A Popular History of Astronomy During the Nineteenth Century Part 56

A research of striking merit into the origin of binary stars was published in 1892 by Dr. T. J. J. See, in the form of an Inaugural Dissertation for his doctor's degree in the University of Berlin. The main result was to show the powerful effects of tidal friction in prescribing the course of their development from double nebulae, revolving almost in contact, to double suns, far apart, yet inseparable.

The high eccentricities of their eventual orbits were shown to result necessarily from this mode of action, which must operate with enormous strength on closely conjoined, nearly equal ma.s.ses, such as the rapidly revolving pairs disclosed by the spectroscope. That these are still in an early stage of their life-history is probable in itself, and is re-affirmed by the exceedingly small density indicated for eclipsing stars by the ratio of phase-duration to period.

Stellar photometry, initiated by the elder Herschel, and provided with exact methods by his son at the Cape, by Steinheil and Seidel at Munich, has of late years a.s.sumed the importance of a separate department of astronomical research. Two monumental works on the subject, compiled on opposite sides of the Atlantic, were thus appropriately coupled in the bestowal of the Royal Astronomical Society's Gold Medal in 1886. Harvard College Observatory led the way under the able direction of Professor E.

C. Pickering. His photometric catalogue of 4,260 stars,[1606]

constructed from nearly 95,000 observations of light-intensity during the years 1879-82, const.i.tutes a record of incalculable value for the detection and estimation of stellar variability. It was succeeded in 1885 by Professor Pritchard's "Uranometria Nova Oxoniensis," including photometric determinations of the magnitude of all naked-eye stars, from the pole to ten degrees south of the equator to the number of 2,784. The instrument employed was the "wedge photometer," which measures brightness by resistance to extinction. A wedge of neutral-tint gla.s.s, accurately divided to scale, is placed in the path of the stellar rays, when the thickness of it they have power to traverse furnishes a criterion of their intensity. Professor Pickering's "meridian photometer," on the other hand, is based upon Zollner's principle of equalization effected by a polarising apparatus. After all, however, as Professor Pritchard observed, "the eye is the real photometer," and its judgment can only be valid over a limited range.[1607] Absolute uniformity, then, in estimates made by various means, under varying conditions, and by different observers, is not to be looked for; and it is satisfactory to find substantial agreement attainable and attained.

Only in an insignificant fraction of the stars common to the Harvard and Oxford catalogues discordances are found exceeding one-third of a magnitude; a large proportion (71 per cent.) agree within one-fourth, a considerable minority (31 per cent.) within one-tenth of a magnitude.[1608] The Harvard photometry was extended, on the same scale, to the opposite pole in a catalogue of the magnitudes of 7,922 southern stars,[1609] founded on Professor Bailey's observations in Peru, 1889-91. Measurements still more comprehensive were subsequently executed at the primary establishment. With a meridian photometer of augmented power, the surprising number of 473,216 settings were made during the years 1891-98, nearly all by the indefatigable director himself, and they afforded materials for a "Photometric Durchmusterung,"

published in 1901, including all stars to 75 magnitude north of declination -40.[1610] A photometric zone, 20 wide, has for some time been in course of observation at Potsdam by MM. Muller and Kempf. The instrument employed by them is constructed on the polarising principle as adapted by Zollner.

Photographic photometry has meanwhile risen to an importance if anything exceeding that of visual photometry. For the usefulness of the great international star-chart now being prepared would be gravely compromised by systematic mistakes regarding the magnitudes of the stars registered upon it. No entirely trustworthy means of determining them have, however, yet been found. There is no certainty as to the relative times of exposure needed to get images of stars representative of successive photometric ranks. All that can be done is to measure the proportionate diameters of such images, and to infer, by the application of a law learned from experience, the varied intensities of light to which they correspond. The law is, indeed, neither simple nor constant. Different investigators have arrived at different formulae, which, being purely empirical, vary their nature with the conditions of experiment. Probably the best expedient for overcoming the difficulty is that devised by Pickering, of simultaneously photographing a star and its secondary image, reduced in brightness by a known amount.[1611] The results of its use will be exhibited in a catalogue of 40,000 stars to the tenth magnitude, one for each square degree of the heavens. A photographic photometry of all the lucid stars, modelled on the visual photometry of 1884, is promised from the same copious source of novelties. The magnitudes of the stars in the Draper Catalogue were determined, so to speak, spectrographically. The quant.i.ty measured in all cases was the intensity of the hydrogen line near G. By the employment of this definite and uniform test, results were obtained, of special value indeed, but in strong disaccord with those given by less exclusive determinations.

Thought, meantime, cannot be held aloof from the great subject upon the future ill.u.s.tration of which so much patient industry is being expended.

