HELIUM STARS.
Helium stars are often palpably connected with nebulæ. The entire Orion
region, where they brilliantly congregate, is pervaded with cosmic fog;
cosmic fog enwraps the Pleiades; and individual instances of the same
association abound, and are likely to multiply as exploration proceeds.
It is, however, visibly closer in some stars of the class than in
others; and these nebulous gradations appear to correspond with spectral
gradations of a very interesting kind, accurately represented in the
progressive order of Miss Maury’s groups. The earliest are strongly
impressed, not only with helium and ordinary hydrogen lines, but with
the Pickering series as well, noted by Mr. McClean to characterise a
primitive stellar condition.[346] Satisfactory evidence of oxygen
absorption in them was adduced by him in 1897, and lines of nitrogen and
silicon are recognisable besides. Among metals only calcium and
magnesium make a feeble effect,[347] the one with a just discernible K,
the second with the “high-temperature” line in the indigo (λ 4481). No
sharp rays are found in these spectra. Their shadings take the form of
hazy streaks.
The chief of a triple group in Monoceros is a specimen peculiarly worthy
of consideration. It has the uncommon property, for a helium star, of
being variable in a short period, whence its catalogue title of S
Monocerotis; and it dominates a small cluster (N.G.C. 2264), measured by
Bruno Peter in 1880.[348] An involving nebula, evasive of telescopic
vision, came out fully in Professor Barnard’s photographs of 1894,[349]
and was described by him as “a very wonderful object, irregular in
outline but quite well defined, with numerous black gaps running into
it, and conforming in general with the peculiarities of the Milky Way in
that region.”[350] A picture on a larger scale was taken by Dr. Roberts
a year later.[351] That S Monocerotis will prove to be a spectroscopic
binary revolving in three days and ten hours, may be judged probable
from the analogy of its fellows in variability.
One of its contemporaries is the third magnitude star, ι Orionis (N.G.C.
1980). Sir John Herschel perceived it to be wrapt in a large feeble
nebulosity, and a divergent streak, linking it to the great “trapezium”
nebula, disclosed itself photographically to Professor W. H.
Pickering.[352] It is widely triple, but sensibly stationary. Although
its spectrum is in the main a copy of that of S Monocerotis, important
distinctions may present themselves to scrutinising inquirers. Both the
Wolf-Rayet blue bands show by absorption in these “early Orion” stars,
the upper one being especially pronounced.
The three stars forming the Belt of the Giant are slightly more
“advanced.” Helium has gained strength in them relatively to the
Pickering series; the reversal of Rydberg’s azure band verges towards
effacement, and in the middle star, ε Orionis, the spectral lines are
fairly well defined. Oxygen, nitrogen, and silicon contribute each its
quota of absorption. A great stream of nebulous matter sweeps through
the Belt, and its two lower gems, ζ and ε Orionis, claim besides shining
appurtenances of their own.[353] An analogous object, σ Scorpii, came
out on Professor Barnard’s photographs with a couple of nebulous
“prongs” attached to it,[354] and also as a focus of marked condensation
in the great nebulous field near Antares. The spectrum is perfectly
similar to that of ε Orionis.
[Illustration:
PLATE XIV.
SPECTRUM OF γ CASSIOPEIÆ, DRAWN FROM A PHOTOGRAPH TAKEN ON MARCH 7,
1898.
STONYHURST COLLEGE OBSERVATORY.
]
Among southern helium stars one deserves special mention if only for its
association with a notable discovery. This is Mr. McClean’s “oxygen
star,” β Crucis. His identification in 1897[355] of numerous lines in
its spectrum as due to the absorptive action of our vital gas was fully
confirmed two years later by Sir David Gill,[356] who employed for the
purpose the splendid apparatus bestowed by Mr. McClean upon the Cape
observatory. Silicon is also present, and the usual reversals of the
indigo line of magnesium and of the violet K of calcium appear
distinctly. Nitrogen, however, is not evident, nor the Pickering series
of hydrogen. The spectra of β and ε Canis Majoris, and of β Centauri are
of the same stamp. One of the brilliants of the Southern Cross, β Crucis
lies immersed in the Milky Way, at an unmeasured, perhaps an
immeasurable distance from the earth. We have thus no means of
estimating its actual radiance, which must, however, greatly exceed that
of the sun. A secular proper motion of fourteen seconds is ascribed to
it, and Sir David Gill finds it to be receding from the sun at the rate
of about eleven miles a second.
Bellatrix, in the shoulder of Orion, is a typical helium star, the chief
representative of Miss Maury’s fourth group. No Pickering lines have
been found in its spectrum, but Professor Keeler noticed the comparative
prominence of a subordinate nebular ray at λ 4390.[357] Nitrogen and
oxygen were identified in it by Sir William and Lady Huggins,[358] and
silicon by Sir Norman Lockyer[359] and Mr. Lunt.[360] The effacement of
iron, remarkable in nearly all helium stars, subsists also in Bellatrix,
and is explained by Sir Norman Lockyer as an effect of transcendental
temperature. It has a spectroscopic _alter ego_ in the _lucida_ of the
Southern Cross.
