STARS AND NEBULÆ
That a science of stellar chemistry should not only have become possible, but should already have made material advances, is assuredly one of the most amazing features in the swift progress of knowledge our age has witnessed. Custom can never blunt the wonder with which we must regard the achievement of compelling rays emanating from a source devoid of sensible magnitude through immeasurable distance, to reveal, by its distinctive qualities, the composition of that source. The discovery of revolving double stars assured us that the great governing force of the planetary movements, and of our own material existence, sways equally the courses of the farthest suns in space; the application of prismatic analysis certified to the presence in the stars of the familiar materials, no less of the earth we tread, than of the human bodies built up out of its dust and circumambient vapours.
We have seen that, as early as 1823, Fraunhofer ascertained the generic participation of stellar light in the peculiarity by which sunlight, spread out by transmission through a prism, shows numerous transverse rulings of interrupting darkness. No sooner had Kirchhoff supplied the key to the hidden meaning of those ciphered characters than it was eagerly turned to the interpretation of the dim scrolls unfolded in the spectra of the stars. Donati made at Florence in 1860 the first efforts in this direction; but with little result, owing to the imperfections of the instrumental means at his command. His comparative failure, however, was a prelude to others' success. Almost simultaneously, in 1862, the novel line of investigation was entered upon by Huggins near London, by Father Secchi at Rome, and by Lewis M. Rutherfurd in New York. Fraunhofer's device of using a cylindrical lens for the purpose of giving a second dimension to stellar spectra was adopted by all, and was, indeed, indispensable. For a luminous point, such as a star appears, becomes, when viewed through a prism, avariegated line, which, until broadened into a band by the intervention of a cylindrical lens, is all but useless for purposes of research. This process ofrolling outinvolves, it is true, much loss of light—a scanty and precious commodity, as coming from the stars; but the loss is an inevitable one. And so fully is it compensated by the great light-grasping power of modern telescopes that important information can now be gained from the spectroscopic examination of stars far below the range of the unarmed eye.
The effective founders of stellar spectroscopy, then (since Rutherfurd shortly turned his efforts elsewhither), were Father Secchi, the eminent Jesuit astronomer of the Collegio Romano, where he died, February 26, 1878, and Sir William Huggins, with whom the late Professor W. A. Miller was associated. The work of each was happily directed so as to supplement that of the other. With less perfect appliances, the Roman astronomer sought to render his extensive rather than precise; at Tulse Hill searching accuracy over a narrow range was aimed at and attained. To Father Secchi is due the merit of having executed the first spectroscopic survey of the heavens. Above 4,000 stars were passed in review by him, and classified according to the varying qualities of their light. His provisional establishment (1863-67) of four types of stellar spectra[1369]has proved a genuine aid to knowledge through the facilities afforded by it for the arrangement and comparison of rapidly accumulating facts. Moreover, it is scarcely doubtful that these spectral distinctions correspond to differences in physical condition of a marked kind.
The first order comprises more than half the visible and probably an overwhelming proportion of the faintest stars. Sirius, Vega, Regulus, Altair, are amongst its leading members. Their spectra are distinguished by the breadth and intensity of the four dark bars due to the absorption of hydrogen, and by the extreme faintness of the metallic lines, of which, nevertheless, hundreds are disclosed by careful examination. The light of these "Sirian" orbs is white or bluish; and it is found to be rich in ultra-violet rays.
Capella and Arcturus belong to the second, or solar type of stars, which is about one-sixth less numerously represented than the first. Their spectra are quite closely similar to that of sunlight, in being ruled throughout by innumerable fine dark lines; and they share its yellowish tinge.
The third class includes most red and variable stars (commonly synonymous), of which Betelgeux in the shoulder of Orion, and"Mira" in the Whale, are noted examples. Their characteristic spectrum is of the "fluted" description. It shows like a strongly illuminated range of seven or eight variously tinted columns seen in perspective, the light falling from the red end towards the violet. Thiskindof absorption is produced by the vapours of metalloids or of compound substances.
To the fourth order of stars belongs also a colonnaded spectrum, butreversed; the light is thrown the other way. The three broad zones of absorption which interrupt it are sharp towards the red, insensibly gradated towards the violet end. The individuals composing Class IV. are few and apparently insignificant, the brightest of them not exceeding the fifth magnitude. They are commonly distinguished by a deep red tint, and gleam like rubies in the field of the telescope. Father Secchi, who in 1867 detected the peculiarity of their analyzed light, ascribed it to the presence of carbon in some form in their atmospheres; and this was confirmed by the researches of H. C. Vogel,[1370]director of the Astro-physical Observatory at Potsdam. The hydro-carbon bands, in fact, seen bright in comets, are dark in these singular objects—the only ones in the heavens (save one bright-line star and a rare meteor)[1371]which display a cometary analogy of the fundamental sort revealed by the spectroscope.
