What compact radio source marks the Milky Way's central black hole?

The compact radio source marking the Milky Way’s central black hole is Sagittarius A*, usually abbreviated Sgr A*.

Sagittarius A* lies at the Galactic Center, in the direction of the constellations Sagittarius and Scorpius. It is not the black hole’s visible surface—black holes emit no light from inside their event horizons—but a compact source of radio and other electromagnetic emission associated with hot gas and material near the black hole.

Astronomers established the object’s enormous mass by tracking stars orbiting it, especially the star S2. Those stellar orbits show that about 4.3 million solar masses are packed into a very small region, strong evidence for a supermassive black hole. The radio source was discovered in 1974 by Bruce Balick and Robert Brown; the asterisk was added later to emphasize its compact, unusual emission.

A common mix-up is confusing Sagittarius A* with Sagittarius A, the larger complex radio structure containing it. In 2022, the Event Horizon Telescope released the first image of Sagittarius A*’s surrounding emission and shadow.