Bruce Draine

Bio/Description

Bruce Draine, a versatile theoretical astrophysicist and renowned expert on the interstellar medium, transferred to emeritus status on July 1, 2024, after forty-two years on the faculty of the Department of Astrophysical Sciences. Everyone can appreciate the beauty of the stars and planets in the night sky, but only astronomy connoisseurs can fully appreciate the wisps of gas and grains of dust strewn across the light years between stars. They are the raw material from which stars and planets are made, and the medium through which radiation, magnetic fields, and shock waves propagate. Understand the interstellar medium, and you will understand the conditions that prevail in most of the universe’s volume. In his career, Bruce has increased our understanding of the interstellar medium by applying his physical insight and his ability to link theory with a complex corpus of observations.

Bruce was born in 1947 in Kolkata, then known as Calcutta, in the same year as the birth of an independent India, when his father was on an assignment there for Union Carbide. During Bruce’s childhood, Union Carbide also brought his family to California, Brazil, and Mexico. Bruce completed high school in New Jersey and attended Swarthmore College. Upon graduating in 1969, he entered the Peace Corps and was dispatched to teach math and physics for two years at a secondary school in the Kwahu region of central Ghana.

After returning to the United States, he entered the doctoral program in physics at Cornell University in 1971. He went on to earn a master of science degree in experimental physics in 1975, choosing later to switch to theory. Bruce approached Ed Salpeter, who was working on one of the flashiest topics in astrophysics: quasars. He wanted to join Salpeter’s effort to explain quasars as relativistic accretion disks surrounding supermassive black holes. Salpeter had other ideas, impelling Bruce to study astrophysical “dust,” the microscopic specks of solid material that reveal themselves by blocking starlight and glowing at infrared wavelengths. Although initially disappointed by the assignment, before long Bruce became fascinated by the physics of dust and, more broadly, the diverse physical problems associated with the interstellar medium. He set out to understand the complete life cycle of dust grains, beginning by studying the process of nucleation of dust grains from cooling gas, as well as the physical processes that charge and destroy dust grains through collisions and shock waves.

After completing graduate school, Bruce obtained a postdoctoral fellowship at the Harvard-Smithsonian Center for Astrophysics. A second postdoctoral position at the Institute for Advanced Study brought him to Princeton, where he has remained since 1979. As a postdoctoral fellow, Bruce became increasingly interested in interstellar shock waves, realized that magnetic effects lead to different predictions for the properties of the “shocked” gas than in standard models, and proposed that this could account for some previously unexplained emission from molecular clouds. 

Bruce was appointed an assistant professor at Princeton’s astrophysical sciences department in 1981. This was just before the retirement of Lyman Spitzer (1914-97), a pioneer in the study of the interstellar medium, and author of the book Physical Processes in the Interstellar Medium (1978), the standard text on the subject at that time. Bruce’s appointment injected new energy into this research area and allowed the department to remain at the forefront of the field. Today, Bruce’s book Physics of the Interstellar and Intergalactic Medium (2011) is the standard text.

In 1984, with graduate student Hyung Mok Lee (now a professor at Seoul National University), Bruce published “Optical Properties of Graphite and Silicate Grains,” a paper with a lasting legacy. The authors synthesized laboratory studies and astronomical observations to provide a model of interstellar dust that has proven extremely useful for calculating the spectrum of radiation from dust particles. An early success of the model—the first of many—was achieving a good match to the data that had just begun arriving from NASA’s Infrared Astronomical Satellite (IRAS).

Bruce was awarded an Alfred P. Sloan Research Fellowship in 1982 and earned tenure in 1984. He continued his work on shock waves and dust and began teaching Princeton students the joys of the interstellar medium through the next four decades. In 1988, he improved on and applied a computational technique called the Discrete Dipole Approximation for calculating absorption and scattering of light by small particles. Decades later, his public-domain code DDSCAT (developed jointly with atmospheric scientist Piotr Flatau) is widely used by astronomers, atmospheric scientists, nanotechnologists, marine biologists, and others.

From 1996 to 1998, Bruce served as chair of the department. On the research front, with his students and postdoctoral researchers, Bruce focused attention on the very smallest dust grains—smaller than a few nanometers. He helped the community realize that these nanoparticles are more abundant than previously thought and are crucial for interpreting observations. A dust grain of such small size, after absorbing even a single photon, can be briefly heated to hundreds of degrees Kelvin, causing it to radiate profusely in the mid-infrared and make a conspicuous contribution to the observed spectrum. 

Bruce’s collaboration with then-postdoctoral researcher Alex Lazarian (now a professor at the University of Wisconsin-Madison) deserves special mention as a fortuitous and fascinating connection between theory and observation. In the late 1990s, they were studying the rotation of dust grains to understand why they are found preferentially aligned with magnetic fields. Meanwhile, observations of the cosmic microwave background radiation revealed unexpected microwave emission from interstellar matter within the galaxy. The emission is associated with dust clouds but is more than an order of magnitude brighter than had been predicted. The community was perplexed, until Bruce and Alex realized that the microwaves could result from rapid rotation—billions of revolutions per second—of the smallest dust grains. Their “spinning dust” theory remains the favored explanation for the so-called “anomalous microwave emission.”

Bruce wrote two influential and highly cited articles for the Annual Review of Astronomy and Astrophysics, one on shock waves with Chris McKee (1993) and the other on interstellar dust (2003), both of which continue accruing citations. His contributions were recognized by the Dannie Heineman Prize for Astrophysics in 2004, by his election to the National Academy of Sciences in 2007, and by his designation as Legacy Fellow of the American Astronomical Society in 2020. He has held distinguished lectureships bearing the names of Jan Oort, Bruno Rossi, Cecilia Payne-Gaposchkin, and Tommy Gold. He spent four sabbaticals in Arcetri, Florence, including a 2023 sabbatical during which a conference on interstellar dust was held in his honor.

Following surprising observations by the European Space Agency’s Planck satellite, in 2021, Bruce and his former student Brandon Hensley proposed a new model for interstellar dust that breaks with fifty years of tradition by reimagining the composition of the dust grains. Dust was traditionally thought to come in two principal varieties—silicate grains and carbonaceous grains. Brandon and Bruce argued that dust destruction and regrowth are rapid enough that the boundary between these two categories should be blurred, forming grains with both silicate and carbonaceous domains. Their model, dubbed “astrodust,” is consistent with the Planck observations.

As the beneficiary of a sound primary and secondary education in the 1950s and 1960s, mainly while living in Mexico City, Bruce became concerned by what he saw as shortcomings in public education in the 1990s in the United States. Bruce and his wife, Dina Gutkowicz-Krusin, were among the founders of Princeton Charter School in 1997. He helped develop the school’s science curriculum and served for two years as the (unpaid) school registrar.

Bruce’s students, postdoctoral researchers, and faculty colleagues have long appreciated his integrity, breadth of knowledge, generosity with his time, and enthusiasm for all areas of physics and astronomy. They also describe him as a kind and gentle soul. They treasure their continuing interactions with Bruce in this new phase of his life and await the new insights into the interstellar medium that Bruce will achieve in retirement.

Written by members of the Department of Astrophysical Sciences faculty.