CHAPTER XII

_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, a variegated line, which,

until broadened into a band by the intervention of a cylindrical lens,

is all but useless for purposes of research. This process of _rolling

out_ involves, 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. This _kind_ of absorption

is produced by the vapours of metalloids or of compound substances.

To the fourth order of stars belongs also a colonnaded spectrum, but

_reversed_; 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 the

_youngest_ because 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 the _caput mortuum_ entitled

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 promptly recognised in a considerable number of

white stars, among them the Pleiades and 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--Beta Lyræ

and Gamma Cassiopeiæ--were noticed by Father Secchi at the outset

of his spectroscopic inquiries. Both show _bright_ lines 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 as _faculous_. That is to say,

brilliant formations, such as have been photographed by Professor Hale

on the sun's disc,[1387] cover, perhaps, the whole, instead of 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 Gamma 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 group _b_ showed 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 C _is 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 Gamma 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] No

fluctuations in the hydrogen-spectrum were betrayed by them; but

subordinate lines of unknown origin showed alternate fading and

vivification.

The spectrum of Beta 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, Eta 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 the course 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

Gamma 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 Gamma 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. _a_ and 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 a _single_ star-spectrum would be the work of some

years. Of two, however--those of Betelgeux and Aldebaran--he was able to

furnish detailed and accurate 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 slit

1/350 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 was _partially_ represented, 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

exposure _forty times_ that 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 Zeta Puppis,[1422] and the

identification was 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 Lambda 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 Beta 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 Beta 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 Professor Pickering'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 a _naturally_ pure 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 preparation was 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 slightly _shoved_ hither 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 stars were 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 Zeta Ursæ Majoris

(Mizar), taken in 1887, and again in 1889, the K line was seen to be

double; while on other plates it appeared single. A careful study of

Miss A. C. Maury of a series of seventy impressions indicated for the

doubling a period of fifty-two days, and showed it to affect all the

lines in the spectrum.[1440] The only available, and no doubt the true,

explanation of the phenomenon was that two similar and nearly equal

stars are here merged into one telescopically indivisible; their

combined light giving a single or double spectrum, according as their

orbital velocities are directed across or along our line of sight. The

movements of a revolving pair of stars must always be opposite in sense,

and proportionately equal in amount. That is, they at all times travel

with speeds in the inverse ratio of their masses. Hence, unless the

plane of their orbits be perpendicular to a plane passing through the

eye, there must be two opposite points where their velocities in the

line of sight reach a maximum, and two diametrically opposite points

where they touch zero. The lines in their common spectrum would thus

appear alternately double and single twice in the course of each

revolution. To that of Mizar, at first supposed to need 104 days for its

completion, a period of only twenty days fourteen hours was finally

assigned by Vogel.[1441] Anomalous spectral effects, probably due to the

very considerable eccentricity of the orbit, long impeded its

satisfactory determination. The mean distance apart of the component

stars, as now ascertained, is just twenty-two million miles, and their

joint mass quadruples that of the sun. But these are minimum estimates.

For if the orbital plane be inclined, much or little, to the line of

sight, the dimensions and mass of the system should be proportionately

increased.

An analogous discovery was made by Miss Maury in 1889. But in the

spectrum of Beta 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 Beta 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 (Alpha Virginis), was announced by Dr.

Vogel, April 24, 1890,[1445] to belong to the novel category, with the

difference, however, of possessing a nearly dark, instead of a

brilliantly lustrous companion. In this case, accordingly, the tell-tale

spectroscopic variations consist merely in a slight swinging to and fro

of single lines. No second spectrum leaves a legible trace on the plate.

Spica revolves in four days at the rate of fifty-seven miles a

second,[1446] or quicker, in proportion as its orbit is more inclined to

the line of sight, round a centre at a minimum distance of three

millions of miles. But the position of the second star being unknown,

the mass of the system remains indeterminate. The lesser component of

the splendid, slowly revolving binary, Castor, is also closely double.

Its spectral lines were found by Bélopolsky in 1896[1447] to oscillate

once in nearly three days, the secondary globe being apparently quite

obscure. Further study of the movements thus betrayed elicited the fact

that the major axis of the eclipse traversed revolves in a period of

2,100 days, as a consequence, most likely, of the flattened shape of the

stars.[1448] Still more unexpected was the simultaneous assignment, by

Campbell and Newall, of a duplex character to Capella.[1449] Here both

components shine, though with a different quality of light, one giving a

pure solar spectrum, the other claiming prismatic affinity with Procyon.

