The no-hair theorem of general relativity predicts that the spacetime around a black hole can be expressed in terms of only three parameters: the black hole mass, spin, and charge. Since it is hard to see how a real astrophysical black hole could sustain a large electric charge, the no-hair theorem predicts that the black hole can be characterized by its mass and spin alone.
The strong curvature of spacetime near a black hole produces a dark shadow surrounded by a bright photon ring. The shape of this shadow is roughly circular. Detecting the shadow of a black hole and establishing that it is indeed circular would constitute an observational test of general relativity.
The diameter of the shadow is proportional to the mass of the black hole and is mostly insensitive to the value of the black hole spin. Detecting the shadow would also allow astronomers to obtain a direct estimate of the ratio of the mass of a black hole to its distance from the observer.
General relativity predicts that the shadow of a black hole should be circular (middle panel), but a black hole that violates the no-hair theorem could have a prolate (left) or oblate (right) shadow. Future EHT images of nearby supermassive black holes will be able to test this prediction. (figures courtesy D. Psaltis and A. Broderick)