The members of all four orders are, however, emphatically suns. They possess, it would appear, photospheres radiating all kinds of light, and differ from each other mainly in the varying qualities of their absorptive atmospheres. The principle that the colours of stars depend, not on the intrinsic nature of their light, but on the kinds of vapours surrounding them, and stopping out certain portions of that light, was laid down by Huggins in 1864.[1372]The phenomena of double stars seem to indicate a connection between the state of the investing atmospheres, by the action of which their often brilliantly contrasted tints are produced, and their mutual physical relations. A tabular statement put forward by Professor Holden in June, 1880,[1373]made it, at any rate, clear that inequality of magnitude between the components of binary systems accompanies unlikeness in colour, and that stars more equally matched in one respect are pretty sure to be so in the other. Besides, blue and green stars of a decided tinge are never solitary; they invariably form part of systems. So that association has undoubtedly a predominant influence upon colour.
Nevertheless, the crude notion thrown out by Zöllner in 1865,[1374]that yellow and red stars are simply white stars in various stages of cooling, obtained for a time undeserved currency. D'Arrest, indeed, protested against it, and Ångström, in 1868,[1375]substituted atmospheric quality for mere colour[1376]as a criterion of age and temperature. His lead was followed by Lockyer in 1873,[1377]and by Vogel in 1874.[1378]The scheme of classification due to the Potsdam astro-physicist differed from Father Secchi's only in presenting his third and fourth types as subdivisions of the same order, and in inserting three subordinate categories; but their variety was "rationalised" by the addition of the seductive idea of progressive development. Thus, the white Sirian stars were represented as theyoungestbecause the hottest of the sidereal family; those of the solar pattern as having already wasted much of their store by radiation, and being well advanced in middle life; while the red stars with banded spectra figured as effete suns, hastening rapidly down the road to final extinction.
Vogel's scheme is, however, incomplete. It traces the downward curve of decay, but gives no account of the slow ascent to maturity. The present splendour of Vega, for instance, was prepared, according to all creative analogy, by almost endless processes of gradual change. What was its antecedent condition? The question has been variously answered. Dr. Johnstone Stoney advocated, in 1867, the comparative youth of red stars;[1379]A. Ritter, of Aix-la-Chapelle, divided them, in 1883,[1380]into two squadrons, posted, the one on the ascending, the other on the descending branch of the temperature-curve, and corresponding, presumably, with Secchi's third and fourth orders of stars with banded spectra. Whether, in the interim, they should display spectra of the Sirian or of the solar type was made to depend on their greater or less massiveness.[1381]But the relation actually existing perhaps inverts that contemplated by Ritter. Certainly, the evidence collected by Mr. Maunder in 1891 strongly supports the opinion that the average solar star is a weightier body than the average Sirian star.[1382]
On November 17, 1887, Sir Norman Lockyer communicated to the Royal Society the first of a series of papers embodying his "Meteoritic Hypothesis" of cosmical constitution, stated and supported more at large in a separate work bearing that name, published in 1890. The fundamental proposition wrought out in it was that "all self-luminous bodies in the celestial space are composed either of swarms of meteorites or of masses of meteoric vapour produced by heat."[1383]On the basis of this supposed community of origin, sidereal objects were distributed in seven groups along a temperature-curve ascending from nebulæ and gaseous, or bright-line stars, through red stars of the third type, and a younger division of solar stars, to the high Sirian level; then descending through the more strictly solar stars to red stars of the fourth type ("carbon-stars"), below which lay only thecaput mortuumentitled Group vii. The ground-work of this classification was, however, insecure, and has given way. Certain spectroscopic coincidences, avowedly only approximate, suggesting that stars and nebulæ of every species might be formed out of variously aggregated meteorites, failed of verification by exact inquiry. And spectroscopic coincidences admit of no compromise. Those that are merely approximate are, as a rule, unmeaning.
In his Presidential Address at the Cardiff Meeting of the British Association in 1891, Dr. Huggins adhered in the main to the line of advance traced by Vogel. The inconspicuousness of metallic lines in the spectra of the white stars he attributed, not to the paucity, but to the high temperature of the vapours producing them, and the consequent deficiency of contrast between their absorption-rays and the continuous light of the photospheric background. "Such a state of things would more probably," in his opinion, "be found in conditions anterior to the solar stage," while "a considerable cooling of the sun would probably give rise to banded spectra due to compounds." He adverted also to the influential effects upon stellar types of varying surface gravity, which being a function of both mass and bulk necessarily gains strength with wasting heat and consequent shrinkage. The same leading ideas were more fully worked out in "An Atlas of Representative Stellar Spectra," published by Sir William and Lady Huggins in 1899. They were, moreover, splendidly illustrated by a set of original spectrographic plates, while precision was added to the adopted classification by the separation of helium from hydrogen stars. The spectrum of the exotic substance terrestrially captured in 1895 is conspicuous by absorption, as Vogel, Lockyer, and Deslandres promptlyrecognised ina considerable number of white stars, among them the Pleiadesand most of the brilliants in Orion. Mr. McClean, whose valuable spectrographic survey of the heavens was completed at the Cape in 1897, found reason to conclude that they are in the first stage of development from gaseous nebulæ;[1384]and in this the Tulse Hill investigators unhesitatingly concur.
