  In general relativity, isolated black holes are invisible due to an
infinitely large redshift of photons propagating from the event horizon to the
remote observer. However, the dark shadow (silhouette) of a black hole can be
visible on the background of matter radiation lensed by the gravitational field
of black holes. The black hole shadow is the celestial sphere projection of the
cross section of photon capture by the black hole. If the illuminating
background is far behind the black hole (at a distance much greater than the
event horizon radius), a classic black hole shadow of a maximal size can also
be observed. A minimal-size shadow can be observed if the same black hole is
illuminated by the inner part of the accretion disk adjacent to the event
horizon. In this case, the shadow of an accreting black hole is a lensed image
of the northern or southern hemisphere of the event horizon, depending on the
orientation of the black hole spin axis. A dark silhouette of the southern
hemisphere of the event horizon is seen in the first image of the supermassive
black hole M87* presented by the Event Horizon Telescope. The brightness of
accretion matter is much higher than the corresponding one of the usual
astrophysical stationary background in the form of numerous stars or extensive
hot gas clouds. For this reason, it is improbable that a black hole shadow can
be observed in the presence of very luminous accretion matter.
