Page "Open cluster" Paragraph 9

Micrometer measurements of the positions of stars in clusters were made as early as 1877 by the German astronomer E. Schönfeld and further pursued by the American astronomer E. E. Barnard prior to his death in 1923.

No indication of stellar motion was detected by these efforts.

However, in 1918 the Dutch-American astronomer Adriaan van Maanen was able to measure the proper motion of stars in part of the Pleiades cluster by comparing photographic plates taken at different times.

As astrometry became more accurate, cluster stars were found to share a common proper motion through space.

By comparing the photographic plates of the Pleiades cluster taken in 1918 with images taken in 1943, van Maanen was able to identify those stars that had a proper motion similar to the mean motion of the cluster, and were therefore more likely to be members.

Spectroscopic measurements revealed common radial velocities, thus showing that the clusters consist of stars bound together as a group.