Nor are partial glimpses denied to us of relations fully discoverable, perhaps, only through centuries of toil. Some important points in cosmical economy have, indeed, become quite clear within the last fifty years, and scarcely any longer admit of a difference of opinion. One of these is that of the true status of nebulae.

This was virtually settled by Sir J. Herschel's description in 1847 of the structure of the Magellanic clouds; but it was not until Whewell, in 1853, and Herbert Spencer, in 1858,[1612] enforced the conclusions necessarily to be derived therefrom that the conception of the nebulae as remote galaxies, which Lord Rosse's resolution of many into stellar points had appeared to support, began to withdraw into the region of discarded and half-forgotten speculations. In the Nubeculae, as Whewell insisted,[1613] "there coexist, in a limited compa.s.s and in indiscriminate position, stars, cl.u.s.ters of stars, nebulae, regular and irregular, and nebulous streaks and patches. These, then, are different kinds of things in themselves, not merely different to us. There are such things as nebulae side by side with stars and with cl.u.s.ters of stars. Nebulous matter resolvable occurs close to nebulous matter irresolvable."

This argument from coexistence in nearly the same region of s.p.a.ce, reiterated and reinforced with others by Mr. Spencer, was urged with his accustomed force and freshness by Mr. Proctor. It is unanswerable. There is no maintaining nebulae to be simply remote worlds of stars in the face of an agglomeration like the Nubecula Major, containing in its (certainly capacious) bosom _both_ stars and nebulae. Add the facts that a considerable proportion of these perplexing objects are gaseous, and that an intimate relation obviously subsists between the mode of their scattering and the lie of the Milky Way, and it becomes impossible to resist the conclusion that both nebular and stellar systems are parts of a single scheme.[1614]

As to the stars themselves, the presumption of their approximate uniformity in size and brightness has been effectually dissipated.

Differences of distance can no longer be invoked to account for dissimilarity in l.u.s.tre. Minute orbs, altogether invisible without optical aid, are found to be indefinitely nearer to us than such radiant objects as Canopus, Betelgeux, or Rigel. Moreover, intensity of light is perceived to be a very imperfect index to real magnitude. Brilliant suns are swayed from their course by the attractive power of ma.s.sive yet faintly luminous companions, and suffer eclipse from obscure interpositions. Besides, effective l.u.s.tre is now known to depend no less upon the qualities of the investing atmosphere than upon the extent and radiative power of the stellar surface. Red stars must be far larger in proportion to the light diffused by them than white or yellow stars.[1615] There can be no doubt that our sun would at least double its brightness were the absorption suffered by its rays to be reduced to the Sirian standard; and, on the other hand, that it would lose half its present efficiency as a light-source if the atmosphere partially veiling its splendours were rendered as dense as that of Aldebaran.

Thus, variety of all kinds is seen to abound in the heavens; and it must be admitted that the consequent insecurity of all hypotheses as to the relative distances of individual stars singularly complicates the question of their allocation in s.p.a.ce. Nevertheless, something has been learnt even on that point; and the tendency of modern research is, on the whole, strongly confirmatory of the views expressed by Herschel in 1802. He then no longer regarded the Milky Way as the mere visual effect of an enormously extended stratum of stars, but as an actual aggregation, highly irregular in structure, made up of stellar clouds and groups and nodosities. All the facts since ascertained fit in with this conception, to which Proctor added arguments favouring the view, since adopted by Barnard[1616] and Easton,[1617] that the stars forming the galactic stream are not only situated more closely together, but are also really, as well as apparently, of smaller dimensions than the lucid orbs studding our skies. By the laborious process of isographically charting the whole of Argelander's 324,000 stars, he brought out in 1871[1618] signs of relations.h.i.+p between the distribution of the brighter stars and the complex branchings of the Milky Way, which has been stamped as authentic by Newcomb's recent statistical inquiries.[1619] There is, besides, a marked condensation of stars, especially in the southern hemisphere, towards a great circle inclined some twenty degrees to the galactic plane; and these were supposed by Gould to form with the sun a subordinate cl.u.s.ter, of which the components are seen projected upon the sky as a zone of stellar brilliants.[1620] The zone has, however, galactic rather than solar affinities, and represents, perhaps, not a group, but a stream.

The idea is gaining ground that the Milky Way is designed, in its main outlines, on a spiral pattern, and that its various branches and sections are consequently situated at very different distances from ourselves. Proctor gave a preliminarily interpretation of their complexities on this principle, and Easton of Rotterdam[1621] has renewed the attempt with better success.

