodic variable stars are those which undergo in- crease and diminution of light at regular in- tervals. Thus the star Mira or o Ceti varies in lustre, in a period of 33 1 days, from the second magnitude to a faintness such that the star can only be seen with a powerful tele- scope, and thence to the second magnitude again. It shines for about a fortnight as a star of the second magnitude, and then re- mains invisible for five months, the decrease of lustre occupying about three months, the increase about seven weeks. " Such," says Sir J. Herschel, " is the general course of its phases. It does not always, however, return to the same degree of brightness, nor increase and diminish by the same gradations; nei- ther are the successive intervals of its maxi- ma equal. From recent observations and in- quiries into its history by Argelander, the lean period would appear to be subject to a pclical fluctuation, embracing 88 such peri- " 3, and having the effect of gradually length- ling and shortening alternately those inter- nals to the extent of 25 days one way and the "ler. The irregularities in the degree of )rightness attained at the maximum are prob- ")ly also periodical." These irregularities are msiderable. Thus between October, 1672, id December, 1676, Mira was never visible the naked eye, while on Oct. 5, 1839, it is half a magnitude above its usual bright- outshining a Ceti and p Aurigso, which lally are brighter than Mira at its maxi- mm. It suggests a probable explanation of lese changes of brightness, that when the is near its minimum its color changes )m white to a full red, which, from what know of the spectra of colored stars (see JPEOTKUM ANALYSIS), seems to indicate that loss of brightness is due to the forma- ion of many spots over the surface of this listant sun. Algol (or the Demon) is anoth- er remarkable variable, passing however much more rapidly through all its changes. It is ordinarily a second magnitude star, but during about seven hours in each period of 69 hours its lustre first diminishes until the star is re- duced to the fourth magnitude, and after it has remained 20 minutes at its minimum, its lustre is gradually restored. Thus Algol re- mains a second magnitude star for about 62 hours in each period of 69 hours. These changes seem to correspond to what might be expected if a large opaque orb is circling around this distant sun in a period of 69 hours, transiting its disk at regular intervals. The star /5 Lyrco has a full period of 12d. 22h., di- vided into two periods of 6d. llh., in each of which the star has a maximum brightness of about the 3 magnitude, but in one period the minimum is about the 4 magnitude, while in the other it is about the 4 magnitude. This peculiarity points to an opaque orb with a sat- ellite, the satellite being occulted by the pri- mary in the alternate transits, and therefore the total loss of light less. The star 6 Cephei varies in a period of 5d. 8h. 48m. from the fifth to the 3 '5 magnitude, taking Id. 14h. in passing from minimum to maximum of bright- ness, while it occupies 3d. 19h. in passing from maximum to minimum. Two or three hundred variable stars are already known. Among irregular variables the most remarka- ble is the star rj Argus. In 1677 Halley cata- logued it as of the fourth magnitude; in 1761 Lacaille estimated it as of the second. Be- tween 1811 and 1815 the star was of the fourth magnitude, and from 1822 to 1826 of the sec- ond ; on Feb. 1, 1827, it had increased to the first magnitude; it fell again to the second magnitude, and remained so till 1837; in 1838 it increased in brightness, till it nearly equalled a Centauri ; and it diminished again till 1843, when, however, it was still of the first magni- tude. In April, 1843, it rapidly increased " until it nearly equalled Sirius in splendor." At pres- ent it is barely visible to the naked eye, and though it has lately been slightly increasing in brightness, it is still only of the sixth magni- tude. The star a Orionis is another remarka- bly irregular variable, but amid all its changes it never descends below the first magnitude. Temporary stars include the so-called new stars, as well as those which were formerly recorded in the catalogues of astronomers, but can no longer be seen, or have at least so changed in brightness as not to be recognized. The most remarkable instance of a new star is that which appeared in 1572 and was observed by Tycho Brahe. " It suddenly shone forth in the constellation Cassiopeia with a splendor exceeding that of stars of the first magnitude, or even Jupiter and Venus at their brightest, and could be seen with the naked eye on the me- ridian in full day. Its brilliancy gradually di- minished from the time of its first appearance, and at the end of 16 months it entirely disap- peared, and has never been seen since. Du j ring the whole time of its apparition, its place in the heavens remained unaltered, and it had no annual parallax ; BO that its distance, was of the same order as that of the fixed stars. Its color, however, underwent considerable variations. Tycho described it as having been at first of a bright white ; afterward of a red- dish yellow, like Mars or Aldebaran ; and lastly of a leaden white, like Saturn." A somewhat similar instance occurred in 1604, when a first magnitude star suddenly appeared in the right foot of Ophiuchus. It presented appearances resembling those shown by the former, and disappeared after a few months. In 1866 a star appeared in the Northern Crown, the observations of which threw great light on the subject of so-called new stars. In the first place, it was found that where this new star appeared there had been a tenth magnitude star ; the new star then was in reality a star
