[Footnote 1: "Determination of the Relative Positions of the
Principal Stars in the Group of the Pleiades." By William L.
Elkin. Transactions of the Astronomical Observatory of Yale
University, Vol. I., Part I. (New Haven: 1887.)]
The Messrs. Repsold have established, and for the present seem likely
to maintain, a practical monopoly in the construction of heliometers.
That completed by them for the observatory of Yale College in 1882
leaves so little to be desired as to show excellence not to be the
exclusive result of competition. In mere size it does not indeed take
the highest rank. Its aperture is of only six inches, while that of
the Oxford heliometer is of seven and a half; but the perfection of
the arrangements adapting it to the twofold function of equatorial and
micrometer stamps it as a model not easy to be surpassed. Steel has
been almost exclusively used in the mounting. Recommended as the
material for the objective cell by its quality of changing volume
under variations of temperature nearly _paripassu_ with glass, its
employment was extended to the telescope tube and other portions of
the mechanism. The optical part of the work was done by Merz, Alvan
Clark having declined the responsibility of dividing the object lens.
Its segments are separable to the extent of 2°, and through the
contrivance of cylindrical slides (originally suggested by Bessel)
perfect definition is preserved in all positions, giving a range of
accurate measurement just six times that with a filar micrometer.
(Gill, "Encyc. Brit.," vol. xvi., p. 253; Fischer, _Sirius_, vol.
xvii., p. 145.)
This beautiful engine of research was in 1883 placed in the already
practiced and skillful hands of Dr. Elkin. He lost no time in fixing
upon a task suited both to test the powers of the new instrument and
to employ them to the highest advantage.
The stars of the Pleiades have, from the earliest times, attracted the
special notice of observers, whether savage or civilized. Hence, on
the one hand, their prominence in stellar mythology all over the
world; on the other, their unique interest for purposes of scientific
study and comparison. They constitute an undoubted cluster; that is to
say, they are really, and not simply in appearance, grouped together
in space, so as to fall under the sway of prevailing mutual
influences. And since there is, perhaps, no other stellar cluster so
near the sun, the chance of perceptible displacements among them in a
moderate lapse of time is greater than in any other similar case.
Authentic data regarding them, besides, have now been so long garnered
that their fruit may confidently be expected at least to begin to
ripen.
Dr. Elkin determined, accordingly, to repeat the survey of the
Pleiades executed by Bessel at Konigsberg during about twelve years
previous to 1841. Wolf and Pritchard had, it is true, been beforehand
with him; but the wide scattering of the grouped stars puts the filar
micrometer at a disadvantage in measuring them, producing minute
errors which the arduous conditions of the problem render of serious
account. The heliometer, there can be no doubt, is the special
instrument for the purpose, and it was, moreover, that employed by
Bessel; so that the Konigsberg and Yale results are comparable in a
stricter sense than any others so far obtained.
One of Bessel's fifty-three stars was omitted by Dr. Elkin as too
faint for accurate determination. He added, however, seventeen stars
from the Bonn _Durchmusterung_, so that his list comprised sixty-nine,
down to 9.2 magnitude. Two independent triangulations were executed by
him in 1884-85. For the first, four stars situated near the outskirts
of the group, and marking the angles of quadrilateral by which it was
inclosed, were chosen as reference points. The second rested upon
measures of distance and position angle outward from Alcyone ([eta]
Tauri). Thus, two wholly unconnected sets of positions were secured,
the close accordance of which testified strongly to the high quality
of the entire work. They were combined, with nearly equal weights, in
the final results. A fresh reduction of the Konigsberg observations,
necessitated by recent improvements in the value of some of the
corrections employed, was the preliminary to their comparison with
those made, after an interval of forty-five years, at Yale College.
The conclusions thus laboriously arrived at are not devoid of
significance, and appear perfectly secure, so far as they go.
It has been known for some time that the stars of the Pleiades possess
a small identical proper motion. Its direction, as ascertained by
Newcomb in 1878, is about south-southeast; its amount is somewhat less
than six seconds of arc in a century. The double star 61 Cygni, in
fact, is displaced very nearly as much in one year as Alcyone with its
train in one hundred. Nor is there much probability that this slow
secular shifting is other than apparent; since it pretty accurately
reverses the course of the sun's translation through space, it may be
presumed that the _backward_ current of movement in which the Pleiades
seem to float is purely an effect of our own _onward_ traveling.
Now the curious fact emerges from Dr. Elkin's inquiries that six of
Bessel's stars are exempt from the general drift of the group. They
are being progressively left behind. The inference is obvious that
they do not in reality belong to, but are merely accidentally
projected upon, it; or, rather, that it is projected upon them; for
their apparent immobility (which, in two of the six, may be called
absolute) shows them with tolerable certainty to be indefinitely more
remote--so remote that the path, moderately estimated at
21,000,000,000 miles in length, traversed by the solar system during
the forty-five years elapsed since the Konigsberg measures dwindles
into visual insensibility when beheld from them. The brightest of
these six far-off stars is just above the eighth (7.9) magnitude; the
others range from 8.5 down to below the ninth.