A most suggestive delineation of the Milky Way, completed in 1889, after five years of labour, by Dr. Otto Boed.i.c.ker, Lord Rosse's astronomer at Parsonstown, was published by lithography in 1892. It showed a curiously intricate structure, composed of dimly luminous streams, and shreds, and patches, intermixed with dark gaps and channels. Ramifications from the main trunk ran out towards the Andromeda nebula and the "Bee-hive"

cl.u.s.ter in Cancer, involved the Pleiades and Hyades, and, winding round the constellation of Orion, just attained the Sword-handle nebula. The last delicate touches had scarcely been put to the picture, when the laborious eye-and-hand method was, in this quarter, as already in so many others, superseded by a more expeditious process. Professor Barnard took the first photographs ever secured of the true Milky Way, July 28, August 1 and 2, 1889, at the Lick Observatory. Special conditions were required for success; above all, a wide field and a strong light-grasp, both complied with through the use of a 6-inch portrait-lens. Even thus, the sensitive plate needed some hours to pick out the exceedingly faint stars collected in the galactic clouds. These cannot be photographed under the nebulous aspect they wear to the eye; the camera takes note of their real nature, and registers their const.i.tuent stars rank by rank.

Hence the difficulty of disclosing them. "In the photographs made with the 6-inch portrait-lens," Professor Barnard wrote, "besides myriads of stars, there are shown, for the first time, the vast and wonderful cloud-forms, with all their remarkable structure of lanes, holes, and black gaps, and sprays of stars. They present to us these forms in all their delicacy and beauty, as no eye or telescope can ever hope to see them."[1622] In Plate VI. one of these strange galactic landscapes is reproduced. It occurs in the Bow of Sagittarius, not far from the Trifid nebula, where the aggregations of the Milky Way are more than usually varied and characteristic. One of their distinctive features comes out with particular prominence. It will be noticed that the bright ma.s.s near the centre of the plate is tunnelled with dark holes and furrowed by dusky lanes. Such interruptions recur perpetually in the Milky Way. They are exemplified on the largest scale in the great rift dividing it into two branches all the way from Cygnus to Crux; and they are reproduced in miniature in many cl.u.s.ters.

PLATE VI.

[Ill.u.s.tration: Region of the Milky Way in Sagittarius--showing a double black aperture.

Photographed by Professor E. E. Barnard.]

Mr. H. C. Russell, at Sydney in 1890, successfully imitated Professor Barnard's example.[1623] His photographs of the southern Milky Way have many points of interest. They show the great rift, black to the eye, yet densely star-strewn to the perception of the chemical retina; while the "Coal-sack" appears absolutely dark only in its northern portion. His most remarkable discovery, however, was that of the spiral character of the two Nubeculae. With an effective exposure of four and a half hours, the Greater Cloud came out as "a complex spiral, with two centres"; while the similar conformation of its minor companion developed only after eight hours of persistent actinic action. The revelation is full of significance.

Scarcely less so, although after a different fas.h.i.+on, is the disclosure on plates exposed by Dr. Max Wolf, with a 5-inch lens, in June, 1891, of a vastly extended nebula, bringing some of the leading stars in Cygnus into apparently organic connection with the piles of galactic star-dust likewise involved by it.[1624] Barnard has similarly found great tracts of the Milky Way to be photographically nebulous, and the conclusion seems inevitable that we see in it a prodigious mixed system, resembling that of the Pleiades in point of composition, though differing widely from it in plan of structure. Of corroborative testimony, moreover, is the discovery independently resulting from Gill's and Pickering's photographic reviews, that stars of the first type of spectrum largely prevail in the galactic zone of the heavens.[1625] With approach to that zone, Kapteyn noticed a steady growth of actinic intensity relative to visual brightness in the stars depicted on the Cape Durchmusterung plates.[1626] In other words, stellar light is, in the Milky Way, _bluer_ than elsewhere. And the reality of the primitive character hence to be inferred for the entire structure was, in a manner, certified by Mr. McClean's observation that Helium stars--the supposed immediate products of nebulous matter--crowd towards its medial plane.

The first step towards the unravelment of the tangled web of stellar movements was taken when Herschel established the reality, and indicated the direction of the sun's journey. But the gradual s.h.i.+fting backward of the whole of the celestial scenery amid which we advance accounts for only a part of the observed displacements. The stars have motions of their own besides those reflected upon them from ours. All attempts, however, to grasp the general scheme of these motions have hitherto failed. Yet they have not remained wholly fruitless. The community of slow movement in Taurus, upon which Madler based his famous theory, has proved to be a fact, and one of very extended significance.

