• Question: How does a black hole work?

    Asked by jill8oi to Claire, Kate, Matt, Rob, Sam on 20 Jun 2013.
    • Photo: Claire Lee

      Claire Lee answered on 20 Jun 2013:

      Hi jill8oi!

      So there are 2 ways that I think about black holes, one involves bedsheets and bowling balls, and we can get to that later if you like. But let’s start a bit closer to home…

      Ok. First, let’s think about the Earth for a second. It’s pretty big, it’s pretty dense. It has mass, it has gravity. Now, I’m not sure what year you are, but perhaps you know already that the gravitational force between two things decreases as the two things get further apart (actually it’s by distance^2). (if not you do now).

      So that means that as you go further away from Earth the gravitational force on you decreases. If you work backwards, then you can calculate the speed that your rocket would have to go to break free of Earth’s gravitational pull. It’s called escape velocity, and it works out to about 11km per second. Cool, that’s doable.

      Now imagine you’re on a more massive object, like the sun. The sun has more mass, so it’s gravitational pull on you will be stronger, so you’d have to go faster to break free of the sun (assuming you were in a super-rocket that wouldn’t melt or anything). The escape velocity of the sun is about 600 km per second. A bit more difficult, but still doable I guess.

      Now imagine you’re on something called a neutron star – it’s basically an old star that has died, and is small, but really, really dense, and it’s core is made up of neutrons (because they’re so dense that the protons and electrons of the atoms have been squished together to form neutrons). They’re around 30km across, but one teaspoonful of neutron star would weigh more than all the people on Earth! Basically, neutron stars are the badasses of the universe.

      Because they are so dense, they have a huge gravitational pull, so their escape velocity is about 100 000 km per second. So, pushing aside the small fact that you wouldn’t actually survive being on one, getting off it is still theoretically possible as long as you had a spaceship that could take you that fast.

      Do you see where I’m going yet?

      Ok. Now lets imagine you take a neutron star and squeeze it smaller (or just add a bunch more mass to it). As the object gets more and more massive, its escape velocity will get higher and higher. Eventually you will get something whose escape velocity is more than 300 000 km per second.

      Do you recognise that number? It’s the speed of light!

      Now we run into a fundamental problem. All before, it was just our technology that would decide whether we could get off the object. But now the laws of physics themselves are preventing us from going faster than the speed of light. So, no matter what, we will never be able to get to the escape velocity of this object – we will never be able to break free of it’s gravitational pull. Not even light will be able to break free of it’s gravitational pull (because not even light can go faster than light).

      This type of object is called a “singularity”, and it is (what we expect is) at the centre of a black hole. Anything that comes within it’s gravitational pull is doomed – there’s no way you could get away. As long as you stay far enough away from it though you’ll be fine, but come too close and sorry for you! The point at which you are doomed is called the “event horizon”.

      (If the sun was suddenly replaced by a black hole of exactly the same mass, besides us not having a light/warmth source anymore, nothing would happen to us. The Earth would still go round in its orbit as per normal – we wouldn’t get “sucked in” or anything, because we’d be outside the event horizon.)

      (TL;DR) – A black hole is basically an object that is so dense, to break free of it’s gravitational pull would mean you would have to go faster than light. Which you, or even light, can’t do. So you’re stuck there

    • Photo: Sam Geen

      Sam Geen answered on 20 Jun 2013:

      If you want to know how a black hole works once it’s there, Claire’s explanation is pretty good!

      Black holes are normally made by dying stars. When our Sun dies, it’ll turn into a white dwarf, which is a very dense ball of protons and electrons. When bigger stars die, their core (the bit at the centre) gets heavier, and the gravity in the core gets stronger. When you get to a star 8 times more massive than our Sun, the pressure on the core from gravity squeezes the electrons and protons together, and you get a neutron star, which is a big ball of neutrons about 20km across. When you get to a star 25 times more massive than our Sun, the pressure on the neutrons is overcome and the core collapses into a black hole. Supernovae are exploding stars caused by the energy from the collapsing core somehow bouncing back and exploding the rest of the star. This only really happens for neutron stars – when you form a black hole, it sucks up the core with it and not very much energy can escape.

      Another thing to think about is the chemical composition of the star. Normally stars are mostly hydrogen, with some helium. But as a star burns, it produces other elements – carbon, nitrogen, oxygen, etc, all the way up to iron. Then in the extreme heat of supernovae the heavier elements (like gold, lead and silver) are made. Astronomers like to call elements heavier than helium “metals”. Very old stars have very few metals because they were made from gas that was never in stars before. However, newer stars have metals in them from older stars. As they get more metals, interesting things happen – the more complex atoms are better at trapping light with their electrons, which causes a “stellar wind”, where the outer layer of the star is blown away by the light from inside it. You get this from the Sun, but it’s very weak. For very massive stars that produce a lot of light, the wind is very strong. For stars that were formed recently with a lot of metals, the star can lose most of its mass that way. Then, when it dies, it has far less mass than it started with – so it doesn’t make a black hole, even if it’s 100 times the mass of our Sun at the start, because it’s already lost most of its mass to these winds. So it’s only older stars above 25 times the mass of our Sun that make a black hole.

      Then there’s the supermassive black hole at the centre of our galaxy, which is about four million times more massive than the Sun. We don’t really know why this is there – maybe it’s lots of small black holes merged together, or maybe it’s something else.

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