Weighing a star with light

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, exaggerated for illustrative purposes, is depicted in the video in the online version of this story.) From the ellipse’s dimensions the researchers could infer Stein 2051 B’s mass, roughly two-thirds that of the Sun. The new result agrees with astrophysical models of white dwarfs and may help settle a long-running discrepancy. Previous analyses based on estimates of the orbital motion of Stein 2051 B and its binary companion had suggested—erroneously, it now seems—that the star was much lighter. The new study marks the first time that apparent shifts in the position of a star due to gravitational lensing have been used to determine the mass of a body outside our solar system. (K. C. Sahu et al., Science 356, 1046, 2017, doi:10.1126/science.aal2879 .)