Chapter IV: Hydrogen Stars

Helium and hydrogen stars cannot be quite definitely set apart. The two classes commingle. A transition specimen of uncommon interest is found in η Leonis, which combines some of the peculiarities of α Cygni with a powerful development of the hydrogen series.[372] Another is presented by θ Aquilæ, placed by Mr. McClean beside such clearly characterised helium stars as Algol and Pleione, but by Miss Maury in the group with Sirius and Vega. Even Vega, although a perfectly normal member of the hydrogen class, preserves a vestige of helium absorption in the typical “Orion line,” λ 4472; and its frequent companion, λ 4026, emerges to view in stars like ζ Aquilæ, in which hydrogen approaches a maximum of strength. Both these lines are in the laboratory immediately associated with D_{3}, as members of the first subordinate series of the “yellow” helium set. None of the special Wolf-Rayet lines occur in hydrogen stars; the Pickering series is unrepresented; the “blue bands” have no dark counterparts; oxygen and nitrogen lines seem to have died out. Between the broad black bars of the Huggins series, however, crowds of ghost-like metallic rays are discernible. More than one hundred and thirty of these nascent markings were counted by Miss Maury in the photographed spectrum of Sirius, and her search, owing to the limited range of the plates, could only be partial.

Sirius is perhaps a slightly “older” star than Vega. Helium has entirely disappeared from its spectrum, and more familiar elements take its place—sodium, iron, magnesium, calcium, silicon, with titanium, vanadium, barium, and perhaps chromium and nickel.[373] A remarkable group of lines high up in the ultra-violet is of unsurmised origin. Photographed by Sir William and Lady Huggins 4th April 1890,[374] their approximate wave-lengths are λλ 3338, 3311, 3278, 3254, 3226, 3199, while the head of the hydrogen series stands at λ 3646. They are accordingly more refrangible than any possible hydrogen line; nor could impressions of them be obtained with apparatus including glass prisms or lenses, for which reason they are to be found only on the Tulse Hill spectrographs. The interpretation of these recondite characters offers an alluring problem. Although absent from the light of Vega, they will doubtless be recognised, when duly sought, in other Sirian stars; but exposures of the requisite kind are laborious, and seldom undertaken. Yet just such special investigations are likely to be the most fruitful.

Sirius is the best-known luminary of its class. This for two reasons. First, because of its vicinity. Light travels from the star to the earth in rather less than nine years. Next, because of our fairly complete acquaintance with the nature of its binary revolutions. They have now been closely observed during forty years, and their period is fifty-two. Hence the mass of the system has been determined, and it has, moreover, been apportioned with satisfactory exactness between the members. Their disparity in gravitative power proves to be small compared with their enormous inequality in lustre. The companion is a mere point of light outshone 36,000 times by its radiant primary, which is, nevertheless, more massive only in the proportion of 2·36 to 1·1. This quasi-obscurity of the Sirian satellite is very curious; but our present concern is with the majestic orb, in the blaze of which it is almost lost to view. From Dr. See’s orbital elements, combined with Sir David Gill’s parallax, a mass is deduced for it just two and a half times that of the sun. If, then, it were a body of the same average density and surface luminosity it should give nearly twice (1·84 times) as much light. In actual fact Sirius is of at least twenty-one times the solar brilliancy. This estimate is arrived at by taking α_{2}, the brighter component of α Centauri, as an intermediary. Sir David Gill has shown that its spectrum is a replica of the solar spectrum; its revolutions prove it to be of equal mass with the sun, and it is hence assumed with the highest probability to emit sensibly the same amount of light. It may accordingly, in comparisons of stellar brightness, be substituted for the sun, the uncertainty attending the direct confrontation of enormously unequal light-sources being thus avoided. Now the distances from the earth, and the photometric magnitudes both of α_{2} Centauri and of Sirius, are well known, so that it is easy to calculate how one star would appear in the place of the other. Sirius is twice as far off as the southern binary; transferred to that remoteness α_{2} Centauri would then show one-quarter its present brilliancy; it would be of 1·9 magnitude, just matching the chief star in the Plough. It would accordingly be 3·3 magnitudes fainter than Sirius, which is as much as to say that it gives only ¹⁄₂₁ part of its light. And the sun, similarly located, would be of the same faintness.

Thus Sirius, while two and a half times more massive, is twenty-one times more luminous than the sun. Or, putting it otherwise, the solar ratio of light to englobed matter is exceeded more than eleven-fold. Three causes may concur to produce this effect. One of them we know to be present. It is quite certain that the Sirian beams are almost undimmed by self-absorption, whereas those of the sun are reduced probably to one-third their original intensity. A second contributory cause to the brilliancy of this star may be found in its great bulk. It is likely to be much less condensed than the sun, consequently to possess a much larger extent of photosphere relatively to mass. A luminous area multiplied four times would explain the outstanding disparity of brightness, but would involve a reduction of mean density to one-eighth the solar standard, or about one-sixth that of water. There remains the third factor of absolute areal brilliancy; but its value presumably depends upon temperature, and comparative stellar temperatures must for the present be left an open question. We are only able to conclude that _if_ Sirius and the sun be on a par as to intrinsic shining power, then the star is probably about eight times more tenuous.

