Chapter IX: Helium Stars with Bright Lines

Temporary stars, and a few stars variable in short periods, belong, properly speaking, to this class; but for the sake of clearness and convenience, they are reserved for separate treatment. The question of light-change will demand later on our undivided attention; it bristles with difficulties, which we are not at present prepared to encounter.

Alcyone, the chief Pleiad, long passed for an ordinary Orion star. Dark lines of hydrogen and helium were prominent in its spectrum; there seemed no reason to suspect the slightest deviation from normality. Nevertheless, Campbell perceived in 1893[423] the red radiance of C set off by a narrow dark line on its more refrangible side; and this state of incipient emission appears to be permanent.[424] Alcyone might then be counted a linking instance between Classes i. and vii. The discovery was startling that a single substance could show certain of its rays bright, the remainder dark, in one and the same star. But the fact, although highly perplexing,[425] has become so common to experience as to have ceased to be surprising. Examples of its occurrence have been registered by the score as regards both hydrogen and helium. They are all found in Classes vii. and viii. (bright-line helium and Wolf-Rayet stars); Mira-variables seem never to have their hydrogen spectrum thus conditioned. Professor Campbell[426] noticed it as an invariable rule that the bright lines in Orion stars “are those of greater wave-length,” while “the dark lines are those of shorter wave-length.”[427] This applies also to helium, but in a qualified sense. If all the lines in its spectrum are taken indiscriminately, the bright and the dark appear to succeed each other without method; but their consideration by series makes it at once evident that, within the limits of each set of vibrations, the bright members are invariably fundamental. This is important, not only for the better ordering of stellar phenomena, but as regards the theory of spectral series in general.

In Alcyone, then, emission is at a minimum; it could scarcely diminish and remain existent. And it is quite possible that it may be on the wane; after the lapse of some hundreds, or thousands of years, the chief Atlantid will perhaps have lost the distinctive note of its spectrum, and will have sunk to the level of unrelieved absorption. One of its companions at present stands out from the crowd in the same way, but more decidedly. Pleione shows three hydrogen lines pretty strongly bright, and they are inevitably C, F, and Hγ. Centrally superposed upon wide dark bands,[428] they assert by this fact alone the non-correspondence in position of their originating stratum with the glowing hydrogen in stars like Mira. And this teaches us the important lesson that there is no stereotyped recipe for the production of stellar bright lines, but that they may originate diversely in the various spectral classes.

The Pleiades are nebulous collectively, and in many cases individually as well; but they are less closely folded in nebular swaddling-bands than the group of stars forming the nucleus of the great Orion nebula. It consists of four leaders, of about the fourth, fifth, sixth, and seventh magnitudes respectively, two of which have faint companions; and, scattered promiscuously, there are to be found besides four minute stellar points, detected at Lick by Professor Barnard and Mr. Alvan G. Clark. For spectroscopic purposes the “trapezium,” or quartette of bright stars, may be treated as one, since they shine with sensibly the same quality of light, while their scarcely visible associates give radiations negligible in amount. Most difficult questions arise in attempting to decide upon the true nature of the trapezium-spectrum. The prevalent view at first was that the stars were of the ordinary dark-line helium type, bright lines coming in here and there simply as projections from the enormous volume of gaseous stuff interposed between the eye and the stellar nucleus of the formation. But the opinion was grounded on superficial evidence, and has not held its ground. Some lines, bright in the nebula, _refuse_ to cross the thin strip of continuous light due to the star; they stop short on one side of it, and reform on the other,[429] the two sections being divided by a narrow gap of absorption. This proves that the nebular rays do not in all cases show bright against the background of continuous stellar light. The very strongest may do so; but it is just possible that in them the appearance is illusory, and due to a kind of irradiation.

There can, on the other hand, be no reasonable doubt that the trapezium-stars have bright lines of their own. But they are peculiar, and peculiarly conditioned. A spectrograph taken by Sir William and Lady Huggins, 5th February 1888,[430] proved to be crossed in the ultra-violet by at least four groups of fine, faint, bright lines, derived primarily from two stars of the trapezium, but extending, through their influence, as it were, some little way into the adjacent nebula. Their origin is problematical; they have not been recorded elsewhere;[431] they have been only partially verified on later Tulse Hill plates. Yet the original negative survives, and its examination has convinced several experts of the reality of the curious script read from it. Conviction, however, on such a point is apt to share the dim character of gloaming phenomena—phenomena on the border between the seen and the not seen.

