Chapter VIII

STARS WITH FLUTED SPECTRA SHOWING BRIGHT LINES.

Although the display of bright lines in fluted spectra is a sure sign of extensive variability, it is not the light-changes of the stars thus characterised that here concern us. The two classes of phenomena are beyond question intimately related; but as a matter of pure convenience they have to be treated of apart.

Mira Ceti has the advantage over its fellows of rising to more brilliant maxima, and of having received longer attention and more careful study. Since, however, all the members of Class vi. are not copied from one pattern, investigations conducted too exclusively can only lead to partial knowledge; and, indeed, the varieties distinguishing the different specimens are precisely their most instructive feature. Thus they agree in showing _some_ brightened hydrogen lines, but not in the selection of those to be brightened. Then helium rays are vivid in certain of these stars, dark in others, and there are further, less assured diversities. These cannot yet be explained by any single consistent theory, but they may be definitely ascertained and brought into some kind of orderly relationship.

The spectrum of Mira has a prescriptive right to be considered first. It is the model, deviations from which count as exceptional. The bands are profound, the radiations of hydrogen intense during fully one-third of the light-period of eleven months. Detected photographically at Harvard College in 1886, they are now looked for, and rarely missed, in every analogous object as it rises from quasi-extinction. The hydrogen stratum in Mira seems to be in a peculiar condition. It emits only the higher members of the Huggins series, the red and the green lines (C and F) being alike invisible. The blue and the indigo lines, on the other hand, shine with extraordinary brilliancy—a brilliancy “too great to be shown on a drawing or to be safely expressed by a number representing relative intensity”[413] (see Plate XIII. Fig. 1). The fifth hydrogen line, by a rule without exception in Mira variables, is hidden; but eight of its associates in the ultra-violet have been recorded. Very singular, indeed, is the partial presentation in this star of a closely-linked sequence of vibrations. It can hardly result from an extraordinary elevation of temperature; we can better conceive it as due to some subtle form of electrical action not yet evoked in the laboratory. The state of things as regards the hydrogen spectrum is the opposite of that prevailing in the reversing stratum of the sun. Here the upper radiations are suppressed; in Mira the series starts from its third term.

The star has been spectrographically investigated by Vogel, Sidgreaves, and Campbell. The Potsdam plates were exposed during the low maximum of January-February 1896,[414] when the variable scarcely exceeded fourth magnitude; and this is a circumstance to be borne in mind, since there is reason to suspect that emission may differ, not only in degree but in kind, at light crises of different intensities. However this be, _only_ the hydrogen lines were perceptibly vivified in 1896. Of the dark lines measured by Dr. Vogel, many coincided with Fraunhofer rulings, but a goodly proportion seemed unfamiliar.

The maximum of 1897–98 was studied by Father Sidgreaves.[415] It was an improvement upon that observed by Vogel. Mira attained 3·2 magnitude on 30th November. The Stonyhurst plates were isochromatic; their range of sensitiveness extended from high up in the violet to near D in the yellow, and they continued to be exposed until 5th February, when the variable had sunk to the sixth magnitude. But the light remained essentially unchanged in quality, although reduced to one-thirteenth its original amount. Only the continuous spectrum in the blue had faded, relatively as well as absolutely, showing that the star grew redder in its decline.

Professor Campbell’s[416] observations at the maximum of October 1898 had the twofold advantage of being made with a magnificent apparatus and at an exceptionally bright phase. In more ways than one they mark a beginning. They both suggest relations and establish facts. The plates exposed with the Mills spectrograph attached to the great Lick refractor show only the region near the third hydrogen line (Hγ) in the fine detail needed for measures of precision. These were designed primarily for the determination of the star’s radial movement, which proved to be one of recession at a speed of 62 kilometres (38½ miles) per second. It may be regarded as constant. No part of it seems to be due to orbital motion round an invisible companion. It was, however, derived exclusively from the _dark_ lines in the spectrum. The _bright_ lines told a different tale. Four were compared—a hydrogen pair and a pair ascribed to iron—and all showed a much smaller displacement redward than the dark lines. The amount of the discrepancy, moreover, proved subject to fluctuations; but to fluctuations obviously depending upon intrinsic, not upon extrinsic causes. No attempt has been made to explain them on the hypothesis of variable motion. It may be accepted, on the evidence of lines physically in a normal state, that Mira—so far as appears yet—is a solitary body in course of withdrawal from the earth at a uniform rate of 38 miles a second.

Early in October 1898 the star reached 2·6 magnitude, and during the few weeks of its greatest brightness the blue and indigo hydrogen bands were perceived to be broken up each into three unequal components. This remarkable appearance falls into line with symptoms of disturbance in stellar spectra of other types, but had not previously been observed in a Mira variable. It is of very curious interest. In studying the “intensity curves” of the tripled line (see Fig. 19) the conviction becomes almost irresistible that here a “Zeeman effect” is in question. The polarisation test might decide. If the lines are distended and shattered by powerful magnetic action, then the lateral components and the central component must be polarised in planes at right angles to one another, and the rotating of a Nicol’s prism in the field should produce alternating extinction. Professor Campbell was prepared to make the experiment at the maximum of 1899, but the star unfortunately failed to replenish its due measure of light, and gave an imperfectly legible spectrum. Favourable opportunities, however, for applying this simple criterion must frequently recur, and they are well worth watching and waiting for. Positive results of the kind indicated would be of revolutionary importance; obscure phenomena would be illuminated; anomalies would be removed; a boundless region would be thrown open to investigation. The issue, it is scarcely too much to say, is vital to the progress of astrophysics.

