  Astronomers have discovered many candidate black holes in X-ray binaries and
in the nuclei of galaxies. The candidate objects are too massive to be neutron
stars, and for this reason they are considered to be black holes. While the
evidence based on mass is certainly strong, there is no proof yet that any of
the objects possesses the defining characteristic of a black hole, namely an
event horizon. Type I X-ray bursts, which are the result of thermonuclear
explosions when gas accretes onto the surface of a compact star, may provide
important evidence in this regard. Type I bursts are commonly observed in
accreting neutron stars, which have surfaces, but have never been seen in
accreting black hole candidates. It is argued that the lack of bursts in black
hole candidates is compelling evidence that these objects do not have surfaces.
The objects must therefore possess event horizons.
