Part 4 (1/2)
The distances even of the few stars found to have measurable parallaxes are on a scale entirely beyond the powers of the human mind to conceive.
In the attempt both to realize them distinctly, and to express them conveniently, a new unit of length, itself of bewildering magnitude, has originated. This is what we may call the _light-journey_ of one year.
The subtle vibrations of the ether, propagated on all sides from the surface of luminous bodies, travel at the rate of 186,300 miles a second, or (in round numbers) six billions of miles a year. Four and a third such measures are needed to span the abyss that separates us from the nearest fixed star. In other words, light takes four years and four months to reach the earth from Alpha Centauri; yet Alpha Centauri lies some ten billions of miles nearer to us (so far as is yet known) than any other member of the sidereal system!
The determination of parallax leads, in the case of stars revolving in known orbits, to the determination of ma.s.s; for the distance from the earth of the two bodies forming a binary system being ascertained, the seconds of arc apparently separating them from each other can be translated into millions of miles; and we only need to add a knowledge of their period to enable us, by an easy sum in proportion, to find their combined ma.s.s in terms of that of the sun. Thus, since--according to Dr. Doberck's elements--the components of Alpha Centauri revolve round their common centre of gravity at a mean distance nearly 25 times the radius of the earth's...o...b..t, in a period of 88 years, the attractive force of the two together must be just twice the solar. We may gather some idea of their relations by placing in imagination a second luminary like our sun in circulation between the orbits of Neptune and Ura.n.u.s.
But systems of still more majestic proportions are reduced by extreme remoteness to apparent insignificance. A double star of the fourth magnitude in Ca.s.siopeia (Eta), to which a small parallax is ascribed on the authority of O. Struve, appears to be above eight times as ma.s.sive as the central orb of our world; while a much less conspicuous pair--85 Pegasi--exerts, if the available data can be depended upon, no less than thirteen times the solar gravitating power.
Further, the actual rate of proper motions, so far as regards that part of them which is projected upon the sphere, can be ascertained for stars at known distance. The annual journey, for instance, of 61 Cygni _across the line of sight_ amounts to 1,000, and that of Alpha Centauri to 446 millions of miles. A small star, numbered 1,830 in Groombridge's Circ.u.mpolar Catalogue, ”devours the way” at the rate of at least 150 miles a second--a speed, in Newcomb's opinion, beyond the gravitating power of the entire sidereal system to control; and Mu Ca.s.siopeiae possesses above two-thirds of that surprising velocity; while for both objects, radial movements of just sixty miles a second were disclosed by Professor Campbell's spectroscopic measurements.
Herschel's conclusion as to the advance of the sun among the stars was not admitted as valid by the most eminent of his successors. Bessel maintained that there was absolutely no preponderating evidence in favour of its supposed direction towards a point in the constellation Hercules.[78] Biot, Burckhardt, even Herschel's own son, shared his incredulity. But the appearance of Argelander's prize-essay in 1837[79]
changed the aspect of the question. Herschel's first memorable solution in 1783 was based upon the motions of thirteen stars, imperfectly known; his second, in 1805, upon those of no more than six. Argelander now obtained an entirely concordant result from the large number of 390, determined with the scrupulous accuracy characteristic of Bessel's work and his own. The reality of the fact thus persistently disclosed could no longer be doubted; it was confirmed five years later by the younger Struve, and still more strikingly in 1847[80] by Galloway's investigations, founded exclusively on the apparent displacements of southern stars. In 1859 and 1863, Sir George Airy and Mr. Dunkin (1821-1898),[81] employing all the resources of modern science, and commanding the wealth of material furnished by 1,167 proper motions carefully determined by Mr. Main, reached conclusions closely similar to that indicated nearly eighty years previously by the first great sidereal astronomer; which Mr. Plummer's reinvestigation of the subject in 1883[82] served but slightly to modify. Yet astronomers were not satisfied. Dr. Auwers of Berlin completed in 1866 a splendid piece of work, for which he received in 1888 the Gold Medal of the Royal Astronomical Society. It consisted in reducing afresh, with the aid of the most refined modern data, Bradley's original stars, and comparing their places thus obtained for the year 1755 with those a.s.signed to them from observations made at Greenwich after the lapse of ninety years. In the interval, as was to be antic.i.p.ated, most of them were found to have travelled over some small span of the heavens, and there resulted a stock of nearly three thousand highly authentic proper motions. These ample materials were turned to account by M. Ludwig Struve[83] for a discussion of the sun's motion, of which the upshot was to s.h.i.+ft its point of aim to the bordering region of the constellations Hercules and Lyra. And the more easterly position of the solar apex was fully confirmed by the experiments, with variously a.s.sorted lists of stars, of Lewis Boss of Albany,[84] and Oscar Stumpe of Bonn.[85] Fresh precautions of refinement were introduced into the treatment of the subject by Ristenpart of Karlsruhe,[86] by Kapteyn of Groningen,[87] by Newcomb[88] and Porter[89] in America, who ably availed themselves of the copious materials acc.u.mulated before the close of the century. Their results, although not more closely accordant than those of their predecessors, combined to show that the journey of our system is directed towards a point within a circle about ten degrees in radius, having the brilliant Vega for its centre. To determine its rate was a still more arduous problem. It involved the a.s.sumption, very much at discretion, of an average parallax for the stars investigated; and Otto Struve's estimate of 154 million miles as the span yearly traversed was hence wholly unreliable. Fortunately, however, as will be seen further on, a method of determining the sun's velocity independently of any knowledge of star-distances, has now become available.
