If the Sun were a grain of sand, Sagittarius A*—the supermassive black hole at the center of our galaxy—would be a full-sized pickup truck. Both sit in the Milky Way, but one of them contains 4.3 million times the mass of the other, packed into an event horizon you could fit inside Mercury’s orbit around the Sun.
The short answer
A supermassive black hole is an object so massive and dense that nothing—not even light—can escape its gravity once it crosses the event horizon. These cosmic heavyweights, millions to billions of times more massive than the Sun, sit at the centers of most large galaxies, including ours. Sagittarius A* (pronounced “Sagittarius A-star”) is the Milky Way’s supermassive black hole, located 26,673 light-years away in the direction of the constellation Sagittarius. It’s the same object people mean when they say “the Milky Way black hole”—they’re synonymous.
How we found an invisible monster
Here’s the trick: we can’t see Sagittarius A* directly because black holes don’t emit light. But we can watch stars near the galactic center whip around something invisible, and we can clock their speed.
Starting in the early 1990s, astronomers tracked a star called S0-2 as it orbited the galactic center. Every 16.2 years, S0-2 completes one lap, reaching extreme velocities—around 25% the speed of light—at its closest approach. By measuring the star’s orbit, astronomers calculated the mass of the object it was circling: 4.297 million times the mass of the Sun, give or take 0.013 million.
That’s the smoking gun. No visible object could be that massive and that compact without collapsing into a black hole. By the early 2000s, decades of watching dozens of stars dance around the galactic center confirmed it: Sagittarius A* exists, even if we couldn’t photograph it yet.
Then, on May 12, 2022, the Event Horizon Telescope Collaboration released the first direct image of Sagittarius A*‘s shadow—a glowing ring of superhot gas surrounding the black void where the event horizon sits. The image didn’t discover the black hole; it confirmed three decades of orbital math and gave us a portrait of the beast we’d been tracking all along.
What “supermassive” actually means
Let’s ground this in real numbers. The Sun weighs about 2 × 10³⁰ kilograms. Sagittarius A* clocks in at 8.6 × 10³⁶ kilograms—4.3 million solar masses. Its event horizon, the point of no return, stretches millions of kilometers across. For scale, that’s comparable to the size of Mercury’s orbit around the Sun. You could drop Mercury’s entire orbital path into Sagittarius A*‘s event horizon and still have room left over.
Stellar black holes—the kind that form when massive stars die—top out around 3 to 20 solar masses. Supermassive black holes are an entirely different beast, likely formed not from a single star’s collapse but from millions of years of smaller black holes merging and accreting gas at the centers of young galaxies. We still don’t fully understand how they get so big so fast, but we’re confident they grow alongside their host galaxies rather than forming afterward.
Why we can observe this one
Most supermassive black holes sit billions of light-years away in other galaxies, too distant and too faint to study in detail. Sagittarius A* is special because it’s ours, and 26,673 light-years—while vast by human standards—is close enough that we have a dense field of stars orbiting it like a cosmic speed trap. We’ve been watching those stars for over 30 years, and modern radio telescopes can now resolve details at the event horizon itself.
Think of it this way: you can’t see an invisible person directly, but if you watch a tennis ball swerve around something that isn’t there, you know exactly where the person is standing and how big they are. Sagittarius A* bends spacetime so dramatically that stars curve around it, and radio emissions from hot gas spiraling into the black hole give us the glowing “tennis ball” we need to trace its outline.
It’s also relatively quiet. Unlike some supermassive black holes in other galaxies that blaze as quasars (feeding frenzies of infalling matter), Sagittarius A* has a low accretion rate. It’s not gorging itself; it’s stable, and that stability makes it easier to study without glare from superheated jets drowning out the details.
The wrinkle: discovered in 1992, photographed in 2022
Here’s where popular science coverage sometimes muddles the timeline. Sagittarius A* wasn’t a 2022 discovery. Astronomers identified it as a strong radio source back in 1974, and by 1992, stellar orbit measurements had confirmed a supermassive black hole sat at the galactic center. The GRAVITY Collaboration’s 2020 observations refined the mass and distance to remarkable precision: 4.297 ± 0.013 million solar masses, 26,673 ± 6 light-years away.
The Event Horizon Telescope’s 2022 image was the capstone, not the foundation. It gave us the first direct visual confirmation of the event horizon’s shadow, validating Einstein’s general relativity predictions about how light bends around such extreme gravity. But the black hole itself? We’d been measuring it, tracking it, and calculating its properties for three decades before the photo dropped.
What it means for us (and why you’re safe)
Sagittarius A* anchors the galaxy gravitationally, but it’s not pulling us in. At 26,673 light-years away, our solar system is nowhere near its influence. We orbit the galactic center just like the black hole does, both held in stable orbits by the combined mass of hundreds of billions of stars, dark matter, and interstellar gas. The black hole isn’t a cosmic vacuum cleaner; it’s just the heaviest single object in a gravitational ensemble.
If you’ve heard warnings about black holes “consuming the galaxy,” ignore them. Sagittarius A* is in equilibrium. Stars and gas near it are either in stable orbits or falling in at a trickle. It’s not expanding, not approaching, and poses zero threat to Earth or anything else in our neighborhood of the Milky Way.
FAQ
What is a supermassive black hole?
A black hole with a mass between millions and billions of times that of the Sun, typically found at the center of large galaxies. Unlike stellar black holes (which form from dead stars), supermassive black holes likely grew from mergers and gas accretion over cosmic timescales. Most large galaxies have one.
How big is Sagittarius A*?
Its event horizon spans millions of kilometers—comparable in scale to Mercury’s orbit around the Sun. Its mass is 4.3 million times the Sun’s mass, packed into a region of space far smaller than our innermost planet’s orbital path.
Where is Sagittarius A*?
At the center of the Milky Way galaxy, 26,673 light-years from Earth in the direction of the constellation Sagittarius. When astronomers talk about “the Milky Way black hole,” they mean Sagittarius A*—the terms are interchangeable.
How do we know it exists if we can’t see black holes?
We track stars orbiting the galactic center and measure their speeds and paths. The math points to an invisible object of 4.3 million solar masses. The 2022 Event Horizon Telescope image captured the shadow of the event horizon, confirming decades of orbital data.
Is Sagittarius A* dangerous to Earth?
No. It’s 26,673 light-years away, and our solar system orbits the galactic center in a stable path. Sagittarius A* isn’t consuming the galaxy or expanding toward us—it’s a gravitational anchor, not an approaching threat.
What’s the difference between black holes and supermassive black holes?
Stellar black holes form when massive stars collapse and range from 3 to 20 solar masses. Supermassive black holes are millions to billions of solar masses and sit at galaxy centers. They likely formed through mergers and accretion rather than a single star’s death.
Written for general interest and accuracy-checked, but not a substitute for specialist sources.
If you’re curious how black holes form in the first place, What Is a Black Hole, Simply Explained? Gravity, Light, and the Point of No walks through the basics.