In 1870 Mr. Proctor undertook to chart down the directions and proportionate amounts of about 1,600 proper motions, as determined by Messrs. Stone and Main, with the result of bringing to light the remarkable phenomenon termed by him "star-drift."[1627] Quite unmistakably, large groups of stars, otherwise apparently disconnected, were seen to be in progress together, in the same direction and at the same rate, across the sky. An example of this kind of unanimity was alleged by him in the five intermediate stars of the Plough; and that the agreement in thwartwise motion is no casual one is practically demonstrated by the concordant radial velocities determined at Potsdam for four out of the five objects in question. All of these approach the earth at the rate of about eighteen miles a second; and the fifth and faintest, Delta Ursae, though not yet measured, may be held to share their advance. One of them, moreover, Zeta Ursae, alias Mizar, carries with it three other stars--Alcor, the Arab "Rider" of the horse, visible to the naked eye, besides a telescopic and a spectroscopic attendant. So that the group may be regarded as octuple. It is of vast compa.s.s. Dr. Hoffler a.s.signed to it in 1897[1628]--although on grounds more or less hypothetical--a mean parallax corresponding to a light-journey of 192 years, which would give to the marching squadron a total extent of at least fourteen times the distance from the sun to Alpha Centauri, while implying for its brightest member--Eta Ursae Majoris--the l.u.s.tre of six hundred suns. The organising principle of this grand scheme must long remain mysterious.

It is no solitary example. Particular a.s.sociation, indeed--as was surmised by Mich.e.l.l far back in the eighteenth century--appears to be the rule rather than an exception in the sidereal system. Stars are bound together by twos, by threes, by dozens, by hundreds. Our own sun is, perhaps, not exempt from this gregarious tendency. Yet the search for its companions has, up to the present, been unavailing. Gould's cl.u.s.ter[1629] seems remote and intangible; Kapteyn's collection of solar stars proved to have been a creation of erroneous data, and was abolished by his unrelenting industry. Rather, we appear to have secured a compartment to ourselves for our long journey through s.p.a.ce. A practical certainty has, at any rate, been gained that whatever aggregation holds the sun as a const.i.tuent is of a far looser build than the Pleiades or Praesepe. Of all such majestic communities the laws and revolutions remain, as yet, inaccessible to inquiry; centuries may elapse before even a rudimentary acquaintance with them begins to develop; while the economy of the higher order of a.s.sociation, which we must reasonably believe that they unite to compose, will possibly continue to stimulate and baffle human curiosity to the end of time.

FOOTNOTES:

[Footnote 1369: _Report Brit. a.s.soc._, 1868, p. 166. Rutherfurd gave a rudimentary sketch of a cla.s.sification of the kind in December, 1862, but based on imperfect observation. See _Am. Jour. of Sc._, vol. x.x.xv., p. 77.]

[Footnote 1370: _Publicationem_, Potsdam, No. 14, 1884, p. 31.]

[Footnote 1371: Von Konkoly _once_ derived from a slow-moving meteor a hydro-carbon spectrum. A. S. Herschel, _Nature_, vol. xxiv., p. 507.]

[Footnote 1372: _Phil. Trans._, vol. cliv., p. 429.]

[Footnote 1373: _Am. Jour. of Sc._, vol. xix., p. 467.]

[Footnote 1374: _Photom. Unters._, p. 243.]

[Footnote 1375: _Spectre Solaire_, p. 38.]

[Footnote 1376: Mr. J. Birmingham, in the Introduction to his Catalogue of Red Stars, adduced sundry instances of colour-change in a direction the opposite to that a.s.sumed by Zollner to be the inevitable result of time. _Trans. R. Irish Acad._, vol. xxvi., p. 251. A learned discussion by Dr. T. J. J. See, moreover, enforces the belief that Sirius was absolutely _red_ eighteen hundred years ago. _Astr. and Astroph._, vol.

xi., p. 269.]

[Footnote 1377: _Phil. Trans.,_ vol. clxiv., p. 492.]

[Footnote 1378: _Astr. Nach._, No. 2,000.]

[Footnote 1379: _Proc. Roy. Soc._, vols. xvi., p. 31; xvii., p. 48.]

[Footnote 1380: _Annalen der Physik_, Bd. xx., p. 155.]

[Footnote 1381: _Ibid._, p. 153.]

[Footnote 1382: _Knowledge_, vol. xiv., p. 101.]

[Footnote 1383: _Meteoritic Hypothesis_, p. 380.]

[Footnote 1384: _Phil. Trans._, vol. cxci. A., p. 128; _Spectra of Southern Stars_, p. 3.]

[Footnote 1385: See the author's _System of the Stars_, p. 84.]

[Footnote 1386: A designation applied by Sir Norman Lockyer to third-type stars.]

[Footnote 1387: See _ante_, p. 198.]

[Footnote 1388: _Bothkamp Beobachtungen_, Heft ii., p. 146.]

[Footnote 1389: _Astr. Nach._, No. 2,539.]

[Footnote 1390: _Ibid._, No. 2,548; _Observatory_, vol. vi., p. 332.]

[Footnote 1391: _Month. Not._, vol. xlvii., p. 92.]

[Footnote 1392: _Publ. Astr. Pac. Soc._, vol. i., p. 80; _Observatory_, vol. xiii., p. 46.]

[Footnote 1393: _Lockyer, Proc. Roy. Soc._, vol. lvii., p. 173.]