In the spectrum of Rigel some iron lines faintly emerge, and the sodium
D appeared conspicuously on Professor Campbell’s isochromatic plates.
The Huggins series of hydrogen is magnificently displayed from its first
term to the limit[361] (see Plate IX. Fig. 3); helium lines are strong
and numerous; those of nitrogen, oxygen, and silicon come out in
photographs, and they were found by Sir William and Lady Huggins to be
associated with certain distinctive rays of titanium.[362] The lines in
this spectrum contrast markedly by their sharpness with those of other
stars chemically similar, such as Regulus. Rigel belongs to Miss Maury’s
sixth group; it approximates to the stage where helium sinks out of
sight, and yields the sole predominance to hydrogen.
In order to form some idea of its prodigious light-power we must
remember that it has no sensible parallax (Gill), and is all but
stationary in the heavens. This implies that it is so far off as to make
almost no perspective response to the sun’s centennial advance through
space. In other words, a base line some thirty-three thousand millions
of miles in length (allowing for foreshortening) shrinks to little more
than a point as seen from Rigel. Assuming the reality of the minute
proper motion of 1·5″ a century deduced from its catalogued places, and
that it is a parallactic effect of our system’s progress towards an
“apex” on the borders of Hercules and Lyra, at the rate of twelve miles
a second, we must ascribe to the star a distance of at least 367 light
years, corresponding to an annual parallax of ¹⁄₁₁₂ of a second. It
follows that Rigel gives about 8000 times more light than the brighter
component of α Centauri, an orb considered by Sir David Gill to be the
exact match in every respect of our sun. But the sun is dimmed to about
one-third of its native lustre by effects of absorption which are
virtually absent from the star. Hence a total light emission 8000 times
greater would represent a radiating surface only 2667 times more
expansive than the solar photosphere. Rigel, moreover, is certainly not
massive in the proportion of its luminosity. Stars of the helium variety
are composed of highly rarefied materials. This has come to be known
through the study of eclipsing stars. Taking, then, the density of Rigel
to be about that of Algol, or one-fourth that of the sun, we find it
even so to be of no less than 34,000 times the solar mass, while gravity
at its surface is of just thirteen-fold power. Nevertheless its spectrum
indicates extreme tenuity in its gaseous surroundings. Calcium, for
instance, in the reversing layer of Rigel emits violet rays _only_.
There is no trace of the blue line (λ 4227). The vapour exists there in
much the same state as in the solar chromosphere and prominences—that is
to say, in a state of the utmost attenuation, which implies the
counterbalancing of gravity by a strong antagonistic influence,
presumably of an electrical nature. This merely extends an inference
already derived from solar phenomena.
The relationships of Deneb (α Cygni) have been variously assigned. There
is, however, no longer any doubt of its affinity to Rigel.[363] The
lines in its spectrum are numerous and clearly defined. Sir Norman
Lockyer measured 307 on photographs of the section above F,[364] and
there are hundreds besides. Those of helium are of subordinate
importance; they are being replaced in the supposed evolutionary
progression by metallic lines. Magnesium absorption is deeply graven in
the ultramarine (λ 4481), and begins to appear through the green triplet
(_b_). Iron lines of the kind “enhanced” in the spark are fairly
abundant; gallium shows at least one strong line,[365] and titanium
lines are prominent. Among non-metallic substances, besides helium and
hydrogen, only silicon is unquestionably present. It comes, however,
well to the front. Mr. Lunt regards α Cygni, Rigel, and Sirius as some
of “the best examples of silicon stars” yet known.[366] Nevertheless,
spectrographs of them fail to show the three silicon lines most
conspicuous in the β Crucis group, while Lockyer’s enhanced lines
imprint themselves with some emphasis.[367] These celestial
modifications of the silicon spectrum afford “valuable data,” in Mr.
Lunt’s opinion, “for the elucidation of the problem of relative stellar
temperatures.” Their interpretation on current principles would lead to
the conclusion that α Cygni, Rigel, and Sirius are hotter than the
“earlier” suns typified by the “oxygen star” in the Cross. But there is
no real certainty as to what causes the difference in kind between the
luminosity of the spark and arc. Temperature may not be the sole, or
even the chief, agent in its production.
The width and density of K in α Cygni are noted by Sir William and Lady
Huggins as anomalous, calcium absorption usually remaining feeble when
that of helium is visible. The curious thinness of the hydrogen lines
may result in part from their projection upon a photospheric background
of exceptional brilliancy.[368] The star, at any rate, is one of those
which “stand apart through a distinctive individuality,”[369] and it
invites, as such, special attention.