Their mutual circulation is performed in 104 days, and the radius of

their orbit cannot be less, and may be a great deal more, than

51,000,000 miles. Hence the possibility is not excluded that the

star--which has an authentic parallax of 0·08"--may be visually

resolved. Indeed, signs of "elongation" were thought to be perceptible

with the Greenwich 28-inch refractor,[1450] while only round images

could be seen at Lick.[1451] Another noteworthy case is that of Polaris,

found by Campbell to have certainly one, and probably two obscure

attendants.[1452] Through his systematic investigations of stellar

radial velocities with the Mills spectrograph, knowledge in this

department has, since 1897, progressed so rapidly that the spectroscopic

binaries of our acquaintance already number half a hundred, and ten

times as many more doubtless lie within easy range of detection.

Now it is evident that a spectroscopic binary, if the plane of its

motion made a very small angle with the line of sight, would be a

variable star. For, during a few hours of each revolution, some at least

of its light should be cut off by a transit of its dusky companion. Such

"eclipse-stars" are actually found in the heavens.

The best and longest-known member of the group is Algol in the Head of

Medusa, the "Demon-star" of the Arabs.[1453] This remarkable object,

normally above the third magnitude, loses and regains three-fifths of

its light once in 68·8 hours, the change being completed in about twelve

hours. Its definite and limited nature, and punctual recurrence,

suggested to Goodricke of York, by whom the periodicity of the star was

discovered in 1783,[1454] the interposition of a large dark satellite.

But the conditions involved by the explanation were first seriously

investigated by Pickering in 1880.[1455] He found that the phenomena

could be satisfactorily accounted for by supposing an obscure body 0·764

the bright star's diameter to revolve round it in a period identical

with that of its observed variation. This theoretical forecast was

verified with singular exactitude at Potsdam in 1889.[1456] A series of

spectral photographs taken there showed each of Algol's minima to be

preceded by a rapid recession from the earth, and succeeded by a rapid

movement of approach towards it. They take place, accordingly, when the

star is at the furthest point from ourselves of an orbit described round

an invisible companion, the transits of which across its disc betray

themselves to notice by the luminous vicissitudes they occasion. The

diameter of this orbit, traversed at the rate of twenty-six miles a

second, is just 2,000,000 miles; and it is an easy further inference

from the duration and extent of the phases exhibited that Algol itself

must be (in round numbers) one million, its attendant 830,000 miles in

diameter. Assuming both to be of the same density, Vogel found their

respective masses to be four-ninths and two-ninths that of the sun, and

their distance asunder to be 3,230,000 miles.

This singularly assorted pair of stars possibly form part of a larger

system. Their period of revolution is shorter now by six seconds than it

was in Goodricke's time; and Dr. Chandler has shown, by an exhaustive

discussion, that its inequalities are comprised in a cycle of about 130

years.[1457] They arise, in his view, from a common revolution, in that

period, of the close couple about a third distant body, emitting little

or no light, in an orbit inclined 20° to our line of vision, and of

approximately the size of that described by Uranus round the sun. The

time spent by light in crossing this orbit causes an apparent delay in

the phases of the variable, when Algol and its eclipsing satellite are

on its further side from ourselves, balanced by acceleration while they

traverse its hither side. Dr. Chandler derives confirmation for his

plausible and ingenious theory from a supposed undulation in the line

traced out by Algol's small proper motion; but the reality of this

disturbance has yet to be established.[1458] Meanwhile, M.

Tisserand,[1459] late Director of the Paris Observatory, preferred to

account for Algol's inequalities on the principle later applied by

Bélopolsky to those of Castor. That is to say, he assumed a revolving

line of apsides in an elliptical orbit traversed by a pretty strongly

compressed pair of globes. The truth of this hypothesis can be tested by

close observation of the phases of the star during the next few years.

The variable in the Head of Medusa is the exemplar of a class including

26 recognised members, all of which doubtless represent occulting

combinations of stars. But their occultations result merely from the

accident of their orbital planes passing through our line of sight;

hence the heavens must contain numerous systems similarly constituted,

though otherwise situated as regards ourselves, some of which, like

Spica Virginis, will become known through their spectroscopic changes,

while others, because revolving in planes nearly tangent to the sphere,

or at right angles to the visual line, may never disclose to us their

true nature. Among eclipsing stars should probably be reckoned the

peculiar variables, Beta 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

German _Astronomische 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. John

Birmingham 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 it was 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. The _negative_ observations, 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 been _replaced_ by 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 have _accidentally_ occurred just on the

line of sight between us and the central portions of two nebulæ; we

must, then, conclude that they showed _on_ these objects because they

took place _in_ them. 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 fields of space,[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.

