It seems impossible: if a black hole does not emit light, how can it leave a visible silhouette?
The answer does not require that there be a screen behind it or a surface on which a dark figure appears. In astronomy, we talk about a region where less radiation reaches the observer. To understand it, you have to look beyond the object itself and remember that its enormous gravity even affects the path of light rays.
The key is in what surrounds it. Near many black holes, there is extremely hot gas that emits radiation and forms a glowing background. Its gravity bends the paths of light.
This material can rotate around the object and reach very high temperatures. Although the black hole remains invisible on its own, its environment emits signals that specialized instruments can pick up. A part of that luminosity may curve its trajectory before reaching our telescopes, as a consequence of the intense deformation of space-time.
Some rays manage to escape towards us, but others cross the event horizon, a boundary from which nothing can return.
That limit is not a physical wall: it marks the point of no return. That is why the central area appears dark. The image depends both on the distribution of the gas and the angle from which we observe the system.
Therefore, we see a dark area surrounded by a luminous ring: it is the so-called shadow of the black hole.
In 2019, the international Event Horizon Telescope collaboration presented the first image of this phenomenon, obtained through a global network of radio telescopes. The result allowed us to observe the bright structure that surrounds the dark central region of a supermassive black hole.
And there is a surprising detail: that shadow appears larger than the horizon itself, precisely because gravity also deflects light from its surroundings.
REFERENCES
Event Horizon Telescope Collaboration; Kazunori Akiyama et al. “First M87 Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole”. The Astrophysical Journal LettersApril 10, 2019.
Event Horizon Telescope Collaboration; Kazunori Akiyama et al. “First M87 Event Horizon Telescope Results. VI. The Shadow and Mass of the Central Black Hole”. The Astrophysical Journal LettersApril 10, 2019.
