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Bohdan Paczynski, 67, noted astrophysicist

PRINCETON, N.J. -- Princeton University astrophysicist Bohdan Paczynski, who pioneered ways to use gravitational fields in space to learn more about stars and planets, died Thursday in his Princeton home after a three-year battle with brain cancer, the university said yesterday. He was 67.

He is best known for using an effect called gravitational microlensing, in which the gravity of a star passing in front of another star much farther away can magnify the background star's light much like a telescopic lens.

Mr. Paczynski showed that the effect could be used to survey the population of stars in our galaxy and to determine the mass of stars. It can also be used to discover planets around other stars, because the gravity of planets around the foreground star changes the lensing effect.

Mr. Paczynski was part of a consortium of astronomers who founded the Optical Gravitational Lensing Experiment, which has witnessed thousands of events. The project last year discovered one of the smallest planets known to exist outside the solar system.

"His influence on the field was enormous. Entire subfields of astrophysics either would not have existed without him or would have been radically different," said David Spergel, chairman of Princeton's astrophysical sciences department.

Besides his work with microlensing, Mr. Paczynski was also known for championing a once-controversial idea that gamma ray bursts, large bursts of energy observed in outer space, actually originated billions of light-years away, rather than within the Milky Way galaxy.