Their marriage began a close scientific collaboration that would span the next four decades, in which the Royal Astronomical Society said "it is difficult and pointless to separate his achievements from hers". They co-authored a number of academic papers on star clusters, stellar magnitudes, and the structure of the Milky Way. Bok was fiercely interested in the mechanical processes which form star clusters and galaxies, and his paper "The Apparent Clustering of External Galaxies" ("Nature", vol. 133, p. 578) was the first to demonstrate how dissipation of a star cluster relates to its density, and so by examining the clusters in our galaxy he concluded that it cannot be older than about twenty billion years.