THE TWO STAR STREAMS
Consider the ships on the Atlantic voyaging between Europe and America:
at any one time there may be a hundred or more, all bound either east or
west, some moving in interpenetrating groups, individuals frequently
passing each other, but rarely or never colliding. We might say, there
are two great streams of ships, one moving east and the other west.
Now in place of each ship, imagine a hundred ships, and magnify their
distances from each other to the vast distances that the stars are from
each other, and all in motion in two great streams as before. This will
convey some idea of the relatively recent discovery, called by
astronomers "star-streaming."
Early in this century the investigation of moving clusters began to
reveal the fact that the motions of the stars were not at random
throughout the universe, and about 1904 Kapteyn was the first to show
that the stellar motions considered in great groups are very far from
being haphazard, but that the stars tend to travel in two great streams,
or favored directions. This was ascertained by analyzing the proper
motions of stars in the sky, many thousands of them, and correcting all
for the effect which the known motion of the sun would have upon them.
The corrected motion, or part that is left over, is known as the star's
own motion, or _motus peculiaris_.
This important investigation was very greatly facilitated by the general
catalogue of 6,188 stars well distributed over the entire sky, the work
of the late Professor Boss. It was published by the Carnegie Institution
of Washington, and includes all stars down to the sixth magnitude. Boss
was very critical in the matter of stellar positions and proper motions
and his work is the most accurate at present available. Excluding stars
of the Orion type and the known members of moving clusters, Kapteyn's
investigation was based on 5,322 stars, which he divided into seventeen
regions of the sky, each northern region having an antipodal one in the
southern hemisphere.
Mathematical analysis of these regions showed them all in substantial
agreement, with one exception, and enabled Kapteyn to draw the
conclusion that the stars of one stream, called Drift I, move with a
speed of thirty-two kilometers per second, while those of the other,
Drift II, travel with a speed of eighteen kilometers per second. Their
directions are not, like those of east and west bound ships, 180 degrees
from each other, but are inclined at an angle of 100 degrees. Drift I
embraces about three-fifths of the stars, and Drift II the remaining
two-fifths. Quite as remarkable as the drifts themselves is the fact
that the relative motion of the two is very closely parallel to the
plane of the Milky Way.
This epochal research has very great significance in all investigations
of stellar motions, and it has been verified in various ways,
particularly by the Astronomer Royal, Sir Frank Dyson, who limited the
stars under consideration to 1,924 in number, but all having very large
proper motions. In this way the two streams are even more
characteristically marked. But radial velocity determinations afford the
ultimate and most satisfactory test, and Campbell has this investigation
in hand, classifying the stars in their streaming according to the type.
Type A stars are so far found to be confirmatory. Turning to the
question of physical differences between the stars of the two streams,
Eddington inquires into the average magnitude of the stars in both
drifts, and their spectral type. Also whether they are distributed at
the same distance from the sun, and in the same proportion in all parts
of the sky. His conclusion is that there is no important difference in
the magnitudes of the stars constituting the two drifts. Regarding their
spectra, stars of early and late types are found in both streams, with a
somewhat higher proportion of late types among the stars of Drift II
than those of Drift I. Campbell and Moore of the Lick Observatory have
investigated seventy-three planetary nebulae which exhibit the phenomena
of star-streaming, and have motions which are characteristic of the
stars.
Dealing with the very important question whether the two streams are
actually intermingled in space, Eddington finds them nearly at the same
mean distance and thoroughly intermingled, and there is no possible
hypothesis of Drifts I and II passing one behind the other in the same
line of sight. A third drift, to which all the Orion stars belong, is
under investigation, together with comprehensive analysis of the drifts
according to the spectral type of all the stars included.
The farther research on star-streaming is pushed, the more it becomes
evident that a third stream, called Drift O, is necessary, especially
to include B-type stars. The farther we recede from the sun, the more
this drift is in evidence. At the average distances of B-type stars, the
observed motions are almost completely represented by Drift O alone.
Halm of Cape Town concludes from recent investigations that the
double-drift phenomena (Drifts I and II) is of a distinctly _local_
character, and concerns chiefly the stars in the vicinity of the solar
system; while stars at the greatest distances from the sun belong
preeminently to Drift O.
The 60-inch reflector on Mount Wilson gathers sufficient light so that
the spectra of very faint stars can be photographed, and a discussion of
velocities derived in this manner has shown that Kapteyn's two star
streams extend into space much farther than it was possible to trace
them with the nearer stars. Star-streaming, then, may be a phenomenon of
the widest significance in reference to the entire universe.
As to the fundamental causes for the two opposite and nearly equal star
streams, it is early perhaps to even theorize upon the subject.
Eddington, however, finds a possible explanation in the spiral nebulae,
which are so numerous as to indicate the certainty of an almost
universal law compelling matter to flow in these forms. Why it does so,
we cannot be said to know; but obviously matter is either flowing into
the nucleus from the branches of the spiral, or it is flowing out from
the nucleus into the branches. Which of the two directions does not
matter, because in either case there would be currents of matter in
opposite directions at the points where the arms merge in the central
aggregation. The currents continue through the center, because the
stars do not interfere with one another's paths. As Eddington concludes:
"There then we have an explanation of the prevalence of motions to and
fro in a particular straight line; it is the line from which the spiral
branches start out. The two star streams and the double-branched spirals
arise from the same cause."