distance, and partly to a reason which we shall presently have occasion to refer to. (814.) If the component individuals S, s (fig. art. 810.) be (as is often the case) very close to each other, the parallactic variation of their angle of position, or the extreme angle included between the lines A a, C c, may be very considerable, even for a small amount of difference of parallaxes between the large and small stars. For instance in the case of two adjacent stars 15" asunder, and otherwise favourably situated for observation, an annual fluctuation to and fro in the apparent direction of their line of junction to the extent of half a degree (a quantity which could not escape notice in the means of numerous and careful measurements) would correspond to a difference of parallax of only of a second. A difference of 1" between two stars apparently situated at 5" distance might cause an oscillation in that line to the extent of no less than 11°, and if nearer one proportionally still greater. This mode of observation has been applied to a considerable number of stars by Lord Wrottesley, and with such an amount of success, as to make its further application desirable. (Phil. Trans. 1851.*) (815.) The following are some of the principal fixed stars to which parallax has been up to the present time more or less probably assigned : Although the extreme minuteness of the last four of these results deprives them of much numerical reliance, it is at least certain that the parallaxes by no means follow the order of magnitudes, and this is farther shown by the fact that a Cygni, one of M. Peters's stars, shows absolutely no indications of any measurable parallax whatever. * See Phil. Trans. 1826, p. 266. et seq. and 1827, for a list of stars well adapted for such observation, with the times of the year most favourable. - The list in Phil. Trans. 1826, is incorrect. † Groombridge's catalogue of circumpolar stars. (816.) From the distance of the stars we are naturally led to the consideration of their real magnitudes. But here a difficulty arises, which, so far as we can judge of what optical instruments are capable of effecting, must always remain insuperable. Telescopes afford us only negative information as to the apparent angular diameter of any star. The round, well-defined, planetary discs which good telescopes show when turned upon any of the brighter stars are phænomena of diffraction, dependent, though at present somewhat enigmatically, on the mutual interference of the rays of light. They are consequently, so far as this inquiry is concerned, mere optical illusions, and have therefore been termed spurious discs. The proof of this is that telescopes of different apertures and magnifying powers, when applied for the purpose of measuring their angular diameters, give different results, the greater aperture (even with the same magnifying power) giving the smaller disc. That the true disc of even a large and bright star can have but a very minute angular measure, appears from the fact that in the occultation of such a star by the moon, its extinction is absolutely instantaneous, not the smallest trace of gradual diminution of light being perceptible. The apparent or spurious disc also remains perfectly round and of its full size up to the instant of disappearance, which could not be the case were it a real object. If our sun were removed to the distance expressed by our parallactic unit (art. 804.), its apparent diameter of 32' 1"-5 would be reduced to only 0"-0093, or less than the hundredth of a second, a quantity which we have not the smallest reason to hope any practical improvement in telescopes will ever show as an object having distinguishable form. (817.) There remains therefore only the indication which the quantity of light they send to us may afford. But here again another difficulty besets us. The light of the sun is so immensely superior in intensity to that of any star, that it is impracticable to obtain any direct comparison between them. But by using the moon as an intermediate term of comparison it may be done, not indeed with much precision, but sufficiently well to satisfy in some degree our curiosity on the subject. Now a Centauri has been directly compared with the moon by the method explained in Art. 783. By a mean of eleven such comparisons made in various states of the moon, duly reduced and making the proper allowance on photometric principles for the moon's light lost by transmission through the lens and prism, it appears that the mean quantity of light sent to the earth by a full moon exceeds that sent by a Centauri in the proportion of 27408 to 1. Now Wollaston, by a method apparently unobjectionable, found the proportion of the sun's light to that of the full moon to be that of 801072 to 1. Combining these results, we find the light sent us by the sun to be to that sent by a Centauri as 21,955,000,000, or about twenty-two thousand millions to 1. Hence from the parallax assigned above to that star, it is easy to conclude that its intrinsic splendour, as compared with that of our sun at equal distances, is 2.3247, that of the sun being unity.† (818.) The light of Sirius is four times that of a Centauri, and its parallax only 0" 15. (Art. 230.) This in effect ascribes to it an intrinsic splendour equal to 96-63 times that of a Centauri, and therefore 224-7 times that of our sun. ‡ Wollaston, Phil. Trans. 1829. p. 27. Results of Astronomical Observations at the Cape of Good Hope, &c. Art. 278. p. 363. If only the results obtained near the quadratures of the moon (which is the situation most favourable to exactness) be used, the resulting value of the intrinsic light of the star (the sun being unity) is 4.1586. On the other hand, if only those procured near the full moon (the worst time for observation) be employed, the result is 14017. Discordances of this kind will startle no one conversant with Photometry. That a Centauri really emits more light than our sun must, we conceive, be regarded as an established fact. To those who may refer to the work cited it is necessary to mention that the quantity there designated by M, expresses, on the scale there adopted, 500 times the actual illuminating power of the moon at the time of observation, that of the mean full moon being unity. See the work above cited, p. 367.—Wollaston makes the light of Sirius one 20,000-millionth of the sun's. Steinheil by a very uncertain method found = (3286500) x Arcturus. CHAPTER XVI. VARIABLE AND PERIODICAL STARS. LIST OF THOSE ALREADY SANGUINE STARS. (819.) 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. (820.) 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 undergo a more or less regular periodical increase and diminution of lustre, involving in one or two cases a complete extinction and revival. These are called periodical stars. The longest known and one of the most remarkable is the star Omicron, in the constellation Cetus (sometimes called Mira Ceti), which was first noticed as variable by Fabricius in 1596. It appears about twelve times in eleven years, or more exactly in a period of 331d 8h 4m 165; 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 to the naked eye, in which state it remains about five months: and continues increasing during the remainder 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, neither are the successive intervals of its maxima equal. From the recent observations and inquiries into its history by M. Argelander, the mean period above assigned would appear to be subject to a cyclical fluctuation embracing eighty-eight such periods, and having the effect of gradually lengthening and shortening alternately those intervals to the extent of twenty-five days one way and the other. The irregularities in the degree of brightness attained at the maximum are probably also periodical. • Astronom. Nachr. No. 624. ૧ ૨ ૩ |