Chapter XVI: Rotation of the Stars

Sir William Abney adverted, in 1877,[531] to the theoretical effects of rotation on stellar spectra. Quite obviously, they must tend to make the component lines wide and diffuse. For each line integrates the displacements and counter-displacements occasioned by the opposite radial movements of the limbs; while the central and polar sections of the disc, having their velocities directed across the line of sight, send out rays in their normal positions, fringed on either side through the juxtaposition of the shifted rays. The amount of broadening in each particular star depends, first, upon the linear speed of rotation, secondly, upon the position of its axis. If this be erect as viewed from the earth, the motion-shifts will tell to their full extent in spreading the bright or dark spectral lines; they will become less and less effective as the axis is less inclined, and will disappear wholly on its coincidence with the visual ray. Now there can be no doubt that every star has a movement of gyration as well as a movement of translation; and it is no less certain that stellar spectra are modified in accordance with its rapidity and direction. Only the question of degree has to be considered. Are the effects produced likely to be appreciable? And if so, have they been perceived?

The spectrum of α Aquilæ (Altair), noticed for some time back as peculiar, has sometimes been thought to intimate a composite origin. It is of the Sirian type, but with a reinforced contingent of metallic lines; and these run together into hazy bands, the general aspect of which was imitated at Potsdam in 1895 in spectrographs of the sun taken out of focus.[532] The defective nature of the agreement, however, discredited the hypothesis of a double spectrum, marked by diffuse hydrogen absorption proceeding from one source, and by metallic lines _fused_ into bands, proceeding from another. Yet M. Deslandres considered that his measures of the star’s radial motion lent it support. They seemed to indicate velocity variable in a period of forty-two days, with minor fluctuations superadded.[533] But the supposed multiple system is, according to Dr. Vogel, a mere creation of accidental errors,[534] and α Aquilæ must for the present, at any rate, be counted a solitary star.

Its spectrum was, in 1893, commented upon by Professor Pickering.[535] He had recourse, for the explanation of its ill-defined character, to the rotational principle, adding a caveat based on the improbable greatness of the required velocity of about 100 miles per second. Adopted, nevertheless, five years later by Dr. Vogel, it was rendered more plausible by his reduction to 27 kilometres (16·8 miles) of the equatorial speed needed to widen the lines to the observed extent. This rate of movement, which is just double that of a point on Jupiter’s equator, might reasonably be admitted as subsisting in a star. But the view encounters other, and more fundamental objections. If it were true, _all_ the lines in the affected spectrum should be similarly diffuse. Movement acts indiscriminately. Every ray emanating from the advancing or receding surface is, in due measure, displaced. None can be exempt from change of refrangibility. The occurrence, then, of a single sharp line in a stellar spectrum suffices to show that the haziness of its associates must be due to some other cause than rotation. And there are many sharp lines in the spectrum of α Aquilæ. They are faintly discernible, as Sir Norman Lockyer pointed out in 1894,[536] on the South Kensington plates, and are unmistakably apparent in Sir William and Lady Huggins’s spectrographs.[537] Those taken at Potsdam are so limited in range of wave-length that negative conclusions cannot safely be founded on them. The hypothesis of rotation must, accordingly, be regarded as inapplicable to the case of α Aquilæ.

Now α Aquilæ is not without analogues. It belongs to a pretty numerous stellar group, differing in chemical constitution, but agreeing in the diffuseness of the absorption traits significant of it. They form one of Miss Maury’s three collateral series—her “Division _b_.” It embraces no “advanced” stars; only those of the helium and hydrogen types, with a few verging towards the intermediate stage of Procyon, are represented in it. Hazy spectra are thus a sign of cosmic youth. They characterise, without exception, the stars of Miss Maury’s “Group i.,” in which the Pickering series of hydrogen is prominent; they cease to appear, or appear by imperfect indications, soon after the Sirian stage is passed. Their explanation by opposite displacements through axial movement would then involve the consequence that stars, as they develop, lose much of their rotational speed. There is, however, but one recognised agency by which it can be retarded—the agency of tidal friction; and it acts sensibly only on bodies attended by closely-revolving satellites of considerable relative mass. Solitary suns like our own can have spent but little of their energy of rotation. Actual velocity in spinning becomes, in fact, accelerated as contraction proceeds, so that ageing stars should have their spectral lines more broadened by motion than those in a primitive condition. And since the effect is imperceptible in the former, we may feel assured that it has not been observed in the latter.

