THE SUN'S MOTION TOWARD LYRA
If Hipparchus or Galileo should return to earth to-night and look at the
stars and constellations as we see them, there would be no change
whatever discernible in either the brightness of the stars or in their
relative positions. So the name fixed stars would appear to have been
well chosen. Halley in the seventeenth century was the first to detect
that slow relative change of position of a few stars which is known as
proper motion, and all the modern catalogues give the proper motions in
both right ascension and declination. These are simply the small annual
changes in position athwart the line of vision; and, as a whole, the
proper motions of the brighter stars exceed the corresponding motions of
the fainter ones because they are nearer to us. The average proper
motion of the brightest stars is 0".25, and of stars of the sixth
magnitude only one-sixth as great.
A few extreme cases of proper motion have been detected, one as large as
9", of an orange yellow star of the eighth magnitude in the southern
constellation Pictor, and Barnard has recently discovered a star with a
proper motion exceeding 10"; several determinations of its parallax give
0".52, corresponding to a distance of 6.27 light years. Nevertheless,
two centuries would elapse before these stars would be displaced as much
as the breadth of the moon among their neighbors in the sky. The proper
motions of stars are along perfectly straight lines, so far as yet
observed. Ultimately we may find a few moving in curved paths or orbits,
but this is hardly likely.
As for a central sun hypothesis, that pointing out Alcyone in
particular, there is no reliable evidence whatever. Analysis of the
proper motions of stars in considerable numbers, first by Sir William
Herschel, showed that they were moving radially from the constellation
Hercules, and in great numbers also toward the opposite side of the
stellar sphere. Later investigation places this point, called the sun's
goal, or apex of the sun's way, over in the adjacent constellation Lyra;
and the opposite point, or the sun's quit, is about halfway between
Sirius and Canopus. By means of the radial velocities of stars in these
antipodal regions of the sky, it is found that the sun's motion toward
Lyra, carrying all his planetary family along with him, is taking place
at the rate of about 12 miles in every second.
While the right ascensions of the solar apex as given by the different
investigations have been pretty uniform, the declination of this point
has shown a rather wide variation not yet explained. For example, there
is a difference of nearly ten degrees between the declination (+34 deg..3)
of the apex as determined by Boss from the proper motions of more than
6,000 stars, and the declination (+25 deg..3) found by Campbell from the
radial velocities of nearly 1,200 stars. Several investigations tend to
show that the fainter the stars are, the greater is the declination of
the solar apex. More remarkable is the evidence that this declination
varies with the spectral type of the stars, the later types, especially
G and K, giving much more northerly values. On the whole the great
amount of research that has been devoted to the solar motion relative to
the system of the stars for the past hundred years may be said to
indicate a point in right ascension 18h. (270 deg.) and declination 34 deg. N.
as the direction toward which the sun is moving. This is not very far
from the bright star Alpha Lyrae, and the antipodal point from which the
sun is traveling is quite near to Beta Columbae.
So swift is this motion (nearly twenty kilometers per second) that it
has provided a base line of exceptional length, and very great service
in determining the average distance of stars in groups or classes. After
thousands of years the sun's own motion combined with the proper motions
of the stars will displace many stars appreciably from their familiar
places. The constellations as we know them will suffer slight
distortions, particularly Orion, Cassiopeia and Ursa Major. Identity or
otherwise of spectra often indicates what stars are associated together
in groups, and their community of motion is known as star drift. Recent
investigation of vast numbers of stars by both these methods have led to
the epochal discovery of star streaming, which indicates that the stars
of our system are drifting by, or rather through, each other, in two
stately and interpenetrating streams. The grand primary cause underlying
this motion is as yet only surmised.