But there is more. With refined apparatus the same observers succeeded, in 1894 and subsequently,[432] in separately photographing three of these remarkable spectra, and they were now perceived to be rich throughout in bright and dark lines, “with the special character strongly marked of bright bands associated with corresponding dark absorption lines.” Most singular of all, the relative positions of these bright and dark lines were found subject to change. Hydrogen radiations, for instance, which in 1894 lay on the blue sides of the absorption stripes, lay in 1897 on their redward margins. This might be explained on the hypothesis of orbital movement by supposing each star of the trapezium composed of a dark-line and a bright-line member, the spectra of which are periodically shifted through the alternations of their velocities in the line of sight. It remains to be seen, however, whether or not the shiftings are periodical; for by this one condition the explanation stands or falls.

So far as their absorption-elements are concerned, the stars of the trapezium belong to the earliest variety of the Orion type. All the lines are wide and diffuse, and the strongest are members of the Pickering series of hydrogen. Rydberg’s series—if we may call it so under reserve—is represented by the prominent reversal of its solitary ray at λ 4689. Mr. McClean recognised oxygen absorption in these stars; Sir William and Lady Huggins identified nitrogen, silicium, and titanium, and the calcium K shows both bright and dark. Few sidereal objects combine so many points of interest as the multiple star at the heart of the great nebula. The origin and meaning of the throngs of delicate rays, here just tantalising vision, pressingly invite research; nor less the manner of relative displacement exhibited by the bright and dark coupled lines. Do they betray a circulatory period? And if so, is it the same for each member of the group? Or do they rather form independent systems, in subordination to a higher scheme, completing itself in the long leisure of many millenniums? Other problems suggest themselves in immediate connection with these stars; nor is it impossible that they may be proposed over again, perhaps in a modified form, by the multiple stellar nucleus of the Trifid Nebula in Sagittarius. But instruments of no insignificant light-power will be needed for the satisfactory examination of its spectrum.

In one other star besides θ Orionis, the shifting of bright hydrogen lines occurs irrespectively, to all appearance, of binary revolution. Spectroscopic duplicity was at first naturally attributed to 11 Monocerotis when its peculiar character disclosed itself on the Harvard plates. Thus in the years 1888–90 the dark F of hydrogen had an illuminated border lying redward; it was on the blue side in 1891–92.[433] We are not informed whether it has since changed its position; but any attempt to impose a period upon alterations so spasmodical would evidently be hopeless. Like θ Orionis, 11 Monocerotis ( = Σ 919) is compound. It consists of three stars of about fifth and sixth magnitudes, which have maintained a strict relative immobility since Herschel divided them in 1781. Their spectra, photographed as one by Pickering, were separately examined by Campbell in 1894.[434] He found two of them to include the brilliant red ray of hydrogen, while it was absent from the third. Presumably, then, only two of the trio are bright-line stars, and it may be that in these two, significant differences in the mode of emission will be brought to light by detailed and systematic investigation.

The swing of the bright lines observed in θ Orionis and 11 Monocerotis is extremely uncommon. In general, a fixed arrangement prevails, and it is of two alternative varieties. Either the bright lines centrally divide broader dark bands, as in γ Cassiopeiæ, or the bright and dark lines are bracketed in pairs, the bright below, the dark above, as in P Cygni.