The strongest evidence for the primitive state of white stars is found in their nebular relations. The components of groups, still involved and entangled with "silver braids" of cosmic mist, show, perhaps invariably, spectra of the helium type, occasionally crossed by bright rays. Possibly all such stars have passed through a bright-line stage; but further evidence on the point is needed. Relative density furnishes another important test of comparative age, and Sirian stars are, on the whole, undoubtedly more bulky proportionately to their mass than solar stars. The rule, however, seems to admit of exceptions; hence the change from one kind of spectrum to the other is not inevitably connected with the attainment of a particular degree of condensation. There is reason to believe that it is anticipated in the more massive globes, despite their comparatively slow cooling, as a consequence of the greater power of gravity over their investing vaporous envelopes. This conclusion is enforced by the relations of double-star spectra. The fact that, in unequal pairs, the chief star most frequently shows a solar, its companion a Sirian, spectrum can scarcely be otherwise explained than by admitting that, while the sequence of types is pursued in an invariable order, it is pursued much more rapidly in larger than in small orbs. It need not, indeed, be supposed that all stars are identical in constitution, and present identical life-histories.[1385]Individualities in the one, and divergencies in the other, must be allowed for. Yet the main track is plainly continuous, and leads by insensible gradations from nebulæ through helium stars to the Sirian, and onward to the solar type, whence, by an inevitable transition, fluted, or "Antarian,"[1386]spectra develop.
The first-known examples of the class of gaseous stars—β Lyræ and γ Cassiopeiæ—were noticed by Father Secchi at the outset of his spectroscopic inquiries. Both showbrightlines of hydrogen and helium, so that the peculiarity of their condition probably consists in the intense ignition of their chromospheric surroundings. Their entire radiating surfaces might be described asfaculous. That is to say, brilliant formations, such as have been photographed by Professor Hale on the sun's disc,[1387]cover, perhaps, the whole, insteadof being limited to a small portion of the photospheric area. But this state of things is more or less inconstant. Some at least of the bright rays indicative of it are subject to temporary extinctions. Already in 1871-72, Dr. Vogel[1388]suspected the prevalence of such vicissitudes; and their reality was ascertained by M. Eugen von Gothard. After the completion of his new astrophysical observatory at Herény in the autumn of 1881, he repeatedly observed the spectra of both stars without perceiving a trace of bright lines; and was thus taken quite by surprise when he caught a twinkling of the crimson C in γ Cassiopeiæ, August 13, 1883.[1389]A few days later, the whole range including D_3 was lustrous. Duly apprised of the recurrence of a phenomenon he had himself vainly looked for during some years, M. von Konkoly took the opportunity of the great Vienna refractor being placed at his disposal to examine with it the relighted spectrum on August 27.[1390]In its wealth of light C was dazzling; D_3 and the green and blue hydrogen rays shone somewhat less vividly; D and the groupbshowed faintly dark; while three broad absorption-bands, sharply terminated towards the red, diffuse towards the violet, shaded the spectrum near its opposite extremities.
The previous absence of bright lines from the spectrum of this star was, however, by no means so protracted or complete as M. von Gothard supposed. At Dunecht, C was "superbly visible" December 20, 1879[1391]; F was seen bright on October 28 of the same year, and frequently at Greenwich in 1880-81. The curious fact has, moreover, been adverted to by Dr. Copeland, that Cis much more variable than F. To Vogel, June 18, 1872, the first was invisible, while the second was bright; at Dunecht, January 11, 1887, the conditions were so far inverted that C was resplendent, F comparatively dim.
No spectral fluctuations were detected in γ Cassiopeiæ by Keeler in 1889; but even with the giant telescope of Mount Hamilton, the helium-ray was completely invisible.[1392]It made, nevertheless, capricious appearances at South Kensington during that autumn, and again October 21, 1894,[1393]while in September, 1892, Bélopolsky could obtain no trace of it on orthochromatic plates exposed with the 30-inch Pulkowa refractor.[1394]Still more noteworthy is the circumstance that the well-known green triplet of magnesium (b), recorded as dark by Keeler in 1889, came out bright on fifty-two spectrographs of the star taken by Father Sidgreaves during the years 1891-99.[1395]Nofluctuations in the hydrogen-spectrum were betrayed by them; but subordinate lines of unknown origin showed alternate fading and vivification.
The spectrum of β Lyræ undergoes transitions to some extent analogous, yet involving a different set of considerations. First noticed by Von Gothard in 1882,[1396]they were imperfectly made out, two years later, to be of a cyclical character.[1397]This, however, could only be effectively determined by photographic means. Beta Lyræ is a "short-period variable." Its light changes with great regularity from 3·4 to 4·4 magnitude every twelve days and twenty-two hours, during which time it attains a twofold maximum, with an intervening secondary minimum. The question, then, is of singular interest, whether the changes of spectral quality visible in this object correspond to its changes in visual brightness. A distinct answer in the affirmative was supplied through Mrs. Fleming's examination of the Harvard plates of the star's spectrum, upon which, in 1891, she found recorded diverse complex changes of bright and dark lines obviously connected with the phases of luminous variation, and obeying, in the long-run, precisely the same period.[1398]Something more will be said presently as to the import of this discovery.
