Sag A*

Sag A* (more officially, Sgr A*) is the supermassive black hole (SMBH) at the center of the Milky Way. It is not active now, i.e., appreciable accretion is not taking place. Its mass is 4.3 million M_Sun. Reflections on molecular clouds in the region (acting as reflection nebulae) are believed to have revealed some of the SMBH's past activity.

It is a radio source (RS) discovered in 1974 within an area of the sky already noted for its radio signal from the beginning of radio astronomy. It was theorized to be a black hole for some years, followed by considerable confirmation in 2002 with the observation of orbits of stars in its vicinity (e.g., S2), orbiting something unseen that was clearly the mass of an SMBH. The Event Horizon Telescope (EHT), which was developed to resolve its event horizon using millimeter astronomy, released its first image in 2022 after years of effort.