Jesse Leonard Greenstein (October 15 1909 – October 21 2002) was an American astronomer.

He began his career at Yerkes Observatory under Otto Struve and later went to Caltech. With Louis G. Henyey he invented a new spectrograph and a wide-field camera . He directed the Caltech astronomy program until 1972 and later did classified work on military reconnaissance satellites.

With Leverett Davis, Jr , he demonstrated in 1949 that the magnetic field in our galaxy is aligned with the spiral arms. His theoretical work, with Davis, was based the conclusion just reached by William A. Hiltner that the recently detected polarization of starlight was due to scattering off interstellar dust grains aligned by a magnetic field.

He won the Henry Norris Russell Lectureship of the American Astronomical Society in 1970, the Bruce Medal in 1971 and the Gold Medal of the Royal Astronomical Society in 1975.

Greenstein did important work in determining the abundances of the elements in stars, and was, with Maarten Schmidt, the first to recognize quasars as compact, very distant sources as bright as a galaxy. The spectra of the first quasars discovered, radio sources 3C48 and 3C273, were displaced so far to the red due to their redshifts as to be almost unrecognizable, but Greenstein deciphered 3C48 shortly before Schmidt, his colleague at The Hale Observatories worked out the spectrum of 3C273.

The asteroid 4612 Greenstein is named after him.

External links

• Obituary from Caltech
• Caltech oral history interview
• Story of the discovery of quasars
• Bruce Medal page
• Awarding of Bruce medal: PASP 83 (1971) 243
• Awarding of RAS gold medal: QJRAS 16 (1975) 356
• Biography by Robert P. Kraft, former director of Lick Observatory