HELIUM STARS WITH BRIGHT LINES.
Temporary stars, and a few stars variable in short periods, belong,
properly speaking, to this class; but for the sake of clearness and
convenience, they are reserved for separate treatment. The question of
light-change will demand later on our undivided attention; it bristles
with difficulties, which we are not at present prepared to encounter.
Alcyone, the chief Pleiad, long passed for an ordinary Orion star. Dark
lines of hydrogen and helium were prominent in its spectrum; there
seemed no reason to suspect the slightest deviation from normality.
Nevertheless, Campbell perceived in 1893[423] the red radiance of C set
off by a narrow dark line on its more refrangible side; and this state
of incipient emission appears to be permanent.[424] Alcyone might then
be counted a linking instance between Classes i. and vii. The discovery
was startling that a single substance could show certain of its rays
bright, the remainder dark, in one and the same star. But the fact,
although highly perplexing,[425] has become so common to experience as
to have ceased to be surprising. Examples of its occurrence have been
registered by the score as regards both hydrogen and helium. They are
all found in Classes vii. and viii. (bright-line helium and Wolf-Rayet
stars); Mira-variables seem never to have their hydrogen spectrum thus
conditioned. Professor Campbell[426] noticed it as an invariable rule
that the bright lines in Orion stars “are those of greater wave-length,”
while “the dark lines are those of shorter wave-length.”[427] This
applies also to helium, but in a qualified sense. If all the lines in
its spectrum are taken indiscriminately, the bright and the dark appear
to succeed each other without method; but their consideration by series
makes it at once evident that, within the limits of each set of
vibrations, the bright members are invariably fundamental. This is
important, not only for the better ordering of stellar phenomena, but as
regards the theory of spectral series in general.
In Alcyone, then, emission is at a minimum; it could scarcely diminish
and remain existent. And it is quite possible that it may be on the
wane; after the lapse of some hundreds, or thousands of years, the chief
Atlantid will perhaps have lost the distinctive note of its spectrum,
and will have sunk to the level of unrelieved absorption. One of its
companions at present stands out from the crowd in the same way, but
more decidedly. Pleione shows three hydrogen lines pretty strongly
bright, and they are inevitably C, F, and Hγ. Centrally superposed upon
wide dark bands,[428] they assert by this fact alone the
non-correspondence in position of their originating stratum with the
glowing hydrogen in stars like Mira. And this teaches us the important
lesson that there is no stereotyped recipe for the production of stellar
bright lines, but that they may originate diversely in the various
spectral classes.
The Pleiades are nebulous collectively, and in many cases individually
as well; but they are less closely folded in nebular swaddling-bands
than the group of stars forming the nucleus of the great Orion nebula.
It consists of four leaders, of about the fourth, fifth, sixth, and
seventh magnitudes respectively, two of which have faint companions;
and, scattered promiscuously, there are to be found besides four minute
stellar points, detected at Lick by Professor Barnard and Mr. Alvan G.
Clark. For spectroscopic purposes the “trapezium,” or quartette of
bright stars, may be treated as one, since they shine with sensibly the
same quality of light, while their scarcely visible associates give
radiations negligible in amount. Most difficult questions arise in
attempting to decide upon the true nature of the trapezium-spectrum. The
prevalent view at first was that the stars were of the ordinary
dark-line helium type, bright lines coming in here and there simply as
projections from the enormous volume of gaseous stuff interposed between
the eye and the stellar nucleus of the formation. But the opinion was
grounded on superficial evidence, and has not held its ground. Some
lines, bright in the nebula, _refuse_ to cross the thin strip of
continuous light due to the star; they stop short on one side of it, and
reform on the other,[429] the two sections being divided by a narrow gap
of absorption. This proves that the nebular rays do not in all cases
show bright against the background of continuous stellar light. The very
strongest may do so; but it is just possible that in them the appearance
is illusory, and due to a kind of irradiation.
There can, on the other hand, be no reasonable doubt that the
trapezium-stars have bright lines of their own. But they are peculiar,
and peculiarly conditioned. A spectrograph taken by Sir William and Lady
Huggins, 5th February 1888,[430] proved to be crossed in the
ultra-violet by at least four groups of fine, faint, bright lines,
derived primarily from two stars of the trapezium, but extending,
through their influence, as it were, some little way into the adjacent
nebula. Their origin is problematical; they have not been recorded
elsewhere;[431] they have been only partially verified on later Tulse
Hill plates. Yet the original negative survives, and its examination has
convinced several experts of the reality of the curious script read from
it. Conviction, however, on such a point is apt to share the dim
character of gloaming phenomena—phenomena on the border between the seen
and the not seen.
