Bohdan Paczynski, who died April 19 at 67, was a Princeton University astrophysicist who laid claim to two of astronomy's most spectacular advances of the late 20th century: gravitational microlensing and the origins of gamma ray bursts.
Paczynski's insights started with an awareness of variation within the seemingly static heavens born of his study of binary star systems, which can vary dramatically in magnitude when one star briefly eclipses the other.
Applying Albert Einstein's insight that gravity warps the path of light, Paczynski realized that a star that perfectly eclipses another behind it can act as a lens, magnifying objects thousands of light-years away. Gravitational microlensing helped reveal some of the smallest planets ever found outside the solar system — a search still very much in its infancy.
Gamma ray bursts are hugely bright, brief events lasting from a few seconds to a couple of hours. Astronomers are uncertain about what causes them, but speculation had usually tended to assume an origin inside the Milky Way. Otherwise, the theory went, whatever caused them would have to be much more powerful than the largest known event, a supernova.
Paczynski is generally credited with the insight that the fact that gamma ray bursts are randomly distributed in the sky, rather than mostly seeming to emanate from some part of the Milky Way galaxy, means that they probably originate at great distances.
Britain's royal astronomer, Martin Rees, once famously offered Paczynski 100-to-1 odds that the bursts came from inside the Milky Way. An Italian astronomical satellite finally confirmed Paczynski's view in the late 1990s. Mr. Rees is reputed to have said, "We were both fools: I for offering the bet; [Paczynski] for not accepting it."
Paczynski grew up in Poland and by age 14 was studying eclipsing binary stars with an astronomy professor at Warsaw University. After receiving a Ph.D. at Warsaw University in 1964, he became a research assistant at the Lick Observatory in California. He then taught at what is now the Copernicus Astronomical Center at Warsaw University while holding a variety of international posts and collaborating with astronomers around the world.
In 1981, while Paczynski was on a sabbatical leave at the Institute for Advanced Studies at Princeton, General Wojciech Jaruzelski declared martial law in Poland. Paczynski accepted a permanent post at Princeton.
Having never had much in the way of problems with the communist authorities in the past, he came to wonder if he had overreacted by becoming an expatriate. "At the times things looked really bad," he told the New York Times in 1999. "The reports from our friends, whatever we read in the media here, gave an impression that things were actually worse than under Stalin."
Together with another astronomer, Shude Mao of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., Paczynski in 1991 proposed linking medium-size telescopes around the world into a network studying gravitational microlenses. They wanted to survey larger portions of the sky to expand star maps with broader swaths of sky that microlenses could see.
The proposal led to collaborations including the Massive Compact Halo Object and Optical Gravitational Lensing Experiment. OGLE last year discovered one of the smallest planets ever found outside the solar system, just 5.5 times the size of Earth, orbiting a star 20,000 light-years away.
Paczynski spent recent years working on a large-scale project to continuously monitor the sky in what he called an All Sky Automate Survey. The idea was to integrate observations from around the globe via the Internet so that any changes — a supernova, a blinking binary, etc. — could be detected quickly and investigated. "The point is that we just do not know what unexpected things may be there," Paczynski said in the Times interview.
Born February 8, 1940, in Wilno, Poland (now Vilnius, Lithuania); died April 19 of brain cancer; survived by his wife, Hanka, and two children.