Sagittarius A*: The Supermassive Black Hole at the Heart of the Milky Way — Where Stars Orbit at 27 Million km/h

There is a black hole sitting at the exact centre of the Milky Way. It weighs four million times more than the Sun. Stars orbit it the way Earth orbits the Sun — except instead of one lap every 365 days, they complete their circuit every 16 years. At speeds that make a speeding bullet look like it is standing still.

For most of human history, we had no idea it was there. Thousands of light-years of gas and cosmic dust blocked every telescope we had. Then, in the 1990s, two rival teams of astronomers pointed infrared cameras at the centre of our galaxy and watched individual stars trace tight ellipses around an invisible point. The math was unambiguous. What they found would win a Nobel Prize — and change everything we thought we knew about the neighbourhood we live in.

What is Sagittarius A*, and where is it?

Sagittarius A* (pronounced Sagittarius A-star) is a supermassive black hole at the gravitational centre of the Milky Way galaxy. Every star you can see in the night sky — including our Sun — is orbiting it right now. It sits about 26,000 light-years away, in the direction of the constellation Sagittarius, hidden behind such thick clouds of interstellar gas and dust that visible light cannot pass through. Until infrared astronomy matured in the 1990s, the centre of our own galaxy was essentially invisible to us.

Its event horizon — the boundary beyond which nothing, not even light, can escape — spans roughly 24 million kilometres across. That is about 18 times wider than the Sun. On human scales it sounds enormous. On galactic scales, it is a pinprick. The entire Milky Way stretches 100,000 light-years. Sagittarius A* is the silent engine at its exact middle.

How did scientists discover the black hole at the centre of the Milky Way?

Starting in the early 1990s, two competing research teams — one led by Reinhard Genzel at the Max Planck Institute in Germany, the other by Andrea Ghez at UCLA — began tracking individual stars near the galactic centre year by year, photograph by photograph. Through infrared cameras capable of seeing past the dust clouds, they watched those stars trace tight ellipses around a single, invisible point.

The orbits told a story that nothing else could explain: something at that location was immensely massive and physically tiny. Billions of solar masses compressed into a volume smaller than our solar system. Only a black hole produces that.

What stars orbit Sagittarius A*, and how fast are they moving?

The most closely watched is S2 — a star roughly 15 times the mass of our Sun. It orbits Sagittarius A* in just 16 years. That number sounds modest until you compare it with our own situation: the Sun takes approximately 225 million years to complete one orbit of the Milky Way. S2 does it in 16. The reason is proximity: its orbit passes within about 120 astronomical units of the black hole — roughly three times the average distance from the Sun to Pluto — but instead of the Sun's gravity, it is feeling the pull of four million suns at once.

At its closest approach, S2 reaches speeds that strain comprehension.

For scale: a high-powered rifle bullet travels at roughly 1 km/s. S2 moves at 7,650 times that speed. The International Space Station orbits Earth at 7.66 km/s. S2 is about 1,000 times faster than the ISS. At these velocities, relativistic effects become directly measurable — scientists have confirmed a gravitational redshift in S2's light as it dips close to the event horizon, exactly matching Einstein's general relativity predictions. This is not a laboratory experiment. This is fundamental physics playing out live, in real time, around a real black hole, 26,000 light-years from your bedroom window.

How was Sagittarius A* first photographed?

The Event Horizon Telescope is not a single dish. It is a coordinated global network of eight radio observatories — spanning Hawaii, Chile, Arizona, Spain, Mexico, and the South Pole — all recording the same signal simultaneously and combining their data through a technique called Very Long Baseline Interferometry. By linking telescopes spread across the diameter of the Earth, scientists effectively create a single instrument the size of our entire planet. Only something that large has the resolving power to image an object just 52 millionths of an arcsecond across in the sky.

To put that in physical terms: imaging Sagittarius A* is like reading the text on a coin resting on the surface of the Moon — from Earth.

Sagittarius A* was harder to photograph than M87* (the first black hole ever imaged, in 2019). M87* is 1,500 times more massive — physically larger and far slower to change. Sgr A* fluctuates on timescales of minutes as hot gas churns around it. Imaging it was like trying to photograph a campfire in a windstorm, then compositing hundreds of noisy frames into a single coherent picture. The result — a glowing asymmetric ring of heated gas around a dark central shadow — is the silhouette of an object whose interior is cut off from the observable universe.

Is Sagittarius A* dangerous? Could it affect Earth?

No. Sagittarius A* is 26,000 light-years away. At that distance, its gravitational influence on the solar system is negligible — far less than the tug from nearby stars. The Sun remains the dominant gravitational body in our neighbourhood, and Sgr A* cannot meaningfully compete with it from here.

It is also, by galactic standards, unusually quiet. Many large galaxies have active galactic nuclei — supermassive black holes continuously consuming surrounding matter and blasting twin jets of energy visible across billions of light-years. Our black hole is dormant by comparison. Astronomers sometimes describe it as starved: there simply is not much material falling into it at the moment.

However — and this is where it gets genuinely unsettling — Sagittarius A* has not always been this quiet. Scientists believe that roughly 3.5 million years ago, it produced a colossal Seyfert flare. For tens of thousands of years, twin cones of ionised plasma erupted from the galactic centre, blasting through the Milky Way's polar regions. At the time, our direct ancestors — Australopithecus — were walking the African savanna, entirely unaware. The flare would have been invisible to the naked eye at our distance. But the scorch marks are still there, and still growing cooler today.

We call them the Fermi Bubbles: two enormous lobes of high-energy gas, each stretching roughly 25,000 light-years above and below the galactic plane. They were discovered in 2010 by the Fermi Gamma-ray Space Telescope and remain one of the most striking and debated structures in the entire Milky Way. They are, in the most literal sense, the exhaust from the last time Sagittarius A* had a proper meal.

Could Sagittarius A* ever wake up again?

Probably. In 2011, astronomers detected a gas cloud — nicknamed G2 — on what looked like a direct collision course with Sgr A*. The scientific community held its collective breath. Some models predicted a brief feeding event that would light up the galactic centre to millions of times its current brightness. G2 made its closest approach in 2014.

Sagittarius A* barely stirred.

The cloud may have been a star wrapped in a dust envelope, which survived the encounter intact. The black hole took only a small meal. Scientists came away with a data point and a reminder: predicting black hole behaviour is still very much an open problem in astrophysics. A diffuse gas streamer from the galactic centre is still slowly spiralling inward. Future flares are a matter of cosmic probability, not possibility. On human timescales — centuries, millennia — Sagittarius A* is effectively inert. On geological timescales, the next major outburst is a question of when, not whether.

There is something disorienting about sitting with all of this. Every star you have ever seen in the sky. Every constellation, every planet, the smear of the Milky Way on a clear night. All of it locked in a slow orbit around a four-million-solar-mass black hole that spent most of human history invisible to us. Not in a distant galaxy. Right here, at the gravitational centre of everything we call home.

Explore what is currently in orbit around Earth on the SkyLens live tracker. For a deeper look at extreme cosmic physics, visit the learn section. More stories about the Milky Way and the objects within it are in the SkyLens blog.