STARS WITH FLUTED SPECTRA.
The blue light of these stars is powerfully absorbed by an
intensification of the screening effect observed in the sun. They are
accordingly rufous, or red. Their spectra are profoundly scored besides
with metallic rays, generally agreeing in position with, although
differing in relative intensity from, the Fraunhofer lines. Thus the
calcium line in the blue has gained still further upon the great pair in
the violet than in transition stars of the solar type. Finally, banded
absorption has come in. A complete system of ten or eleven flutings,
sharp towards the violet, graduated insensibly towards the red, shadows
nearly the entire visible spectrum. It is printed in stereotype. The
bands are variously impressed, but similarly located, in all members of
the class. This gives strong assurance of an identical origin. We do not
yet know how they are produced, or by what substances, but there can be
no doubt that their explanation in one star will apply to all.
With high dispersion the bands can be resolved into fine lines set very
close together.[386] The fluted effect is due to the crowding of these
lines towards a limiting wave-length prescribed, beyond question, by a
rhythmical law. For that each band represents, as it were, a condensed
series there need be no hesitation in admitting. Indeed, by mere
reduction in scale the hydrogen procession in a white star assumes the
aspect of a genuine fluting. A promising start has even been made in the
research of laws regulating the distribution of lines in bands.[387]
Each of the stellar stripes is then, so far, a separate entity, while
all may be linked into harmony by subordination to some higher unknown
principle. Whether they originate from one or many forms of matter has
still to be determined; nor is there any certainty as to whether
elements or compounds are concerned in their production.
Stars of Secchi’s third type, forming the class just now under
consideration, are divided by Miss Maury into three groups. Antares and
β Andromedæ belong to the first of these (“Group xvii.”). They show all
the characteristic bands, slightly marked, and transparent enough to
allow every detail of the linear spectrum to be clearly visible.
Betelgeux (α Orionis) is the pattern of the next group. Its prismatic
light makes a beautiful and wonderful effect. The usual multiple
absorption is exerted upon it, but with a delicately balanced power. The
blue rays retain appreciable vivacity; the flutings are not so deep as
to obscure the underlying rays; they are finely shaded, yet exquisitely
distinct. Ten were measured by Vogel and Dunér, the strongest of which
are the fifth and seventh, with their _steep_ sides at λ 5453 and λ 5169
respectively. Battalions of dark lines show through them. On the Harvard
plates Miss Maury counted 463, mostly composite, between the sodium D
and the calcium H. The latter and its associate, K, have shrunken
somewhat from their giant dimensions in Arcturus.[388] Iron absorption
predominates. The rays significant of it are more prominent than in the
sun, and some have unilateral shadings—a feature also visible in the
spectra of sun-spots and of metallic oxides, and indicative probably of
a decline in heat.[389] Professor Keeler remarked that the lines in
Antarian stars “are essentially those of the solar spectrum, but the
relative intensities are not the same, and the general aspect of the
spectrum is quite different from that of the spectrum of the sun. The
strong lines are mostly those of iron—apparently the low temperature
lines. Their relatively greater strength in the star spectrum gives to
some well-known solar groups (notably the _b_ group) quite an unfamiliar
aspect.”[390]
Hydrogen-absorption is much more effective in Betelgeux than in any
other spectrum of the fluted kind. It is, however, distinctly, though
feebly represented in all by the four lowest members of the Huggins
series. The rest are either absent or shrouded in overlying vapours.
In ρ Persei, a star capriciously variable between 3·4 and 4·2
magnitudes, the bands are perceptibly deeper than in α Orionis. On the
other hand, the metallic rays seem rather less numerous and intense. But
this diminution may be more apparent than real. The comparative
faintness of the light they interrupt would partly account for it, and
the added density of the associated flutings would help towards
effacement.