Confirmatory evidence is not wanting. There is a certain class of stars which, we have the strongest reason to believe, rotate in very short periods, and on axes almost perpendicular to the line of sight. They combine, accordingly, both the conditions needed for the display of spectra rendered diffuse by motion-shifts. These are occulting variables like Algol. Since they revolve in planes passing very nearly through the earth, and their equators cannot deviate materially from the same level, it is certain that virtually the whole speed of their advancing and receding limbs is radially directed; no considerable part of it is spectroscopically ineffective. Further, although they may rotate faster, they cannot rotate more slowly than they revolve, and their orbital periods are extraordinarily short. The system of Algol, which is by no means one of the quickest eclipsing pairs, circulates in sixty-nine hours. Its equatorial rate of rotation, by a minimum estimate, is thirteen miles a second, or just eleven times the solar. The absorption rays in the light from one limb are accordingly displaced towards the blue, and those from the opposite limb towards the red, eleven times more than the Fraunhofer lines measured by Young and Dunér; and their compounded effect in the stellar spectrum is to widen the lines by an amount corresponding to a speed of twenty-six miles. In other words, each should spread over nearly one quarter the interval between the D-lines in the sun. The alteration is, nevertheless, inconspicuous. The spectrum of Algol does not strike the eye as hazy. The hydrogen series shows the distension proper to the type, no more; the rays of helium, magnesium, and calcium are of the average sharpness. Now some eclipsing stars must rotate much more rapidly than Algol. U Ophiuchi, for instance, has a period of only twenty hours. Yet in none of them have blurred spectra been noticed. Enormous velocities—velocities most probably non-existent—would evidently be indispensable for their production.

Such spectra as that of α Aquilæ must then be accounted for otherwise than by rotation. For the suggested geometrical cause, which proves inadequate, a physical cause has to be substituted. One may be found in excessive pressure. The diffuse lines possibly originate at unusual depths in the stellar atmospheres. Sir William and Lady Huggins advert[538] to the probability of great differences in this respect between various stellar classes. In early stars they say—and none of the members of “Division _b_” are mature—“we may see deep down into the star, and the continuous spectrum may come from a thick region of dense gas, throughout which little, or possibly no condensation to the liquid or the solid state takes place. Under these conditions, the absorbing gases in front of it will not be, as in the sun, of very limited thickness, but will occupy a region of vast extent.”

The solar H and K illustrate the character of lines generated in dense vapours at a high temperature; their “wings,” as we may remind our readers, being added in the immediate vicinity of the photosphere to the comparatively definite lines produced in the upper reversing strata. Now it is a curious fact that distended lines, such as H and K, are apt to be doubly reversed. Dr. Scheiner has noticed symptoms of incipient illumination at the centres of the broad hydrogen bands distinctive of first-type stars, and they are similarly manifest in Wolf-Rayet stars showing mixed series of emission and absorption. Hence the particular significance of M. Deslandres’ detection in α Aquilæ of fine “chromospheric” lines of hydrogen, and occasionally of calcium and iron,[539] superposed upon the dim, dusky bands indicative of the state of those substances in the reversing layer. Diffuse spectra may thus, with some probability, be assigned to abortive bright-line stars. Or they perhaps mark objects just losing the faculty of specific emission. If so, the mode of its departure is different from that exemplified by Alcyone, in which the dark lines have their normal aspect, while one red ray survives as the sole remnant of what was perhaps once a blazing spectrum. The future course of stars resembling α Aquilæ can be traced only by conjecture. But what hints are at hand lead to the supposition that they will proceed by insensible gradations to the solar stage, their absorption rays becoming narrower, more numerous, and better defined with the slow advance of condensation.

The upshot of our inquiry is to bring the conviction that no approach has yet been made towards determining the rotation of any star. Spectra are not rare composed mainly of blurred lines, and so suggesting at first sight diffusion on the principle of movement; the intermixture of sharp lines, visible on closer scrutiny, nevertheless peremptorily negatives the suggestion. Again, rotational velocity in the line of sight should be at a maximum in Algol variables; yet they do not possess specially diffuse spectra. Theory, however, need not be at fault because it fails to be verified by observation. The failure merely informs us that its consequences, by their smallness, elude our means of discovery.