Gravitational bending of light rays near objects of size approaching 2MG/c2 (gravitational radius or ‘event horizon’) is known to be severe. Light rays circle indefinitely at 3MG/c2 as Charles Darwin (the grandson) described in 1958. Gravitational lensing causes more distant objects to appear as arcs or multiple images. As collapsing stars take infinite time to reach 2MG/c2, Trevor Marshall considered light paths (geodesics) from objects sized 2-3MG/c2, as a spherical collapsar or supermassive galactic centre. The figure indicates how a distant observer sees the rear side of the collapsar, with the full image appearing to have a size 2.6 times larger.
Light reaches the exterior from a surface light cone greater than 68.3o (90o in the 3MG/c2 limit), not an infinitesimal light cone as generally assumed. Scattered reflected light or thermal emission from the collapsar would be detectable. Unlike from a hypothetical ‘black hole’, Marshall pointed out, the collapsar is not absolutely ‘black’, though it blanks out background radiation. The M87 galactic centre has a bright accretion disc, giving greater problems in detecting the interior object; our galactic centre (Sgr A*) has only a very weak disc so processing to reveal a non-black centre is more feasible.