Bright hydrogen lines have so far been detected—for the most part photographically at Harvard College—in about sixty stars, including Pleione, the surmised lost Pleiad, P Cygni, noted for instability of light in the seventeenth century, and the extraordinary southern variable, η Carinæ. In most of these objects other vivid rays are associated with those due to hydrogen. A blaze of hydrogen, moreover, accompanies the recurring outbursts of about one hundred and fifty "long-period variables," giving banded spectra of the third type. Professor Pickering discovered the first example of this class, towards the close of 1886, in Mira Ceti; further detections were made visually by Mr. Espin; and the conjunction of bright hydrogen-lines with dusky bands has been proved by Mrs. Fleming's long experience in studying the Harvard photographs, to indicate unerringly the subjection of the stars thus characterised to variations of lustre accomplished in some months.
A third variety of gaseous star is named after MM. Wolf and Rayet, who discovered, at Paris in 1867,[1399]its three typical representatives, close together in the constellation Cygnus. Six further specimens were discovered by Dr. Copeland, five of them in thecourse of a trip for the exploration of visual facilities in the Andes in 1883;[1400]and a large number have been made known through spectral photographs taken in both hemispheres under Professor Pickering's direction. At the close of the nineteenth century, over a hundred such objects had been registered, none brighter than the sixth magnitude, with the single exception of γ Argûs, the resplendent continuous spectrum of which, first examined by Respighi and Lockyer in 1871, is embellished with the yellow and blue rays distinctive of the type.[1401]Here, then, we have a stellar globe apparently at the highest point of sunlike incandescence, sharing the peculiarities of bodies verging towards the nebulous state. Examined with instruments of adequate power, their spectra are seen to be highly complex. They include a fairly strong continuous element, a numerous set of absorption-lines, and a range of emission-lines, more or less completely represented in different stars. Especially conspicuous is a broad effluence of azure light, found by Dr. Vogel in 1883,[1402]and by Sir William and Lady Huggins in 1890,[1403]to be of multiple structure, and hence to vary in its mode of display. Its suggested identification with the blue carbon-fluting was disproved at Tulse Hill. Metallic vapours give no certain sign of their presence in the atmospheres of these remarkable bodies; but nebulum is stated to shine in some.[1404]Hydrogen and helium account for a large proportion of their spectral rays. Thirty-two Wolf-Rayet stars were investigated, spectroscopically and spectrographically, by Professor Campbell with the great Lick refractor in 1892-94;[1405]and several disclosed the singularity, already noticed by him in γ Argûs, of giving out mixed series, the members of which change from vivid to obscure with increase of refrangibility. It is difficult to imagine by what chromospheric machinery this curious result can be produced. Alcyone in the Pleiades presents the same characteristic. Alone among the hydrogen lines, crimson C glows in its spectrum, while all the others are dark. Luminosity of the Wolf-Rayet kind is particularly constant, both in quantity and quality. It seems to be incapable of developing save under galactic conditions. All the stars marked by it lie near the central line of the Milky Way, or in the Magellanic Clouds. They tend also to gather into groups. Circles of four degrees radius include respectively seven in Argo, eight in Cygnus.
The first spectroscopic star catalogue was published by Dr. Vogel at Potsdam in 1883.[1406]It included 4,051 stars, distributed over a zone of the heavens extending from 20° north to 20° south of the celestial equator.[1407]More than half of these were white stars, while red stars with banded spectra occurred in the proportion of about one-thirteenth of the whole. To the latter genus, M. Dunér, then of Lund, now Director of the Upsala Observatory, devoted a work of standard authority, issued at Stockholm in 1884. This was a catalogue with descriptive particulars of 352 stars showing banded spectra, 297 of which belong to Secchi's third, 55 to his fourth class (Vogel's iii.aand iii.b). Since then discovery has progressed so rapidly, at first through the telescopic reviews of Mr. Espin, then in the course of the photographic survey carried on at Harvard College, that considerably over one thousand stars are at present recognised as of the family of Betelgeux and Mira, while about 250 have so far exhibited the spectral pattern of 19 Piscium. One fact well ascertained as regards both species is the invariability of the type. The prismatic flutings of the one, and the broader zones of the other, are as if stereotyped—they undergo, in their fundamental outlines, no modification, though varying in relative intensity from star to star. They are always accompanied by, or superposed upon, a spectrum of dark lines, in producing which sodium and iron have an obvious share; and certain bright rays, noticed by Secchi with imperfect appliances as enhancing the chiaroscuro effects in carbon-stars, came out upon plates exposed by Hale and Ellerman in 1898 with the stellar spectrograph of the Yerkes Observatory.[1408]Their genuineness was shortly afterwards visually attested by Keeler, Campbell, and Dunér;[1409]but no chemical interpretation has been found for them.