But there is more. With refined apparatus the same observers succeeded,
in 1894 and subsequently,[432] in separately photographing three of
these remarkable spectra, and they were now perceived to be rich
throughout in bright and dark lines, “with the special character
strongly marked of bright bands associated with corresponding dark
absorption lines.” Most singular of all, the relative positions of these
bright and dark lines were found subject to change. Hydrogen radiations,
for instance, which in 1894 lay on the blue sides of the absorption
stripes, lay in 1897 on their redward margins. This might be explained
on the hypothesis of orbital movement by supposing each star of the
trapezium composed of a dark-line and a bright-line member, the spectra
of which are periodically shifted through the alternations of their
velocities in the line of sight. It remains to be seen, however, whether
or not the shiftings are periodical; for by this one condition the
explanation stands or falls.
So far as their absorption-elements are concerned, the stars of the
trapezium belong to the earliest variety of the Orion type. All the
lines are wide and diffuse, and the strongest are members of the
Pickering series of hydrogen. Rydberg’s series—if we may call it so
under reserve—is represented by the prominent reversal of its solitary
ray at λ 4689. Mr. McClean recognised oxygen absorption in these stars;
Sir William and Lady Huggins identified nitrogen, silicium, and
titanium, and the calcium K shows both bright and dark. Few sidereal
objects combine so many points of interest as the multiple star at the
heart of the great nebula. The origin and meaning of the throngs of
delicate rays, here just tantalising vision, pressingly invite research;
nor less the manner of relative displacement exhibited by the bright and
dark coupled lines. Do they betray a circulatory period? And if so, is
it the same for each member of the group? Or do they rather form
independent systems, in subordination to a higher scheme, completing
itself in the long leisure of many millenniums? Other problems suggest
themselves in immediate connection with these stars; nor is it
impossible that they may be proposed over again, perhaps in a modified
form, by the multiple stellar nucleus of the Trifid Nebula in
Sagittarius. But instruments of no insignificant light-power will be
needed for the satisfactory examination of its spectrum.
In one other star besides θ Orionis, the shifting of bright hydrogen
lines occurs irrespectively, to all appearance, of binary revolution.
Spectroscopic duplicity was at first naturally attributed to 11
Monocerotis when its peculiar character disclosed itself on the Harvard
plates. Thus in the years 1888–90 the dark F of hydrogen had an
illuminated border lying redward; it was on the blue side in
1891–92.[433] We are not informed whether it has since changed its
position; but any attempt to impose a period upon alterations so
spasmodical would evidently be hopeless. Like θ Orionis, 11 Monocerotis
( = Σ 919) is compound. It consists of three stars of about fifth and
sixth magnitudes, which have maintained a strict relative immobility
since Herschel divided them in 1781. Their spectra, photographed as one
by Pickering, were separately examined by Campbell in 1894.[434] He
found two of them to include the brilliant red ray of hydrogen, while it
was absent from the third. Presumably, then, only two of the trio are
bright-line stars, and it may be that in these two, significant
differences in the mode of emission will be brought to light by detailed
and systematic investigation.
The swing of the bright lines observed in θ Orionis and 11 Monocerotis
is extremely uncommon. In general, a fixed arrangement prevails, and it
is of two alternative varieties. Either the bright lines centrally
divide broader dark bands, as in γ Cassiopeiæ, or the bright and dark
lines are bracketed in pairs, the bright below, the dark above, as in P
Cygni.
