Reference Work Entry

Biographical Encyclopedia of Astronomers

pp 845-848


Greenstein, Jesse Leonard

  • Roy H. GarstangAffiliated withUniversity of Colorado 

BornNew York City, New York, USA, 15 October 1909

DiedDuarte, California, USA, 21 October 2002

American astrophysicist Jesse Greenstein discovered and clarified the properties of the largest sample of white dwarfs found up to that time. An outstanding administrator as well as scientist, he coordinated the most successful of the decadal reports, “Astrophysics for the 1970s.”

Greenstein went to Harvard University at the early age of 16 and majored in astronomy, obtaining his BA in 1929. He had planned to go to the University of Oxford, but a health problem prevented that, and so he remained at Harvard University. His first research was on the temperature scale for O and B stars. Cecilia Payne-Gaposchkin had found that some O and B stars had abnormally low color temperatures in spite of showing high-excitation lines in their spectra. Greenstein showed that the mean color temperatures were lowest in the directions of the Milky Way. His explanation in terms of atmospheric effect was incorrect: He had found the general interstellar reddening caused by interstellar dust discovered by Robert Trumpler very soon after. Harvard University conferred his MA in 1930.

Greenstein participated in his family’s real estate and other businesses through the earliest years of the depression, simultaneously carrying out some astronomical research. He returned to Harvard University in 1934 and completed his Ph.D. in 1937.

Greenstein’s thesis research concerned the interstellar medium and the associated absorption and reddening of starlight. He was particularly interested in the ratio of the extinction of the light from a star to the amount of reddening that the light experienced. Greenstein did calculations on Mie scattering by a distribution of small particles. He observed 38 highly reddened B stars by calibrated photographic spectrophotometry of objective-prism plates obtained with the 24-in. reflector at the Harvard Agassiz Station. The extinction law Greenstein found was λ−0.7. He measured the general absorption in the region of each of the B stars and found the ratio of photographic absorption to color excess to be in the range 4–6.

While at Harvard, Greenstein and Fred Whipple attempted to explain radio emissions from the Milky Way Galaxy, only recently discovered by Karl Jansky , as thermal radiation from dust grains. They concluded that the radio emissions could not be accounted for in that manner. However, Greenstein maintained his interest in radio astronomy and later strongly supported research in that area.

After graduating from Harvard University, Greenstein was fortunate to obtain a National Research Council Fellowship for 2 years. He chose to spend these at the Yerkes Observatory of the University of Chicago. The Yerkes Observatory was then entering its great period under director Otto Struve , and its staff was preparing to use the McDonald Observatory in Texas, which was at that time under construction as a joint project of the universities of Chicago and Texas. When his fellowship ended in 1939, Greenstein was appointed to the University of Chicago faculty at Yerkes, where he remained until 1947. During most of that period, he was a research associate at the McDonald Observatory.

At first, Greenstein worked principally on interstellar matter. With Louis Henyey , he studied the scattering of light by dust; an approximate formula that they developed for the particle scattering function later found applications in radiative transfer studies in astrophysics and atmospheric physics. In other collaborations with Henyey, Greenstein studied the diffuse galactic light by setting a photometer on an apparently empty space between the stars. The two astronomers studied spectra of reflection nebulae and emission nebulae and showed that H-α is widely distributed in the Milky Way, not just in bright nebulae.

Greenstein used the new 82-in. reflector at the McDonald Observatory to study several stellar spectra. His first work was helping Struve to obtain coude spectra of τ Scorpii for Albrecht Unsöld to use at Kiel, spectra which became a testing ground for many subsequent developments in the analysis of stellar atmospheres. Greenstein analyzed the spectrum of the supergiant Canopus, the second brightest star in the sky, finding its composition to be normal. He observed υ Sagittarii, which he proved has a hydrogen-poor atmosphere. This was the first of many studies Greenstein made of abnormalities in stellar spectra.

During World War II, Greenstein remained at the Yerkes Observatory and was engaged with Henyey in optical design work for defense purposes. One noteworthy project was their design of a wide-angle camera for military aerial photography. The Henyey-Greenstein camera was later used by Donald Osterbrock and Stewart Sharpless to take several remarkable photographs of the Milky Way, the zodiacal light, the gegenschein, and the aurorae. The Milky Way really did look like an edge-on spiral with a dust lane!