As might have been expected, speculation has not been idle regarding the purpose and goal of the strange voyage of discovery through s.p.a.ce upon which our system is embarked; but altogether fruitlessly. The variety of the conjectures hazarded in the matter is in itself a measure of their futility. Long ago, before the construction of the heavens had as yet been made the subject of methodical inquiry, Kant was disposed to regard Sirius as the ”central sun” of the Milky Way; while Lambert surmised that the vast Orion nebula might serve as the regulating power of a subordinate group including our sun. Herschel threw out the hint that the great cl.u.s.ter in Hercules might prove to be the supreme seat of attractive force;[90] Argelander placed his central body in the constellation Perseus;[91] Fomalhaut, the brilliant of the Southern Fish, was set in the post of honour by Boguslawski of Breslau. Madler (who succeeded Struve at Dorpat in 1839) concluded from a more formal inquiry that the ruling power in the sidereal system resided, not in any single prepondering ma.s.s, but in the centre of gravity of the self-controlled revolving mult.i.tude.[92] In the former case (as we know from the example of the planetary scheme), the stellar motions would be most rapid near the centre; in the latter, they would become accelerated with remoteness from it.[93] Madler showed that no part of the heavens could be indicated as a region of exceptionally swift movements, such as would result from the presence of a gigantic (though possibly obscure) ruling body; but that a community of extremely sluggish movements undoubtedly existed in and near the group of the Pleiades, where, accordingly, he placed the centre of gravity of the Milky Way.[94] The bright star Alcyone thus became the ”central sun,” but in a purely pa.s.sive sense, its heads.h.i.+p being determined by its situation at the point of neutralisation of opposing tendencies, and of consequent rest.
By an avowedly conjectural method, the solar period of revolution round this point was fixed at 18,200,000 years.
The scheme of sidereal government framed by the Dorpat astronomer was, it may be observed, of the most approved const.i.tutional type; deprivation, rather than increase of influence accompanying the office of chief dignitary. But while we are still ignorant, and shall perhaps ever remain so, of the fundamental plan upon which the Galaxy is organised, recent investigations tend more and more to exhibit it, not as monarchical (so to speak), but as federative. The community of proper motions detected by Madler in the vicinity of the Pleiades may accordingly possess a significance altogether different from what he imagined.
Bessel's so-called ”foundation of an Astronomy of the Invisible” now claims attention.[95] His prediction regarding the planet Neptune does not belong to the present division of our subject; a strictly a.n.a.logous discovery in the sidereal system was, however, also very clearly foreshadowed by him. His earliest suspicions of non-uniformity in the proper motion of Sirius dated from 1834; they extended to Procyon in 1840; and after a series of refined measurements with the new Repsold circle, he announced in 1844 his conclusion that these irregularities were due to the presence of obscure bodies round which the two bright Dog-stars revolved as they pursued their way across the sphere.[96] He even a.s.signed to each an approximate period of half a century. ”I adhere to the conviction,” he wrote later to Humboldt, ”that Procyon and Sirius form real binary systems, consisting of a visible and an invisible star.
There is no reason to suppose luminosity an essential quality of cosmical bodies. The visibility of countless stars is no argument against the invisibility of countless others.”[97]
An inference so contradictory to received ideas obtained little credit, until Peters found, in 1851,[98] that the apparent anomalies in the movements of Sirius could be completely explained by an orbital revolution in a period of fifty years. Bessel's prevision was destined to be still more triumphantly vindicated. On the 31st of January, 1862, while in the act of trying a new 18-inch refractor, Mr. Alvan G. Clark (one of the celebrated firm of American opticians) actually discovered the hypothetical Sirian companion in the precise position required by theory. It has now been watched through nearly an entire revolution (period 494 years), and proves to be very slightly luminous in proportion to its ma.s.s. Its attractive power, in fact, is nearly half that of its primary, while it emits only 1/10000th of its light. Sirius itself, on the other hand, possesses a far higher radiative intensity than our sun. It gravitates--admitting Sir David Gill's parallax of 038” to be exact--like two suns, but s.h.i.+nes like twenty. Possibly it is much distended by heat, and undoubtedly its atmosphere intercepts a very much smaller proportion of its light than in stars of the solar cla.s.s.
