Einstein’s general theory of relativity, first proposed more than a century ago, predicts that in the gravitational field of a massive body, light rays should bend by an angle that depends on the body’s mass. Researchers at the Space Telescope Science Institute have now exploited that effect, known as gravitational lensing, to determine the mass of a star. For two years, the team tracked white dwarf Stein 2051 B as it crossed in front of a distant background star. At their closest, the stars were separated by a mere 10th of an arcsecond—roughly the angle subtended in the sky by Pluto.
As the stars came into alignment, gravitational lensing by the white dwarf subtly distorted the apparent position of the background star. Specifically, the background star appeared to trace an ellipse a couple of milliarcseconds wide, even though its actual position in the sky all but remained fixed. (The illusion,...
