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.
[Illustration:
FIG. 19.—Intensity-Curves of Hγ in Spectrum of Mira (Campbell).
]
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.