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The non-appearance of . Ceti, during four years, has already been noticed ; and to this instance we may add that of y 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 Aquilæ, 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 in. stances 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. 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 observ. ateurs curieux ... Un jour viendra, peut-être, où les sciences auront assez d'amateurs pour qu'on puisse suffire à ces détails." --Lalande, art. 824.Surely that day is now arrived.

constructed by Sir Wm. Herschel, with the express object of facilitating these researches, and the reader will find them, and a full account of his method of com. parison, in the Phil. Trans. 1796, and subsequent years.

(598.) We come now to a class of phænomena of quite a different character, and which give us a real and positive insight into the nature of at least some among the stars, and enable us unhesitatingly to declare them subject to the same dynamical laws, and obedient to the same power of gravitation, which governs our own system. Many of the stars, when examined with telescopes, are found to be double, i. e. to consist of two (in some cases three) individuals placed near together. This might be attributed to accidental proximity, did it occur only in a few instances; but the frequency of this companionship, the extreme closeness, and, in many cases, the near equality of the stars so conjoined, would alone lead to a strong suspicion of a more near and intimate relation than mere casual juxtaposition. The bright star Castor, for example, when much magni. fied, is found to consist of two stars of between the third and fourth magnitude, within 5” of each other. Stars of this magnitude, however, are not so common in the heavens as to render it at all likely that, if scattered at random, any two would fall so near. But this is only one out of numerous such instances. Sir Wm. Herschel has enumerated upwards of 500 double stars, in which the individuals are within half a minute of each other ; and to this list Professor Struve of Dorpat, prosecuting the enquiry by the aid of instruments more conveniently mounted for the purpose, has recently added nearly five times that number. Other observers have still further extended the catalogue, already so large, with. out exhausting the fertility of the heavens. Among these are great numbers in which the interval between the centers of the individuals is less than a single second, of which & Arietis, Atlas Pleiadum, y Coronæ, y Coronæ, 9 and 8 Herculis, and < and a Ophiuchi, may

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be cited as instances. They are divided into classes according to their distances the closest forming the first class.

(599.) When these combinations were first noticed, it was considered that advantage might be taken of them, to ascertain whether or not the annual motion of the earth in its orbit might not produce a relative apparent displacement of the individuals constituting a double star. Supposing them to lie at a great distance one behind the other, and to appear only by casual juxtaposition nearly in the same line, it is evident that any motion of the earth must subtend different angles at the two stars so juxtaposed, and must therefore produce different parallactic displacements of them on the surface of the heavens, regarded as infinitely distant. Every star, in consequence of the earth's annual motion, should appear to describe in the heavens a small ellipse, (distinct from that which it would appear to describe in consequence of the aberration of light, and not to be confounded with it,) being a section, by the concave surface of the heavens, of an oblique elliptic cone, having its vertex in the star, and the earth's orbit for its

base; and this section will be of less dimensions, the more distant is the star. If, then, we regard two stars, apparently situated close beside each other, but in reality at very different distances, their parallactic ellipses will be similar, but of different dimensions. Sup. pose, for instance, S and s to be the positions of two stars of such an apparently or optically double star as seen from the sun, and let A B CD, abcd, be their parallactic ellipses ; then, since they will be at all times similarly situated in these ellipses, when the one star is seen at A, the other will be seen at a. When the earth has made a quarter of a revolution in its orbit, their apparent places will be B b; when another quarter, Cc; and when another, D d. If, then, we measure carefully, with micrometers adapted for the purpose, their appa. rent situation with respect to each other, at different times of the year, we should perceive a periodical change, both in the direction of the line joining them, and in the distance between their centers. For the lines A a and C c cannot be parallel, nor the lines B b and Dd equal, unless the ellipses be of equal dimensions, i. e. unless the two stars have the same parallax, or are equi. distant from the earth.

(600.) Now, micrometers, properly mounted, enable us to measure very exactly both the distance between two objects which can be seen together in the same field of a telescope, and the position of the line joining them with respect to the horizon, or the meridian, or any other determinate direction in the heavens. The meridian is chosen as the most convenient; and the situation of the line of junction between the two stars of a double star is referred to its direction, by placing in the focus of the eye-piece of a telescope, equatorially mounted, two cross wires making a right angle, and adjusting their position so that one of the two stars shall just run along it by its diurnal motion, while the telescope remains at rest; noting their si. tuation; and then turning the whole system of wires round in its own plane by a proper mechanical movement, till the other wire becomes 'exactly parallel to their line of junction, and reading off on a divided circle the angle the wires have moved through. Such an ar

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