On the 8th of December, 1891, Dr. Max Wolf took a photograph of the

region about Chi Aurigæ. No stranger so bright as the eighth

magnitude was among the stars depicted upon it. On the 10th,

nevertheless, a stellar object of the fifth magnitude, situated a couple

of degrees to the north-east of Beta Tauri and previously

unrecorded, where eleventh magnitude stars appeared, imprinted itself

upon a Harvard negative. Subsequent photographs taken at the same place

showed it to have gained about half a magnitude by the 20th; but the

plates were not then examined, and the discovery was left to be modestly

appropriated by an amateur, the Rev. Dr. Anderson of Edinburgh, by whom

it was announced, February 1, 1892, through the medium of an anonymous

postcard, to Dr. Copeland, the Astronomer Royal for Scotland.[1484] By

him and others, the engines of modern research were promptly set to

work. And to good purpose. Nova Aurigæ was the first star of its kind

studied by the universal chemical method. It is the first, accordingly,

of which authentic records can be handed down to posterity. They are of

a most remarkable character. The spectrum of the new object was

photographed at Stonyhurst and South Kensington on February 3; a few

days later, at Harvard and Lick in America, at Potsdam and Hérény on the

Continent of Europe. But by far the most complete impression was

secured, February 22, with an exposure of an hour and three-quarters, by

Sir William and Lady Huggins, through whose kindness it is reproduced in

Plate V., Fig. 1. The range of bright lines displayed in it is of

astonishing vividness and extent. It includes all the hydrogen rays dark

in the spectrum of Sirius (separately printed for comparison), besides

many others still more refrangible, as yet unidentified. Very

significant, too, is the marked character of the great prominence lines

H and K. The visual spectrum of the Nova was splendidly effective.

A quartette of brilliant green rays, two of them due to helium,

caught the eye; and they had companions too numerous to be easily

counted. The hydrogen lines were broad and bright; C blazed, as Mr.

Espin said, "like a danger-signal on a dark night"; the sodium pair were

identified at Tulse Hill, and the yellow helium ray was suspected to

lurk close beside them. Fig. 2 in the same plate shows the spectrum as

it was seen and mapped by Lady Huggins, February 2 to 6, together with

the spectra employed to test the nature of the emissions dispersed in

it. One striking feature will be at once remarked. It is that of the

pairing of bright with dark lines. Both in the visible and the

photographic regions this singular peculiarity was unmistakable; and

since the two series plainly owned the same chemical origin, their

separate visibility implied large displacement. Otherwise they would

have been superposed, not juxtaposed. Measurements of the bright rays,

accordingly, showed them to be considerably pushed down towards the red,

while their dark companions were still more pushed up towards the blue

end. Thus the spectrum of Nova Aurigæ, like that of Beta Lyræ,

with which it had many points in common, appeared to be really double.

It was supposed to combine the light of two distinct bodies, one, of a

gaseous nature, moving rapidly away from the earth, the other, giving a

more sunlike spectrum, approaching it with even higher speed. The

relative velocity determined at Potsdam for these oppositely flying

masses amounted to 550 miles a second.[1485] And this prodigious rate of

separation was fully maintained during six weeks! It did not then

represent a mere periastral rush-past.[1486] To the bodies exhibiting

its effects, and parting company for ever under its stress, it must have

belonged, with slight diminution, in perpetuity. The luminous outburst

by which they became visible was explained by Sir William Huggins, in a

lecture delivered at the Royal Institution, May 13, 1892, on the tidal

theory of Klinkerfues and Wilsing. Disturbances and deformations due to

the mutual attraction of two bulky globes at a close approach would, he

considered, "give rise to enormous eruptions of the hotter matters from

within, immensely greater, but similar in kind, to solar eruptions; and

accompanied, probably, by large electrical disturbances." The multiple

aspect and somewhat variable character of both bright and dark lines

were plausibly referred to processes of "reversal," such as are nearly

always in progress above sun-spots; but the long duration of the star's

suddenly acquired lustre did not easily fit in with the adopted

rationale. A direct collision, on the other hand, was out of the

question, since there had obviously been little, if any, sacrifice of

motion; and the substitution of a nebula for one of the "stars"[1487]

compelled recourse to scarcely conceivable modes of action for an

explanation of the perplexing peculiarities of the compound spectrum.

