We may assume that the stars are all in motion. It is true that only
a comparatively small number of stars have been actually seen to
be in motion; but as some motion exists in nearly every case where
observations would permit of its being determined, we may assume the
rule to be universal. Moreover, if a star were at rest at one time it
would be set in motion by the attraction of other stars.
Statements of the motion from different points of view illustrate in
a striking way the vast distance of the stars and the power of modern
telescopic research. If Hipparchus or Ptolemy should rise from their
sleep of 2,000 years--nay, if the earliest priests of Babylon should
come to life again and view the heavens, they would not perceive any
change to have taken place in the relative positions of the stars. The
general configurations of the constellations would be exactly that to
which they were accustomed. Had they been very exact observers they
might notice a slight difference in the position of Arcturus; but as a
general rule the unchangeability would have been manifest.
In dealing with the subject, the astronomer commonly expresses the
motion in angular measurement as so many seconds per year or per
century. The keenest eye would not, without telescopic aid, be able to
distinguish between two stars whose apparent distance is less than 2′
or 120″ of arc. The pair of stars known as (ε) Lyræ are 3′ apart; yet,
to ordinary vision they appear simply as a single star. To appreciate
what 1″ of arc means we must conceive that the distance between
these two stars is divided by 200. Yet this minute space is easily
distinguished and accurately measured by the aid of a telescope of
ordinary power.
On the other hand, if we measure the motions by terrestrial standards
they are swift indeed. Arcturus has been moving ever since the time of
Job at the rate of probably more than 200 miles per second--possibly
300 miles. Generally, however, the motion is much smaller, ranging
from an imperceptible quantity up to 5, 10 or 20 miles a second. Slow
as the angular motion is, our telescopic power is such that the motion
in the course of a very few years (with Arcturus the motion in a few
days) can be detected. As accurate determinations of positions of the
stars have been made only during a century and a half, no motions can
be positively determined except those which would become evident to
telescopic vision in that period. Only about 3,000 stars have been
accurately observed so long as this. In the large majority of cases the
interval of observation is so short or the motion so slow that nothing
can be asserted respecting the law of the motion.
The great mass of stars seem to move only a few seconds per century,
but there are some whose motions are exceptionally rapid. The general
rule is that the brighter stars have the largest proper motions. This
is what we should expect, because in the general average they are
nearer to us, and therefore their motion will subtend the greatest
angle to the eye. But this rule is only one of majorities. As a matter
of fact, the stars of largest proper motion happen to be low in the
scale of magnitude. It happens thus because the number of stars of
smaller magnitudes is so much greater than that of the brighter ones
that the very small proportion of large proper motions which they offer
over-balances those of the brighter stars.
The discovery of the star of greatest known proper motion was made by
Kapteyn, of Groningen, in 1897, coöperating with Gill and Innes, of the
Cape Observatory. While examining the photographs of the stars made
at this institution, Kapteyn was surprised to notice the impression
of a star of the eighth magnitude which at first could not be found
in any catalogue. But on comparing different star lists and different
photographs it soon became evident that the star had been previously
seen or photographed, but always in slightly varying positions. An
examination of the observed positions at various times showed that the
star had a more rapid proper motion than any other yet known. Yet,
great though this motion is, it would require nearly 150,000 years for
the star to make a complete circuit of the heavens if it moved round
the sun uniformly at its present rate.
The fact that the stars move suggests a very natural analogy to the
solar system. In the latter a number of planets revolve round the sun
as their center, each planet continually describing the same orbit,
while the various planets have different velocities. Around several
of the planets revolve one or more satellites. Were civilized men
ephemeral, observing the planets and satellites only for a few minutes,
these bodies would be described as having proper motions of their own,
as we find the stars to have. May it not then be that the stars also
form a system; that each star is moving in a fixed orbit performing
a revolution around some far-distant center in a period which may be
hundreds of thousands or hundreds of millions of years? May it not be
that there are systems of stars in which each star revolves around a
center of its own while all these systems are in revolution around a
single center?
This thought has been entertained by more than one contemplative
astronomer. Lambert’s magnificent conception of system upon system will
be described hereafter. Mädler thought that he had obtained evidence
of the revolution of the stars around Alcyone, the brightest of the
Pleiades, as a center. But, as the proper motions of the stars are
more carefully studied and their motion and direction more exactly
ascertained, it becomes very clear that when considered on a large
scale these conceptions are never realized in the actual universe as a
whole. But there are isolated cases of systems of stars which are shown
to be in some way connected by their having a common proper motion. We
shall mention some of the more notable cases.
The Pleiades are found to move together with such exactness that up
to the present time no difference in their proper motions has been
detected. This is true not only of the six stars which we readily see
with the naked eye, but of a much larger number of fainter ones made
known by the telescope. It is an interesting fact, however, that a
few stars apparently within the group do not partake of this motion,
from which it may be inferred that they do not belong to the system.
But there must be some motion among themselves, else the stars would
ultimately fall together by their mutual attraction. The amount and
nature of this motion cannot, however, be ascertained except by
centuries of observation.
Another example of the same sort is seen in five out of the seven
stars of Ursæ Major, or The Dipper. The stars are those lettered β, γ,
δ, ε and θ. All five have a proper motion in R. A. of nearly 8″ per
century, while in declination the movements are sometimes positive and
sometimes negative; that is to say, some of the stars are apparently
lessening their distance from the pole, while others are increasing
it. But when we project the motions on a map we find that the actual
direction is very nearly the same for all five stars, and the reason
why some move slightly to the north and others slightly to the south is
due to the divergence of the circles of right ascension. It is worthy
of remark that the community of motion is also shown by spectroscopic
observations of the radial motions described below.
The five stars in question are all of the second magnitude except δ,
which is of the third. It is a curious fact that no fainter stars than
these five have been found to belong to the system.
From a study of these motions Höffler has concluded that the five
stars lie nearly in the same plane and have an equal motion in one
and the same direction. From this hypothesis he has attempted to make
a determination of their relative and actual distances. The result
reached in this way cannot yet, however, be regarded as conclusive.
There are three stars in Cassiopeia, β, η and μ each having a large
proper motion in so nearly the same direction that it is difficult to
avoid at least a suspicion of some relation between them. The angular
motions are, however, so far from equal that we cannot regard the
relation as established.
In the constellation Taurus, between Aldebaran and the Pleiades, most
of the stars which have been accurately determined seem to have a
common motion. But these motions are not yet so well ascertained that
we can base anything definite upon them. They show a phenomenon which
Proctor very aptly designated as star-drift.
The systems we have just described comprise stars situated so far apart
that, but for their common motion, we should not have suspected any
relation between them. The community of origin which their connection
indicates is of great interest and importance, but the question belongs
to a later chapter.