As regards Procyon, visual verification was awaited until November 13, 1896, when Professor Schaeberle, with the great Lick refractor, detected the long-sought object in the guise of a thirteenth-magnitude star. Dr.
See's calculations[99] showed it to possess one-fifth the ma.s.s of its primary, or rather more than half that of our sun.[100] Yet it gives barely 1/20000th of the sun's light, so that it is still nearer to total obscurity than the dusky satellite of Sirius. The period of forty years a.s.signed to the system by Auwers in 1862[101] appears to be singularly exact.
But Bessel was not destined to witness the recognition of ”the invisible” as a legitimate and profitable field for astronomical research. He died March 17, 1846, just six months before the discovery of Neptune, of an obscure disease, eventually found to be occasioned by an extensive fungus-growth in the stomach. The place which he left vacant was not one easy to fill. His life's work might be truly described as ”epoch-making.” Rarely indeed shall we find one who reconciled with the same success the claims of theoretical and practical astronomy, or surveyed the science which he had made his own with a glance equally comprehensive, practical, and profound.
The career of Friedrich Georg Wilhelm Struve ill.u.s.trates the maxim that science _differentiates_ as it develops. He was, while much besides, a specialist in double stars. His earliest recorded use of the telescope was to verify Herschel's conclusion as to the revolving movement of Castor, and he never varied from the predilection which this first observation at once indicated and determined. He was born at Altona, of a respectable yeoman family, April 15, 1793, and in 1811 took a degree in philology at the new Russian University of Dorpat. He then turned to science, was appointed in 1813 to a professors.h.i.+p of astronomy and mathematics, and began regular work in the Dorpat Observatory just erected by Parrot for Alexander I. It was not, however, until 1819 that the acquisition of a 5-foot refractor by Troughton enabled him to take the position-angles of double stars with regularity and tolerable precision. The resulting catalogue of 795 stellar systems gave the signal for a general resumption of the Herschelian labours in this branch. His success, so far, and the extraordinary facilities for observation afforded by the Fraunhofer achromatic encouraged him to undertake, February 11, 1825, a review of the entire heavens down to 15 south of the celestial equator, which occupied more than two years, and yielded, from an examination of above 120,000 stars, a harvest of about 2,200 previously unnoticed composite objects. The ensuing ten years were devoted to delicate and patient measurements, the results of which were embodied in _Mensurae Micrometricae_, published at St. Petersburg in 1837.
This monumental work gives the places, angles of position, distances, colours, and relative brightness of 3,112 double and multiple stars, all determined with the utmost skill and care. The record is one which gains in value with the process of time, and will for ages serve as a standard of reference by which to detect change or confirm discovery.
It appears from Struve's researches that about one in forty of all stars down to the ninth magnitude is composite, but that the proportion is doubled in the brighter orders.[102] This he attributed to the difficulty of detecting the faint companions of very remote orbs. It was also noticed, both by him and Bessel, that double stars are in general remarkable for large proper motions. Struve's catalogue included no star of which the components were more than 32” apart, because beyond that distance the chances of merely optical juxtaposition become considerable; but the immense preponderance of extremely close over (as it were) loosely yoked bodies is such as to demonstrate their physical connection, even if no other proof were forthcoming. Many stars previously believed to be single divided under the scrutiny of the Dorpat refractor; while in some cases, one member of a supposed binary system revealed itself as double, thus placing the surprised observer in the unexpected presence of a triple group of suns. Five instances were noted of two pairs lying so close together as to induce a conviction of their mutual dependence;[103] besides which, 124 examples occurred of triple, quadruple, and multiple combinations, the reality of which was open to no reasonable doubt.[104]
It was first pointed out by Bessel that the fact of stars exhibiting a common proper motion might serve as an unfailing test of their real a.s.sociation into systems. This was, accordingly, one of the chief criteria employed by Struve to distinguish true binaries from merely optical couples. On this ground alone, 61 Cygni was admitted to be a genuine double star; and it was shown that, although its components appeared to follow almost strictly rectilinear paths, yet the probability of their forming a connected pair is actually greater than that of the sun rising to-morrow morning.[105] Moreover, this tie of an identical movement was discovered to unite bodies[106] far beyond the range of distance ordinarily separating the members of binary systems, and to prevail so extensively as to lead to the conclusion that single do not outnumber conjoined stars more than twice or thrice.[107]
In 1835 Struve was summoned by the Emperor Nicholas to superintend the erection of a new observatory at Pulkowa, near St. Petersburg, destined for the special cultivation of sidereal astronomy. Boundless resources were placed at his disposal, and the inst.i.tution created by him was acknowledged to surpa.s.s all others of its kind in splendour, efficiency, and completeness. Its chief instrumental glory was a refractor of fifteen inches aperture by Merz and Mahler (Fraunhofer's successors), which left the famous Dorpat telescope far behind, and remained long without a rival. On the completion of this model establishment, August 19, 1839, Struve was installed as its director, and continued to fulfil the important duties of the post with his accustomed vigour until 1858, when illness compelled his virtual resignation in favour of his son Otto Struve, born at Dorpat in 1819. He died November 23, 1864.
