Sagittarius A*
/ SAJ-ih-TAIR-ee-us A-star /
At the very center of our galaxy, in the constellation Sagittarius, lurks a compact source of radio waves so unusual that radio astronomers in the 1970s tagged it with an asterisk to mark it as special. They named it Sagittarius A*, pronounced 'Sagittarius A-star,' and for decades its true nature was uncertain. Today we know it is the supermassive black hole at the heart of the Milky Way — the dark, dense anchor about which our entire galaxy turns.
Sagittarius A* has a mass of roughly 4 million Suns crammed into a region smaller than the orbit of Mercury. We know its mass with remarkable precision not by seeing it — a black hole emits no light — but by watching stars orbit it. Over more than two decades, astronomers tracked individual stars, especially one called S2, looping around an invisible point on tight elliptical orbits; S2 whips around once every 16 years, reaching speeds of thousands of kilometers per second at closest approach. The only thing that can anchor such fast, tight orbits in such a tiny volume is a black hole of about 4 million solar masses. In 2022 the Event Horizon Telescope released an image of the glowing ring of gas around its shadow, confirming the picture directly.
Sagittarius A* matters as the nearest supermassive black hole, our best laboratory for testing how these giants work and how Einstein's gravity behaves in the strong-field regime. Its stellar orbits gave the cleanest proof that supermassive black holes are real, earning the 2020 Nobel Prize. Two cautions are worth stating plainly. First, it is remarkably quiet: it is not a blazing quasar but a faint, barely-feeding black hole. Second, 'nothing escapes' refers only to its event horizon, a tiny sphere at the very center; beyond that boundary it is not vacuuming up the galaxy — the Sun, 26,000 light-years away, orbits it no differently than it would orbit the same mass in any other form.
The star S2 swings around Sagittarius A* on a 16-year orbit, passing within about 17 light-hours of the black hole at closest approach — roughly four times the distance from the Sun to Neptune — and racing at some 3 percent of the speed of light. Tracking that single orbit pinned the central mass at about 4 million Suns and revealed Einstein's predicted shift in the star's path.
We weigh Sagittarius A* by timing stars that orbit it — most famously S2, which laps it every 16 years.
Sagittarius A* is quiet, not a quasar, and it is not a cosmic vacuum cleaner. 'Nothing escapes' applies only at its event horizon; far outside, its gravity is just that of 4 million Suns.