FIG. 19.—Intensity-Curves of Hγ in Spectrum of Mira (Campbell).
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The production of multiple hydrogen-lines in the spectrum of Mira may quite possibly be restricted to brilliant phases. Metallic emission almost certainly is. Thus the iron rays (λλ 4376, 4308) registered as bright in 1898 were, the one strongly dark, the other either dark or invisible in 1896 and 1897. This gives a hint of the diversities to be looked for in the future, and lends enhanced interest to minutiæ of observation which in themselves might seem trivial.

The spectrum of Mira includes a good many dark lines closely adjacent to, if not actually coincident with, rays of helium. None have been seen bright. Calcium absorption is very prominent. The line in the blue, which develops with increase of pressure, comes out as a black grooving; the giant pair in the violet are of surprising intensity. The less refrangible, as in all such stars, smothers the hydrogen emission of nearly its own wave-length; and this circumstance demonstrates some unexpected relations. The calcium-layer, plainly under considerable pressure, must be located, as in the sun, quite close to the photosphere. But the glowing hydrogen necessarily lies lower still, stoppage of light implying superincumbence of the arresting vapour, and there seems no room for it except in the very interstices of the photosphere itself. The overlaying of a light by a heavy substance is indeed anomalous, yet no other arrangement is consistent with the spectral phenomena of Mira and its congeners. Besides calcium, iron, magnesium, strontium, titanium, manganese, and chromium are easily recognised as absorptive constituents of its atmosphere.

The mode of hydrogen-radiation characterising Mira does not recur in all stars of its type. Some show the two lowest lines conspicuously bright, and they are often accompanied by the glimmering of the yellow helium ray. R Aquilæ is an example.[417] Other members of the class have F for their chief bright line, C being invisible, as in R Andromedæ and S Cassiopeiæ, or dim as in V Boötis. Hydrogen in these stars appears to exist in its nebular condition.[418] Analogous to them in some respects, R Cygni may in others be divergent. Its chief bright lines are F and D;[419] but the nature of the accompanying banded absorption appears somewhat indeterminate.[420] It might be definitely ascertained by a few well-timed observations. An important spectrographic investigation of χ Cygni was carried out by M. Eberhard at Potsdam in 1901.[421] It disclosed phenomena closely analogous to those detected by Campbell in Mira. Thus the maximum of lustre was attained, in the hydrogen series, by its fourth member (Hδ); iron lines, both bright and dark, were abundantly visible; above all, the absorption and emission-spectra were relatively displaced, just as in Mira in 1898. The bright lines, that is to say, were pushed towards the blue, while the dark lines deviated in an opposite sense, though very slightly, from their normal places. This surprising feature may then prove common to the whole of this class of stars, and doubtless depends upon some essential peculiarity of their constitution.

About two hundred variable stars with fluted spectra are known to emit bright lines, and this kind of spectrum is a distinctive badge of variability. Mrs. Fleming’s classification of them[422] is based mainly upon differences of hydrogen-emission. For the typical star of the first of her eleven groups she chose R Lyncis, in which Hβ and Ηγ are brilliant, while Ηδ—sometimes the brightest line in Mira—is scarcely visible. She then traced a continuous sequence of change to R Leonis, the exemplar of her last group, in which Hβ is imperceptible, Ηγ excessively faint, Ηδ conspicuous. But this order of relative lustre is not in R Leonis permanently maintained. The effacement of the green ray is only transient. In April 1895 Dr. Krüger, observing with the eleven-inch Bamberg refractor, found it to dominate the spectrum; and MM. Gruss and Laska saw in the same star, 6th May 1894, Hα doubtfully, Hβ and D_{3} unmistakably, although two nights later Hα shone alone, while on 28th May Hβ was similarly isolated. Such changes, inexplicable as they are, cannot be set aside as incredible. Their further investigation is most desirable. Meanwhile, the relative brilliancy of the hydrogen lines in variables evidently supplies a highly insecure basis for their arrangement.

Mira is the only member of its family which has been at all adequately studied. A good beginning has been made with χ Cygni; but about most of the remaining couple of hundred, particulars are wholly lacking. The great majority, having been registered in sweeping spectrographic surveys, were pigeon-holed for future reference, after brief inquiry into the history of their recorded light-changes; and in their pigeon-holes they have been mostly allowed to rest. Enough is known, however, to whet curiosity as to what remains unknown. Spectral changes of a remarkable kind affect these stars; their thorough verification and the unravelment of their tangled relationships are essential to progress. The work may be difficult, but it is of profound interest. The elucidation of the hydrogen-spectrum in one variable star may indeed open the door to unexpected and far-reaching discoveries.

The helium-spectrum is equally significant, but more evasive. The emergence of the yellow ray seems to accompany the brightening of the two lower hydrogen rays; but its shining may be comparatively transient. The important point, however, is that it does not seem to occur at all in stars showing, like Mira, a mutilated hydrogen series. Then there is the further question whether D_{3}, when it does shine out, shines alone. Are all its numerous associates invisible, or are they dark, as some of them appear to be in Mira? Finally, we know very little as yet about the lighting-up of metallic rays in such spectra. It is nevertheless certain that some regulating principle governs the selection of those brightened; and only by detailed study can the nature of that principle be ascertained. All this, and much more, needs prolonged and extended inquiry; but in a field that will yield ample return for the expended labour.