Greenstein moved to California in 1948 when he was appointed professor (and chairman of the astronomy department) at the California Institute of Technology (Caltech), ending as the Lee A. DuBridge Professor of Astrophysics. He officially retired at the end of 1979. Greenstein was also a staff member of the Hale Observatories and remained in that position from 1948 until 1980.

Greenstein was asked to go to Pasadena to help Caltech prepare for the operation of the Palomar Observatory, to gather the scientific staff for the Palomar Observatory, and to set up an outstanding astronomy graduate program at Caltech. He had to handle the complications of the joint operation by the Carnegie Institution of Washington and Caltech of the Mount Wilson and Palomar observatories. He was one of only two astronomers on the Caltech faculty, the other being Fritz Zwicky .

Soon after his arrival in California, Greenstein had an important collaboration with Leverett Davis. The polarization of starlight by the interstellar medium had just been discovered by William Hiltner and John Hall . Interstellar grains absorbed and reddened the light: To produce polarization required elongated grains, and the grains must be aligned over a large volume of space. Davis and Greenstein suggested that the grains contain small amounts of iron compounds and would be paramagnetic. The grains would be spinning rapidly because of collisions with hydrogen molecules in space. They suggested that an interstellar magnetic field of order 10−5 G must exist to align the grains and the field lie along the spiral arms of the Galaxy. Paramagnetic spinning grains produce magnetic energy dissipation, which in turn leads to a torque, and this makes the grains spin around their shortest axis. Other astronomers studied other mechanisms for producing the grain alignment, but Davis and Greenstein’s basic conclusions about the galactic magnetic field were correct.

Planetary nebulae had been observed to have a continuum in the visual spectral region. Recombination of hydrogen had been shown as not being the source of the continuum. Greenstein and Thornton Page considered the possibility that the capture continuum of the negative hydrogen ion might be the source, but that turned out to be too weak. The source was found by Greenstein and Lyman Spitzer , who showed that two-photon emission from the 2s state of hydrogen provided sufficient intensity. The 2s state was populated both by electron capture directly onto the 2s state and by electron collision transfer from the 2p state to the 2s state. The effect was to reduce the size of the Balmer discontinuity and reduce the calculated electron temperatures of the planetary nebulae.

After his move to California, Greenstein started very extensive studies of the chemical composition of stellar atmospheres. He continued these studies for more than 10 years and, with many collaborators, published about 60 papers in this field. Much of this work was related to studies of the origin of the elements and complemented work on nuclear reaction cross sections being done at Caltech. Greenstein studied the isotope ratios 13C/12C and 3He/4He and the nuclei 6Li, 7Li, 9Be, and 98Tc. The 13C/12C ratio in most stars seemed about the same as in the Sun. Comet C/1963 A1 (Ikeya) was observed, whose 13C/12C ratio was also about the solar value. The Li/H ratio was higher in young stars, and 3He/4He was high in some peculiar stars. Detailed interpretation of many of these observations proved more difficult than had been anticipated. An important paper with H. Larry Helfer and George Wallerstein determined hydrogen to metal ratios in two K-type giant stars in globular clusters and in one high-velocity field star. The hydrogen to metal ratios were from 20 to 100 times the solar values. The ratios of other elements to iron were within a factor of five of solar values. Subsequent analyses of several other field giant stars indicated still more extreme metal deficiencies, up to factors of 800 less than the solar abundance. Some stars also showed peculiarities in the abundances of individual elements. Greenstein later commented that, after many years of work, the subject was clearly much more complicated than had been thought when he started.

White dwarf stars are faint objects, and in consequence they had been little studied in earlier years. The new equipment at the Palomar Observatory allowed Greenstein to initiate an extensive series of studies on white dwarf stars and their colors, spectra, compositions, magnetic fields, and evolution. A joint paper with Olin J. Eggen listed 166 white dwarf stars, mostly with new spectroscopic and photometric data. Greenstein developed a classification system for white dwarfs; his publications showed the large variations in the characteristics of white dwarfs. Some have hydrogen-rich and metal-poor surfaces, while others have helium-rich atmospheres. Some of his spectra showed unidentified very broad features.

Greenstein was interested in other kinds of faint stars, including subdwarfs and brown dwarfs. Working with Lawrence Aller , Greenstein analyzed three G-type subdwarfs and found metal deficiencies ranging from 20 to 100.

