You and I have lived on planet Earth long enough to know that if you want to launch something into space, it needs to travel fast enough to escape the pull of Earth’s gravity. Launch it with too slow of a speed, and it crashes back into Earth. Launch it with a little more speed, and you can send it into orbit (like a satellite). But launch it fast enough, and it can escape from Earth’s gravity altogether. The speed to completely escape from Earth’s gravity is pretty fast: about 11.2 km/s, or 7 miles per second!
Make something denser and more massive, and you’ll have to go faster and faster to escape from it. To escape from the surface of the Sun, for instance, you’d have to go at 617 km/s, or 383 miles per second. What if there were something so massive and so dense that, in order to escape from it, you’d have to exceed the speed of light, or 186,000 miles per second? Well, that’s what a Black Hole is, something so massive and so dense that even light can’t escape. Since we’ve already talked about how nothing can travel faster than the speed of light, we know that if light can’t get out, nothing else can, either. This means that black holes are giant vacuum cleaners in space, sucking up everything that gets in their way and never letting them out again.
So how can you make one? We can find them easily enough, but how are they made? Let’s start with something you know: the Sun.
The reason the Sun is so bright and so large is because it’s burning its fuel. What does the Sun run on? Hydrogen. The Sun runs on nuclear fusion of Hydrogen where it takes Hydrogen atoms at the center and, under the incredible pressure of gravity, fuses them into a heavier element, such as Helium. The Sun has been around for over 4.5 billion years, and it’s about halfway through the phase of its life where it burns hydrogen into helium.
What’s going to happen when all the hydrogen has been burned into helium? Right now, the Sun is held up by all the energy produced by fusion, which balances out the force of gravity, which tries to collapse it. Well, when the hydrogen stops burning in the core, all that’s left is gravity, and the Sun will start to collapse. It will get dense enough and the pressure and temperature will get high enough that the Helium will now start to fuse into heavier elements like Carbon and Oxygen. Helium burning happens at much higher temperatures than Hydrogen burning does, so the Sun will expand, becoming a red giant.
The Sun will become so large that it will engulf the planets Mercury, Venus, and possibly even Earth!
But Helium burns relatively quickly, and when the Sun is done burning Helium, it isn’t massive enough to fuse Carbon and Oxygen into heavier elements. When this happens, the Sun will collapse again, and the remaining Helium will “flash burn”, where a burst of fusion can cause part of the surface of our star to be ejected, eventually forming a planetary nebula on the outside with a white dwarf star on the inside. When our Sun does this, it will wind up looking like this picture, known as the Cat’s Eye Nebula:
But our Sun is a pretty typical star. What would happen if we were a more massive star? (After all, even though they’re relatively rare, stars can have masses up to about 100 times the mass of our Sun.) Well, the more massive the star gets, the greater the pressure and temperature gets in the center. This means two things for us:
- Bigger stars burn their fuel faster, and therefore live shorter lives than less massive stars. (This is like Blade Runner, where the mad scientist Dr. Tyrell tells his proud creation, “The light that burns twice as bright burns half as long – and you have burned so very, very brightly, Roy.”)
- More massive stars can burn and fuse heavier elements than the Sun can.
Carbon and Oxygen, in a heavy enough star, can be fused into elements like Silicon, and if the star is massive enough, Silicon can be fused into metals like Iron. What can Iron be fused into? Absolutely nothing! Iron is the most stable element, so once you get Iron in your core, it can’t undergo either fusion or fission anymore. Just before it’s done burning all of its fuel, a massive star might look like the image below, segregated by density of its various elements.
Just like with our Sun, once any star is done burning all the fuel it can burn, it starts to collapse. Also just like before, the outer layers get blown off, producing a planetary nebula in the end, and the inner layers collapse even further, producing a compact object. If the star is of a comparable mass to our own (or a little bit greater), the pressure of the inner, compact object won’t be enough to destroy the atoms inside, and you’ll get a white dwarf. But if the star is significantly more massive, the pressure from the collapse becomes so great that it can actually smash the electrons and protons in the atoms together with enough energy that they will fuse to produce neutrons in a process known as electron capture! (Electron capture also produces neutrinos, for you detail-sticklers.) The explosion that this results in is called a supernova, which will leave you with either a neutron star, if the collapsed core is less than about 2 or 3 times the mass of the Sun, or a black hole, if the core is more massive than that. In order for this to happen, the original star needs to be at least 8 times as massive as our Sun, and possibly more.
Do we see this happening anywhere nearby? Yes. In 1054, a supernova occurred in the constellation of the Crab, and it was so bright that it was visible on Earth during the day. It’s almost 1000 years later, and this supernova has become the Crab Nebula, also known as M1, or the very first object in the Messier catalogue:
Guess what we see at the center of the Crab Nebula? A pulsar, which is a special type of neutron star! The nebula itself, as shown in this image, is 10 light years across, meaning that the shockwave of the matter ejected during the supernova explosion is moving at about 1% the speed of light on average!
So, where are the black holes? There’s a list here, but the first one we’ve found is Cygnus X-1, where there’s a collapsed mass about 9 times as massive as our Sun orbiting a very bright star about 30 times as massive as our Sun. It’s only a matter of time before that one becomes a black hole, too! The other place to find black holes is at the center of galaxies, including our own. Those black holes can be as massive as a few billion times our Sun. Check out M87 at the center of the Virgo Cluster, for example:
But, to be honest, we aren’t yet sure how those supermassive black holes form.
In any case, now you know how black holes are made, and what you need to make one. Just keep ’em away from me!