THE SPECTRA OF DOUBLE STARS.
The spectra of double stars stand in the closest relations to their
colours. This, indeed, is almost a truism, since spectra merely show in
detail what is summarised in mixed tints. Yet the two forms of statement
are not tautological. The result of prismatic analysis cannot be wholly
anticipated from the visual impression. The eye makes no attempt to
reduce its sensations to their elements. Totally different rays may be
blended and balanced so as to produce an identical sum-total to the
perception of the optic nerve. Nor would it be in all cases easy to
pronounce upon the colour of a star from the simple inspection of its
spectrum. One cannot tell beforehand, so to speak, how the eye will take
things. Some scarcely measurable reinforcement of selective stoppage, a
few rays of absorption added or removed, may make the difference between
rosy and golden, or purple and pink. Thus neither direct nor prismatic
observations are superfluous; but the latter, as affording
scientifically accurate and—through photographic means—permanent
records, are by far the more important.
The spectra of double stars unlike in colour are usually of different
types; and here a remarkable rule applies. Contrasted pairs are, with
few and doubtful exceptions, notably unequal in brightness, and the
warmer tint invariably belongs to the larger component. Blue, green, or
violet stars are always the satellites of red or yellow primaries; and,
in accordance with these indications, they give first-type spectra,
while their brighter and more ruddy neighbours show Antarian flutings or
solar lines. We are thus led to the unexpected conclusion that, of two
globes simultaneously contracting, the larger, which should naturally
cool more gradually, and therefore run through its evolutionary stages
at a more leisurely pace, attains solar standing while its companion
still remains a “white star.” This relation is the very crux of cosmic
growth-theories; something more will be said about it in the next
chapter.
The separate spectral examination of coupled stars is far from easy, and
has indeed rarely been attempted. Only by Sir William and Lady Huggins
has the subject been prosecuted systematically and with success. Their
application to it of photography, rendered possible by the completion,
in 1897, of an ingeniously devised reflecting slit, constituted in
itself an immense advance. Previously, only the superposed spectra of
double stars had been chemically recorded, and these, for discriminatory
purposes, were of no more than provisional use. One coloured pair,
however, presents less difficulty in this respect than the rest. The
components of β Cygni lie far enough apart to give distinct
spectrographic images, formed by an objective prism, on the Draper
Memorial plates.[515] Specially inviting as well to direct scrutiny,
they were among the earliest objects subjected to Sir William Huggins’s
light-analysis.
The pair consists of a third-magnitude “topaz” star and a
fifth-magnitude “sapphire” at 34″. The unaltered value of this interval
since Bradley’s measurement of it in 1755 almost assures us that they
drift together through space under the stress of a physical bond. For
their proper motion, though very small, would have sufficed, in the
course of a century and a half, to produce unmistakable relative
displacement. Blue stars, besides, are never solitary; and the
companionship upon which their uncommon hue depends must evidently be
real, not simply optical. Plate IX. Fig. iv. shows the spectra of these
stars as photographed by Sir William and Lady Huggins.
Their complete diversity is apparent at a glance.[516] The hydrogen
series is writ large and strong on that of the minor luminary; helium
absorption is not apparent; the Sirian type is pronounced. Of solar
quality, no less decidedly, is the golden light of the primary. Yet it
cannot escape notice that the photographed spectra do not explain the
vivid colouring of β Cygni; they might have been taken (speaking
broadly) from any two stars of the types represented. This, indeed, was
just what should have been expected, since the special absorption
differentiating them from the common run of stars was known to lie
outside the range of sensitiveness of ordinary plates. As regards the
blue member of the pair, at any rate, there could be no doubt of the
fact. A set of dark bands, cutting out a goodly proportion of its yellow
and orange rays, were observed by Sir William Huggins in 1864,[517] and
again by Dr. Vogel in 1872,[518] and they correspond with, and fully
explain, its chromatic peculiarity. The topaz hue of the primary cannot
be so directly associated with the subtraction of particular qualities
of light; it is more probably due to an enhancement of that veiling of
the higher spectral reaches to which sunshine owes its primrose tinge.
Further investigation is, however, desirable; above all, the
photographic registration on isochromatic plates of the unfamiliar
absorption-bands from which the companion of β Cygni derives its
distinction as an azure star.
