[2] Before leaving Harvard, Greenstein was involved in a project with Fred Lawrence Whipple to explain Karl Jansky's discovery of radio waves from the Milky Way and to propose a source.
His theoretical work with Davis was based on the conclusion just reached by William A. Hiltner that the recently detected polarization of starlight was due to dichroic extinction by interstellar dust grains aligned with the ambient magnetic field.
For the 1965 book Galactic Structure, edited by Blaauw and Schmidt, Greenstein wrote an important chapter on subluminous blue stars.
[4] Greenstein did important work in determining the abundances of the elements in stars, and was, with Maarten Schmidt, among the first to recognize quasars as compact, very distant sources as bright as a galaxy.
The spectra of the first quasars discovered, radio sources 3C 48 and 3C 273, were displaced so far to the red due to their redshifts as to be almost unrecognizable, but Greenstein deciphered 3C 48 shortly before Schmidt, his colleague at the Hale Observatories worked out the spectrum of 3C 273.