  In recent years, the evidence for the existence of an ultra-compact
concentration of dark mass associated with the radio source Sgr A* in the
Galactic Center has become very strong. However, an unambiguous proof that this
object is indeed a black hole is still lacking. A defining characteristic of a
black hole is the event horizon. To a distant observer, the event horizon casts
a relatively large ``shadow'' with an apparent diameter of ~10 gravitational
radii due to bending of light by the black hole, nearly independent of the
black hole spin or orientation. The predicted size (~30 micro-arcseconds) of
this shadow for Sgr A* approaches the resolution of current
radio-interferometers. If the black hole is maximally spinning and viewed
edge-on, then the shadow will be offset by ~8 micro-arcseconds from the center
of mass, and will be slightly flattened on one side. Taking into account
scatter-broadening of the image in the interstellar medium and the finite
achievable telescope resolution, we show that the shadow of Sgr A* may be
observable with very long-baseline interferometry at sub-millimeter
wavelengths, assuming that the accretion flow is optically thin in this region
of the spectrum. Hence, there exists a realistic expectation of imaging the
event horizon of a black hole within the next few years.