The theory of “composite stellar spectra” was proposed by Professor
Pickering in 1891.[519] Spectrographic impressions showing a mixture of
types should, he explained, result from the superposition of dissimilar
spectra derived from close or telescopically indivisible stars. The
forecast was verified by Miss Maury’s detection of eighteen
self-imprinted images of the compound sort.[520] “In spectra of this
class,” she writes, “the K-line appears either unduly narrow or
overspread with a peculiar haziness. This appears to be due to the
presence of an additional star, having a spectrum which belongs to some
group earlier in the series. It is also significant that in such spectra
the first-type characteristics predominate in the ultra-violet, the
second or third-type features in the green and blue.” These duplex
effects demonstrably own, in certain of the instances enumerated, a
duplex cause; for they include γ Andromedæ, ε Boötis, and α Piscium, all
three remarkable pairs. The presumption is accordingly strong that
spectra appearing hybrid in small-scale delineations really emanate from
a double source, although visual evidence of duplicity be wanting.
Indeed, several of Miss Maury’s _crypto-doubles_, ο Leonis, ο Andromedæ,
and α Equulei among the number, have been spectroscopically resolved by
Professor Campbell into unlike pairs. And even should the motion-test
fail, it need not be inferred that the star recalcitrant to it is
single; for a negative result may signify merely that the method is
inapplicable owing to the high inclination of the plane in which coupled
stars revolve.
One of the show-objects of the heavens is γ Andromedæ, composed of a
chrome-yellow star of 2·2 magnitude, and a sea-green fifth-magnitude
attendant at 11″. The attendant itself can be divided with a good
telescope into a blue and a green star, considerably advanced along an
elliptic track since their first observation by Otto Struve in 1842;
while the wide pair, discovered by Christian Mayer in 1777, remains
relatively fixed, although their systematic union is attested by an
identical secular progress of about 7″. Their spectra, photographed at
Tulse Hill, closely resemble those of the components of β Cygni, the
different patterns of absorption stamped on them forming almost as
striking a contrast in the negatives as the colours of the original
objects do in the sky. A similar combination is offered by ε Boötis, but
on a reduced scale. The ultramarine satellite is here poised at a
distance of only 3″ from its golden primary. Their spectra have, indeed,
been no more than inferentially distinguished. Miss Maury’s scrutiny of
the joint impression left by them upon the Harvard plates made it,
however, fairly certain that, as usual, the blue star is of Sirian, the
yellow star of solar quality; so that a relation of development is again
indicated just the converse of that held, on _à priori_ grounds, to be
probable.
The theoretical incongruity is, in some cases, heightened by the
substitution for the sun-like primary of a red star giving a fluted
spectrum. Such a pair is α Herculis. An “emerald” star of the sixth
magnitude, at a distance of 5″ from its glowing leader-orb, yielded to
Sir William Huggins’s early examination a spectrum of precisely the same
character as that of the satellite to β Cygni. Antares, too, is quite
similarly coupled with a green star, the spectrum of which, judging by
the duplex impressions obtained at Harvard College, resembles that of
Sirius, with, it may be, some approximation to that of Procyon.[521]
Mr. Burnham performed in 1881 the unprecedented feat of dividing a
third-type star into a very close pair. He detected a satellite of the
ninth magnitude situated within just one second of arc of η Geminorum, a
fine red star, variable in a period of 229 days, although its maxima are
unmarked by any signs of gaseous emission, doubtless because of the
comparatively slight extent of the light-change. The spectrum of the
small star cannot, of course, be directly observed, but its nature may
be indicated by colour-observations. Should a glint of blue or green be
caught under favourable circumstances, the inference that it proceeds
from a source of the Sirian quality can be unhesitatingly drawn. A
particular interest attaches to η Geminorum as the only Antarian star
with a companion likely to prove sensibly mobile within a reasonable
lapse of time.
The spectra of double stars that are unstable in colour have an
importance both evidential and explanatory. They illustrate and tend to
expound chromatic phenomena. The diversity in light-quality of 95
Herculis is then of extreme significance. These stars, as our readers
will remember, are now perfectly matched. They are of equal brightness,
and of the same yellow shade. But half a century ago they displayed
brilliant complementary radiance in red and green. And their spectra
correspond, not to their present uniformity, but to their historic
contrast. Vogel in 1899 recorded for one component—presumably the star
formerly green—a Sirian, for its twin a solar spectrum. Additional
weight is thus lent to the old observations; and a hint, not to be
neglected, is given as to the probability of future change.
It is less surprising to meet with spectral dissimilarity in the
components of γ Delphini. For they differ in magnitude, and very
markedly in colour, notwithstanding past intervals of agreement. And it
was just during one of their periods of agreement, in 1883, that Vogel
found the larger star to be of solar type, while its companion, now
green, but then colourless, gave a Sirian spectrum. Here again, as in 95
Herculis, spectral distinctions seem to persist while chromatic
distinctions are alternately effaced and restored.
