A Popular History of Astronomy During the Nineteenth Century / Fourth Edition

by Agnes M. Clerke

Numerically, indeed, these results were encompassed with uncertainty. Thus, Arcturus is now fully ascertained to be travelling towards the sun at the comparatively slow pace of less than five miles a second; and Sirius moves twice as fast in the same direction. The great difficulty of measuring so distended a line as the Sirian F might, indeed, well account for some apparent anomalies. The scope of Sir William Huggins's achievement was not, however, to provide definitive data, but to establish as practicable the method of procuring them. In this he was thoroughly successful, and his success was of incalculable value. Spectroscopic investigations of stellar movements may confidently be expected to play a leading part in the unravelment of the vast and complex relations which we can dimly detect as prevailing among the innumerable orbs of the sidereal world; for it supplements the means which we possess of measuring by direct observation movements transverse to the line of sight, and thus completes our knowledge of the courses and velocities of stars at ascertained distances, while supplying for all a valuable index to the amount of perspective foreshortening of apparent movement. Thus some, even if an imperfect, knowledge may at length be gained of the revolutions of the stars—of the systems they unite to form, of the paths they respectively pursue, and of the forces under the compulsion of which they travel.

The applicability of the method to determining the orbital motions of double stars was pointed out by Fox Talbot in 1871;[1439] but its use for their discovery revealed itself spontaneously through the Harvard College photographs. In "spectrograms" of ζ Ursæ Majoris (Mizar), taken in 1887, and again in 1889, the K line was seen to be double; while on other plates it appeared single. A careful study of Miss A. C. Maury of a series of seventy impressions indicated for the doubling a period of fifty-two days, and showed it to affect all the lines in the spectrum.[1440] The only available, and no doubt the true, explanation of the phenomenon was that two similar and nearly equal stars are here merged into one telescopically indivisible; their combined light giving a single or double spectrum, according as their orbital velocities are directed across or along our line of sight. The movements of a revolving pair of stars must always be opposite in sense, and proportionately equal in amount. That is, they at all times travel with speeds in the inverse ratio of their masses. Hence, unless the plane of their orbits be perpendicular to a plane passing through the eye, there must be two opposite points where their velocities in the line of sight reach a maximum, and two diametrically opposite points where they touch zero. The lines in their common spectrum would thus appear alternately double and single twice in the course of each revolution. To that of Mizar, at first supposed to need 104 days for its completion, a period of only twenty days fourteen hours was finally assigned by Vogel.[1441] Anomalous spectral effects, probably due to the very considerable eccentricity of the orbit, long impeded its satisfactory determination. The mean distance apart of the component stars, as now ascertained, is just twenty-two million miles, and their joint mass quadruples that of the sun. But these are minimum estimates. For if the orbital plane be inclined, much or little, to the line of sight, the dimensions and mass of the system should be proportionately increased.

An analogous discovery was made by Miss Maury in 1889. But in the spectrum of β Aurigæ, the lines open out and close up on alternate days, indicating a relative orbit[1442] with a radius of less than eight million miles, traversed in about four days. This implies a rate of travel for each star of sixty-five miles a second, and a combined mass 4·7 times that of the sun. The components are approximately equal, both in mass and light,[1443] and the system formed by them is transported towards us with a speed of some sixteen miles a second. The line-shiftings so singularly communicative proceed, in this star, with perfect regularity.

This new class of "spectroscopic binaries" could never have been visually disclosed. The distance of β Aurigæ from the earth, as determined, somewhat doubtfully, by Professor Pritchard, is nearly three and a third million times that of the earth from the sun (parallax = 0·06′); whence it has been calculated that the greatest angular separation of the revolving stars is only five-thousandths of a second of arc.[1444] To make this evanescent interval perceptible, a telescope eighty feet in aperture would be required.