A fairly complete preliminary answer to the question, What are the stars made of? was given by Sir William Huggins in 1864.[1410]By laborious processes of comparison between stellar dark lines and the bright rays emitted by terrestrial substances, he sought to assure his conclusions, regardless of cost in time and pains. He averred, indeed, that—taking into account restrictions by weather and position—the thorough investigation of asinglestar-spectrum would be the work of some years. Of two, however—those of Betelgeux and Aldebaran—he was able to furnish detailed andaccurate drawings. The dusky flutings in the prismatic light of the first of these stars have not been identified with the absorption of any particular substance; but associated with them are metallic lines, of which 78 were measured, and a good many identified by Huggins, while the wave-lengths of 97 were determined by Vogel in 1871.[1411]A photographic research, made by Keeler at the Alleghany Observatory in 1897, convinced him that the linear spectrum of third-type stars of the Betelgeux pattern essentially repeats that of the sun, but with marked differences in the comparative strength of its components.[1412]Hydrogen rays are inconspicuously present. That an exalted temperature reigns, at least in the lower strata of the atmosphere, is certified by the vaporisation there of matter so refractory to heat as iron.[1413]
Nine elements—among them iron, sodium, calcium, and magnesium—were recognised by Huggins as having stamped their signature on the spectrum of Aldebaran; while the existence in Sirius, and nearly all the other stars inspected, of hydrogen, together with sundry metals, was rendered certain or highly probable. This was admitted to be a bare gleaning of results; nor is there reason to suppose any of his congeners inferior to our sun in complexity of constitution. Definite knowledge on the subject, however, made little advance beyond the point to which it was brought by Huggins's early experiments until spectroscopic photography became thoroughly effective as a means of research.
In this, as in so many other directions, Sir William Huggins acted as pioneer. In March, 1863, he obtained microscopic prints of the spectra of Sirius and Capella.[1414]But they told nothing. No lines were visible in them. They were mere characterless streaks of light. Nine years later Dr. Henry Draper of New York got an impression of four lines in the spectrum of Vega. Then Huggins attacked the subject again in 1876, when the 18-inch speculum of the Royal Society had come into his possession, using prisms of Iceland spar and lenses of rock crystal; and this time with better success. A photograph of the spectrum of Vega showed seven strong lines.[1415]Still he was not satisfied. He waited and worked for three years longer. At length, on December 18, 1879, he was able to communicate to the Royal Society[1416]results answering to his expectations. The delicacy of eye and hand needed to obtain them may be estimated from the single fact that the image of a star had to be kept, by continual minute adjustments, exactly projected upon a slit1/350 of an inch in width during nearly an hour, in order to give it time to imprint the characters of its analyzed light upon a gelatine plate raised to the highest pitch of sensitiveness. But by this time he had secured in his wife a rarely qualified assistant.
The ultra-violet spectrum of the white stars—of which Vega was taken as the type—was thus shown to be a very remarkable one. A group of broad dark lines intersected it, arranged at intervals diminishing regularly upward, and falling into a rhythmical succession with the visible hydrogen lines. All belonged presumably to the same substance; and the presumption was rendered a certainty by direct photographs of the hydrogen spectrum taken by H. W. Vogel at Berlin a few months earlier.[1417]In them seven of the white-star series of grouped lines were visible; and the full complement of twelve appeared on Cornu's plates in 1886.[1418]
In yellow stars, such as Capella and Arcturus, the same rhythmical series waspartiallyrepresented, but associated with a great number of other lines; their state, as regards ultra-violet absorption, approximating to that of the sun; while the redder stars betrayed so marked a deficiency in actinic rays that from Betelgeux, with an exposureforty timesthat required for Sirius, only a faint spectral impression could be obtained, and from Aldebaran, in the strictly invisible region, almost none at all.
Thus, by the means of stellar light-analysis, acquaintance was first made with the ultra-violet spectrum of hydrogen;[1419]and its harmonic character, as expressed by "Balmer's Law," supplies a sure test for discriminating, among newly discovered lines, those that appertain from those that are unrelated to it. Deslandres' five additional prominence-rays, for instance, were at once seen to make part of the series, because conforming to its law;[1420]while a group of six dusky bands, photographed by Sir William and Lady Huggins, April 4, 1890,[1421]near the extreme upper end of the spectrum of Sirius, were pronounced without hesitation, for the opposite reason, to have nothing to do with hydrogen. Their true affinities are still a matter for inquiry.
As regards the hydrogen spectrum, however, the stars had further information in reserve. Until recently, it was supposed to consist of a single harmonic series, although, by analogy, three should co-exist. In 1896, accordingly, a second, bound to the first by unmistakable numerical relationships, was recognised by Professor Pickering in spectrographs of the 2·5 magnitude star ζ Puppis,[1422]and the identificationwas shortly afterwards extended to prominent Wolf-Rayet emission lines. The discovery was capped by Dr. Rydberg's indication of the Wolf-Rayet blue band at λ 4,688 as the fundamental member of the third, and principal, hydrogen series.[1423]None of the "Pickering lines" (as they may be called to distinguish them from the "Huggins series") can be induced to glimmer in vacuum-tubes. They seem to characterise bodies in a primitive state,[1424]and are in many cases associated with absorption rays of oxygen, the identification of which by Mr. McClean in 1897[1425]was fully confirmed by Sir David Gill.[1426]The typical "oxygen star" is β Crucis, one of the brilliants of the Southern Cross; but the distinctive notes of its spectrum occur in not a few specimens of the helium class. Thus, Sir William and Lady Huggins photographed several ultra-violet oxygen lines in β Lyræ,[1427]and found in Rigel signs of the presence of nitrogen,[1428]which, as well as silicium, proves to be a tolerably frequent constituent of such orbs.[1429]For some unknown reason, metalloids tend to become effaced, as metals, in the normal course of stellar development, exert a more and more conspicuous action.
