black hole shadow

A black hole shadow is the dark region of the sky centered on a black hole when viewed from afar. Light cannot pass through the black hole, and for example, stars situated exactly behind the black hole (to the observer) won't be seen, or at least not at their normal positions; within a certain radius, nothing is seen, i.e., blackness. This shadow can be highly obscured by electromagnetic radiation (EMR) generated in front of the black hole when accretion is taking place, i.e., thermal emission from heating under pressure and various forms of non-thermal emission from gas's interaction with electric fields and magnetic fields. Discerning the shadow requires viewing at a wavelength such that there is a high contrast between EMR from this foreground, and EMR just outside the edge of the shadow (as well as being a wavelength that penetrates clouds further in front of the black hole). For current efforts to view nearby (negligible redshift) supermassive black holes, wavelengths on the order of a millimeter are used (millimeter astronomy, at the border between microwave and infrared).

The shadow's edges are affected by the significant bending of light paths by the strong-field gravity. The size of the shadow is determined by the trajectory of photons, the shadow disk's edge being at the closest points in the sky that photons end up aimed at the observer. The size of a black hole is "by definition", often thought of as the size of the event horizon. The visible dark disk has a larger diameter than the event horizon, for a Schwarzschild black hole, being (27^1/2)/2 (≈ 2.6) times larger. A bright ring (photon ring) around the shadow is light redirected from near and distant sources, some of which has encircled the black hole more than once, which can be discerned through the fine detail of the ring, which consists of multiple concentric rings. The ring's appearance (and presumably the shape and size of the shadow) is also affected by the black hole's rotation as well as transients due to any accretion, and the ring is generally asymmetric (brightest where the rotation is toward us) and is constantly changing.