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.