  General Relativity predicts that a star passing close to a supermassive black
hole should exhibit a relativistic redshift. We test this using observations of
the Galactic center star S0-2. We combine existing spectroscopic and
astrometric measurements from 1995-2017, which cover S0-2's 16-year orbit, with
measurements in 2018 March to September which cover three events during its
closest approach to the black hole. We detect the combination of special
relativistic- and gravitational-redshift, quantified using a redshift
parameter, $\Upsilon$. Our result, $\Upsilon=0.88 \pm 0.17$, is consistent with
General Relativity ($\Upsilon=1$) and excludes a Newtonian model ($\Upsilon=0$
) with a statistical significance of 5 $\sigma$.
