What are black holes?

Kids hear about black holes on science fiction shows and films in which they are shown to be incredibly complex, near-mystical objects with the power to transport you through time, space and into other dimensions. This, of course, piques children’s curiosity and they turn to the nearest adult for more information.

Black holes can actually be described very simply – they are lots of stuff squeezed into a small space. To be a little more precise, a black hole can be the equivalent of ten Suns squashed down to about 10 miles across.

Where does this “stuff” come from? Big old stars! Anything that is very big will have the force of its own gravity trying to crush it. Stars are fueled by hydrogen which gets turned into helium. This powerful nuclear reaction creates pressure which fights against gravity and the star holds it shape. But like any fuel it eventually runs out, at which point no more pressure is generated to push back against gravity and the star’s own gravity crushes it into a small dense object called a black hole. The star has to be big for this to happen – it won’t happen to our Sun, it’s far too small.

When you have so much stuff squeezed into a small space you have a very powerful gravity field. That’s why they are black – even light gets sucked into them. If you fell into one the most likely scenario is that you would be crushed by the gravity. Or you could be stretched out in a process that those funny physics guys call spaghettification.

So, what about all that weird sci-fi stuff? Some black holes spin. In those that don’t spin everything has collapsed to a single point called a singularity, whereas in the spinning ones the spin sort of opens up the singularity into a circle – a hole. Perhaps one you could travel through if you could avoid getting pulled apart…

We’re now approaching the outer edges of human knowledge but this is also the point at which imaginative science fiction writers can come in and excite our curiosity. What’s on the other side of a black hole? We may never know, but we can certainly imagine.

Image credit: ESA/NASA, the AVO project and Paolo Padovani

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