Father Secchi’s notice of γ Cassiopeiæ as a gaseous star goes back to 1866. He noticed the vividness in its spectrum of C, F, and D_{3}, but the helium line has not since held its own with the others. It is subject to prolonged extinctions; nor is it certain that even the hydrogen rays always keep up the same standard of brightness. The variability of the spectrum will, however, be discussed later in connection with other similar instances; here we have to do with its fundamental characteristics. The hydrogen lines in γ Cassiopeiæ are doubly reversed.[435] Wide absorption bands are divided by narrower emission bands, and these again by hair-lines of darkness. Their structure is analogous to that of H and K in the solar spectrum. The radiations fall off in intensity—as Campbell’s rule prescribes—with diminishing wave-length, while the absorptions gain in the same proportion. F is “superlatively bright”;[436] Hε is neutral; no bright lines have been photographed in the ultra-violet. The helium lines are dark, with occasional exceptions; but the green and blue magnesium lines shine by direct emission, and Father Sidgreaves recognises as a probable vanadium line a strong dark-blue ray (λ 4586), which seems to fluctuate in brightness. Another remarkable circumstance relating to this star is the recent effacement from its spectrum of the signs of sodium absorption formerly visible in it. They have, at any rate, escaped notice since Von Konkoly’s record of 15th September 1884.[437] But the immense vogue and value of spectrography have tended to reduce to a minimum the attention bestowed upon the lower spectral sections, and thus unduly to incline the balance of observation. The study of γ Cassiopeiæ might alone furnish a not inadequate task for a well-equipped observer. Only individual enthusiasm is likely to deal successfully with the baffling problems it presents. Spectral variability is, in its case, accentuated by perfect photometric constancy. The star is steadily of 2·3 magnitude. It is purely white in colour, lies immersed in the Milky Way, and has no measurable parallax. Its real size and splendour are then inestimably great.

The spectrum of P Cygni is not known to vary, although the star itself was reckoned a “Nova” on its discovery by Janson in 1600, and by its capricious emergences earned from Huygens, half a century later, the title of the “_revenante_ of the Swan.”[438] Finally, it settled down to fifth-magnitude brightness, which it seems disposed indefinitely to retain. Its spectrum shows an approximately complete set of bright and dark hydrogen and helium rays; but in their arrangement into couples _juxtaposition_ replaces _superposition_—that is to say, the bright lines are in their normal places,[439] while the corresponding dark ones are shifted upward, as if by rapid motion, towards the eye. But there can be no real question of motion, since the relation persists without change year after year. Nor can it be explained on the pressure-principle of altered refrangibility. The action, if exerted at all, would be of the opposite kind to that observed. The displacements in the spectrum of P Cygni are towards the blue; if due to pressure, they should be towards the red. The phenomenon of the relative displacement of bright and dark lines in the same spectrum is one of the most interesting in stellar physics, and has received, up to the present, no adequate explanation.

The absorption lines in P Cygni are much sharper and narrower than in γ Cassiopeiæ. Those of calcium, magnesium, and sodium are at once apparent, and Bélopolsky ascribes many of the remainder to nitrogen.

The spectrum of the great southern variable, η Carinæ, resembles that of P Cygni by its inclusion of many bright lines shadowed by dark ones on their blue sides. It has been photographically studied by Sir David Gill, Mr. McClean, and Miss A. J. Cannon.[440]

Among stars nearly related to γ Cassiopeiæ may be mentioned φ Persei (4·2 magnitude), υ Cygni (4·4 magnitude), α Columbæ (2·7 magnitude), δ and μ Centauri (2·8 and 3·4 magnitude). Bright F was detected by Mr. Espin[441] in the spectrum of the star in Perseus, and was found by Campbell[442] to be accompanied by a much brighter C. The total absence of K is surprising, but may not be permanent if the spectral variability suspected at Potsdam[443] be substantiated. In υ Cygni there appear to be double reversals of helium as well as of hydrogen.[444] Of α Columbæ it is only known that F is a broad dark line bisected by a narrow bright one. The spectra of the two stars in Centaur are thought to be almost identical.[445] Hydrogen emissions in them are strong and numerous, but none others have been recognised. The helium lines are all dark; metallic lines are inconspicuous.

Bright hydrogen and helium lines seem like relics of past conflagrations. In a few cases we know them to be such, and it is possible that in all they have the same implications. For any of these stars may have undergone prehistoric vicissitudes of lustre, after which they would have settled down into stability; although the recurrence of such incidents in the future can alone afford secure grounds for inferring that they diversified stellar biographies in earlier times. Bright-line helium stars are for the most part situated in the Milky Way. They are subject to the influences exercised by that strange aggregation.