Chapter V: Solar Stars

The transition from hydrogen to solar stars is effected as gradually as the transition from helium to hydrogen stars. Metallic absorption comes more and more to the front in successive objects, while the Huggins series retires into the background. There are no definite stopping-places; the course of change flows on continuously. At a certain stage of progress, however, the characters distinctive respectively of the condition that has been, and of the condition that is about to be, appear evenly balanced. The hydrogen lines, although reduced to about one-quarter their Sirian intensity,[379] still muster strong even in the ultra-violet, the metallic spectrum being at the same time pronounced and crowded. This medium state can be studied to advantage in Procyon, the lesser Dog star. So perfect is the blend of types shown by it, that Professor Pickering found it difficult to decide whether the spectrum was actually intermediate, or combined the Sirian and the solar light of two separate, but closely conjoined stars.[380] Either alternative is possible, but the former is the more probable. Nevertheless, the presence of the full complement of Huggins lines, together with a K-band of ten-fold the intensity possessed by it in Castor, must be regarded as somewhat anomalous.

Procyon is one of our nearer neighbours in space. Dr. Elkin has measured for it a parallactic shift of 0·325″, corresponding to a light-journey of ten years. And the revolutions of a faint companion complete the data requisite for finding the mass of the system. It comes out 2·7 times that of the sun, and we cannot be far wrong in assigning to the brilliant component twice the solar quantum of matter. It gives nearly quadruple the solar light; yet the disparity between light and mass is notably reduced from the Sirian standard. Absorption has increased as condensation has progressed. The rays of Procyon are perceptibly tinted with yellow.

A similar spectrum is shown by the splendid Canopus. The extreme remoteness of this orb, which is second only to Sirius in apparent lustre, compels us to attribute to it a prodigious real light-power. It has no measurable parallax, and no sensible proper motion. Only a minimum estimate then of its magnitude is practicable. Sir David Gill attached to the zero representing its parallax a “probable error” of 0·011″. Hence the measures executed do not exclude a parallax of this amount, although they are just as consistent with an equal negative value. Canopus then may be no further off, but cannot be nearer than a light-journey of 296 years. Admitting, for the sake of illustration, that it is in fact at this distance, which is thirty times that of Procyon, we obtain the astonishing result that it gives no less than 3600 times its radiance. And since the spectra of the two stars agree nearly line for line, this figure must represent approximately the ratio of their photospheric areas, that of their cubical contents being 216,000 to one. In other words, 216,000 bodies of Procyon’s size would go to make up one such globe as the star of prehistoric Egypt. Yet Procyon, as we have seen, is a sun constructed on a larger scale than our own. The existence of a luminary so vast as Canopus, although bewildering to imagination, need not appear incredible when we consider the immense scope of creation, and the boundless resources variously displayed throughout the ethereal spaces populous with stars.

Another interesting specimen of the Procyon variety is γ Cygni. Visually of 2·3, it is only of 3·2 photographic magnitude. This implies blue absorption to an extent unusual in the presence of the ultra-violet hydrogen series. It is accompanied by a disproportionately strong K, well brought out in Mr. McClean’s spectrograms.[381] The star resembles α Cygni in the definite character of its lines, although their chemical meanings are very different. They have, however, as yet been most imperfectly deciphered.[382] The spectroscopic relations of γ Cygni derive added importance from its apparent connection with a far-spreading galactic nebulosity photographed by Wolf and Barnard. But the star may be merely seen in projection upon it. The peculiarities of its light recur with less accentuation in that of Polaris.

A very close approach is made to the solar spectrum by χ Orionis; virtual identity is reached by Capella, η Boötis, and α_{2} Centauri. It is scarcely compromised in Arcturus, or any of its numerous associates in Group xv.; the same lines subsist, only drawn somewhat more heavily, and there is an added shade of ultra-violet absorption. The steadiness with which the solar type is maintained, all but unmodified throughout a large collection of objects, is very remarkable. Of the 681 bright stars investigated by Miss Maury, 19 are Capellans, 111 Arcturians; the latter are barely distinguishable one from the other, the former only by the finest grades of difference.[383] This seems to indicate a particularly stable phase of stellar existence. Our sun’s constitution, we can infer, is adjusted to a high degree of permanence; he is moving along a nearly level tract of his evolutionary journey, and will decline with extreme slowness from his actual state.

Solar stars are to be found of all sizes, their variety in this respect forming an instructive commentary upon their spectral similarity. Consider Arcturus. Dr. Elkin, from a long series of skilfully planned observations, assigned to it in 1897 a parallax so small (0·024″) that its light cannot reach us in less than 136 years. And since at this abysmal remoteness it outshines the sun’s twin, α_{2} Centauri, by one-third of a magnitude, the actual excess of its brightness must be at the very least thirteen hundred-fold. In view, then, of its spectral identity, Arcturus may confidently be asserted to possess a photosphere 1300 times more extensive than the sun’s. The globe it encompasses is, accordingly, about 47,500 times more voluminous, and in the same proportion (assuming equal mean densities) more massive. It follows that gravity exercises over the surroundings of Arcturus thirty-six times its solar power. Yet its spectrum bears no trace of sensibly augmented pressure. We are confronted everywhere in sidereal physics with this seeming inconsistency between the nominal force of gravity and its effective action.

