The Sagittarius A* Shadow Diameter Against Its Orbital Mass and Distance

Stellar orbits give the mass of Sagittarius A* and the distance to the Galactic Centre with a precision no extragalactic target can match. Propagating those values through the Schwarzschild shadow formula predicts a shadow diameter of 53.3 microarcseconds, with the gravitational radius subtending 5.12 microarcseconds. The measured emission-ring diameter of 51.8 plus or minus 2.3 microarcseconds is 0.973 times the predicted shadow diameter, and the inferred shadow diameter of 48.7 plus or minus 7.0 microarcseconds is 0.914 times it, differing from the prediction by 0.65 standard deviations at its own quoted width. The ring is not the shadow, and the few-per-cent offset between them is in the direction and of the size that emission outside the critical curve produces. Because the mass and distance are known to a few per cent or better, this target, unlike M87*, is limited by the imaging and by the ring-to-shadow conversion, not by its inputs. Computed from published values; simulated, not measured.