Regulus—an intermediate specimen—and β Centauri are perhaps the only
helium stars at determined distances from the earth. For the latter Sir
David Gill found a parallax of 0·046″, equivalent to a light-journey of
seventy-one years, so that it is by no means a near neighbour. In its
place the sun would be just perceptible to the naked eye; it would
appear of sixth magnitude, while the star is only a couple of grades
below the first rank (its photometric magnitude is 1·2). Its emissions,
in fact, surpass the solar radiance rather more than 150 times. We may
then allow that they proceed from a photospheric expanse fifty times
ampler than the sun’s, which must encompass a globe 342 times more
voluminous. Assuming further for β Centauri (as for Rigel) a density
one-fourth the solar, we obtain the result that it is of 85 times the
solar mass. Owing, however, to the comparative remoteness of its surface
from its centre, gravity has there less than twice the power which it
exercises on the sun. Putting it otherwise, the acceleration of a
falling body on β Centauri is about 750 feet a second. The value of this
“constant” is probably of essential importance in determining the
character of stellar spectra; hence attempts at its estimation, despite
the uncertainties that hamper them, are worth making.
The Pleiades are tolerably mature helium stars; Alcyone was selected by
Miss Maury as the type of her fifth group. Some of its associates show
hazy, others sharp lines. Algol approaches still closer to the boundary
of the Sirian class, its “Orion lines” being quite secondary to the
hydrogen set. Regulus is of nearly the same standing, but its spectral
markings are dim and diffuse; those of inferior intensity thus make no
appreciable impression, and the star’s rays are all but exempt from
absorptive encroachments.
They indeed tell but slightly throughout the entire class of helium
stars, and this seems to indicate the absence of any strong contrast in
temperature between their photospheres and the encompassing incandescent
vapours. That these are exceedingly tenuous is rendered almost certain
(as already pointed out) by the non-reversal of the blue line of
calcium. As to their relative temperatures, no dogmatic assertion is
possible. The effects of transcendental heat evade inquiry. We cannot
without hesitation assume that known rules apply under unknown
conditions. The “temperature” of the electric spark is a purely
conventional expression; to what state of matter it actually
corresponds, can barely be surmised. Yet it is important to remember
that the progression of helium stars—if the testimony of their silicon
lines be credible—is _towards_ this state from a lower degree of
molecular excitement; while their “high-temperature” magnesium ray,
present at the start, gains prominence by accordant gradations.
The inverse relationship between helium and metallic absorption is
extremely significant. They seem to be almost incompatible; one tends to
effacement with the incoming of the other. Yet it must be borne in mind
that both subsist together in sun-spots, where a condition of things
temporarily arises enabling helium to exert its proper stoppage upon
light. Spot-spectra thus approximate, so far, to stellar spectra of the
“Orion” stamp. Here, no doubt, we hold the clue to some profound
physical analogy, the investigation of which may help to dissolve part
of the mystery shrouding the “process of the suns.” Moreover, oxygen,
nitrogen, and “cosmic” hydrogen (if we may so call the modified gas
giving the Pickering lines) are even more sensitive than helium to the
adverse influence of metals. This statement is scarcely impugned by the
fact that a trace of oxygen-absorption survives in the sun.
The closest connections of early helium stars are with members of the
Wolf-Rayet family. But for the dusky lines of magnesium and calcium
apparent in them, their spectra might indeed be said to reverse the
Wolf-Rayet radiations. Later on, when nebular symptoms disappear, when
helium fades, and faint iron lines crowd in, they slide imperceptibly
into the Sirian stage of existence. No halt is cried. The frontier is
crossed without advertence.
Helium stars are not equably scattered over the sphere. Their
condensation towards the plane of the Milky Way, first noticed by
Pickering,[370] was strongly emphasised by McClean’s southern survey.
“In the contiguous constellations of Musca, Crux, Centaurus, and
Scorpio,” he tells us,[371] “there are twenty-seven helium stars out of
a total of thirty-six” brighter than 3·5 magnitude, and the proportion
in Perseus, Taurus, and Orion is fifteen out of nineteen. It would be
desirable to ascertain whether objects of inferior lustre are similarly
swayed by this galactic attraction. But up to the present nothing is
certainly known as to the prevalence of the type among faint stars.
Below the sixth magnitude, its distinctive marks are hardly
recognisable. The conjecture, however, is plausible that Milky Way
aggregations are composed mainly of helium suns, large and small. But
since their great intrinsic brilliancy renders them visible at distances
completely quenching the rays of solar stars of the same size, they
should preponderate in the Milky Way for this reason alone, apart from
any real numerical superiority. So that the question of their
distribution, like most others in stellar physics, has complex bearings.
Helium stars are plunged, without any known exception, in abysmal depths
of space. None have been found within a radius measured by about seventy
years of light-travel. They frequent a sidereal region different from
ours, where nebulæ linger and stars with blazing chromospheres have
their habitat.