PLATE V.

[Illustration: Photographic and Visual Spectrum of Nova Aurigæ.

Fig. 1.--From a Photograph taken by Sir William and Lady Huggins, Feb.

22, 1892.

Fig. 2.--From a Drawing made by Lady Huggins, Feb. 2 to 6, 1892.]

An unexpected _dénouement_, however, threw all speculations off the

track. The Nova contained most of its brightness, fluctuations

notwithstanding, until March 9; after which date it ran swiftly and

uniformly down towards what was apprehended to be total extinction. No

marked change of spectrum attended its decline. When last examined at

Tulse Hill, March 24, all the more essential features of its prismatic

light were still faintly recognisable.[1488] The object was steadily

sinking on April 26, when a (supposed) final glimpse of it was caught

with the Lick 36-inch.[1489] It was then of about the sixteenth

magnitude. But on August 17 it had sprung up to the tenth, as Professors

Holden, Schaeberle, and Campbell perceived with amazement on turning the

same instrument upon its place. And to Professor Barnard it appeared,

two nights later, not only revived, but transformed into the nucleus of

a planetary nebula, 3" across.[1490] The reality of this seeming

distension, however, at once disputed, was eventually disproved. It

unquestionably arose from the imperfect focussing power of the telescope

for rays of unusual quality.[1491]

The rekindled Nova was detected in this country by Mr. H. Corder, on

whose notification Mr. Espin, on August 21, examined its nearly

monochromatic spectrum.[1492] The metamorphosis of Nova Cygni seemed

repeated.[1493] The light of the new object, like that of its

predecessor, was mainly concentrated in a vivid green band, identified

with the chief nebular line by Copeland,[1494] Von Gothard,[1495] and

Campbell.[1496] The second nebular line was also represented. Indeed,

the last-named observer recognised nearly all the eighteen lines

measured by him in the Nova as characteristic of planetary nebulæ.[1497]

Of particular interest is the emergence in the star-spectrum

photographed by Von Gothard of an ultra-violet line originally

discovered at Tulse Hill in the Orion nebula, which is also very strong

in the Lyra annular nebula.

Obviously, then, the physical constitution of Nova Aurigæ became

profoundly modified during the four months of its invisibility. The

spectrum of February was or appeared compound; that of August was

simple; it could be reasonably associated only with a single

light-source. Many of the former brilliant lines, too, had vanished, and

been replaced by others, at first inconspicuous or absent. As a result,

the solar-prominence type, to which the earlier spectrum had seemed to

conform, was completely effaced in the later. The cause of these

alterations remains mysterious, yet its effects continue. The chromatic

behaviour of the semi-extinct Nova, when scrutinised with great

refractors, shows its waning light to be distinctly nebular.[1498] Like

nearly all its congeners, the star is situated in the full stream of the

Milky Way, and we learn without surprise that micrometrical measures by

Burnham and Barnard[1499] failed to elicit from it any sign of

parallactic shifting. It is hence certain that the development of light,

of which the news reached the earth in December, 1891, must have been on

a vast scale, and of ancient date. Nova Aurigæ at its maximum assuredly

exceeded the sun many times in brightness; and its conflagration can

scarcely have occurred less, and may have occurred much more, than a

hundred years ago.

By means of the photographic surveys of the skies, carried on in both

hemispheres under Professor Pickering's superintendence, such amazing

events have been proved to be of not infrequent occurrence. Within six

years five new stars were detected from Draper Memorial, or chart-plates

by Mrs Fleming, besides the retrospective discovery of a sixth which had

rapidly burnt itself out, eight years previously, in Perseus.[1500] Nova

Normæ was the immediate successor of Nova Aurigæ; Nova Carinæ and Nova

Centauri lit up in 1895, the latter in a pre-existent nebula; Nova

Sagittarii and Nova Aquilæ attained brief maxima in 1898 and 1899

respectively. Now, three out of these five stars reproduced with

singular fidelity the spectrum of Nova Aurigæ; they displayed the same

brilliant rays shadowed, invariably on their blue sides, by dark ones.

