Chapter III: 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.

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.