Part 23 (2/2)
Recent researches on the proper motions of stars have brought to light many groups of stars whose individual members have equal and parallel velocities. Eddington calls these moving cl.u.s.ters. The component stars are not exceptionally near to each other, and it often happens that other stars not belonging to the group are actually interspersed among them. They may be likened to double stars which are permanent neighbors, with some orbital motion, though exceedingly slow.
The connection is rather one of origin; occurring in the same region of s.p.a.ce, perhaps, from a single nebula. They set out with the same motion, and have ”shared all the accidents of the journey together.” Their equality of motion is intact because any possible deflections by the gravitative pull of the stellar system is the same for both. Mutual attraction may tend to keep the stars together, but their community of motion persists chiefly because no forces tend to interfere with it. In this way physically connected pairs may be separated by very great distances.
So with the moving cl.u.s.ters: their component stars may be widely separate on the celestial sphere, but equality of their motions affords a clue to their a.s.sociation in groups. The Hyades, a loose cl.u.s.ter in Taurus, is a group of thirty-nine stars, within an area of about 15 degrees square, which has been pretty fully investigated, especially by the late Professor Lewis Boss; and no doubt many fainter stars in the same region will ultimately be found to belong to the same group.
If we draw arrows on a chart representing the amount and direction of the proper motions of these stars, these arrows must all converge toward a point. This shows that their motions are parallel in s.p.a.ce. It is a relatively compact group, and the close convergence shows that their individual velocities must agree within a small fraction of a kilometer per second. Radial velocity measures of six of the component stars are in very satisfactory accord, giving 45.6 kilometers per second for the entire group.
We can get the transverse velocity, and therefrom the distances of the stars, which are among the best known in the heavens, because the proper motions are very accurately known. The mean parallax of the group by this indirect method comes out 0”.025, agreeing almost exactly with the direct determination by photography, 0”.023, by Kapteyn, De Sitter, and others.
Eddington concludes that this Taurus group is a globular cl.u.s.ter with a slight central condensation. Its entire diameter is about ten pa.r.s.ecs, and its known motion enables us to trace its past and future history. It was nearest the sun 800,000 years ago, when it was at about half its present distance. Boss calculated that in 65 million years, if the present motion is maintained, this group will have receded so far as to appear like an ordinary globular cl.u.s.ter 20' in diameter, its stars ranging from the ninth to the twelfth apparent magnitude. We may infer that the motion will likely continue undisturbed, because there are interspersed among the group many stars not belonging to it, and these have neither scattered its members nor sensibly interfered with the parallelism of their motion.
Another moving cl.u.s.ter, the similarity of proper motion of whose component stars was first pointed out by Proctor, is known as the Ursa Major system, which embraces primarily Beta, Gamma, Delta, Epsilon, and Zeta Ursae Majoris, or five of the seven stars that mark the familiar Dipper. But as many as eight other stars widely scattered are thought to belong to the same system, including Sirius and Alpha Coronae Borealis.
The absolute motion amounts to 28.8 kilometers per second, and is approximately parallel to the Galaxy. Turner has made a model of the cl.u.s.ter, which has the form of a flat disk.
Among stars of the Orion type of spectrum are several examples of moving cl.u.s.ters. The Pleiades together with many fainter stars form another moving cl.u.s.ter; as also do the brighter stars of Orion, together with the faint cloudlike extensions of the great nebula in Orion, whose radial velocity agrees with that of the stars in the constellation.
Still another very remarkable moving cl.u.s.ter is in Perseus, first detected by Eddington, and embracing eighteen stars, the brightest of which is Alpha Persei.
The further discovery of moving cl.u.s.ters is most important in the future development of stellar astronomy, because with their aid we can find out the relative distribution, luminosity, and distance of very remote stars. So far the stars found a.s.sociated in groups are of early types of spectrum; but the Taurus cl.u.s.ter embraces several members equally advanced in evolution with the sun, and in the more scattered system of Ursae Major there are three stars of Type F.
”Some of these systems,” Eddington concludes, ”would thus appear to have existed for a time comparable with the lifetime of an average star. They are wandering through a part of s.p.a.ce in which are scattered stars not belonging to their system--interlopers penetrating right among the cl.u.s.ter stars. Nevertheless, the equality of motion has not been seriously disturbed. It is scarcely possible to avoid the conclusion that the chance attractions of stars pa.s.sing in the vicinity have no appreciable effect on stellar motions; and that if the motions change in course of time (as it appears they must do) this change is due, not to the pa.s.sage of individual stars, but to the central attraction of the whole stellar universe, which is sensibly constant over the volume of s.p.a.ce occupied by a moving cl.u.s.ter.”
CHAPTER LVI
THE TWO STAR STREAMS
Consider the s.h.i.+ps 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 pa.s.sing each other, but rarely or never colliding. We might say, there are two great streams of s.h.i.+ps, one moving east and the other west.
Now in place of each s.h.i.+p, imagine a hundred s.h.i.+ps, 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 cl.u.s.ters 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 a.n.a.lyzing 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 Inst.i.tution of Was.h.i.+ngton, 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 cl.u.s.ters, 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 a.n.a.lysis 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 s.h.i.+ps, 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, cla.s.sifying 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 const.i.tuting 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.
<script>