We know the mass of the Milky Way's central black hole to within 1%: Sagittarius A* weighs 4,297,000 Suns, give or take 42,000.
Astronomers have pinned the mass of Sagittarius A*, the black hole at the heart of the Milky Way, to 4,297,000 Suns — with a combined uncertainty of only about 42,000, roughly one percent.
We know the weight of the most massive single object in our galaxy to within about one percent. Sagittarius A*, the supermassive black hole anchoring the center of the Milky Way, comes in at 4,297,000 solar masses — 4.297 million times the mass of the Sun. The uncertainty on that number is about 42,000 solar masses, which sounds enormous until you notice it's one part in a hundred of the total.
That figure is not a rounded guess. It's written as M = (4.297 ± 0.012 ± 0.040) × 10⁶ solar masses, and the two separate error bars tell you exactly how confident the measurement is and where the doubt still lives.
How you weigh a black hole you cannot see
You never see Sagittarius A* directly — a black hole emits no light. Instead, astronomers watch the stars whipping around it. A cluster of stars orbits the galactic center at speeds up to a few percent of the speed of light, and each orbit is a tiny natural scale: track the shape and period of the loop, apply Kepler's laws as sharpened by general relativity, and the mass of whatever they're circling falls out of the math.
The definitive measurement came from the GRAVITY Collaboration, led by Reinhard Genzel's group at the Max Planck Institute for Extraterrestrial Physics, using the GRAVITY instrument on the Very Large Telescope Interferometer in Chile. By combining the light of four separate telescopes, GRAVITY resolves the positions of these stars with astonishing precision. In their 2022 paper in *Astronomy & Astrophysics* — "Mass distribution in the Galactic Center based on interferometric astrometry of multiple stellar orbits" (Abuter et al., A&A 657, L12) — the team tracked not just one star but several, and reported M = (4.297 ± 0.012) × 10⁶ solar masses. In the same work they nailed the distance to the galactic center at 8,277 parsecs, with an uncertainty of just 9.
Why there are two error bars, not one
The two numbers after the mass measure two different kinds of doubt. The first, ±0.012 million (about 0.3%), is the statistical error — how much the answer wobbles because of the finite number of measurements and the natural scatter in the data. The second, ±0.040 million (about 0.9%), is systematic, and it traces to something surprisingly mundane: optical aberrations inside the GRAVITY instrument itself. Small imperfections in how the telescopes' light is combined can nudge apparent star positions, and the collaboration folded that into an honest budget rather than pretending it away.
Add the two in quadrature and you get a combined uncertainty near 42,000 solar masses — the "give or take" in the headline. The Max Planck Institute for Extraterrestrial Physics quoted the clean 4.297-million figure in its 2023 press release, "Ultracompact: The Black Hole at the center of our Milky Way."
This number then traveled. When the Event Horizon Telescope Collaboration published its 2023 paper "First Sagittarius A* EHT Results. VI: Testing the Black Hole Metric," it adopted the GRAVITY value — writing it in full as (4.297 ± 0.012 ± 0.040) × 10⁶ — as an independent anchor for its own tests of whether the object behaves the way Einstein's equations demand.
The vivid part is the leverage. A star named S2 completes a full orbit of Sagittarius A* every 16 years, swinging in as close as about 120 astronomical units — inside that loop, it moves fast enough that its light visibly shifts color from the pull of gravity. Decades of patiently timing that single star, and a handful of its neighbors, is what turned an invisible object into a number good to one percent.
The seam still open: that systematic error, the larger of the two, is the instrument arguing with itself. Sharper optics or a new generation of interferometers could shrink it — and a better-known mass is exactly the tool needed to test whether the shadow the EHT imaged is precisely the size Einstein predicts, or something the theory doesn't yet explain.