that of the sun. Hence it follows, that, upon the lowest possible computation, the light really thrown out by Sirius cannot be so little as double that emitted by the sun; or that Sirius must, in point of intrinsic splendour, be at least equal to two suns, and is in all probability vastly greater.* (592.) Now, for what purpose are we to suppose such magnificent bodies scattered through the abyss of space? Surely not to illuminate our nights, which an additional moon of the thousandth part of the size of our own would do much better, nor to sparkle as a pageant void of meaning and reality, and bewilder us among vain conjectures. Useful, it is true, they are to man as points of exact and permanent reference; but he must have studied astronomy to little purpose, who can suppose man to be the only object of his Creator's care, or who does not see in the vast and wonderful apparatus around us provision for other races of animated beings. The planets, as we have seen, derive their light from the sun; but that cannot be the case with the stars. These doubtless, then, are themselves suns, and may, perhaps, each in its sphere, be the presiding center round which other planets, or bodies of which we can form no conception from any analogy offered by our own system, may be circulating. (593.) Analogies, however, more than conjectural, are not wanting to indicate a correspondence between the dynamical laws which prevail in the remote regions of the stars and those which govern the motions of our own system. Wherever we can trace the law of periodicity — the regular recurrence of the same phænomena in the same times- we are strongly impressed with the idea of rotatory or orbitual motion. Among the stars are several which, though no way distinguishable from others by any apparent change of place, nor by any difference of appearance in telescopes, yet un * Dr. Wollaston, assuming, as we think he is perfectly justified in doing, a much lower limit of possible parallax in Sirius than we have adopted in the text, has concluded the intrinsic light of Sirius to be nearly that of fourteen suns. - or, dergo a regular periodical increase and diminution of lustre, involving, in one or two cases, a complete extinction and revival. These are called periodical stars. One of the most remarkable is the star Omicron, in the constellation Cetus, first noticed by Fabricius in 1596. It appears about twelve times in eleven years, more exactly, in a period of 334 days; remains at its greatest brightness about a fortnight, being then, on some occasions, equal to a large star of the second magnitude; decreases during about three months, till it becomes completely invisible, in which state it remains during about five months, when it again becomes visible, and continues increasing during the remaining three months of its period. Such 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. Hevelius, indeed, relates (Lalande, art. 794.) that during the four years between October, 1672, and December, 1676, it did not appear at all. (594.) Another very remarkable periodical star is that called Algol, or ß Persei. It is usually visible as a star of the second magnitude, and such it continues for the space of 2d 14h, when it suddenly begins to diminish in splendour, and in about 3 hours is reduced to the fourth magnitude. It then begins again to increase, and in 3 hours more is restored to its usual brightness, going through all its changes in 2d 20h 48m, or thereabouts. This remarkable law of variation certainly appears strongly to suggest the revolution round it of some opaque body, which, when interposed between us and Algol, cuts off a large portion of its light; and this is accordingly the view taken of the matter by Goodricke, to whom we owe the discovery of this remarkable fact*, in the year 1782; since which time * The same discovery appears to have been made nearly about the same time by Palitzch, a farmer of Prolitz, near Dresden, - a peasant by station, an astronomer by nature, who, from his familiar acquaintance with the aspect of the heavens, had been led to notice among so many thousand stars this one as distinguished from the rest by its variation, and had ascertained its period. The same Palitzch was also the first to rediscover the predicted comet of Halley in 1759, which he saw the same phænomena have continued to be observed, though with much less diligence, than their high interest would appear to merit. Taken any how, it is an indication of a high degree of activity, in regions where, but for such evidences, we might conclude all lifeless. Our own sun requires nine times this period to perform a revolution on its own axis. On the other hand, the periodic time of an opaque revolving body, sufficiently large, which should produce a similar temporary obscuration of the sun, seen from a fixed star, would be less than fourteen hours. (595.) The following list exhibits specimens