Space is mostly empty, but it's also incredibly crowded with ghosts. When we look up at the night sky, we see the glowing survivors—the stars that haven't blown up yet. But for every few hundred stars you see, there’s a dark, invisible remnant lurking in the shadows. Most people asking how many black holes are there in the Milky Way expect a single, clean number.
The truth is a bit more chaotic.
Estimates aren't just guesses; they are based on the census of star formation over the last 13 billion years. If you ask an astrophysicist at NASA or someone like Dr. James Lattimer at Stony Brook, they’ll tell you the number is staggering. We are talking about millions. Tens of millions. Maybe even a hundred million.
It sounds like a sci-fi nightmare, right? A galaxy riddled with gravity traps. But space is big. Like, really big. Even with 100 million black holes, your chances of running into one are basically zero.
The Massive Census: Counting the Invisible
How do you count something you can't see? You don't count the black holes themselves; you count the stars that were big enough to become them.
Only the "heavyweights" turn into black holes. We're talking about stars with at least 20 times the mass of our Sun. When these monsters run out of fuel, they don't go out quietly. They collapse, explode in a supernova, and leave behind either a neutron star or a stellar-mass black hole.
Astronomers use the Initial Mass Function (IMF) to figure out the ratio of big stars to small stars. Based on the rate of star formation in our galaxy’s history, the consensus is that about 1 in every 1,000 stars is massive enough to end its life as a black hole. Since the Milky Way has roughly 100 billion to 400 billion stars, the math leads us to a range of 10 million to 100 million stellar-mass black holes.
Most of these are "quiet." They aren't eating anything. They aren't glowing. They are just wandering through the vacuum, occasionally bending light from distant stars behind them—a process called gravitational microlensing.
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The Big Boss: Sagittarius A*
While there are millions of small ones, there is only one "Supermassive" black hole at the center. This is Sagittarius A* (pronounced A-star).
It is a beast. It weighs about 4 million times the mass of our Sun.
We’ve actually seen it now. Thanks to the Event Horizon Telescope (EHT), we have a "photo" of the glowing gas swirling around its drain. It’s the anchor of the galaxy. While the millions of smaller black holes are like tiny potholes on a highway, Sagittarius A* is the grand central station. It’s located about 26,000 light-years away from Earth. Close enough to study, far enough that it’ll never bother us.
The Different "Flavors" of Black Holes
Not all black holes are created equal. When we discuss how many black holes are there in the Milky Way, we have to categorize them because they exist in different "weight classes."
Stellar-mass black holes are the most common. These are typically 5 to 50 times the mass of the Sun. They are the ones scattered everywhere.
Intermediate-mass black holes (IMBHs) are the missing link. These would be hundreds or thousands of times the Sun's mass. For a long time, we couldn't find any. They were like the Bigfoot of astronomy. However, recent data from missions like Gaia and LIGO suggests they might be hiding in the centers of globular clusters—dense balls of stars that orbit the galaxy. If they exist in the numbers some theorists suggest, we could add several hundred more to our galactic tally.
Then you have Primordial black holes. These are purely theoretical. Some scientists, like those following the work of the late Stephen Hawking, think they could have formed in the first seconds after the Big Bang. If they exist, they could be as small as an atom but weigh as much as a mountain. There could be trillions of them, and we wouldn't even know. They might even be the answer to what "Dark Matter" actually is.
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How Close is the Nearest One?
This is where people usually get nervous. If there are 100 million of these things, is there one in our backyard?
For a while, we thought Gaia BH1 was the winner. It’s about 1,560 light-years away in the constellation Ophiuchus. To put that in perspective, that’s about 9 quadrillion miles. You aren't getting there in a Tesla.
Then came Gaia BH2, sitting about 3,800 light-years away.
The thing is, we keep finding closer ones because our telescopes are getting better. Are there any closer? Probably. There could be a black hole 100 light-years away and we wouldn't know it unless it was actively ripping a star apart or passing directly in front of a bright light source. But even at 100 light-years, the gravitational pull on Earth is less than the pull of a person standing next to you.
The Search Methods: How We Find Them Today
In 2026, we don't just wait for them to eat something. We listen for them.
LIGO (Laser Interferometer Gravitational-Wave Observatory) changed the game. It detects ripples in spacetime caused by black holes crashing into each other. Since it started operating, we’ve "heard" dozens of these mergers. This has taught us that black holes are often found in pairs.
We also use X-ray binaries. Sometimes a black hole orbits a normal star. It acts like a cosmic parasite, sucking the gas off the star. As the gas falls in, it heats up to millions of degrees and screams in X-rays. Satellites like Chandra and NuSTAR see these "cries" and mark the location of a black hole.
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Why the Number Might Be Higher Than We Think
Some researchers argue that 100 million is a conservative estimate.
The Milky Way has swallowed other, smaller galaxies in its past. Every time we "ate" a neighbor, we likely inherited its black holes. The "Gaia-Enceladus" sausage galaxy, which merged with the Milky Way billions of years ago, probably dumped a massive amount of dark remnants into our stellar halo.
Also, we have to consider "rogue" black holes. These are black holes that were kicked out of their original homes by gravitational slingshots. They are flying through the galaxy at millions of miles per hour. Honestly, it's a bit unnerving, but again, the sheer scale of the galaxy makes a collision virtually impossible.
Common Misconceptions About the Galactic Count
- They are vacuum cleaners. They aren't. If you replaced the Sun with a black hole of the same mass, Earth wouldn't get sucked in. It would just get very cold and keep orbiting in the exact same path.
- They are all in the center. Nope. While Sagittarius A* is the big one at the hub, the millions of stellar-mass ones are spread through the spiral arms like raisins in a muffin.
- They live forever. Technically, they evaporate via Hawking Radiation, but for a stellar-mass black hole, this takes longer than the current age of the universe. For all intents and purposes, the ones we count today are here to stay.
What's Next for the Hunt?
The European Space Agency’s Gaia mission is still the MVP here. It’s tracking the positions and movements of over a billion stars with insane precision. By watching for stars that "wobble" for no apparent reason, Gaia is helping us find the "dark companions"—the black holes that aren't emitting any light.
In the next decade, the Nancy Grace Roman Space Telescope will use microlensing to find even more. We expect the confirmed count to jump from a few dozen to thousands in the next few years.
Actionable Steps for Amateur Observers
You can't see a black hole through a backyard telescope. Don't let anyone tell you otherwise. However, you can see where they live and understand the scale.
- *Locate Sagittarius A:** On a dark summer night, find the "Teapot" asterism in the constellation Sagittarius. The spout of the teapot points roughly toward the center of our galaxy, where the supermassive black hole sits.
- Track Cygnus X-1: This was the first black hole ever discovered. It's in the constellation Cygnus (the Swan). While you can't see the black hole, you can see the star it's orbiting (HDE 226868) with a decent amateur telescope.
- Join Citizen Science: Projects like "Black Hole Hunters" on Zooniverse allow you to look at real data from telescopes to help identify potential black hole candidates through light curves.
- Stay Updated: Follow the Event Horizon Telescope's annual releases. They are currently working on making "movies" of Sagittarius A* to show how it changes in real-time.
Basically, the Milky Way is a graveyard of dead stars. There are likely 100 million of these gravitational anomalies out there. They aren't threats; they are just part of the furniture of a mature galaxy. We're just finally getting the right glasses to see them.