WOLF-RAYET STARS.
A very remarkable star was described by Professor Pickering in
1896.[446] Through the measurement of its absorption lines a companion
hydrogen series to that already known was, for the first time,
recognised; while both the Wolf-Rayet bands in the azure showed in it by
direct emission. Only the presence of a dark K obliges us to separate ζ
Puppis from Wolf-Rayet stars proper, and to consider it as a linking
instance between them and helium stars of the earliest variety. Just
this trace of calcium-absorption differentiates the hybrid spectrum of
the star in the Poop from spectra of Class viii., which include no
legible metallic impressions. Their absence is of especial importance as
extending to nebulæ. Nebular chemistry is entirely non-metallic.
The Pickering series in Wolf-Rayet stars is not infrequently bright in
its lower members, submerged by absorption higher up. The bands in the
blue, on the other hand, are always bright. They form a multiple group,
the mutual relations of which await more complete disentanglement. The
lowest member is, or may be, the “fundamental” of Rydberg’s hydrogen
series at λ 469. Yet the circumstance that it is not really solitary
tends to discountenance this identification.[447] The “lazulite” ray at
λ 465 is never entirely effaced, and Campbell has provided it with
several associates of still shorter wave-lengths.[448] Some of these
fall suspiciously near nitrogen lines. The possibility is not then
excluded that all these enigmatical blue effluences may be the joint
products of glowing hydrogen and nitrogen, although this view, like
every other that can be proposed, is beset by serious difficulties.
Among the less refrangible of the Wolf-Rayet bright lines there are two
(at about λ 569 and 559) which appear to coincide with rays photographed
by Professor Hale in carbon stars. Neither has been chemically
interpreted. One of them, the greenish or “citron” line (λ 569), seems
in some way correlated with the blue band at λ 465. A more definite
connection can be traced between the latter and the yellow line at λ
581. They are emphasised in the same stars, while the brilliancy of the
Rydberg beam at λ 469 is accompanied by a special vivifying of the green
Pickering line at λ 541. This rule is quasi-universal; it may stand, at
least temporarily, as a useful principle of order amid half-intelligible
phenomena.
Hydrogen is most diversely imprinted on the Wolf-Rayet spectra. Its
lines, according to Professor Campbell, “have nearly every known
character. In many of the stars they are dark. Again, they are dark with
bright borders, and suggest strongly that they are doubly reversed. The
bright hydrogen lines vary from faint to very bright, from monochromatic
lines to very broad bands, and from those clearly single to those
apparently multiple.” This splitting-up of widened lines is evidently
analogous to the tripling of the brilliant hydrogen rays observed in
Mira, and once more recalls the possibility that certain peculiar
stellar spectra may be produced in powerful magnetic fields. Helium is
not very prominent in spectra of this type. It frequently shows by
emission in D_{3}, and occasionally, even in the same star, by
absorption at λ 4472; but its display is subordinate to that of other
known and unknown elements.
The grand exemplar of the Wolf-Rayet class is γ Argûs (_alias_ γ
Velorum), a star of 2·4 magnitude, giving a resplendent spectrum ablaze
with yellow and blue lines. It was first effectively studied by
Professor Campbell in 1893–94,[449] although at the Lick Observatory the
star barely attains an altitude of six degrees, and can be observed for
only a few minutes on any one night. The main facts that struck him were
the brilliancy of C, the transitional character of F, and the
unmitigated darkness of all the upper hydrogen lines. Similarly, the
initial term of the Pickering series at λ 541 shows by emission, the
rest by absorption, while a bright D_{3} contrasts with a number of more
refrangible dark associates. As usual, both blue bands are visible, but
the weight of radiation falls upon that of shorter wave-length, the
Rydberg line being comparatively inconspicuous. Miss Cannon found
several of the hydrogen lines in both series to be dark with illuminated
borders, an arrangement, as she remarked, the inverse of that prevailing
in γ Cassiopeiæ and its allies.[450] Mr. McClean recognised oxygen
absorption in this star,[451] and the feature is not unlikely to prove,
on fuller inquiry, common to all the members of its class. The absence
of H and K is more than probable, and suggests comparisons and
reflections.
A 7·5 magnitude star in Cygnus (D.M. + 43° 3571) shows, like γ Argûs, a
mixed succession of hydrogen lines, but modified, perhaps, by double
reversals.[452] With the Lick thirty-six inch Professor Keeler perceived
its spectrum as “an extremely complicated range of absorption bands and
faint bright lines,”[453] the unusual width of which struck both him and
Professor Campbell. Thus the azure bands actually overlap, forming a
single indistinct glow one hundred tenth-metres broad. A spectrograph of
this object, taken by Mr. Ellerman with the forty-inch Yerkes refractor,
is described[454] as totally unlike any spectrum of the fourth type.
Whether the dissimilarity is of a nature to be generalised so as to
exclude all idea of kinship between these stellar families is more than
we can tell at present.
