Relativistic deflection of background starlight measures the mass of a nearby white dwarf star

General relativity weighs a white dwarf Light from a background star is deflected by the gravitational field of the Sun. This effect was used in 1919 to provide some of the first evidence for general relativity. Sahu et al. applied the concept to another star: a nearby white dwarf called Stein 2051 B, which passed close in front of a more distant normal star (see the Perspective by Oswalt). The authors measured the tiny shifts in the apparent position of the background star, an effect called astrometric microlensing. The apparent motion matched the predictions of general relativity, which allowed the authors to determine the mass of the white dwarf. Science, this issue p. 1046; see also p. 1015 General relativity causes starlight to bend, allowing astronomers to weigh a white dwarf star. Gravitational deflection of starlight around the Sun during the 1919 total solar eclipse provided measurements that confirmed Einstein’s general theory of relativity. We have used the Hubble Space Telescope to measure the analogous process of astrometric microlensing caused by a nearby star, the white dwarf Stein 2051 B. As Stein 2051 B passed closely in front of a background star, the background star’s position was deflected. Measurement of this deflection at multiple epochs allowed us to determine the mass of Stein 2051 B—the sixth-nearest white dwarf to the Sun—as 0.675 ± 0.051 solar masses. This mass determination provides confirmation of the physics of degenerate matter and lends support to white dwarf evolutionary theory.