Palpably, then, the arrangement was systematic and significant; it could

not result merely from the casually directed, opposite velocities of

bodies meeting in space. The hypothesis of stellar encounters

accordingly fell to the ground, and has been provided with no entire

satisfactory substitute. Most speculators now fully recognise that

motion-displacements cannot be made to account for the doubled spectra

of Novæ, and seek recourse instead to some kind of physical agency for

producing the observed effect.[1501] And since this is also visible in

certain permanent, though peculiar objects--notably in P Cygni,

Beta Lyræ, and Eta Carinæ--the acting cause must also evidently be

permanent and inherent.

The "new star of the new century"[1502] was a visual discovery. Dr.

Anderson duplicated, with added _éclat_, his performance of nine years

back. In the early morning of February 22, 1901, he perceived that Algol

had a neighbour of nearly its own brightness, which a photograph taken

by Mr. Stanley Williams, at Brighton, proved to have risen from below

the twelfth magnitude within the preceding 28 hours. And it was still

swiftly ascending. On the 23rd, it outshone Capella; for a brief space

it took rank as the premier star of the northern hemisphere. A decline

set in promptly, but was pursued hesitatingly. The light fluctuated

continually over a range of a couple of magnitudes, and with a close

approach, during some weeks, to a three-day periodicity. A year after

the original outburst, the star was still conspicuous with an

opera-glass. The spectrum underwent amazing changes. At first

continuous, save for fine dark lines of hydrogen and helium, it unfolded

within forty-eight hours a composite range of brilliant and dusky bands

disposed in the usual fashion of Novæ. These lasted until far on in

March, when hydrogen certainly, and probably other substances as well,

ceased to exert any appreciable absorptive action. Blue emissions of the

Wolf-Rayet type then became occasionally prominent, in remarkable

correspondence with the varying lustre of the star;[1503] finally, a

band at Lambda 3969, found by Wright at Lick to characterise nebular

spectra,[1504] assumed abnormal importance; and in July the nebular

transformation might be said to be complete. Striking alterations of

colour attended these spectral vicissitudes. White to begin with, the

star soon turned deep red, and its redness was visibly intensified at

each of its recurring minima of light. Blanching, however, ensued upon

the development of its nebulous proclivities; and its surviving rays are

of a steely hue.

All the more important investigations of Nova Persei were conducted by

photographic means. Libraries of spectral plates were collected at the

Yerkes and Lick Observatories, at South Kensington, Stonyhurst, and

Potsdam, and await the more exhaustive interpretation of the future.

Meanwhile, extraordinary revelations have been supplied by immediate

photographic delineation. On August 22 and 23, 1901, Professor Max Wolf,

by long exposures with the 16-inch Bruce twin objectives of the

Königstuhl Observatory (Heidelberg), obtained indications of a large

nebula finely ramified, extending south-east of the Nova;[1505] and the

entire formation came out in four hours with the Yerkes 2-foot

reflector, directed to it by Mr. Ritchey on September 20.[1506] It

proved to be a great spiral encircling, and apparently emanating from,

the star. But if so, tumultuously, and under stress of catastrophic

impulsions. A picture obtained by Mr. Perrine with the Crossley

refractor, in 7h. 19m., on November 7 and 8, disclosed the progress of a

startling change.[1507] Comparison with the Yerkes photograph showed

that during the intervening 48 days four clearly identifiable

condensations had become displaced, all to the same extent of about 90

seconds of arc, and in fairly concordant directions, suggesting motion

_round_ the Nova as well as away from it. The velocity implied, however,

is so prodigious as virtually to exclude the supposition of a bodily

transport of matter. It should be at the rate of no less than twenty

thousand miles a second, admitting the object to be at a distance from

us corresponding to an annual parallax of one-tenth of a second, and

actual measurements show it to be indefinitely more remote. The fact of

rapid variations in the nebula was reaffirmed, though with less

precision, from Yerkes photographs of November 9 and 13, Mr. Ritchey

inferring a general expansion of its southern portions.[1508] Much

further evidence must be at hand before a sane judgment can be formed as

to the nature of the strange events taking place in that secluded corner

of the Galaxy.[1509] And it is highly probable that the illumination of

the nebulous wreaths round the star will prove no less evanescent than

the blazing of the star itself.

