|
Main
- books.jibble.org
My Books
- IRC Hacks
Misc. Articles
- Meaning of Jibble
- M4 Su Doku
- Computer Scrapbooking
- Setting up Java
- Bootable Java
- Cookies in Java
- Dynamic Graphs
- Social Shakespeare
External Links
- Paul Mutton
- Jibble Photo Gallery
- Jibble Forums
- Google Landmarks
- Jibble Shop
- Free Books
- Intershot Ltd
|
books.jibble.org
Previous Page
| Next Page
Page 45
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.
Previous Page
| Next Page
|
|