Dr. Scheiner's spectrographic researches at Potsdam in 1890 and subsequently, exemplify the immense advantages of self-registration. In a restricted section of the spectrum of Capella, he was enabled to determine nearly three hundred lines with more precision than had then been attained in the measurement of terrestrial spectra. This star appeared to be virtually identical with the sun in physical constitution, although it emits, according to the best available data, about 140 times as much light, and is hence presumably 1,600 times more voluminous. An equally close examination of the spectrum of Betelgeux showed the predominance in it of the linear absorption of iron;[1430]but its characteristic flutings do not extend to the photographic region. Spectra of the second and third orders are for this reason not easily distinguished on the sensitive plate.
A spectrographic investigation of all the brighter northern stars was set on foot in 1886 at the observatory of Harvard College, under the form of a memorial to Dr. H. Draper, whose promising work in that line was brought to a close by his premature death in 1882. No individual exertions could, however, have realized a tithe of what has been and is being accomplished under ProfessorPickering's able direction, with the aid of the Draper and other instruments, supplemented by Mrs. Draper's liberal provision of funds. A novel system was adopted, or, rather, an old one—originally used by Fraunhofer—was revived.[1431]The use of a slit was discarded as unnecessary for objects like the stars, devoid of sensible dimensions, and giving hence anaturallypure spectrum; and a large prism, placed in front of the object-glass, analysed at once, with slight loss of light, the rays of all the stars in the field. Their spectra were taken, as it were, wholesale. As many as two hundred stars down to the eighth magnitude were occasionally printed on a single plate with a single exposure. No cylindrical lens was employed. The movement of the stars themselves was turned to account for giving the desirable width to their spectra. The star was allowed—by disconnecting or suitably regulating the clock—to travel slowly across the line of its own dispersed light, so broadening it gradually into a band. Excellent results were secured in this way. About fifty lines appear in the photographed spectrum of Aldebaran, and eight in that of Vega. On January 26, 1886, with an exposure of thirty-four minutes, a simultaneous impression was obtained of the spectra (among many others) of close upon forty Pleiades. With few and doubtful exceptions, they all proved to belong to the same type. An additional argument for the common origin of the stars forming this beautiful group was thus provided.[1432]
The "Draper Catalogue" of stellar spectra was published in 1890.[1433]It gives the results of a rapid analytical survey of the heavens north of 25° of southern declination, and includes 10,351 stars, down to about the eighth magnitude. The telescope used was of eight inches aperture and forty-five focus, its field of view—owing to the "portrait-lens" or "doublet" form given to it—embracing with fair definition no less than one hundred square degrees. An objective prism eight inches square was attached, and exposures of a few minutes were given to the most sensitive plates that could be procured. In this way the sky was twice covered in duplicate, each star appearing, as a rule, on four plates. The registration of their spectra was sought to be made more distinctive than had previously been attempted, Secchi's first type being divided into four, his second into five subdivisions; but the differences regarded in them could be confidently established only for stars above the sixth magnitude. The work supplies none the less valuable materials for general inferences as to the distribution and relations of the spectral types. The labour of its actual preparationwas borne by a staff of ladies under the direction of Mrs. Fleming. Materials for its completion to the southern pole have been accumulated with the identical instrument used at Cambridge, transferred for the purpose in 1889 to Peru, and the forthcoming "Second Draper Catalogue" will comprise 30,000 stars in both hemispheres. As supplements to this great enterprise, two important detailed discussions of stellar spectra were issued in 1897 and 1901 respectively.[1434]The first, by Miss A. C. Maury, dealt with 681 bright stars visible in the northern hemisphere; the second, by Miss A. J. Cannon, with 1,122 southern stars. Both authors traced, with care and ability, the minute gradations by which the long process of stellar evolution appears to be accomplished.
The progress of the Draper Memorial researches was marked by discoveries of an unexampled kind.
The principle upon which "motion in the line of sight" can be detected and measured with the spectroscope has already been explained.[1435]It depends, as our readers will remember, upon the removal of certain lines, dark or bright (it matters not which), from their normal places by almost infinitesimal amounts. The whole spectrum of the moving object, in fact, is very slightlyshovedhither or thither, according as it is travelling towards or from the eye; but, for convenience of measurement, one line is usually picked out from the rest, and attention concentrated upon it. The application of this method to the stars, however, is encompassed with difficulties. It needs a powerfully dispersive spectroscope to show line-displacements of the minute order in question; and powerful dispersion involves a strictly proportionate enfeeblement of light. This, where the supply is already to a deplorable extent niggardly, can ill be afforded; for which reason the operation of determining a star's approach or recession is, even apart from atmospheric obstacles, an excessively delicate one.
