VARIABLE STARS. This term has been applied to a class of stars which exhibit variations of brightness when observed from time to time. The branch of astronomy which takes cognisance of such objects is entirely of modern origin. The first star of which the light was found to be variable is a small star in the constellation of the Whale, usually designated in the catalogues of astronomers as o Ceti, or omicron Ceti. This star was observed by Daniel Fabricius, on the 13th of August,1596, and noted by him as a star of the third magnitude. In the month of October of the same year, the star had vanished from observation, as if it had been extinguished, and for some time after wards it does not seem to have attracted the notice of observers. Bayer, in his Uranometria,' published in 1603, has inserted the star, but he makes no allusion to its previous disappearance. The discovery of the variability of its light is due to Holwarda, a Dutch astronomer. In the month of December, 1638, Holwarda perceived the star during an eclipse of the moon, when it exceeded in brightness a star of the third magnitude. About the middle of the following summer be was unable to discover the slightest trace of it. However, on the 7th of November, 1639, he again detected it in its original position.
The star was now carefully observed by several individuals, among the rest, by the famous Hevelius. The discovery of the period of its variations is due to Bouillaud, who found that an interval of 333 days elapsed between two successive disappearances or reappearances. The results of modern observation indicate the exact period to be 331d 15° 7".
In addition to the star to which we have just been referring a great many other stars have been found to be variable, and the number is rapidly increasing every year. This important result is due in a great degree to the practice of carefully scrutinising small stars in connection with the search for asteroids which has been so assiduously prosecuted in recent years by a number of individuals in different countries. We shall now allude briefly to the peculiarities of two or threo other ex amples of variable stars.
/3 Persei. This star, usually termed Algol, which is situate in the head of Medusa, was found by Montanari and Maraldi to exhibit strange fluctuations of brightness, but the period of its variations was first established by Goodricke in 1782. It generally appears as a star of the second magnitude. In the short space of three hours and a half it descends to the fourth magnitude, and then in an equal interval of time regains its usual brightness. It shines as a star of the second magnitude during the space of two days; thirteen hours, and three quarters, and it consequently passes through the complete cycle of its changes in two days, twenty hours, and three quarters. According to Argelauder, the exact period is 2d 20° 48. 52'.
13 Lyme. This star was first found to be variable by Goodricke in 1784. It is an object of great interest, inasmuch as it possesses a double maximum and a double minimum. When it arrives at its maximum brightness, it resembles a star of the third magnitude. At one of its minima it appears between the third and fourth magnitude, and at the other, between the fourth and fifth magnitude. Argelauder has found that it passes through its variations in 12d 21° 53. 10'.
a Cephei. This interesting star was also first discovered to be variable by Goodrieke in 1784. Argelander determined its period to be 5d 8° 47. 39.5"; but the late Mr. Johnson, director of the Radcliffe Obser vatory, Oxford, fixed the period at 5d 6" 42' 184'. At its minimum it is equal to a star of the fifth magnitude, and it hence increases until it resembles a star between the third and fourth magnitude at its maximum. The iuterval which elapses between the maximum and the
minimum is 3d 19°, while between the minimum and the maximum the interval is only id 14".
Some stars have been discovered to be variable, but their fluctuations are so irregular that it has been hitherto found impossible to reduce them to any fixed law. A remarkable example of this kind is furnished by the bright star in the southern hemisphere, denominated Argils. In 1677, Halley, during his residence at St. Helena, classed it among the stars of the fourth magnitude. In 1751, Lacaille estimated it to be of the second magnitude; however Burchell, who resided in South Africa from 1811 to 1815, again ranked it among the stars of the secoud magnitude. From 1822 to 1826, it was estimated to be of the second magnitude by Brisbane and Fallows, who observed it, the former at New South Wales, and the latter at the Cape of Good Hope. In 1827, Burehell, while residing at St Paul's, Brazil, estimated it to be of the first magnitude, and almost equal to a Crueis ; but in the following year he found from observations made at Geyer, that it had again descended to the second magnitude. Johnson, who observed the star at St. Helena between 1829 and 1833, estimates it to be of the second magnitude; and Taylor's observations at Madras during the same period indicate the same fact. Sir John Herschel also, from the time of his arrival at the Cape of Good Hope in 1834 till 1837, esti mated It invariably to be between the first and eeeond magnitude. But on the 16th of December, 1837, while engaged in making photo metric observations of the small stars in its vicinity, he was hurpri.ed to find that it had rapidly Increased in brightness. It was now equal to a Centauri, and far surpassed in brightness all the other fixed stars except Osiopna and Sirius. It attained its maximum brightness on the 2n41 of January. 1633. Shortly afterwards It, grew fainter, and it oatinueni to diminish in brightness till March, 1313. In the following mouth it again rapidly increased iu brightness. According to the ob eervations of 31aelear at the Capo of tlood Hope, and of Mackay at eUleutta, It. now surpassed Canopus in brightness, and almost rivalled Sirius. It continued for several years to exhibit this great degree of brightness, when it began to fluctuate as before.
The new stars which have appeared in the heavens, and of which several instances are recorded In history, probably belong to the class of variable stars. The most notable objecte of this &ascription aro the new star which appeared in 1572, of which Tyoho Braho has given a detailed account, and the new star of 1604, which was observed by Galileo and Kepler.
The following table of variable stars, drawn up by Mr. Pogson, is extracted from voL xvii. of the ' Observations made at the Radcliffe Observatory,' Oxford. alr. region is known as ono of the most suc cessful explorers of this interesting field of astronomy.
No satisfactory explanation of the phenomena of variable stars has hitherto been advanced by any inquirer. Some astronomers have suggested that the variations of light may be due to dark bodies circulating around the stars. According to others, the fluctuations of brightness may arise from the pretence of dark tracts on the surfaces of the stars, which are periodically turned towards the earth by the revolution of the stars on their axes. The phenomena, however, are in fIrneral so Irregular, that neither of these hypotheses is capable of satisfacterily accounting for them.