Black holes are the sharks of the universe. There’s a lot of unjustified fear of them and it's difficult to actually encounter one. Space is very big and stellar black holes are not much larger than a small island, so to study them you’re gonna need a bigger telescope.
When we talk about black holes in the Milky Way, we tend to picture Sagittarius A* – the behemoth at the core of our galaxy. Yes, it weighs over 4 million times our Sun, but it is only about 6,000 times larger. It is also 26,000 light-years away, so not really in our neighborhood.
Sagittarius A* is a supermassive black hole and it’s the only one of its kind in the Milky Way. The rest are stellar black holes that are never heavier than 15 times our Sun.
The closest to Earth is V616 Monocerotis, which is 3,000 light-years away and nine to 13 times the mass of the Sun. Then there’s Cygnus X-1, located 6,000 light-years away and 15 solar masses. Relatively nearby at 7,800 light-years, GRO J0422+32 is the smallest stellar black hole discovered.
Those three black holes have something in common beyond being the closest known black holes. They all have companion stars. That’s how we detected them. The companion star loses material that gets eaten by the black hole, but before it plunges through the event horizon, it heats up and starts emitting X-rays.
Black holes are, as the name suggests, black. Light doesn’t escape and so we are stuck with ways to look for them. They do bend spacetime, so ideally we could look for the microlensing effects, tiny variations in the light of more distant stars. But the chance of that is minuscule. We need to look at the right time when a nearby black hole and a star are aligned. Chances of that are close to zero.
So how can we estimate how close the nearest black hole is? A research paper last year discovered a black hole faintly emitting radio waves, but no X-rays. The study looked at a tiny patch of sky and discovered a "quiet" black hole 7,200 light-years from Earth, suggesting there could be millions of black holes out there.
Could this really be the case and how close would this make the closest black hole to Earth?
Currently, we can estimate the number of black holes based on the number of supernovae. Researchers estimate about 20,000 stellar explosions have happened over the last million years in the Milky Way. Assuming the rate hasn’t changed over its 13 billion years, and even considering that only the most massive stars end up being black holes, tens of millions of black holes should be hiding in the Milky Way.
The Milky Way is 100,000 light-years across and with an average thickness of 1,000 light-years. Simplistically, it has a volume of 7.86 trillion cubic light-years. If we assume that there are just 1 million black holes in our galaxy, it means there is one every 125 light-years.
Obviously, this is a coarse approximation: Black holes might be rarer, they could have merged into larger objects, and they are definitely not evenly distributed in space.
Nevertheless, there is a large undiscovered population of black holes out there. It won’t be discovered overnight, but a mixture of serendipitous observations and dedicated campaigns might uncover more and more of these fascinating objects.