A good many yellow stars have purplish attendants of dubious
spectroscopic standing. Their quality remains untried, and is difficult
to conjecture. One of the best examples is η Cassiopeiæ, a revolving
pair consisting of a 3·5 and a 7·5 star 5″ apart. The primary emits
golden light marked with the solar stamp of absorption; its satellite
has been variously described as violet, rosy, and garnet. These notes of
colour, indeed, supply no hint as to the nature of the corresponding
spectrum; but some indication that it is more “advanced” than that of
the large star may be gathered from the mass-relations of the pair.
Their gravitational disparity, as determined by Otto Struve, is 3·7 to
1, while their light disparity is 40 to 1. In other words, the satellite
is nearly seventeen times less luminous than it would be if of the same
mean density with its primary, and of equal areal lustre. In reality it
is probably both more compressed and less brilliant. But these
properties belong to a comparatively late stage of growth, and should be
associated with a strongly absorptive atmosphere. The precise type of
absorption characterising dependent stars of a violet hue it would be
rash to predict, but it is very desirable to ascertain.
A pair closely resembling η Cassiopeiæ is ξ Boötis. Again in this case a
yellow primary of solar type has a rose-purple attendant actively
circulating round it. With it may be classed a couple in Pisces (Σ
3036), coloured “very little yellow and dusky red,”[522] and probably β
Cephei, composed of a sulphur-tinted helium star of 3·4 magnitude,[523]
and an eighth-magnitude violet attendant at 14″.
The great majority of double stars present, however, the same or similar
tints; they are white and creamy, or sulphur-coloured and golden, and
the spectra derived from them accord entirely with these indications.
They are almost always variants of one type. But the rule observed in
contrasted pairs that the smaller is the earlier star is here inverted.
The subordinate members of systems undistinguished for colour often show
signs of having progressed further on the road towards extinction than
the larger orbs. This principle is finely illustrated by the grand
southern binary, α Centauri. Now these stars are almost exactly equal in
mass, yet one gives only a quarter of the other’s light. It is also more
deeply tinted with yellow; we may, indeed, safely infer that it is
dimmer mainly because of the additional absorption to which its colour
testifies. The spectra of the pair, splendidly delineated in Sir David
Gill’s photographs, are both of the solar class, but with a difference.
That of the brilliant component is an exact copy of the Fraunhofer
spectrum (see Plate X. Fig. 2), while that of the inferior star might be
called post-Arcturian, manifesting traces of affinity with the fluted
type of Betelgeux.[524] The spectral relations of α Centauri doubtless
prevail in many other systems, but they do not arise inevitably, even
under quite similar conditions. Thus the unequal stars of γ Leonis give
virtually identical spectra of the Arcturian or post-Arcturian
species;[525] and the equal stars of γ Virginis, though of Sirian type,
are unmarked by the smallest difference in the mode of absorption. It
would then appear that, while two globes cast, as it were, in the same
mould, like those united in α Centauri and γ Virginis, frequently
proceed _pari passu_ along the life-course of suns, one may outrun the
other under the influence of unknown circumstances. Couples unassorted
in size comport themselves differently; but here, too, allowance has to
be made for original diversities of constitution, or supervening
incidents of development.
To resume. The colours of double stars afford preliminary indications
as to the nature of their spectra, but cannot, in all cases, be
interpreted with much confidence. Blue and green stars are, as a
nearly invariable rule, the satellites of red or yellow primaries.
They belong to the Sirian type, modified, probably, by special
absorption serving to lend predominance to the more refrangible rays,
and so produce their unusual tints. “Purple” attendant stars have also
been observed; the quality of light, however, corresponding to this
designation remains unknown. It may prove to be stamped with strong
absorption, such as would be symptomatic of advanced condensation; and
if so, purplish or violet satellites are a radically distinct class of
bodies from azure stars; for they might be inferred to be
proportionately more massive and less luminous than their primaries,
while the inverse relation doubtless holds good in gold and green as
well as in topaz and turquoise combinations. Stellar pairs of equal
magnitudes are, with the rarest exceptions, alike in colour and
spectrum. They are primrose-tinted—scarcely ever pure white—and of
solar or Sirian type.
The spectra of couples no more than two seconds apart can be separately
photographed with the Tulse Hill apparatus; and indications of duplicity
are often obtained from the composite nature of the spectral impressions
given by apparently single stars. Only dissimilar components, however,
are capable of being thus distinguished; superposed spectra disclose
themselves as such by their differences, among which opposite
motion-displacements are occasionally met with. The discrimination of
mixed qualities of light in single spectrographic records is a branch of
research promising further developments.