of periodical stars of every variety of period, so far as they can be considered to be at present ascertained : Discoverers. The variations of these stars, however, appear to be affected, perhaps in duration of period, but certainly in extent of change, by physical causes at present unknown. nearly a month before any of the astronomers, who, armed with their telescopes, were anxiously watching its return. These anecdotes carry us back to the era of the Chaldean shepherds. *These letters B. Fl. and M. refer to the Catalogues of Bode, Flamsteed, and Mayer. The non-appearance of o Ceti, during four years, has already been noticed; and to this instance we may add that of x Cygni, which is stated by Cassini to have been scarcely visible throughout the years 1699, 1700, and 1701, at those times when it ought to have been most conspicuous. (596.) These irregularities prepare us for other phænomena of stellar variation, which have hitherto been reduced to no law of periodicity, and must be looked upon, in relation to our ignorance and inexperience, as altogether casual; or, if periodic, of periods too long to have occurred more than once within the limits of recorded observation. The phænomena we allude to are those of temporary stars, which have appeared, from time to time, in different parts of the heavens, blazing forth with extraordinary lustre ; and after remaining awhile apparently immovable, have died away, and left no trace. Such is the star which, suddenly appearing in the year 125 B. C., is said to have attracted the attention of Hipparchus, and led him to draw up a catalogue of stars, the earliest on record. Such, too, was the star which blazed forth, A. D. 389, near a Aquila, remaining for three weeks as bright as Venus, and disappearing entirely. In the years 945, 1264, and 1572, brilliant stars appeared in the region of the heavens between Cepheus and Cassiopeia; and, from the imperfect account we have of the places of the two earlier, as compared with that of the last, which was well determined, as well as from the tolerably near coincidence of the intervals of their appearance, we may suspect them to be one and the same star, with a period of about 300, or, as Goodricke supposes, of 150 years. The appearance of the star of 1572 was so sudden, that Tycho Brahe, a celebrated Danish astronomer, returning one evening (the 11th of November) from his laboratory to his dwelling-house, was surprised to find a group of country people gazing at a star, which he was sure did not exist half an hour before. This was the star in question. It was then as bright as Sirius, and continued to increase till it surpassed Jupi ter when brightest, and was visible at mid-day. It began to diminish in December of the same year, and in March, 1574, had entirely disappeared. So, also, on the 10th of October, 1604, a star of this kind, and not less brilliant, burst forth in the constellation of Serpentarius, which continued visible till October, 1605. (597.) Similar phænomena, though of a less splendid character, have taken place more recently, as in the case of the star of the third magnitude discovered in 1670, by Anthelm, in the head of the Swan; which, after becoming completely invisible, re-appeared, and, after undergoing one or two singular fluctuations of light, during two years, at last died away entirely, and has not since been seen. On a careful re-examination of the heavens, too, and a comparison of catalogues, many stars are now found to be missing; and although there is no doubt that these losses have often arisen from mistaken entries, yet in many instances it is equally certain that there is no mistake in the observation or entry, and that the star has really been observed, and as really has disappeared from the heavens.* This is a branch of practical astronomy which has been too little followed up, and it is precisely that in which amateurs of the science, provided with only good eyes, or moderate instruments, might employ their time to excellent advantage.t It holds out a sure promise of rich discovery, and is one in which astronomers in established observatories are almost of necessity precluded from taking a part by the nature of the observations required. Catalogues of the comparative brightness of the stars in each constellation have been *The star 42 Virginis is inserted in the Catalogue of the Astronomical Society from Zach's Zodiacal Catalogue. I missed it on the 9th May, 1828, and have since repeatedly had its place in the field of view of my 20 feet reflector, without perceiving it, unless it be one of two equal stars of the 9th magnitude, very nearly in the place it must have occupied.Author. +"Ces variations des étoiles sont bien dignes de l'attention des observateurs curieux... Un jour viendra, peut-être, où les sciences auront assez d'amat urs pour qu'on puisse suffire à ces détails."— Lalande, art. 824.— Surely that day is now arrived. |