Vega is so much more remote than Sirius that it may safely be stated to quadruple its emissions. Its mass, however, remains undetermined, since it sways no detected companion with a measurable force. That it is small compared with its light can hardly be doubted. Indeed, throughout the hydrogen class, this rule prevails to all appearance universally.

The temperature of the stars, as already remarked, is one on which dogmatic assertions are best avoided. All authorities agree nevertheless that the conditions governing light-production in such orbs as Sirius and Vega approximate in many important respects to those present in a disruptive electric discharge. One important item of evidence to this effect is the prominence in spectra of this class of metallic lines weak in the arc, but strong in the spark. “The general result,” Sir Norman Lockyer says,[375] “of the investigation of the enhanced iron lines in stellar spectra confirms the view that the absorbing regions of the hottest stars exist at a higher temperature than is attainable in laboratory experiments.” Concurrent testimony was derived from variations of relative intensity in the magnesium and calcium lines shown by particular stars. But no allowance was made for modifications resulting from differences of pressure, which the Tulse Hill researches had proved to be highly influential. Hence the absence in Sirian stars of the “blue” line of calcium (λ 4227) tells nothing by itself as to their temperature. The special value of Dr. Scheiner’s magnesium-test is that the _opposite_ behaviour of the two lines considered (λ 4481 and λ 4352) excludes the density-factor. For increase of heat may occasion the weakening of individual lines concurrently with the strengthening of others; but changes of pressure must always act in the same direction—though not necessarily to the same extent—on every element of the spectrum affected by them.[376] Professor Keeler[377] suggested that by means of the magnesium triplet, _b_, inferences as to temperature in stars might be extended to grades beyond the possibility of artificial production. This group is conspicuous alike in the flame, arc, and spark; it cannot be experimentally abolished, yet it fails (as we have seen) to appear in Rigel, and emerges very feebly in Sirius and Vega. Now it belongs to a subordinate series due to a special molecular arrangement, which could not easily persist in an extreme stage of heat. And a break-up of the arrangement would be marked by the effacement of the triplet in the green. “If this reasoning is correct,” Professor Keeler wrote, “the aspect of the _b_-lines in stellar spectra gives us an extension of the method proposed by Scheiner, and it shows that the temperature of certain stars exceeds that of the most powerful electric spark.”

The long range of powerful hydrogen lines in Sirian spectra, on the other hand, cannot be regarded as a sure symptom of excessive heat. It seems rather to indicate an approach to homogeneity in the originating stratum. The abridgment and enfeeblement of the series, and the development of metallic absorption, follow the same course, which is certainly not prescribed by thermal change. This inverse relation has not so far been satisfactorily accounted for. The most plausible hypothesis regarding it is that of Sir William and Lady Huggins, who connect it with the inevitable gain of effective gravity in condensing globes.

Nor can the intensity of the higher spectral sections be taken as an unequivocal sign that hydrogen stars are hotter than the sun. For it may be caused not by the intrinsic emissive superiority of their photospheres, but by their unveiled condition. We know that the sun’s more refrangible rays would, through the removal of his absorbing atmosphere, acquire strength enough to turn the balance of colour from yellowish to bluish, and it is amply possible that its spectrum, displayed to equal advantage with that of Vega, might rival or outdo its actinic compass.

Fomalhaut (α Piscis Austrini) is a fine example of an advanced hydrogen star. Metallic lines are considerably more developed in it than in Vega or Sirius; the McClean spectrograms show a profoundly grooved K-line, and its blue associate (λ 4227) is faintly reversed. The “spark ray” of magnesium (λ 4481) is prominent. Miss Maury prints a table of wave-lengths[378] measured from the Harvard spectrograms of this star which deserves particular attention as a record, perhaps, of a transition epoch in stellar growth. Lines of iron, titanium, and silicon are readily identifiable in it, but most of the entries have no obvious meaning. Fomalhaut is of 1·3 magnitude, and Sir David Gill has determined for it a parallax of 0·13″ showing it to be almost six times more remote than α Centauri. Its real brightness is hence easily found to be fourteen and a half times that of the sun. Its mass and density, however, remain entirely unknown.

As illustrating the physical differentiation of bodies to all appearance chemically similar, two stars may be singled out. These are Castor and γ Ursæ Majoris, the third of the Plough. Both are included in Miss Maury’s Group viii., and both show deep and broad furrows of hydrogen. The spectra, in fact, bear the same inscription, only printed from dissimilar types. In γ Ursæ, the spectrum of Castor is viewed, as it were, out of focus. The lines distinct in the one are hazy and diffuse in the other; none probably are really missing, though a good many are effaced by expansion. This peculiarity is met with in a considerable number of helium and hydrogen stars forming Miss Maury’s “division _b_.” In “division _c_,” on the contrary, of which α Cygni is the best exemplar, the lines are notably sharp and narrow, while in “division _a_” they are of normal appearance, some thin, others fringed or winged. The cause of these variations is obscure. It would naturally be connected with differences of pressure in the stellar reversing layers; and this again must depend in great measure upon the locus of absorption, which probably varies, not only from star to star, but for each separate substance in the same star. So that the conditions to be regarded, even from this point of view alone, are highly complex.