Father Secchi’s notice of γ Cassiopeiæ as a gaseous star goes back to
1866. He noticed the vividness in its spectrum of C, F, and D_{3}, but
the helium line has not since held its own with the others. It is
subject to prolonged extinctions; nor is it certain that even the
hydrogen rays always keep up the same standard of brightness. The
variability of the spectrum will, however, be discussed later in
connection with other similar instances; here we have to do with its
fundamental characteristics. The hydrogen lines in γ Cassiopeiæ are
doubly reversed.[435] Wide absorption bands are divided by narrower
emission bands, and these again by hair-lines of darkness. Their
structure is analogous to that of H and K in the solar spectrum. The
radiations fall off in intensity—as Campbell’s rule prescribes—with
diminishing wave-length, while the absorptions gain in the same
proportion. F is “superlatively bright”;[436] Hε is neutral; no bright
lines have been photographed in the ultra-violet. The helium lines are
dark, with occasional exceptions; but the green and blue magnesium lines
shine by direct emission, and Father Sidgreaves recognises as a probable
vanadium line a strong dark-blue ray (λ 4586), which seems to fluctuate
in brightness. Another remarkable circumstance relating to this star is
the recent effacement from its spectrum of the signs of sodium
absorption formerly visible in it. They have, at any rate, escaped
notice since Von Konkoly’s record of 15th September 1884.[437] But the
immense vogue and value of spectrography have tended to reduce to a
minimum the attention bestowed upon the lower spectral sections, and
thus unduly to incline the balance of observation. The study of γ
Cassiopeiæ might alone furnish a not inadequate task for a well-equipped
observer. Only individual enthusiasm is likely to deal successfully with
the baffling problems it presents. Spectral variability is, in its case,
accentuated by perfect photometric constancy. The star is steadily of
2·3 magnitude. It is purely white in colour, lies immersed in the Milky
Way, and has no measurable parallax. Its real size and splendour are
then inestimably great.
The spectrum of P Cygni is not known to vary, although the star itself
was reckoned a “Nova” on its discovery by Janson in 1600, and by its
capricious emergences earned from Huygens, half a century later, the
title of the “_revenante_ of the Swan.”[438] Finally, it settled down
to fifth-magnitude brightness, which it seems disposed indefinitely to
retain. Its spectrum shows an approximately complete set of bright and
dark hydrogen and helium rays; but in their arrangement into couples
_juxtaposition_ replaces _superposition_—that is to say, the bright
lines are in their normal places,[439] while the corresponding dark
ones are shifted upward, as if by rapid motion, towards the eye. But
there can be no real question of motion, since the relation persists
without change year after year. Nor can it be explained on the
pressure-principle of altered refrangibility. The action, if exerted
at all, would be of the opposite kind to that observed. The
displacements in the spectrum of P Cygni are towards the blue; if due
to pressure, they should be towards the red. The phenomenon of the
relative displacement of bright and dark lines in the same spectrum is
one of the most interesting in stellar physics, and has received, up
to the present, no adequate explanation.
The absorption lines in P Cygni are much sharper and narrower than in γ
Cassiopeiæ. Those of calcium, magnesium, and sodium are at once
apparent, and Bélopolsky ascribes many of the remainder to nitrogen.
The spectrum of the great southern variable, η Carinæ, resembles that of
P Cygni by its inclusion of many bright lines shadowed by dark ones on
their blue sides. It has been photographically studied by Sir David
Gill, Mr. McClean, and Miss A. J. Cannon.[440]
Among stars nearly related to γ Cassiopeiæ may be mentioned φ Persei
(4·2 magnitude), υ Cygni (4·4 magnitude), α Columbæ (2·7 magnitude), δ
and μ Centauri (2·8 and 3·4 magnitude). Bright F was detected by Mr.
Espin[441] in the spectrum of the star in Perseus, and was found by
Campbell[442] to be accompanied by a much brighter C. The total absence
of K is surprising, but may not be permanent if the spectral variability
suspected at Potsdam[443] be substantiated. In υ Cygni there appear to
be double reversals of helium as well as of hydrogen.[444] Of α Columbæ
it is only known that F is a broad dark line bisected by a narrow bright
one. The spectra of the two stars in Centaur are thought to be almost
identical.[445] Hydrogen emissions in them are strong and numerous, but
none others have been recognised. The helium lines are all dark;
metallic lines are inconspicuous.
Bright hydrogen and helium lines seem like relics of past
conflagrations. In a few cases we know them to be such, and it is
possible that in all they have the same implications. For any of these
stars may have undergone prehistoric vicissitudes of lustre, after which
they would have settled down into stability; although the recurrence of
such incidents in the future can alone afford secure grounds for
inferring that they diversified stellar biographies in earlier times.
Bright-line helium stars are for the most part situated in the Milky
Way. They are subject to the influences exercised by that strange
aggregation.