Along with Donald Osterbrock and Stewart Sharpless, he used distance measurements of O and B type stars to show the existence of spiral arms in the Milky Way Galaxy.
Zajedno s Donaldom Osterbrockom i Stewartom Sharplessom, pomoću mjerenja udaljenosti zvijezda O i B tipa dokazao je postojanje spiralnih krakova u galaksiji Mliječni put.