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As the name suggests, black holes don’t give off any light, because their gravity is so strong that even light can’t escape being sucked in. The reason their gravity is so strong is that they are incredibly dense. The most common type of black hole would typically weigh about 5 times as much as the Sun, but only be about 30km wide, which is about the size of a medium-sized city. This makes it 100 million times smaller than the Earth and a hundred trillion times smaller than then Sun (that’s 100000000000000!), even though it’s heavier than the sun! The type of black holes that I study are much, much heavier though! They weigh between a million and a billion times more than the Sun, with sizes between about the same size as the Sun and a thousand times bigger.

You might wonder how you can find anything out about something that doesn’t give off any light, and the only way that you can is when other things in space, mostly gas, gets too close and get sucked towards the black holes. As the gas falls in, it makes a huge, swirling disk which gets very hot, and shoots out huge galaxy-sized jets of gas into space. All of this gives off a lot light, from the visible light we can see up to Ultraviolet and X-rays and down to Radio waves. All of this is watched by a huge range of telescopes designed to look at specific types of light. I mostly work with X-ray and UV telescopes, which have to be in space (like the Hubble telescope) to see anything, because the Earth’s atmosphere blocks most of the UV and X-rays from getting to the ground (luckily for us!). I also use radio telescopes, which are made of lots of huge radio dishes which all point at the same object in space at the same time, so that we can make pictures of things out of the radio waves coming from it.

Here’s a artists impression of a supermassive black hole with it’s disk of gas and jet shooting out into space:

The point of all this is to find out what happens to stuff that falls onto black holes and use that to work out how black holes work. Even though the closest black holes are thousands of light years away (billions of miles), it’s important to know how they work because they are so strange. It would be impossible to make something as dense as a black hole in a lab on Earth, so looking at black holes is the only way we can see what happens when gravity is as strong as it is around a black hole. This lets us test what we know about gravity to it’s limits, in particular the theory that Einstein came up with to describe it (General Relativity). If we didn’t know about this, things like GPS and satellite TV wouldn’t work, so it can affect you directly. It has also been found that supermassive black holes have a huge effect on how the galaxy they’re inside grows and develops, meaning they have a direct impact on how the whole Universe has changed over time. Plus, they’re worth looking at just because they’re really interesting!

The main ways we try to work out what’s going on around black holes is by looking at pictures and spectra (plural of spectrum) of them in different types of light. X-rays only come from the very hottest things, so they’re good at looking at things which are right next to the black hole, optical and UV light are good for looking at the disc of material which is swirling into the black hole, and radio waves come from the cooler gas further out, in particular the huge jets of gas that they shoot out. As the black holes are so far away, we can only see really big details of the stuff around supermassive black holes, even with our best telescopes – they’re mainly good for looking at the jets, which can be bigger than the width of the whole galaxy which the black hole is inside. Most of our information comes from the spectra.

Here’s an artists impression of NASA’s X-ray Space Telescope, called Swift, whose data I use a lot:

A spectrum is just the different colours in light split up, the same as a rainbow, when visible light is split by raindrops, or by a prism. By looking at how much of each colour there is in the light, we can tell things like what gases make up the surroundings of the black hole, how fast the gas is moving, how hot the gas is and whether there is anything blocking some of the light from getting to us. You can even sometimes work out things like how strong the gravity is or how powerful any magnetic fields are. When you piece all of these bits of information together, you can start to build a picture of what the gas around the black hole is doing, how big and heavy the black hole is, how much energy they’re producing and more!

Whilst we know quite a lot about these things, there are still a lot of questions which haven’t been answered yet. It’s my job to try and answer some of these questions!

As an Astronomer, space is my lab, so unlike a lot of scientists I don’t spend any time in a lab doing experiments. The black holes I study are my experiments, set up by the universe for us to study how it works, and telescopes are the instruments I use to measure it. Whilst some Astronomers would go to a mountain somewhere for a few weeks and collect some data themselves with a telescope, the telescopes that I use are in space, so I can’t do that! Instead, I ask for the telescope to be pointed at something I want to look at, then a signal is sent to the telescope in space and it automatically turns at looks at it, then sends the data back down to me electronically (via NASA or ESA, or whoever is controlling the telescope!).

A lot of my time is spent looking at telescope data (images or spectra) on a computer and trying to work out the physics of what is going on in the object I’m looking at. This involves things like writing computer programmes to do different things to the data, plotting graphs to see what it looks like in different ways and making computer models of what the black hole is doing, to see if the data fits my idea of what is going on. Once I have a good idea and some results that are worth sharing with other scientists, I’ll write a scientific paper, which is a report of what I did, what the results were and what I think the Science behind them is. This is then published in a Scientific journal, on the internet and on paper for libraries, so that other scientists and anyone else can read them.

Other things I often do during a normal day are read others’ papers to see what other people have previously found out about the thing that I’m studying at that particular time, sending emails to other people I’m working with, or talking to them if they’re in the same place as me.

I usually spend several hours a week ‘demonstrating’ which is helping to teach undergraduate university students Physics, helping them with the Physics questions they’re set, or programming, in the labs, or with Astronomy projects.

Every week there are presentations to go to by Astronomers from other Universities and institutes where they talk about the latest results and theories. I also spend a lot of time writing things down on paper and thinking of ideas to explain what I see in the data!

This is a picture of me in my office, at my computer, where I spend a lot of my time. I have two screens, so I can do twice as much work…