Milky Way supermassive black hole archeology
May 1st, 2026
NuSTAR image of the Galactic center region. Sgr A* is the position of supermassive black hole at the center of the Milky Way galaxy. Green dashed ellipses show the areas of giant molecular clouds. The distance from the Bridge to Sgr A* is 200 light years. Credit: Mori 2015

At the center of our Milky Way galaxy is a black hole with a mass more than a million times the mass of the Sun, called Sgr A*. This has been directly confirmed by detailed radio imaging of material close to the black hole by the event horizon telescope as well as the motions of stars in the center of the galaxy effected by the gravitational pull of this supermassive black hole. The low luminosity of Sgr A* indicates that the system is in a relatively quiescent state compared to Active Galactic Nuclei (AGN) in other galaxies which may harbor more massive black holes. However, this has not always been the case, and evidence of higher luminosity in the past is indicated by the increasing X-ray brightness of giant molecular clouds near Sgr A*. Over the past four years observations by NuSTAR of regions close to the center of our galaxy have confirmed that X-ray emission from one of these clouds, called “The Bridge” has been increasing and is likely due to reflection of X-rays from a past Sgr A* outburst approximately 200 years ago. NuSTAR observations last week of The Bridge will add to the detailed investigation of the full profile of this Sgr A* illumination event. Characterizing past Sgr A* outbursts is a necessary step towards understanding the physical mechanisms that triggered major outbursts from a quiescent supermassive black hole, possibly similar to a tidal disruption event seen in other AGN. Observations of The Bridge will continue in 2027, for a proposal selected to be part of cycle 12 of the NuSTAR General Observer program.