What Are Black Holes Made Of? | Unveiled

What Are Black Holes Made Of?

Black Holes are among the most fascinating and mysterious objects in the known universe. They’re thought to hold many secrets about how space and time work, and some say that they could even be used to create wormholes through galaxies one day. But what creates these incredible, destructive and iconic forces of nature?

This is Unveiled and today we’re answering the extraordinary question; What are black holes made of?

Black holes are so strange and powerful that they can feel like an almost unreal concept straight out of a science fiction novel. But these things are definitely real, and they’re crucial to our reality! Because of their mysterious properties, though, scientists have trouble even detecting them at all. They effectively pull in all light that comes close, so the only way to know a black hole is there is to notice how it interacts with other matter around it. Which means that if a black hole ever wasn’t near anything else, then we’d have no way to know that it’s there at all! In fact, it’s thought that most stellar black holes are isolated right now - impossible to detect and therefore totally unknown to us. It’s perhaps a scary thought for interstellar space travellers of the future; that a black hole they were oblivious to could unfortunately consume them. Especially because black holes aren’t exactly rare either… We can’t count them directly, but we know they’re formed from certain types of large star, leading to estimates that there could be between ten million and a billion black holes in just our own Milky Way Galaxy.

Black holes form out of collapsing stars, but not just any stars. Different stars go through different processes depending on how large they are, and only the most massive form black holes at the end of their life cycles. The stars are initially kept alive by burning fuel in their core, but as soon as they run out of fuel their own gravity turns against them, causing them to implode and eventually transform into a stellar black hole.

But a stellar black hole is only one of three types of black hole that scientists believe exist. Another is primordial black holes, which are thought to have formed right after the big bang. These mind-boggling structures are the smallest, and according to NASA are the size of an atom with all the mass of a mountain. But then there are supermassive black holes which, as their name implies, are extremely massive (potentially billions of times as massive as our own sun). Even the experts aren’t one-hundred-percent sure how these things form… They could be the result of thousands of smaller black holes merging together, or just a single black hole devouring much more matter than it can reasonably handle. Other theories have supermassive black holes forming out of dark matter but, in any case, these vast and sprawling constructs are usually found at the centre of galaxies. Theoretically, there should exist a “middle ground” between black hole types too - where a black hole is bigger than stellar but not so big to be categorised as supermassive - but we haven’t conclusively proven it yet. So, for now, science mostly sticks with the general understanding that black holes are simply formed out of stars; that at their beginning they’re the product of supernovas.

But, can we truly say that black holes are made of stars? Carl Sagan famously said of human beings; “We’re made of star stuff,” meaning that the elements that constitute a human, such as carbon, hydrogen, nitrogen, and oxygen, all came from stars going supernova and spreading them across the universe. So, in that sense we’re made of the same matter that black holes are, and we’re both offshoots of the same thing - a star’s self-destruction.

But there’s where the comparison between us and a black hole ends. Because, while we know lots about how the human body works, we know very little about how a black hole does… and we’re still not even close to confidently knowing what happens at the centre of one. The problem is that most of a black hole isn’t really made of anything at all, and that’s because any matter that gets close enough is devoured... into apparent nothingness. Black holes are, at their ominous and unusual hearts, massive distortions in spacetime. They pack large amounts of matter incredibly densely into an inconceivably small point. So much so that our understanding via conventional physics breaks down near the centre, or the singularity.

Turn the issue on its head, though, and we could say that black holes are actually made of anything and everything. They’re formed out of collapsing stars, but stars form in the first place from gases and collapsing nebulas. These are fundamental, cosmological processes but dial them back far enough and they - along with everything else in the universe - begin with the same raw materials. In theory, then, if you had a way to compress matter far enough, you could turn anything into a black hole. It’s not at all likely, or even physically, practically possible, but the idea hinges on the concept of the Schwarzschild Radius, proposed by Karl Schwarzschild, which gives an equation to know how far down something has to be compressed to form a black hole. Ultimately, if an object is compressed to the point where its physical radius is smaller than its Schwarzschild radius, then it will do exactly that!

So, extremely hypothetically speaking, anything could be a black hole; buildings, cars, baseballs, pencils, even humans ourselves if all of our mass was compressed far enough. Earth could become a black hole, but all of its mass would need to be compressed down to a speck of just 8.7 millimetres before that happens… In reality, though, we don’t have to worry. There’s no conceivable way that an Earth-induced black hole could happen in nature unless Earth somehow grew far more massive than it currently is. Even our own sun isn’t massive enough to ever actually form one.

Not that such seemingly standard facts of life have discouraged people from trying to make tiny black holes of their very own - or variations of. In 2016, for example, physicist Jeff Steinhauer got around the apparent impossibilities by creating imitation black holes in his laboratory using sound waves - rather than light. A spot out of which sound can’t escape, it’s a simpler concept (relatively speaking) but impressive nonetheless!

If anything, efforts like Steinhauer’s show just how mysterious genuine black holes are. Anything could be happening inside of one, but there’s no way of getting even a glimpse ourselves. What we do know is that black holes are formed from regular matter initially, but only when it forms into the universe’s largest stars, and only when those collapse. In the backs of the minds of most black hole enthusiasts there is the question of dark matter, but that’s another enigma entirely! Until we can get to grips with that which we can’t see, they’re the fascinating product of perishing stars. And that’s what black holes are made of.