A pulsar in orbit with two other stars—the first trio of its kind ever discovered—could soon be used to put the general theory of relativity, Einstein's theory of gravity, to an unprecedented test. Astronomers at the Robert C. Byrd Green Bank Telescope in West Virginia announced the discovery of the exotic stellar system this week. A pulsar is a spinning neutron star, the superdense leftover core of a supernova. As it whirls, it emits a beam of radio waves that sweeps the sky as steadily as the ticking of an atomic clock. By tracking tiny variations in the flashing, astronomers can tell the masses and motions of other bodies in orbit around it. Researchers hope to use that information to see whether Einstein correctly described the nature of mass, a basic tenet of general relativity.