Critical Summary

At the beginning of this twentieth century we knew very little about the motions of the stars in the line of sight. A. A. Belopolski at Poulkova, W. W. Campbell and his associates at the Lick Observatory and in Chile, Frank Schlesinger and associates at the Allegheny Observatory, and the Potsdam observers were among the early workers in this exciting project of using the Doppler principle to interpret blue and red shifts of the spectral lines as indicators of speeds in the line of sight. Spectroscopically detected double stars attracted much attention then as now. But so far as stellar motions in bulk were concerned, it was the component of velocity at right angles to the line of sight, the proper motion, that was principally exploited.

To prepare for the future work on proper motion, great catalogues of star positions were undertaken in a fine display of international cooperation and of trust in the future. To make these catalogues, the photographic plate some 80 years ago began to displace the human eye, as Frank Schlesinger explains in selection 20.

One of the impressive catalogues of positions and proper motions is described below by R. E. Wilson who was deeply involved in its construction. But long before the "Boss" catalogues were available the star motions had been analyzed by J. C. Kapteyn, the discoverer of the "two star streams" (selection 19), and by many others. Now that the rotation of the galaxy has been deduced, chiefly by B. Lindblad and J. H. Oort, we hear little of the two star streams; but in the first two decades of the century the Kapteyn hypothesis was of much use in guiding observation and theory.

Two types of star motion other than radial velocities and proper motions (and double star orbital motion) are now under study; they are the axial rotations, which are detected and measured spectroscopically, and Ambartsumian’s "expanding stellar associations." The latter represents motion by inference only, based on the present distribution of groups of stars with peculiar spectra. Both of these developments are rich in implications concerning stellar evolution.

To invade the distant realm of faint stars, the Lick Observatory, as reported

in selection 24 by W. H. Wright, has undertaken the long-term project of determining very precisely the present positions of millions of stars. The stars are stored as images on photographic plates that will be duplicated many years hence—perhaps in 20 years, perhaps 100, probably both. These pairs of plates will make it possible to derive the motions of the millions of stars within their range, and thereby provide a gold mine that will one day buy good new knowledge of the structure and behavior of our galactic system.

A similar program is in progress in Russia.