Their still greater opacity in α Herculis occasions effects of contrast
with the vividly tinted bright zones, described as “singular and
magnificent” by Father Secchi, one of their earliest observers (see
Plate XI. Fig. 1). Professor Keeler, whose study of fluted spectra was
based on photographs of high dispersion showing manifold details, found
the dark groovings, plentiful in α Orionis, to be present only as a
comparatively scanty survival in α Herculis. Miss Maury reached a
similar conclusion. Yet Dr. Vogel was struck with the richness in
absorption-lines of this spectrum and the analogous one of β
Pegasi.[391] Both these stars vary irregularly, α Herculis from 3·1 to
3·9, β Pegasi from 2·2 to 2·7 magnitude. Next to Antares, the brightest
specimens of this class in the southern hemisphere are γ Crucis and β
Gruis. Mr. McClean obtained spectrographs of all three at the Cape in
1897.[392]
Uncertainty has often been expressed as to the true nature of the
luminosity in the open spaces of fluted spectra. Are they simply
intervals of unshaded photospheric radiance, or is their brilliancy
reinforced by the addition of bright lines? Just where they meet the
black edges of the shafts of absorption, their splendour exceeds, in the
opinion of some observers, what could be produced by contrast alone. It
seems, nevertheless, unlikely that rays of emission should occur in
these positions and nowhere else in the same spectra. Moreover, illusory
impressions of the kind, both visual and photographic, are common and
pertinacious. Nothing, however, is more inimical to truth than dogmatic
denial; we must be ready to admit much that we should beforehand have
deemed impossible, even the reality of far-fetched coincidences; for
anticipation is often belied by fact. Professor Keeler, whose mind was
singularly free from prepossessions, found it “impossible to avoid the
conclusion” that in the spectrum of α Herculis “the edges of the zones
bordering on the dark bands are bright—much brighter, that is, than the
average continuous spectrum—and that they are due to a real predominance
of emission at the regions of the spectrum in which they occur. In the
case,” he wrote further, “of stars like α Orionis, of a less pure type,
such a conclusion could not be safely drawn; yet the superior brightness
of the spectrum at these places is obvious, and it can be traced even in
second-type stars. May there not, after all, be bright regions in the
solar spectrum, such as Draper supposed he had found in the places of
the bright oxygen lines? And what is the relation between the dark bands
in third-type stars and the bright zones which border on them?”[393]
Questions more easily asked than answered. They suggest doubts, not at
once to be set at rest, as to the nature of so-called “continuous”
spectra. May they not in certain cases include several maxima of
radiation? The possibility is at least not excluded of individual
differences in this respect between stellar photospheres. Yet of true
gaseous emissions there seems to be no trace in Antarian stars. The
admission of its presence could not be made for one without being
extended to all; and many spectra of the class are clearly exempt from
abrupt intensifications.
Chemical recognition has not been carried far in them. The familiar
lines of only five substances—iron, calcium, magnesium, sodium, and
hydrogen—are entirely unmistakable. The rest await more searching
scrutiny. It will be of especial interest to determine whether titanium
and its usual associate, vanadium, retain in these objects any share of
their importance in sun-spots. Helium-absorption, too, which
occasionally emerges to view in spot-spectra, might be looked for with
some prospect of success, but is likely to be inconspicuous. The
fundamental problem, however, in this connection relates to the origin
of the flutings. It ought to prove capable of a definite and, so to
speak, a simultaneous solution. For the members of the system show the
coherence of a structural design. They form a marshalled array, an
interdependent order. Their occurrence piecemeal need not then be
expected. They will be recognised together or not at all.
Stars with fluted spectra have a fixity significant of immeasurable
remoteness. Yet two—Antares and Betelgeux—are of the first order of
apparent brightness. Their real magnitude must hence be prodigious. An
approximate estimate of it can be arrived at in the case of Antares,
which has an ostensible parallax of 0·021″, corresponding to a light
journey of 155 years. This, at least, was the outcome of Mr. Finlay’s
measurements at the Cape; but it is so small as to lie but slightly
outside the margin allowed for their probable error. Its genuineness can
then only be assumed for the purpose of fixing ideas; since the star may
be indefinitely further off, while unlikely to be appreciably nearer,
than this minute annual shift asserts it to be. Under this reserve we
may compare Antares with our standard star, α_{2} Centauri, which, as we
have seen, is equivalent to comparing it with the sun. The result is to
show that four hundred suns, in its place, would barely supply the light
we receive from the _alter ego_ of Mars.[394] And this is only what
remains after a heavy absorption-toll has been levied—a toll of probably
twice the amount paid by the sifted solar beams. The photospheric extent
of Antares may then be set down as at least eight hundred times that of
the sun, while the immense sphere it covers may be held, on good
grounds, to have a comparatively high mean density. Even on the basis of
equality in this respect, its mass would exceed that of the sun more
than 22,000 times, while gravity at the surface of this unimaginable
globe must possess at least twenty-eight times its solar power. Similar
reasonings apply to Betelgeux, only with still further enlargement in
measure of the conclusions they lead to. For Betelgeux is a more
brilliant luminary than Antares, and its immobility in the sky is, if
anything, more nearly absolute.
The processes of interior circulation in such bodies are extremely
difficult to realise. The intensity of radiation must depend—other
things being equal—upon the promptitude of delivery at the photospheric
level of heated stuff from within. This must increase with the force of
gravity, which is the driving power on ascending and descending
currents; but it must, on the other hand, fall off as the sphere they
traverse grows more compact. The manner in which the balance is struck
in each individual star between these opposing influences transcends
every rational conjecture. We can only see that it must vary widely, and
that its variations necessarily affect the photospheric composition and
the radiative characteristics of the globes in which they prevail.
Antarian stars obey no special law of distribution. They are scattered
at large over the heavens, and usually in isolated positions. They show
as a rule no tendency to gather into groups. A collection of nine
specimens, located in the intervening space between the two grand
clusters in Perseus, is perhaps unique. Discovered by Mr. Espin in
1891,[395] this nest of red stars appears like a garnet clasp linking
together a pair of diamond aigrettes. They are of about the eighth
magnitude, but may nevertheless, since they are assuredly vastly remote,
be most majestic orbs. Not that we should ascribe to all stars of this
spectral type the colossal dimensions of Betelgeux and Antares. The same
degree of variety may be supposed to exist among them as among solar
stars. Yet the analogy may not hold. It is conceivable that very great
mass is a pre-requisite for the development of a fluted spectrum.