Pioneering astrophysicist; Born February 8, 1940; Died April 19, 2007. Astrophysicist Bohdan Paczynski, who pioneered ways to use gravitational fields in outer space to learn more about stars and planets, has died aged 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.

Paczynski spent 20 years at what is now Warsaw's Copernicus Astronomical Centre, becoming a professor in 1979. He moved to Princeton, New Jersey, in 1982.

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.

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

Besides his work with microlensing, Paczynski was also known for his championing of 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.