We explore a simple spherical model of optically thin accretion on a Schwarzschild black hole , and study the properties of the image as seen by a distant observer .
We show that a dark circular region in the center — a shadow — is always present .
The outer edge of the shadow is located at the photon ring radius b _ { ph } \equiv \sqrt { 27 } r _ { g } , where r _ { g } = GM / c ^ { 2 } is the gravitational radius of the accreting mass M .
The location of the shadow edge is independent of the inner radius at which the accreting gas stops radiating .
The size of the observed shadow is thus a signature of the spacetime geometry and it is hardly influenced by accretion details .
We briefly discuss the relevance of these results for the Event Horizon Telescope image of the supermassive black hole in M87 .