FIGURE 1. Hungry mouth.

The event horizon of a black hole (the bright area at the centre of the image) completely absorbs emissions from matter behind the black hole when viewed from Earth. The result is a darker circle that could be seen in a sufficiently high-resolution image. In the case shown here — a theoretical calculation of the event-horizon shadow of the source Sgr A^*, which is the subject of Shen and colleagues' study¹ — the black hole is assumed to be rotating rapidly. The tendency of photons to be flung around the black hole in the direction of its rotation brings about an event-horizon shadow that is off-centre, to the right in the image. A brighter ring around the shadow is formed by light rays that are strongly deflected by the gravitational pull of the black hole without being absorbed by it. (Figure courtesy of Eric Agol, Univ. Washington.)