Weighing the Invisible: How We Measured the Black Hole at the Center of the Galaxy

At the center of the Milky Way, about 27,000 light-years from here, there is an object that emits almost no light of its own and yet weighs as much as four million Suns. We call it Sagittarius A*, usually shortened to Sgr A*, and for decades it was a suspicion rather than a fact. The remarkable thing is not just that it exists. It is that we managed to weigh something we cannot see, sitting behind so much dust that no ordinary telescope can look at it, and that we now have its picture. The methods behind those two achievements are worth understanding, because they show how astronomers extract certainty from very little.

The first problem is the dust. The plane of our galaxy is thick with gas and dust that blocks visible light almost completely along the line of sight to the center. If you point an optical telescope at the constellation Sagittarius, you see a crowded star field and nothing obviously special. The breakthrough came from observing in the infrared, which slips through dust far more easily than visible light, and from a technology called adaptive optics that cancels out the blurring caused by Earth’s atmosphere by flexing a mirror hundreds of times a second. Starting in the 1990s, two groups, one led by Reinhard Genzel in Germany using the Very Large Telescope in Chile, and one led by Andrea Ghez in California using the Keck telescopes in Hawaii, began taking sharp infrared images of the galactic center year after year.

What they watched for was motion. A cluster of stars sits very close to Sgr A*, and over years of observation these stars visibly move, tracing out loops on the sky. One star in particular, labeled S2 by one team and S0-2 by the other, turned out to be on a tight elliptical orbit that it completes in about sixteen years. That is fast enough that astronomers patient enough to watch for a decade and a half could record a full circuit. At its closest approach the star whips around the central object at more than 7,000 kilometers per second, a couple of percent of the speed of light, and swings out again.

Here is where the weighing happens, and the physics is almost old-fashioned. A star on a closed orbit obeys the same law Johannes Kepler wrote down for the planets and Newton later explained: the size and period of the orbit are fixed by the mass of whatever it is orbiting. Measure how big the orbit is and how long it takes, and the central mass falls right out of the equation. When Genzel’s and Ghez’s teams plugged in S2’s orbit, the answer was about 4.1 million times the mass of the Sun, packed into a region no larger than our solar system. Nothing known to astrophysics can be that massive and that compact and still avoid collapsing into a black hole. A cluster of dark stars would not fit, and would not be stable. The conclusion, reached by tracking a single star for years, was that the galaxy hosts a supermassive black hole.

The orbit gave a bonus. When S2 made its closest approach in 2018, instruments precise enough to catch it detected the light from the star being stretched to redder wavelengths as it climbed out of the black hole’s gravitational well, an effect predicted by Einstein’s general relativity and never before seen in such a strong field around such an object. The theory passed. In 2020 the Nobel Prize in Physics recognized this body of work, with one half going to Roger Penrose for proving mathematically that black holes are a natural consequence of general relativity, and the other half shared by Genzel and Ghez for the discovery of the supermassive compact object governing the orbits at the galaxy’s heart.

For all that, no one had seen the thing itself. That came in May 2022, when the Event Horizon Telescope collaboration released an image of Sgr A*: a fuzzy bright ring wrapped around a dark center, the shadow cast by the black hole on the glowing gas swirling around it. The Event Horizon Telescope is not a single instrument but a network of radio observatories scattered across the planet, from Hawaii to Spain to the South Pole, linked so that they function together as a telescope effectively the size of the Earth. Only an aperture that large has the sharpness to resolve something as small, on the sky, as Sgr A*‘s shadow, which is comparable to seeing a doughnut on the surface of the Moon.

Imaging Sgr A* was harder than it sounds, and harder than the collaboration’s first target. In 2019 the same network had produced the first-ever image of a black hole, the giant at the center of the galaxy M87, which is more than a thousand times more massive than ours. Counterintuitively, the bigger black hole was the easier one. Gas orbiting a black hole circles it in a time that scales with the black hole’s size, so around the enormous M87 object the gas takes days to go around, holding still long enough for a clean exposure. Around the much smaller Sgr A*, the same gas completes an orbit in minutes, so the source flickers and changes while you are trying to photograph it, like attempting a long exposure of a child who will not sit still. It took years of work and new analysis techniques to average through that variability and recover the ring.

The size and shape of that ring matched what general relativity predicts for a black hole of 4 million solar masses, an independent confirmation of the mass that the star orbits had already supplied. Two completely different methods, one tracking starlight over sixteen years and one assembling radio waves from across the Earth, pointed to the same object with the same heft. That kind of agreement is what turns a strong inference into settled knowledge.

The broader lesson reaches past our own galaxy. Sgr A* is not a freak. Astronomers now think nearly every large galaxy harbors a supermassive black hole at its center, and the masses of those black holes track the properties of their host galaxies closely enough that the two seem to have grown up together. Our own is on the quiet side, feeding slowly and shining faintly, which is part of why it took so long to pin down. A more active black hole would have announced itself with floods of radiation. Ours simply sat there, 27,000 light-years away, bending the paths of a handful of stars and waiting for instruments good enough to notice. We built them, watched a star complete its long loop, and weighed the dark thing it was falling around. Then we took its picture.