  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.
