PRINCETON - Princeton University astrophysicist Bohdan Paczynski, 67, who pioneered ways to use gravitational fields in outer space to learn more about stars and planets, has died.

He died Thursday at his Princeton home after a three-year battle with brain cancer, the university said yesterday.

Mr. Paczynski, a native of Poland who came to Princeton in 1982, 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.

He 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.

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