A chart of the relative displacements indicated for Bessel's stars by
the differences in their inter-mutual positions as determined at
Konigsberg and Yale accompanies the paper before us. Divergences
exceeding 0.40" (taken as the limit of probable error) are regarded as
due to real motion; and this is the case with twenty-six stars besides
the half dozen already mentioned as destined deserters from the group.
With these last may be associated two stars surmised, for an opposite
reason, to stand aloof from it. Instead of tarrying behind, they are
hurrying on in front.
An excess of the proper movement of their companions belongs to them;
and since that movement is presumably an effect of secular parallax,
we are justified in inferring their possession of an extra share of it
to signify their greater proximity to the sun. Hence, of all the stars
in the Pleiades these are the most likely to have a measurable annual
parallax. One is a star a little above the seventh magnitude,
distinguished as _s_ Pleiadum; the other, of about the eighth, is
numbered 25 in Bessel's list. Dr. Elkin has not omitted to remark that
the conjecture of their disconnection from the cluster is confirmed by
the circumstance that its typical spectrum (as shown on Prof.
Pickering's plates) is varied in _s_ by the marked character of the K
line. The spectrum of its fellow traveler (No. 25) is still
undetermined.
It is improbable, however, that even these nearer stars are
practicable subjects for the direct determination of annual parallax.
By indirect means, however, we can obtain some idea of their distance.
All that we want to know for the purpose is the _rate_ of the sun's
motion; its _direction_ we may consider as given with approximate
accuracy by Airy's investigation. Now, spectroscopic measurements of
stellar movements of approach and recession will eventually afford
ample materials from which to deduce the solar, velocity; though they
are as yet not accurate or numerous enough to found any definitive
conclusion upon. Nevertheless, M. Homann's preliminary result of
fifteen miles a second as the speed with which our system travels in
its vast orbit inspires confidence both from the trustworthiness of
the determinations (Mr. Seabroke's) serving as its basis and from its
intrinsic probability. Accepting it provisionally, we find the
parallax of Alcyone = about 0.02', implying a distance of
954,000,000,000,000 miles and a light journey of 163 years. It is
assumed that the whole of its proper motion of 2.61' in forty-five
years is the visual projection of oar own movement toward a point in
R.A. 261°, Decl. +25°.
Thus the parallax of the two stars which we suspect to lie between us
and the stars forming the genuine group of the Pleiades, at perhaps
two-thirds of their distance, can hardly exceed 0.03'. This is just
half that found by Dr. Gill for [xi] Toucani, which may be regarded
as, up to this, the smallest annual displacement at all satisfactorily
determined. And the error of the present estimate is more likely to be
on the side of excess than of defect. That is, the stars in question
can hardly be much nearer to us than is implied by an annual parallax
of 0.03", and they may be considerably more remote.
Dr. Elkin concludes, from the minuteness of the detected changes of
position among the Pleiades, that "the hopes of obtaining any clew to
the internal mechanism of this cluster seem not likely to be realized
in an immediate future;" remarking further: "The bright stars in
especial seem to form an almost rigid system, as for only one is there
really much evidence of motion, and in this case the total amount is
barely 1 per century." This one mobile member of the naked eye group
is Electra; and it is noticeable that the apparent direction of its
displacement favors the hypothesis of leisurely orbital circulation
round the leading star. The larger movements, however, ascribed to
some of the fainter associated stars are far from harmonizing with
this preconceived notion of what they ought to be.
On the contrary, so far as they are known at present, they force upon
our minds the idea that the cluster may be undergoing some slow
process of disintegration. M. Wolf's impression of incipient
centrifugal tendencies among its components certainly derives some
confirmation from Dr. Elkin's chart. Divergent movements are the most
strongly marked; and the region round Alcyone suggests, at the first
glance, rather a very confused area of radiation for a flight of
meteors than the central seat of attraction of a revolving throng of
suns.
There are many signs, however, that adjacent stars in the cluster do
not pursue independent courses. "Community of drift" is visible in
many distinct sets; while there is as yet no perceptible evidence,
from orbital motion, of association into subordinate systems. The
three eighth-magnitude stars, for instance, arranged in a small
isosceles triangle near Alcyone, do not, as might have been expected
_a priori_, constitute a real ternary group. They are all apparently
traveling directly away from the large star close by them, in straight
lines which may, of course, be the projections of closed curves; but
their rates of travel are so different as to involve certain
progressive separation. Obviously, the order and method of such
movements as are just beginning to develop to our apprehension among
the Pleiades will not prove easy to divine.--_A.M. Clerke, in Nature._
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