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(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 phaenomena 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' 3" 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 L 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, f

(818.) The light of Sirius is four times that of a Centauri and its parallax only 0"*230. (Art 230.) This in effect ascribes to it an intrinsic splendour equal to 63*02 times that of a Centauri, and therefore 146*54 times that of our sun.|

• Wollaston, Phil. Trans. 1829. p. 27.

f Results of Astronomical Observations at the Cape of Good Hope, jrc 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 1 -4017. 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 Q = (3286500)* x Arcturus.

CHAPTER XVI.

VARIABLE AND PERIODICAL STARS. — LIST OF THOSE ALREADY

KNOWN. IRREGULARITIES IN THEIR PERIODS AND LUSTBE WHEN

BRIGHTEST. IRREGULAR AND TEMPORARY STARS ANCIENT

CHINESE RECORDS OF SEVERAL. MISSING STARS.—DOUBLE STARS.

THEIR CLASSIFICATION. SPECIMENS OF EACH CLASS.— BINARY

SYSTEMS. REVOLUTION ROUND EACH OTHER. DESCRIBE ELLIPTIC ORBITS UNDER THE NEWTONIAN LAW OF GRAVITY. ELEMENTS OF ORBITS OF SEVERAL. ACTUAL DIMENSIONS OF THF.IR

ORBITS. COLOURED DOUBLE STARS. PHENOMENON OF COMPLEMENTARY COLOURS. — SANGUINE STARS.— PROPER MOTION OF THE STARS. —PARTLY ACCOUNTED FOR BY A REAL MOTION OF THE SUN.— SITUATION OF THE SOLAR APEX. AGREEMENT OF SOUTHERN AND NORTHERN STARS IN GIVING THE SAME RESULT.— PRINCIPLES ON WHICH THE INVESTIGATION OF THE SOLAR MOTION DEPENDS. — ABSOLUTE VELOCITY OF THE SUN'S MOTION. SUPPOSED REVOLUTION OF THE WHOLE SIDEREAL SYSTEM ROUND A

COMMON CENTER. SYSTEMATIC PARALLAX AND ABERRATION.

EFFECT OF THE MOTION OF LIGHT IN ALTERING THE APPARENT PERIOD OF A BINARY STAR.

(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 phenomena 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 33 ld 15h 7m; 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.

Hevelius relates* that during the four years between October 1672 and December 1676 it did not appear at all. It was unusually bright on October 5. 1839 (the epoch of its maximum for that year according to M. Argelander's observations) when it exceeded a. Ceti and equalled /3 Aurigae in lustre.

(821.) Another very remarkable periodical star ia that called Algol, or /3 Persei. It is usually visible as a star of the second magnitude, and such it continues for the space of 2d 13£h, when it suddenly begins to diminish in splendour, and in about Z\ hours is reduced to the fourth magnitude, at which it continues about 15TM. 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 58'-5. 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 f, in the year 1782; since which time the same phenomena have continued to be observed, but with this remarkable additional point of interest; viz. that the more recent observations as compared with the earlier ones indicate a diminution in the periodic time. The latest observations of Argelander, Heis, and Schmidt, even go to prove that this diminution is not uniformly progressive, but is actually proceeding with accelerated rapidity, which however will probably not continue, but, like other cyclical combinations in astronomy, will by degrees relax, and then be changed into an increase, according to laws of periodicity which, as well as their causes, remain to be discovered. The

• Lalande's Astronomy, Art. 794.

f The same discovery appears to have been made nearly about the same time by Palitzch, a farmer of Prolitt, 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 re-discover the predicted comet of Halley in 1759, which he saw nearly a month before any of the astronomers, who, armed with their telescopes, were anxiously Hatching its return. These anecdote* carry us back to the era of the Chaldean shepherds.

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