How to make a blackhole

A light hearted look at how you might make a blackhole if you happen to have a large star handy.

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Yesterday, if you followed the instructions, you made a star with a big patch of cold hydrogen gas. Today you’re going to take that star and make a blackhole. Why? because it might come in handy one day, especially if you need it to make a wormhole to travel around the universe.

WARNING, blackholes can be dangerous if not handled correctly so make sure you know what you’re doing and follow all of the instructions carefully.

The first thing you will need is your star that you made yesterday, if it’s not big enough then you’ll have to make another one because blackholes only form from rather large stars, if you get one that’s about 20 times the mass of the Sun then that should be big enough, you don’t want to end up with a neutron star, they’re just nasty. The only problem with stars that are this big is that they don’t last very long so you’ll have to work quickly.

Make sure you’ve got plenty of room and have cleared out enough space around the star, you don’t want some binary white dwarf nicking your star’s mass when it gets to the supergiant phase or you might end up with a type Ia supernova, by surprise. That’s bad because it will stop your star turning into a black hole.

A stellar-mass black hole in orbit with a companion star located about 6,000 light years from Earth.
Why you need space around your star – in case a blackhole nicks some of it (Credit: NASA/CXC/M.Weiss)

Depending on the mass of the star you’ve got, you’ll have a bit of time to make yourself a cup of coffee, read a good book or mow the lawns. In fact your should have a couple of hundred million years – just make sure you keep the space clear around your star – a couple of light years should be enough. If you keep an eye on the star you should start to notice that it gets a bit cooler and bigger all of a sudden. Nothing to worry about yet it’s just finished fusing hydrogen in the core and is now starting to fuse helium. There’s a shell of hydrogen around the core that’s also starting to fuse. Be careful though, because the core is up around 100 million degrees, so a bit too hot to touch.

Things happen pretty quickly now, as the star fuses heavier and heavier elements in the core. If you were able to cut your star in half, and colour code the different shells fusing, it would look a bit like an onion. At this point you should be glad you made a large star because the solar wind from the star will be ejecting material from the outer layers and, in doing so, be losing a bit of mass in total. It’s not too bad though as it’s the mass of the core and the surrounding shell that we’re really interested in. If you have a look inside the core you’ll notice that there’s a fair bit of iron building up. Keep an eye on the build up of this iron because when there’s enough of it to be about 1.4 times the mass of the Sun then you’re in for some fireworks, and you’ll need to stand back. You star is about to go supernova and will briefly be the brightest light source in the galaxy, you probably want to put your sun glasses on and move away (20,000-30,000 light years should be enough).

As the iron is building up the core is getting hotter and hotter and denser and denser and the force of gravity is trying to squash it more and more. There’s a point reached where gravity just can’t squash the iron anymore, this is called electron degeneracy, basically everything is as close as it can be. There’s more and more iron being added all the time so the core is approaching the magic number of 1.4 times the mass of the Sun. When this happens gravity can overcome electron degeneracy and the rapidly causes the core of iron to collapse. It does this by basically turning the protons in the core into neutrons and to do this it needs all of the electrons – hence there’s no need to worry about electron degeneracy anymore, because there’s hardly any electrons. When the core does this big change it also releases a huge amount of neutrinos that expand out from the core in a big shockwave. Normally neutrinos don’t interact with matter but because there’s so many of them rushing out from the core, scientists think this might be one of the causes of the star blowing itself apart into a supernova. The other cause might be when the core collapse hits the next barrier, neutron degeneracy, and then it bounces back from the new super dense core and sends the shockwave out from the core blowing the star apart.

Immediately after this big explosion we’ve got a neutron star in the core. You have to be quite careful with the next bit, and remember its happening very quickly, as you don’t want to be stuck with the neutron star – they cause all sorts of problems and just aren’t worth the effort. Hopefully our supernova hasn’t blown away all of the material around the core and some of it’s still in falling into the neutron star we’ve got at the moment. When this ball of neutrons get’s big enough, about 2-3 times the mass of the Sun, gravity will be able to break the neutron degeneracy that’s holding the neutron star together. This is when your blackhole is born, the neutron star suddenly disappears as no photons can escape the event horizon of this new blackhole. Inside the event horizon the material is collapsing down to what scientists call a singularity where physics gets very weird.

And there you have it, a brand new blackhole. If you set up a nice accretion disk and keep feeding it material (like other stars, planets, gas, dust and frozen chickens) then you can grow a super massive black hole. Good luck, and be careful.