US astronomer who took part in the discovery of the interstellar magnetic field and the discovery and interpretation of quasars. His early work involved the spectroscopic investigation of stellar atmospheres; later work included a study of the structure and composition of white dwarf stars.

Greenstein was born in New York and studied at Harvard. In 1948, he joined the California Institute of Technology and also the staff of the Mount Wilson and Palomar Observatories. He became professor of astrophysics 1971. During the 1970s he guided both the US space agency NASA and the National Academy of Sciences in their policies.
He confirmed the hypothesis of US astronomer Maarten Schmidt (1929- ) that the emission lines of quasars could be explained by a shift in wavelength. In collaboration with Schmidt, Greenstein proposed a detailed physical model of the size, mass, temperature, luminosity, magnetic field, and high-energy particle content of quasars.
By 1978 Greenstein had discovered some 500 white dwarf stars. His research enabled him to pinpoint the problems of explaining the evolutionary sequence that links red giant stars with white dwarfs. This initiated spectroscopic studies of such stars from space.