We have been compelled somewhat to anticipate our narrative as regards

inquiries into the nature of nebulæ. The excursions of opinion on the

point were abruptly restricted and defined by the application to them of

the spectroscope. On August 29, 1864, Sir William Huggins sifted through

his prisms the rays of a bright planetary nebula in Draco.[1510] To his

infinite surprise, they proved to be mainly of one colour. In other

words, they avowed their origin from a mass of glowing vapour. As to

what _kind_ of vapour it might be by which Herschel's conjecture of a

"shining fluid" diffused at large throughout the cosmos was thus

unexpectedly verified, an answer only partially satisfactory could be

afforded. The conspicuous bright line of the Draco nebula seemed to

agree in position with one emitted by nitrogen, but has since proved to

be distinct from it; of its two fainter companions, one was unmistakably

the F line of hydrogen, while the other, in position intermediate

between the two, still remains unidentified.

By 1868 Huggins had satisfactorily examined the spectra of about seventy

nebulæ, of which one-third displayed a gaseous character.[1511] All of

these gave the green ray fundamental to the nebular spectrum, and

emanating from an unknown form of matter named by Sir William Huggins

"nebulum." It is associated with seven or eight hydrogen lines, with

three of "yellow" helium, and with a good many of undetermined origin.

The absence of the crimson radiation of hydrogen--perceived with

difficulty only in some highly condensed objects--is an anomaly very

imperfectly explained as a physiological effect connected with the

extreme faintness of nebular light.[1512] An approximate coincidence

between the chief nebular line and a "fluting" of magnesium having been

alleged by Lockyer in support of his meteoritic hypothesis of nebular

constitution, it became of interest to ascertain its reality. The task

was accomplished by Sir William and Lady Huggins in 1889 and 1890,[1513]

and by Professor Keeler, with the advantages of the Mount Hamilton

apparatus and atmosphere, in 1890-91.[1514] The upshot was to show a

slight but sure discrepancy as to place, and a marked diversity as to

character, between the two qualities of light. The nebular ray

(wave-length 5,007 millionths of a millimetre) is slightly more

refrangible than the magnesium fluting-edge, and it is sharp and fine,

with no trace of the unilateral haze necessarily clinging even to the

last "remnant" of a banded formation.

Planetary and annular nebulæ are, without exception, gaseous, as well as

those termed "irregular," which frequent the region of the Milky Way.

Their constitution usually betrays itself to the eye by their blue or

greenish colour; while those yielding a continuous spectrum are of a

dull white. Among the more remarkable of these are the well-known nebula

in Andromeda, and the great spiral in Canes Venatici; and, as a general

rule, the emissions of all such nebulæ as present the appearance of

star-clusters grown misty through excessive distance are of the same

kind. It would, however, be eminently rash to conclude thence that they

are really aggregations of sun-like bodies. The improbability of such an

inference has been greatly enhanced by the occurrence, at an interval of

a quarter of a century, of stellar outbursts in the midst of two of

them. For it is practically certain that the temporary stars were

equally remote with the hazy formations they illuminated; hence, if the

constituent particles of the latter be suns, the incomparably vaster

orbs by which their feeble light was well-nigh obliterated must, as was

argued by Mr. Proctor, have been on a scale of magnitude such as the

imagination recoils from contemplating. Nevertheless, Dr. Scheiner, not

without much difficulty, obtained, in January, 1899, spectrographic

prints of the Andromeda nebula, indicative, he thought, of its being a

cluster of solar stars.[1515] Sir William and Lady Huggins, on the other

hand, _saw_, in 1897, bright intermixed with dark bands in the spectrum

of the same object.[1516] And Mr. Maunder conjectures all "white" nebulæ

to be made up of sunlets in which the coronal element predominates,

while chromospheric materials assert their presence in nebulæ of the

"green" variety.[1517]

Among the ascertained analogies between the stellar and nebular systems

is that of variability of light. On October 11, 1852, Mr. Hind

discovered a small nebula in Taurus. Chacornac observed it at Marseilles

in 1854, but was confounded four years later to find it vanished.