A star of about the same brightness, distinguished as
“Argelander-Oeltzen, 17,681,” was swept up in Sagittarius by Pickering
in 1881. In its spectrum the golden ray at λ 581 and the lazulite beam
at λ 465 predominate almost exclusively. Vogel could see no others with
the great Vienna refractor in 1883;[455] nevertheless, Campbell
succeeded, ten years later, in measuring twenty bright lines in this
wonderful spectrum.[456] Only one among them, and that of secondary
importance, can be attributed to helium; but many due to that substance
may be included in the unexplored absorption spectrum of “A.O. 17,681.”
A star of 6·4 magnitude in Canis Major, catalogued as “Lalande 13,412,”
shows, instead of the unknown blue and yellow rays at λ 465 and λ 581,
the “new” hydrogen lines at λ 469 and λ 541.[457] The spectrum includes,
besides, a more refrangible blue band, centred about λ 461, but diffuse
and divided. These multiple azure effulgences in the Wolf-Rayet stars
offer a problem of singular interest. They possess none of the structure
of genuine flutings; they seem apt to spread unsymmetrically. Is this an
effect of pressure on the emitting vapour? Or does it arise from some
property inherent in it, or some mode of action exerted upon it? The
answers may be long postponed, but cannot fail to prove interesting.
A unique specimen of this class was photographically detected in Cygnus
in the course of the “Draper Memorial” surveys.[458] It is extremely
faint—below the ninth magnitude—and was enrolled in the Bonn
Durchmusterung as D.M. + 30° 3639. Nevertheless, its spectrum offers
more than common facilities for exact observation, owing to the
sharpness of its component rays. Thirty were measured by Campbell in
1893,[459] and they include, with many common to the type, several that
appear to be individual to the star. The two brightest lines, however,
are F and λ 569, the “citron” line strong in γ Argûs. The Pickering and
Rydberg series are faint, while the alternative blue band at λ 465 glows
intensely. But the distinctive feature of the star is that it is
_spectroscopically nebulous_. Observed on the F-line like a solar
prominence, Professor Campbell found it to present a very appreciable
disc,[460] which, on narrowing the slit, became reduced to a line, as
shown in Figure 20. The length of this line is about 5″ of arc, and it
measures the apparent diameter of the incandescent envelope of hydrogen
which surrounds the body of the star. Only the hydrogen lines behave
thus exceptionally; all the other spectral rays show as mere bright
points upon the continuous background, which they do not transcend by a
hair’s-breadth. That is to say, hydrogen is the sole glowing constituent
of the enormous appendage revealed by the powerful appliances available
at the Lick Observatory. It has been seen nowhere else, but Runge[461]
and Keeler[462] separately verified its existence.
[Illustration:
FIG. 20.—Hydrogen-Envelope of the Wolf-Rayet Star, D.M. + 30° 3639
(Campbell).
]
There are a few circumstances worth noting in connection with this
extraordinary appearance. First, the vast spread of incandescent
hydrogen round the star has no effect in thickening the representative
lines of that substance in the star’s spectrum. They are, on the
contrary, particularly fine and narrow. The envelope, we can hence
infer, is not an atmosphere; there is no appreciable downward pressure
of its strata. Again, it must be—in the ordinary sense—hotter than the
photosphere it surrounds; for the bright lines emanating from it are not
reversed where they cross the prismatic thread due to the nucleus, as
they should be if the nucleus were at a higher temperature than its
envelope. Finally, it may be possible, by researches into the parallax
and proper motion of this star, to form some estimate of its actual
distance, and consequently of the real extent of its gaseous
surroundings. Thus might be opened a novel line of inquiry destined to
lead future students of the skies far afield.
The chemistry of the Wolf-Rayet stars—judging from partial
interpretations of the disclosing script—is of the simplest. They have
as unfailing constituents hydrogen and helium; oxygen is at least
occasionally present, and the detection of nitrogen may be expected with
some confidence. Sir William and Lady Huggins showed in 1890[463] the
inadmissibility of a carbon origin for any of the blue bands, and
defined their positions by exact measurements. Professor Campbell’s
observations, visual and photographic, of thirty-two members of the
class contributed materially to promote acquaintance with their
peculiarities; yet they stimulated, rather than satisfied curiosity.
The distribution of these objects is strongly selective. They are
virtually confined to the Milky Way. The rule is emphasised by its
apparent exceptions, for the single specimen of the type deviating
considerably in position from the galactic plane has proved, on closer
inquiry, to be situated in a galactic offset; while the twenty-two
grouped in the Magellanic Clouds belong to aggregations of the galactic
order, and subject presumably to galactic conditions. The Milky Way,
then, and the Nubeculæ afford analogous and exclusive facilities for the
development of such bodies. They seem, moreover, to be provided more
freely in some regions than in others, since the objects in question
tend to collect into knots or groups, the finding of one Wolf-Rayet star
being generally the prelude to additional detections in the same
neighbourhood. Yet they rarely or never form binary combinations. They
are loosely associated without any suggestion of mutual circulation. Nor
has any of them, so far, given signs of spectroscopic duplicity. They
are singularly steady light-givers. No Wolf-Rayet star is under the
slightest suspicion of variability. Atmospheric incandescence on the
largest scale is compatible in them with the perfectly uniform working
of arrangements for the transference of heated matter from the interior
to the surface of the radiating bodies. Considering the frequent and
extreme instability of many bright-line stars of other varieties, this
fact can hardly be too strenuously insisted upon.