Sunday Science: Black Holes

An artist's drawing a black hole named Cygnus X-1. It formed when a large star caved in. This black hole pulls matter from blue star beside it. Credits: NASA/CXC/M.Weiss

Black holes are incredibly dense regions of space that exert such a massive gravitational force, even light cannot escape from them.

That’s a lot of science in one sentence.
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So, let’s break the science of black holes down.

Small and dense

​A black hole is a large amount of matter that is squashed to a tiny region of space.

​Imagine fitting every piece of Lego in the world (that's 400 billion bricks) into one brick.

But a black hole doesn't look like a Lego brick or a star, or any other astronomical object out there.

We actually don't have a clue what a black hole looks like because it doesn't have a surface like a star does.

What it does have is tremendous mass.

Let's go back to our Lego example. If we squashed every Lego brick in the world into one brick, that's going to be one immensely dense Lego brick. It'll fall through the desk and have a huge gravitational field because the more mass an object has, the greater its gravitational pull.

The gravitational field of a black hole is so large that even light cannot escape its gravitational pull.

The boundary of this region where nothing useful can escape the black hole's grasp* is called the event horizon. In other words, if light crosses the event horizon, it won't be seen again.

It also means you can't photograph or even see a black hole. 

If we can't see black holes, how do we know they're there?

Black holes are detected by observing how the material surrounding them behaves.

For instance, black holes exert a powerful gravitational pull on nearby stars. If you look more closely at the stars' movements, you can calculate the mass of its invisible partner - and this could be a black hole if the inferred mass is large enough.

You can also investigate the gas and dust surrounding a black hole. This is funnelled by gravity into a disk and, as this material swirls around the black hole, it is moving so fast that it heats up and emits X-rays. We can detect these X-rays from Earth.

Another hint of a black hole is that light from the stars behind a black hole will be bent by its gravitational pull. You can measure the deflection of this light to derive the existence of a black hole - this bending is a process called gravitational lensing.

Black holes can also collide and create gravitational waves. Gravitational waves were detected last year and helped to prove one of Einstein's most fundamental theories - the General Theory of Relativity.

Yet, our understanding of black holes and their potential to help us understand our universe is still far from understood.

To quote theoretical physicist John Wheeler: “[The black hole] teaches us that space can be crumpled like a piece of paper into an infinitesimal dot, that time can be extinguished like a blown-out flame, and that the laws of physics that we regard as ‘sacred,’ as immutable, are anything but.”

Extra reading and watching

NASA's explanation on black holes is a great first port of call, Wired gives more details on how black holes form and this article from Space gives you more information on the different types of black hole in our universe.

And you can't go wrong with this brilliant Black Hole overview from New Scientist or this video explaining black holes:

* 30 years ago, Stephen Hawking suggested that black holes should release heat. He, more recently, argued that information could, technically, escape from a black hole.

What is Sunday Science?

​Hello. I’m the freelance writer who gets tech. I have two degrees in Physics and, during my studies, I became increasingly frustrated with the complicated language used to describe some outstanding scientific principles. Language should aid our understanding — in science, it often feels like a barrier.
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So, I want to simplify these science sayings and this blog series “Sunday Science” gives a quick, no-nonsense definition of the complex-sounding scientific terms you often hear, but may not completely understand. 

If there’s a scientific term or topic you’d like me to tackle in my next post, fire an email to [email protected] or leave a comment below. If you want to sign up to our weekly newsletter, click here.