Pollux (β Geminorum) conforms strictly to the spectral pattern of Arcturus. It is, however, a full magnitude fainter and at only half its distance; it must accordingly be a much smaller body. Its superficial area is, in fact, one-tenth that of Arcturus. Nevertheless it contains fifteen times more matter than the sun, and gravity at the surface of Pollux has more than eleven-fold its solar power. Planets revolving round this star would have, at the same distances, periods about one-quarter the length of those belonging to the earth and its sister worlds, our year, for instance, being reduced to ninety days, so that a whole summer would be consumed in a brief holiday excursion.

But there are small as well as large solar stars. An insignificant object in the Great Bear, catalogued as “Groombridge 1618,” and noted for its rapid proper motion, is, according to Sir Robert Ball’s measures, comparatively near the earth, its light reaching us in 10·2 years. The sun, however, in that position would be four and a half magnitudes brighter, for it radiates fifty times more powerfully. The spectrum of 1618 Groombridge is of the Arcturian sub-class, so that the proportion of its mass may, under reserve, be taken to follow the proportion of its light. About 350 such stars, then, should be put into the scale to balance one sun, and gravity at its surface has one-seventh its value at the photospheric level. Another minor sun is “Bradley 3077” in Cassiopeia, although the inferiority is here slighter, since Bradley’s star emits perhaps ten times more copiously than Groombridge’s. Further examples of the kind will certainly come to be known when some progress has been made with the investigation of faint spectra. But this is most baffling work, subject to the illusions that everywhere haunt the limits of distinct visibility.

Enough has been said to make it clear that the Fraunhofer spectrum is exactly copied in orbs of most various dimensions. This points, in Dr. Scheiner’s opinion,[384] to the closest agreement, not only in the percentage of the chemical elements entering into their composition, but also in conditions of temperature and pressure. How such uniformity can be combined with widely different gravitational constants, is extremely hard to understand. The Tulse Hill experiments, already referred to, showed the predominant influence of pressure in altering spectral characters. Since, then, they are the same in Arcturus and Groombridge 1618, there is practical certainty that the calcium envelopes (for instance) of both stars do not differ appreciably in tenuity. Yet the compulsive force acting upon one is 252 times more powerful than that exerted on the other. The persuasion that it is somehow neutralised is irresistible. We might even venture tentatively to define solar stars as bodies in which the ratio is the same between gravity and electrical repulsion. In the course of time, doubtless, it will change; one or the other force will gain relatively to the other, and the spectral type will vary to correspond. Presumably the augmentation of strength will be on the attractive side; but cosmical electricity is still an unexplored region.

The symptoms of approach towards the fluted description of spectrum set in gradually, and are of two kinds. General absorption of the more refrangible rays spreads and deepens, and specific absorption becomes intensified in certain dusky lines. Conspicuous among these is the “blue” line of calcium (λ 4227), the stress laid upon which unquestionably signifies increased density in the absorbing vapour. This is just what might be expected to accompany the progress of cooling and contraction, through which _domestic_ gravity gains advantage, as acting in a steadily narrowing sphere. The symptoms described are visible in α Hydræ and β Cancri; they are particularly well marked in Aldebaran. The last is a glaringly red star, its blue emissions being mostly arrested by its own atmosphere. Incipient flutings, too, are traceable. It is the “type star” of Miss Maury’s sixteenth group, which includes twenty-three objects scarcely to be discriminated as regards the quality of their light. “From the greatly-increased width in them,” she writes[385] of the line at λ 4227, “it would appear to be complex, and to include lines weak or absent in the stars of the solar type.”

Aldebaran has a parallax of one-tenth of a second, and is of standard first magnitude. Its real brightness is then certainly twenty-eight times greater than that of the sun; and since it has suffered much more heavily from absorptive encroachments, the emitting surface must proportionately exceed the spread of the photosphere. Even apart from any allowance for increased density, the Taurus luminary may be considered by a quite moderate estimate to be of 200 times the solar mass. On the other hand, the primary member of the famous pair, 61 Cygni, which as to spectrum is a faint duplicate of Aldebaran, ranks very low in the hierarchy of suns, emitting, in fact, only ¹⁄₃₆₄ the light of that great red orb. Here, again, the lesson is enforced that the widest variety of size and mass may consist with spectral identity. Aldebaran is encompassed by gaseous strata apparently no denser and no hotter than the absorbing layers in 61 Cygni. This circumstance is evidently of vital moment in stellar natural history.