  A supermassive black hole in the nucleus of a galaxy will be revealed when a
star passes close enough to be torn apart by tidal forces and a flare of
radiation is emitted by the stream of stellar debris that plunges into the
black hole. Since common active galactic nuclei have accreting black holes that
can also produce flares, a convincing demonstration that a stellar tidal
disruption has occurred generally begins with a ``normal'' galaxy that has no
evidence of prior nuclear activity. Here we report a luminous UV flare from an
elliptical galaxy at z = 0.37 in the Groth field of the GALEX Deep Imaging
Survey that has no evidence of a Seyfert nucleus from optical spectroscopy and
X-ray imaging obtained during the flare. Multiwavelength data collected at the
time of the event, and for 2 years following, allow us to constrain, for the
first time, the spectral energy distribution of a candidate tidal disruption
flare from optical through X-rays. The luminosity and temperature of the
radiation and the decay curve of the flare are in excellent agreement with
theoretical predictions for the tidal disruption of a star, and provide the
strongest empirical evidence for a stellar disruption event to date.