D'Arrest missed it October 3, and redetected it December 29, 1861. It

was easily seen in 1865-66, but invisible in the most powerful

instruments from 1877 to 1880.[1518] Barnard, however, made out an

almost evanescent trace of it, October 15, 1890, with the great Lick

telescope,[1519] and saw it easily in the spring of 1895, while six

months later it evaded his most diligent search.[1520] Then again, on

September 28, 1897, the Yerkes 40-inch disclosed it to him as a mere

shimmer at the last limit of visibility; and it came out in three

diffuse patches on plates to which, on December 6 and 27, 1899, Keeler

gave prolonged exposures with the Crossley reflector.[1521] Moreover, a

fairly bright adjacent nebula, perceived by O. Struve in 1868, and

observed shortly afterwards by d'Arrest, has totally vanished, and was

most likely only a temporary apparition. These are the most authentic

instances of nebular variability. Many others have been more or less

plausibly alleged;[1522] but Professor Holden's persuasion, acquired

from an exhaustive study of the records since 1758,[1523] that the

various parts of the Orion nebula fluctuate continually in relative

lustre, has not been ratified by photographic evidence.

The case of the "trifid" nebula in Sagittarius, investigated by Holden

in 1877,[1524] is less easily disposed of. What is certain is that a

remarkable triple star, centrally situated, according to the

observations of both the Herschels, 1784-1833, in a dark space between

the three great _lobes_ of the nebula, is now, and has been since 1839,

densely involved in one of them; and since the hypothesis of relative

motion is on many grounds inadmissible, the change that has apparently

taken place must be in the distribution of light. One no less

conspicuous was adduced by Mr. H. C. Russell, director of the Sydney

Observatory.[1525] A particularly bright part of the great Argo nebula,

as drawn by Sir John Herschel, has, it would seem, almost totally

disappeared. He noticed its absence in 1871, using a 7-inch telescope,

failed equally later on to find it with an 11-1/2-inch, and his

long-exposure photographs show no vestige of it. The same structure is

missing from, or scarcely traceable in, a splendid picture of the nebula

taken by Sir David Gill in twelve hours distributed over four nights in

March, 1892.[1526] An immense gaseous expanse has, it would seem, sunk

out of sight. Materially it is no doubt there; but the radiance has left

it.

Nebulæ have no ascertained proper motions. No genuine change of place in

the heavens has yet been recorded for any one of them. All equally hold

aloof, so far as telescopic observation shows, from the busy journeyings

of the stars. This seeming immobility is partly an effect of vast

distance. Nebular parallax has, up to the present, proved evanescent,

and nebular parallactic drift, in response to the sun's advance through

space, remains likewise imperceptible.[1527] It may hence be presumed

that no nebulæ occur within the sphere occupied by the nearer stars. But

the difficulty of accurately measuring such objects must also be taken

into account. Displacements which would be conspicuous in stars might

easily escape detection in ill-defined, hazy masses. Thus the measures

executed by d'Arrest in 1857[1528] have not yet proved effective for

their designed purpose of contributing to the future detection of proper

motions. Some determinations made by Mr. Burnham with the Lick refractor

in 1891,[1529] will ultimately afford a more critical test. He found

that nearly all planetary nebulæ include a sharp stellar nucleus, the

position of which with reference to neighbouring stars could be fixed no

less precisely than if it were devoid of nebulous surroundings. Hence,

the objects located by him cannot henceforward shift, were it only to

the extent of a small fraction of a second, without the fact coming to

the knowledge of astronomers.

The spectroscope, however, here as elsewhere, can supplement the

telescope; and what it has to tell, it tells at once, without the

necessity of waiting on time to ripen results. Sir William Huggins made,

in 1874,[1530] the earliest experiments on the radial movements of

nebulæ. But with only a negative upshot. None of the six objects

examined gave signs of spectral alteration, and it was estimated that

they must have done so had they been in course of recession from or

approach towards the earth by as much as twenty-five miles a second.

With far more powerful appliances, Professor Keeler renewed the attempt

at Lick in 1890-91. His success was unequivocal. Ten planetary nebulæ

yielded perfectly satisfactory evidence of line-of-sight motion,[1531]

the swiftest traveller being the well-known greenish globe in

Draco,[1532] found to be hurrying towards the earth at the rate of forty

miles a second. For the Orion nebula, a recession of about eleven miles

was determined,[1533] the whole of which may, however, very well belong

to the solar system itself, which, by its translation towards the

constellation Lyra, is certainly leaving the great nebula pretty rapidly

behind. The anomaly of seeming nebular fixity has nevertheless been

removed; and the problem of nebular motion has begun to be solved

through the demonstrated possibility of its spectroscopic investigation.

