Astrophys. J. 684, 364–372 (2008)

Christopher Thom of the University of Chicago in Illinois and his colleagues used a high-resolution spectrometer at the Keck Observatory in Hawaii to look at stars in the direction of a cloud called Complex C to assess its distance from Earth.

The spectra of some stars showed absorption lines that could be ascribed to the cloud, proving that they lay behind it; the spectra of others had no such feature. Because the distances to the stars are known, this provided a bracket for the distance to the cloud. Subsequent estimates of its mass suggest that it could be a significant source of fresh material to the galactic disc.