Astronomy lost one of the great figures of this century with the passing, on 31 March 1997, of Lyman Spitzer, Jr. He will be remembered as a major contributor to understanding of the interstellar medium, plasma physics, the structure of the Milky Way and other galaxies, and the dynamics of star clusters; as the father of the Copernicus ultraviolet satellite and the Hubble Space Telescope; and as the guiding figure of the Princeton Astronomy Department for more than a third of a century.
Spitzer was one of the foremost figures in each of the four areas to which he most lent his insights and his efforts. Few would dispute his position as the leading expert and contributor in the world-wide effort to understand the physics of the interstellar medium. He was one of the first to suggest that the stars themselves are born out of interstellar material. He emphasized the importance of dust grains and magnetic fields in star formation; more generally, he pioneered the study of the physical properties of dust grains in the ISM. He anticipated the existence of multiple phases of temperature and pressure in the ISM, and considerations of pressure balance led him to postulate the existence of Galactic coronal gas. More important, he had a fundamental understanding of the physics of the ISM. In addition to his papers, his monograph Physical Processes in the Interstellar Medium has served for nearly two decades as the standard for the teaching of this topic.
In stellar dynamics, Spitzer, concurrently with Ambartsumyan, calculated the rate at which a star cluster ejects stars as a result of stellar encounters. He and collaborators refined the cutoff distance for two-body encounters in plasmas and consequently in stellar systems. Spitzer introduced two important mechanisms into the dynamics of star clusters: impulsive, tidal shocks by interstellar clouds, and the equipartition instability, in which high- and low-mass stars are unable to come into equilibrium but go into a runaway collapse of the former. For the evolution that results from the earlier known core-collapse instability, Spitzer exploited Henon's Monte Carlo simulation technique in a series of papers more realistic than any that had preceded; and he then encouraged Princeton student Haldan Cohn to develop the Fokker-Planck method of simulation of the dynamical evolution of star clusters. The culmination of Spitzer's star-cluster work was his monograph, Dynamical Evolution of Globular Clusters. In addition, Spitzer and Schwarzschild were the first to suggest that interstellar cloud complexes (we would now say giant molecular clouds) are responsible for the secular increase in stellar velocity dispersions; and Baade and Spitzer introduced the idea of gravitational interaction between galaxies.
Spitzer was also in the forefront of research in plasma physics, where his foresight was instrumental in the development of the Princeton Plasma Physics Lab. He was a leader in research in the basis properties of plasmas—physicists world-wide use the "Spitzer" rates for Coulomb collisional processes in a fully ionized gas. He was one of the first to promote research into multiple methods of controlled hydrogen fusion, to replace our dwindling supply of fossil fuels. He was a leader of the group that persuaded the Atomic Energy Commission to support a controlled fusion program, and to this he contributed a scheme of his own (the "Stellerator"). In the general field of plasma physics, his monograph, Physics of Fully Ionized Gases, has been a classic for many decades.
In the field of space research, Spitzer was a leader and an innovator. He will be remembered for the Copernicus UV spectroscopy satellite, and especially as the father of the Hubble Space Telescope, which he saw launched more than 40 years after he first proposed it. In the intervening period, he served as its major advocate, shepherding the project through both NASA planning and congressional scrutiny. He served for many years as Chair of the Space Telescope Institute Council, in preparation for that momentous launch.
Even outside these areas, Spitzer contributed original and important ideas. He dealt the definitive blow to the "tidal hypothesis" that, in the earlier part of this century, would have had the Solar System formed from material torn from the sun by the tidal force of a passing star. What Spitzer showed was that any such gas would dissipate explosively. He was also the discoverer of the Earth's exosphere, reasoning that the atmosphere would have a much higher helium content than it does were it not for a high-temperature layer that promotes escape of the lighter gases.
Lyman Spitzer, Jr. was educated at Phillips Academy, Andover, and at Yale. He did graduate work at Cambridge and at Princeton, where he completed his PhD under Henry Norris Russell in 1938. After a postdoctoral fellowship at Harvard, he joined the faculty at Yale. In 1947 he moved to Princeton, where he spent the rest of his career. He remained deeply involved in research, even on the final day of his life.
Spitzer's achievements were recognized during his lifetime by the award of the US National Medal of Science, the Crafoord Prize of the Royal Swedish Academy, and many other awards. Other obituaries have appeared in Nature (387, 244) and the New York Times (Wednesday, April 2, 1997) and will appear in the Biographical Memoirs of the National Academy of Sciences. Oral history material from 1977 and 1978 is on file at the Niels Bohr Library of the American Institute of Physics, and his papers will be archived at Princeton.
We are most fortunate that, during the last year of Spitzer's life, he and J.P. Ostriker were able to put together Dreams, Stars, and Electrons, Selected Writings of Lyman Spitzer, Jr. (Princeton University Press, 1997). This obituary could not do better than to end by quoting from the closing sentence of Ostriker's preface (to which I am indebted for many of the facts above):
"Universally admired as a scientist and cherished as a friend and mentor by many, Spitzer's life and work will serve as a model for generations to come."
Photo (available in PDF version) courtesy Robert Matthews, Princeton Communications Department.