Keeler's were the first trustworthy determinations of radial motion

obtained visually. That the similar work on the stars begun at Greenwich

in 1874, and carried on for thirteen years, remained comparatively

unfruitful, was only what might have been expected, the instruments

available there being altogether inadequate for the attainment of a high

degree of accuracy.

The various obstacles in the way of securing it were overcome by the

substitution of the sensitive plate for the eye. Air-tremors are thus

rendered comparatively innocuous; and measurements of stellar lines

displaced by motion with reference to fiducial lines from terrestrial

sources, photographed on the same plates, can be depended upon within

vastly reduced limits of error. Studies for the realisation of the

"spectrographic" method were begun by Dr. Vogel and his able assistant,

Dr. Scheiner, at Potsdam in 1887. Their preliminary results,

communicated to the Berlin Academy of Sciences, March 15, 1888, already

showed that the requirements for effective research in this important

branch were at last about to be complied with. An improved instrument

was erected in the autumn of the same year, and the fifty-one stars,

bright enough for determination with a refractor of 11 inches aperture,

were promptly taken in hand. A list of their motions in the line of

sight, published in 1892,[1534] was of high value, both in itself and

for what it promised. One noteworthy inference from the data it

collected was that the eye tends, under unfavourable circumstances, to

exaggerate the line-displacements it attempts to estimate. The

velocities photographically arrived at were of much smaller amounts than

those visually assigned. The average speed of the Potsdam stars came out

only 10·4 miles a second, the quickest among them being Aldebaran, with

a recession of thirty miles a second. More lately, however, Deslandres

and Campbell have determined for Zeta Herculis and Eta Cephei

respectively approaching rates of forty-four and fifty-four miles

a second.

The installation, in 1900, of a photographic refractor 31-1/2 inches in

aperture, coupled with a 20-inch guiding telescope, will enable Dr.

Vogel to investigate spectrographically some hundreds of stars fainter

than the second magnitude; and the materials thus accumulated should

largely help to provide means for a definite and complete solution of

the more than secular problem of the sun's advance through space. The

solution should be complete, because including a genuine determination

of the sun's velocity, apart from assumptions of any kind. M. Homann's

attempt, in 1885,[1535] to extract some provisional information on the

subject from the radial movements of visually determined stars gave a

fair earnest of what might be done with materials of a better quality.

He arrived at a goal for the sun's way shifted eastward to the

constellation Cygnus--a result congruous with the marked tendency of

recently determined apexes to collect in or near Lyra; and the most

probable corresponding velocity seemed to be about nineteen miles a

second, or just that of the earth in its orbit. A more elaborate

investigation of the same kind, based by Professor Campbell in

1900[1536] upon the motions of 280 stars, determined with extreme

precision, suffered in completeness through lack of available data from

the southern hemisphere. The outcome, accordingly, was an apex most

likely correctly placed as regards right ascension, but displaced

southward by some fifteen degrees. The speed of twelve miles a second,

assigned to the solar translation, approximates doubtless very closely

to the truth.

A successful beginning was made in nebular spectrography by Sir William

Huggins, March 7, 1882.[1537] Five lines in all stamped themselves upon

the plate during forty-five minutes of exposure to the rays of the

strange object in Orion. Of these, four were the known visible lines,

and a fifth, high up in the ultra-violet, at wave-length 3,727, has

evidently peculiar relationships, as yet imperfectly apprehended. It is

strong in the spectra of many planetaries; it helped to characterise the

nebular metamorphosis of Nova Aurigæ, yet failed to appear in Nova

Persei. Two additional hydrogen lines, making six in all, were

photographed at Tulse Hill, from the Orion nebula, in 1890;[1538] and

Dr. Copeland's detection in 1886[1539] of the yellow ray D_3 gave the

first hint of the presence of helium in this prodigious formation. Nor

are there wanting spectroscopic indications of its physical connection

with the stars visually involved in it. Sir William and Lady Huggins

found a plate exposed February 5, 1888, impressed with four groups of

fine bright lines, originating in the continuous light of two of the

trapezium-stars, but extending some way into the surrounding

nebula.[1540] And Dr. Scheiner[1541] argued a wider relationship from

the common possession, by the nebula and the chief stars in the

constellation Orion, of a blue line, bright in the one case, dark in the

others, since identified as a member of one of the helium series.

The structural unity of the stellar and nebular orders in this extensive

region of the sky has also, by direct photographic means, been

unmistakably affirmed.