After serving in World War II in the OSS, he returned to the university and received his Ph.D. in astrophysics in 1948.

He is best known for a seminal 1962 paper with Donald Lynden-Bell and Allan Sandage which suggested for the first time that the Milky Way Galaxy had collapsed out of a gas cloud.

He pioneered the now-accepted notion of moving groups, and introduced the idea that these may originate from dissolved open clusters.

[1] For the 1965 book Galactic Structure, edited by Blaauw and Schmidt, Eggen wrote an important chapter on moving groups of stars.

[4] The University of Wisconsin-Madison retains a collection of Eggen's personal papers and correspondence.