The zodiacal star, Spica (α Virginis), was announced by Dr. Vogel, April 24, 1890,[1445] to belong to the novel category, with the difference, however, of possessing a nearly dark, instead of a brilliantly lustrous companion. In this case, accordingly, the tell-tale spectroscopic variations consist merely in a slight swinging to and fro of single lines. No second spectrum leaves a legible trace on the plate. Spica revolves in four days at the rate of fifty-seven miles a second,[1446] or quicker, in proportion as its orbit is more inclined to the line of sight, round a centre at a minimum distance of three millions of miles. But the position of the second star being unknown, the mass of the system remains indeterminate. The lesser component of the splendid, slowly revolving binary, Castor, is also closely double. Its spectral lines were found by Bélopolsky in 1896[1447] to oscillate once in nearly three days, the secondary globe being apparently quite obscure. Further study of the movements thus betrayed elicited the fact that the major axis of the eclipse traversed revolves in a period of 2,100 days, as a consequence, most likely, of the flattened shape of the stars.[1448] Still more unexpected was the simultaneous assignment, by Campbell and Newall, of a duplex character to Capella.[1449] Here both components shine, though with a different quality of light, one giving a pure solar spectrum, the other claiming prismatic affinity with Procyon. Their mutual circulation is performed in 104 days, and the radius of their orbit cannot be less, and may be a great deal more, than 51,000,000 miles. Hence the possibility is not excluded that the star—which has an authentic parallax of 0·08′—may be visually resolved. Indeed, signs of "elongation" were thought to be perceptible with the Greenwich 28-inch refractor,[1450] while only round images could be seen at Lick.[1451] Another noteworthy case is that of Polaris, found by Campbell to have certainly one, and probably two obscure attendants.[1452] Through his systematic investigations of stellar radial velocities with the Mills spectrograph, knowledge in this department has, since 1897, progressed so rapidly that the spectroscopic binaries of our acquaintance already number half a hundred, and ten times as many more doubtless lie within easy range of detection.

Now it is evident that a spectroscopic binary, if the plane of its motion made a very small angle with the line of sight, would be a variable star. For, during a few hours of each revolution, some at least of its light should be cut off by a transit of its dusky companion. Such "eclipse-stars" are actually found in the heavens.

The best and longest-known member of the group is Algol in the Head of Medusa, the "Demon-star" of the Arabs.[1453] This remarkable object, normally above the third magnitude, loses and regains three-fifths of its light once in 68·8 hours, the change being completed in about twelve hours. Its definite and limited nature, and punctual recurrence, suggested to Goodricke of York, by whom the periodicity of the star was discovered in 1783,[1454] the interposition of a large dark satellite. But the conditions involved by the explanation were first seriously investigated by Pickering in 1880.[1455] He found that the phenomena could be satisfactorily accounted for by supposing an obscure body 0·764 the bright star's diameter to revolve round it in a period identical with that of its observed variation. This theoretical forecast was verified with singular exactitude at Potsdam in 1889.[1456] A series of spectral photographs taken there showed each of Algol's minima to be preceded by a rapid recession from the earth, and succeeded by a rapid movement of approach towards it. They take place, accordingly, when the star is at the furthest point from ourselves of an orbit described round an invisible companion, the transits of which across its disc betray themselves to notice by the luminous vicissitudes they occasion. The diameter of this orbit, traversed at the rate of twenty-six miles a second, is just 2,000,000 miles; and it is an easy further inference from the duration and extent of the phases exhibited that Algol itself must be (in round numbers) one million, its attendant 830,000 miles in diameter. Assuming both to be of the same density, Vogel found their respective masses to be four-ninths and two-ninths that of the sun, and their distance asunder to be 3,230,000 miles.

This singularly assorted pair of stars possibly form part of a larger system. Their period of revolution is shorter now by six seconds than it was in Goodricke's time; and Dr. Chandler has shown, by an exhaustive discussion, that its inequalities are comprised in a cycle of about 130 years.[1457] They arise, in his view, from a common revolution, in that period, of the close couple about a third distant body, emitting little or no light, in an orbit inclined 20° to our line of vision, and of approximately the size of that described by Uranus round the sun. The time spent by light in crossing this orbit causes an apparent delay in the phases of the variable, when Algol and its eclipsing satellite are on its further side from ourselves, balanced by acceleration while they traverse its hither side. Dr. Chandler derives confirmation for his plausible and ingenious theory from a supposed undulation in the line traced out by Algol's small proper motion; but the reality of this disturbance has yet to be established.[1458] Meanwhile, M. Tisserand,[1459] late Director of the Paris Observatory, preferred to account for Algol's inequalities on the principle later applied by Bélopolsky to those of Castor. That is to say, he assumed a revolving line of apsides in an elliptical orbit traversed by a pretty strongly compressed pair of globes. The truth of this hypothesis can be tested by close observation of the phases of the star during the next few years.

The variable in the Head of Medusa is the exemplar of a class including 26 recognised members, all of which doubtless represent occulting combinations of stars. But their occultations result merely from the accident of their orbital planes passing through our line of sight; hence the heavens must contain numerous systems similarly constituted, though otherwise situated as regards ourselves, some of which, like Spica Virginis, will become known through their spectroscopic changes, while others, because revolving in planes nearly tangent to the sphere, or at right angles to the visual line, may never disclose to us their true nature. Among eclipsing stars should probably be reckoned the peculiar variables, β Lyræ and V Puppis, each believed to consist of a pair of bright stars revolving almost in contact.[1460] Three stars, on the other hand, distinguished by rapid and regular fluctuations, have been proved by Bélopolsky to be attended by non-occulting satellites, which circulate, nevertheless, in the identical periods of light-change.