It was first executed by Sir William Huggins early in 1868.[1436]Selecting the brightest star in the heavens as the most promising subject of experiment, he considered the F line in the spectrum of Sirius to be just so much displaced towards the red as to indicate (the orbital motion of the earth being deducted) recession at the rate of twenty-nine miles a second; and the reality and direction of the movement were ratified by Vogel and Lohse's observation, March 22, 1871, of a similar, but even more considerable displacement.[1437]The inquiry was resumed by Huggins with improved apparatus in the following year, when the velocities of thirty starswere approximately determined.[1438]The retreat of Sirius, which proved slower than had at first been supposed, was now announced to be shared, at rates varying from twelve to twenty-nine miles, by Betelgeux, Rigel, Castor, Regulus, and five of the principal stars in the Plough. Arcturus, on the contrary, gave signs of rapid approach, as well as Pollux, Vega, Deneb in the Swan, and the brightness of the Pointers.
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 theirorbital 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 itsmotion 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 abouta 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.
Gore's "Catalogue of Known Variables"[1461]included, in 1884, 190 entries, and the number was augmented to 243 on its revision in 1888.[1462]Chandler's first list of 225 such objects,[1463]published about the same time, received successive expansions in 1893 and 1896,[1464]and finally included 400 entries. A new "Catalogue of Variable Stars,"still wider in scope, will shortly be issued by the GermanAstronomische Gesellschaft. Mr. A. W. Roberts's researches on southern variables[1465]have greatly helped to give precision, while adding to the extent of knowledge in this branch. Dr. Gould held the opinion that most stars fluctuate slightly in brightness through surface-alterations similar to, but on a larger scale than those of the sun; and the solar analogy might be pushed somewhat further. It perhaps affords a clue to much that is perplexing in stellar behaviour. Wolf pointed out in 1852 the striking resemblance in character between curves representing sun-spot frequency and curves representing the changing luminous intensity of many variable stars. There were the same steep ascent to maximum and more gradual decline to minimum, the same irregularities in heights and hollows, and, it may be added, the same tendency to a double maximum, and complexity of superposed periods.[1466]It is impossible to compare the two sets of phenomena thus graphically portrayed without reaching the conclusion that they are of closely related origin. But the correspondence indicated is not, as has often been hastily assumed, between maxima of sun-spots and minima of stellar brightness, but just the reverse. The luminous outbursts, not the obscurations of variable stars, obey a law analogous to that governing the development of spots on the sun. Objects of the kind do not, then, gain light through the closing-up of dusky chasms in their photospheres, but by an actual increase of surface-brilliancy, together with an immense growth of these brilliant formations—prominences and faculæ—which, in the sun, accompany, or are appended to spots. A comparison of light-curves with curves of spot-frequency leaves no doubt on this point, and the strongest corroborative evidence is derived from the emergence of bright lines in the spectra of long-period variables rising to their recurring maxima.
Every kind and degree of variability is exemplified in the heavens. At the bottom of the scales are stars like the sun, of which the lustre is—tried by our instrumental means—sensibly steady. At the other extreme are ranged the astounding apparitions of "new," or "temporary" stars. Within the last thirty-six years eleven of these stellar guests (as the Chinese call them) have presented themselves, and we meet with a twelfth no farther back than April 27, 1848. But of the "new star" in Ophiuchus found by Mr. Hind on that night, little more could be learnt than of the brilliant objects of the same kind observed by Tycho and Kepler. The spectroscope had not then been invented. Let us hear what it had to tell of later arrivals.
About thirty minutes before midnight of May 12, 1866, Mr. JohnBirmingham of Millbrook, near Tuam, in Ireland, saw with astonishment a bright star of the second magnitude unfamiliarly situated in the constellation of the Northern Crown. Four hours earlier, Schmidt of Athens had been surveying the same part of the heavens, and was able to testify that it was not visible there. That is to say, a few hours, or possibly a few minutes, sufficed to bring about a conflagration, the news of which may have occupied hundreds of years in travelling to us across space. The rays which were its messengers, admitted within the slit of Sir William Huggins's spectroscope, May 16, proved to be of a composition highly significant as to the nature of the catastrophe. The star—which had already declined below the third magnitude—showed what was described as a double spectrum. To the dusky flutings of Secchi's third type four brilliant rays were added.[1467]The chief of these agreed in position with lines of hydrogen; so that the immediate cause of the outburst was inferred to have been the eruption, or ignition, of vast masses of that subtle kind of matter, the universal importance of which throughout the cosmos is one of the most curious facts revealed by the spectroscope.
T Coronæ (as the new star was called) quickly lost its adventitious splendour. Nine days after its discovery it was again invisible to the naked eye. It is now a pale yellow, slightly variable star near the tenth magnitude, and finds a place as such in Argelander's charts.[1468]It was thus obscurely known before it made its sudden leap into notoriety.
The next "temporary," discovered by Dr. Schmidt at Athens, November 24, 1876, could lay no claim to previous recognition even in that modest rank. It was strictly a parvenu. There was no record of its existence until it made its appearance as a star of nearly the third magnitude, in the constellation of the Swan. Its spectrum was examined, December 2, by Cornu at Paris,[1469]and a few days later by Vogel and O. Lohse at Potsdam.[1470]It proved of a closely similar character to that of T Coronæ. A range of bright lines, including those of hydrogen, and probably of helium, stood out from a continuous background impressed with strong absorption. It may be presumed that in reality the gaseous substances, which, by their sudden incandescence, had produced the apparent conflagration, lay comparatively near the surface of the star, while the screen of cooler materials intercepting large portions of its light was situated at a considerable elevation in its atmosphere.