An inquiry into the laws of stellar distribution, undertaken during the early years of his residence at Pulkowa, led Struve to confirm in the main the inferences arrived at by Herschel as to the construction of the heavens. According to his view, the appearance known as the Milky Way is produced by a collection of irregularly condensed star-cl.u.s.ters, within which the sun is somewhat eccentrically placed. The nebulous ring which thus integrates the light of countless worlds was supposed by him to be made up of stars scattered over a bent or ”broken plane,” or to lie in two planes slightly inclined to each other, our system occupying a position near their intersection.[108] He further attempted to show that the limits of this vast a.s.semblage must remain for ever shrouded from human discernment, owing to the gradual extinction of light in its pa.s.sage through s.p.a.ce,[109] and sought to confer upon this celebrated hypothesis a definiteness and certainty far beyond the aspirations of its earlier advocates, Cheseaux and Olbers; but arbitrary a.s.sumptions vitiated his reasonings on this, as well as on some other points.[110]
In his special line as a celestial explorer of the most comprehensive type, Sir William Herschel had but one legitimate successor, and that successor was his son. John Frederick William Herschel was born at Slough, March 17, 1792, graduated with the highest honours from St.
John's College, Cambridge, in 1813, and entered upon legal studies with a view to being called to the Bar. But his share in an early compact with Peac.o.c.k and Babbage, ”to do their best to leave the world wiser than they found it,” was not thus to be fulfilled. The acquaintance of Dr. Wollaston decided his scientific vocation. Already, in 1816, we find him reviewing some of his father's double stars; and he completed in 1820 the 18-inch speculum which was to be the chief instrument of his investigations. Soon afterwards, he undertook, in conjunction with Mr.
(later Sir James) South, a series of observations, issuing in the presentation to the Royal Society of a paper[111] containing micrometrical measurements of 380 binary stars, by which the elder Herschel's inferences of orbital motion were, in many cases, strikingly confirmed. A star in the Northern Crown, for instance (Eta Coronae), had completed more than one entire circuit since its first discovery; another, Tau Ophiuchi, had _closed up_ into apparent singleness; while the motion of a third, Xi Ursae Majoris, in an obviously eccentric orbit, was so rapid as to admit of being traced and measured from month to month.
It was from the first confidently believed that the force retaining double stars in curvilinear paths was identical with that governing the planetary revolutions. But that ident.i.ty was not ascertained until Savary of Paris showed, in 1827,[112] that the movements of the above-named binary in the Great Bear could be represented with all attainable accuracy by an ellipse calculated on orthodox gravitational principles with a period of 58-1/4 years. Encke followed at Berlin with a still more elegant method; and Sir John Herschel, pointing out the uselessness of a.n.a.lytical refinements where the data were necessarily so imperfect, described in 1832 a graphical process by which ”the aid of the eye and hand” was brought in ”to guide the judgment in a case where judgment only, and not calculation, could be of any avail.”[113]
Improved methods of the same kind were published by Dr. See in 1893,[114] and by Mr. Burnham in 1894;[115] and our acquaintance with stellar orbits is steadily gaining precision, certainty, and extent.
In 1825 Herschel undertook, and executed with great a.s.siduity during the ensuing eight years, a general survey of the northern heavens, directed chiefly towards the verification of his father's nebular discoveries.
The outcome was a catalogue of 2,306 nebulae and cl.u.s.ters, of which 525 were observed for the first time, besides 3,347 double stars discovered almost incidentally.[116] ”Strongly invited,” as he tells us himself, ”by the peculiar interest of the subject, and the wonderful nature of the objects which presented themselves,” he resolved to attempt the completion of the survey in the southern hemisphere. With this n.o.ble object in view, he embarked his family and instruments on board the _Mount Stewart Elphinstone_, and, after a prosperous voyage, landed at Cape Town on the 16th of January, 1834. Choosing as the scene of his observations a rural spot under the shelter of Table Mountain, he began regular ”sweeping” on the 5th of March. The site of his great reflector is now marked with an obelisk, and the name of Feldhausen has become memorable in the history of science; for the four years' work done there may truly be said to open the chapter of our knowledge as regards the southern skies.