After the discovery, by Maarten Schmidt, of the large redshift δλ/λ = 0.16 of the quasar 3C273, Greenstein and Thomas Matthews confirmed this by showing that the previously unidentifiable lines in 3C48 could be explained by lines of common elements with a redshift of 0.367. Greenstein and Schmidt showed that the redshifts of quasars could not be gravitational, so that, unless new physics intervened, these sources must be very distant and very bright.

Greenstein was fortunate to be permitted to continue observing at the Palomar Observatory for some years after he retired. He had many collaborators, including James W. Liebert, J. Beverly Oke, Harry L. Shipman, and Edward M. Sion. Greenstein continued to make spectroscopic observations of white dwarfs and of many other stars.

Greenstein collaborated with a large number of astronomers in the compilation of a spectroscopic atlas of white dwarfs, which was published in 1993. This atlas showed in great detail the incredible variety of white dwarf spectra. It illustrated the refinements that had been made in the classification of these stars as well as the little-understood peculiarities in individual spectra. Many white dwarfs had previously been classified as of type DC, the C indicating a continuous spectrum showing virtually no lines. Greenstein’s later work reduced the apparent number of DC stars by using improved equipment at the Palomar Observatory. He demonstrated the presence of weak C2 bands or weak He[I] lines in many of these stars. The star G 141-2 shows only a broad H-α line and apparently nothing else. The well-known white dwarf 40 Eri B could be observed in the ultraviolet and showed strong Lyman alpha and a strong line at 1391 Å which could possibly be Si[IV] or possibly molecular hydrogen. Among individual stars, GD 356 is unique; it has both H-α and H-β in emission, and both lines show Zeeman splitting corresponding to a magnetic field, if a dipole, of 20 MG. The magnetic star Grw +70° 8247 has an effective temperature of about 14,500 K and is a very small star, with a radius of only 0.0066 solar radius, making it one of the heaviest white dwarf stars known.

In other papers, Greenstein studied binary stars with both stars degenerate. In six pairs he found that the components were similar in luminosity and temperature; the white dwarfs are near twins. There must be many more such pairs to be discovered.

Greenstein also studied binary stars that contain one normal star and one white dwarf. He concluded that duplicity has not changed the evolution of either the white dwarf or the main-sequence star. Each star evolves in isolation. These binaries are separated by many times the average separation of binaries with two main-sequence stars, so presumably there must be many more of these binaries to be discovered.

Greenstein obtained many spectra of other kinds of stars, including the star PC0025 + 0047, which is an unusual M-type star and which he observed over a very wide range of wavelengths. It has the strongest water vapor bands and the strongest vanadium oxide bands in any known dwarf star. Its effective temperature must be as low as 1,900 K. It may be an old hydrogen-burning star with a mass of about 0.08 solar masses, or it may be a young brown dwarf.

Greenstein’s total scientific output was prodigious, about 380 papers and articles in all. His later papers on white dwarfs list large numbers of these fascinating objects with strange characteristics, which should serve as a starting point for many future investigations.

Greenstein served on many national committees, starting soon after World War II ended. He was involved in the first grants committee for astronomy of the Office of Naval Research. He was on the first advisory committee of the National Science Foundation when it was considering its first astronomy grants. Greenstein was chair of the National Academy of Sciences Astronomy Survey Committee and produced the second survey (1972) in what has become a series of decadal surveys. He was on National Aeronautics and Space Administration (NASA) committees, where he felt that he helped to bridge the gap between scientists and the NASA management.

Greenstein was a member of the Harvard Board of Overseers for 6 years. At Caltech, Greenstein served as the chairman of the faculty board. He resigned from heading astronomy at Caltech in 1972 but continued his observational work.

Greenstein received many honors, including election to the National Academy of Sciences, the Gold Medal of the Royal Astronomical Society, the Russell Lectureship of the American Astronomical Society, the Bruce Medal of the Astronomical Society of the Pacific, and an honorary D.Sc. from the University of Arizona in 1987.

Greenstein had two sons, one of whom, George (born 1940), has been on the astronomy faculty at Amherst College since 1971. Peter (born 1946) is active in music in California. Greenstein was predeceased by his wife Naomi, whom he met at Harvard and married in 1934.

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