The object, meanwhile, steadily faded. By the end of the year itwas of no more than seventh magnitude. After the second week of March, 1877, strengthening twilight combined with the decline of its radiance to arrest further observation. It was resumed, September 2, at Dunecht, with a strange result. Practically the whole of its scanty light (it had then sunk below the tenth magnitude) was perceived to be gathered into a single bright line in the green, and that the most characteristic line of gaseous nebulæ.[1471]The star had, in fact, so far as outward appearance was concerned, become transformed into a planetary nebula, many of which are so minute as to be distinguishable from small stars only by the quality of their radiations. It is now, having sunk to about the fourteenth magnitude,[1472]entirely beyond the reach of spectroscopic scrutiny.
Perhaps none of the marvellous changes witnessed in the heavens has given a more significant hint as to their construction than the stellar blaze kindled in the heart of the great Andromeda nebula some undetermined number of years or centuries before its rays reached the earth in the month of August, 1885. The first published discovery was by Dr. Hartwig at Dorpat on August 31; but it was found to have been already seen, on the 19th, by Mr. Isaac W. Ward of Belfast, and on the 17th by M. Ludovic Gully of Rouen. Thenegativeobservations, on the 16th, of Tempel[1473]and Max Wolf, limited very narrowly the epoch of the apparition. Nevertheless, it did not, like most temporaries, attain its maximum brightness all at once. When first detected, it was of the ninth, by September 1 it had risen to the seventh magnitude, from which it so rapidly fell off that in March it touched the limit of visibility (sixteenth magnitude) with the Washington 26-inch. Its light bleached very perceptibly as it faded.[1474]During the earlier stages of its decline, the contrast was striking between the sharply defined, ruddy disc of the star, and the hazy, greenish-white background upon which it was projected,[1475]and with which it was inevitably suggested to be in some sort of physical connection.
Let us consider what evidence was really available on this point. To begin with, the position of the star was not exactly central. It lay sixteen seconds of arc to the south-west of the true nebular nucleus. Its appearance did not then signify a sudden advance of the nebula towards condensation, nor was it attended by any visible change in it save the transient effect of partial effacement through superior brightness.
Equally indecisive information was derived from the spectroscope.To Vogel, Hasselberg, and Young, the light of the "Nova" seemed perfectly continuous; but Huggins caught traces of bright lines on September 2, confirmed on the 9th;[1476]and Copeland succeeded, on September 30, in measuring three bright bands with an acute-angled prism specially constructed for the purpose.[1477]A shimmer of F was suspected, and had also been perceived by Mr. O. T. Sherman of Yale College. Still, the effect was widely different from that of the characteristic blazing spectrum of a temporary star, and prompted the surmise that here, too, a variable might be under scrutiny. The star, however, was certainly so far "new" that its rays, until their sudden accession of strength, were too feeble to affect even our reinforced senses. Not one of the 1,283 small stars recorded in charts of the nebula could be identified with it; and a photograph taken by Dr. Common, August 16, 1884, on which a multitude of stars down to the fifteenth magnitude had imprinted themselves, showed the uniform, soft gradation of nebulous light to be absolutely unbroken by a stellar indication in the spot reserved for the future occupation of the "Nova."[1478]
So far, then, the view that its relation to the nebula was a merely optical one might be justified; but it became altogether untenable when it was found that what was taken to be a chance coincidence had repeated itself within living memory. On the 21st of May, 1860, M. Auwers perceived at Königsberg a seventh magnitude star shining close to the centre of a nebula in Scorpio, numbered 80 in Messier's Catalogue.[1479]Three days earlier it certainly was not there, and three weeks later it had vanished. The effect to Mr. Pogson (who independently discovered the change, May 28)[1480]was as if the nebula had beenreplacedby a star, so entirely were its dim rays overpowered by the concentrated blaze in their midst. Now, it is simply incredible that two outbursts of so uncommon a character should haveaccidentallyoccurred just on the line of sight between us and the central portions of two nebulæ; we must, then, conclude that they showedonthese objects because they took placeinthem. The most favoured explanation is that they were what might be called effects of overcrowding—that some of the numerous small bodies, presumably composing the nebulæ, jostled together, in their intricate circlings, and obtained compensation in heat for their sacrifice of motion. But this is scarcely more than a plausible makeshift of perplexed thought. Mr. W. H. S. Monck, on the other hand, has suggested that new stars appear when dark bodies are rendered luminous by rushing through the gaseous fieldsofspace,[1481]just as meteors kindle in our atmosphere. The idea, which has been revived and elaborated by Dr. Seeliger of Munich,[1482]is ingenious, but was not designed to apply to our present case. Neither of the objects distinguished by the striking variations just described is of gaseous constitution. That in Scorpio appears under high magnifying powers as a "compressed cluster"; that in Andromeda is perhaps, as Sir J. Herschel suggested, "optically nebulous through the smallness of its constituent stars"[1483]—if stars they deserve to be called.