Supermassive Black Hole
Sagittarius A*
The supermassive black hole at the center of the Milky Way galaxy. It anchors our galaxy and has a mass 4.3 million times that of our Sun.
Mass4.3 Million Solar Masses
Distance26,670 light-years
Approaching Event Horizon...
Supermassive Black Hole
The supermassive black hole at the center of the Milky Way galaxy. It anchors our galaxy and has a mass 4.3 million times that of our Sun.
The supermassive black hole at the centre of our own galaxy, weighed by watching stars orbit it for thirty years, and photographed in 2022.
Sagittarius A* holds about 4.3 million solar masses at the centre of the Milky Way, roughly 26,000 light-years away. We know that figure with unusual precision, and the way it was obtained is one of the finest pieces of observational astronomy of the last half-century.
Two teams — one led by Reinhard Genzel in Germany, one by Andrea Ghez in the United States — spent nearly thirty years tracking individual stars orbiting the galactic centre. Working in infrared to see through the intervening dust, they measured the positions of stars in the innermost region night after night, year after year, until the orbits closed.
One star, S2, orbits every 16 years. At its closest approach it passes within about 120 astronomical units of the centre and reaches roughly 3% of the speed of light. An orbit that tight around an object that invisible leaves essentially one explanation. Genzel and Ghez shared the 2020 Nobel Prize in Physics for the work.
In May 2022 the Event Horizon Telescope collaboration released an image of Sagittarius A*, three years after publishing M87*. The delay was not for lack of data — both were observed in the same 2017 campaign.
The problem was that Sagittarius A* is small and fast. Gas orbits it in minutes rather than the days or weeks it takes around M87*, so the source changes appearance during the observation itself. Imaging it is like photographing a running child with a long exposure. Reconstructing a usable image required years of new algorithmic work and thousands of simulations.
The result matched the prediction: a dark central shadow surrounded by a bright asymmetric ring of emission, with a size consistent with a 4.3-million-solar-mass black hole. Two black holes differing in mass by a factor of a thousand both behaved exactly as general relativity said they would.
Despite its mass, Sagittarius A* is faint. It accretes very little material — far below the rate that would make it a quasar — and its output is a tiny fraction of what a black hole this size could produce if it were feeding well.
This is fortunate and also somewhat puzzling. There is gas available in the galactic centre, and the question of why so little of it reaches the black hole is an active area of research. Occasional flares are observed in infrared and X-rays, lasting hours, when clumps of material do fall in.
The scale is worth holding onto: 4.3 million solar masses sounds overwhelming, but the event horizon is only about 12 million kilometres across. That would fit comfortably inside the orbit of Mercury. Its gravitational influence dominates only the innermost few light-years of a galaxy 100,000 light-years wide — the Sun orbits the galactic centre because of the combined mass of everything inside our orbit, not because of the black hole.