In Black Hole Hunters, we're asking you to help us find some of the Milky Way galaxy's millions of missing black holes. Using data from the TESS satellite, we'd like you to look at graphs of how the brightness of stars changes over time, looking for an effect called gravitational microlensing. This lensing effect can indicate that a massive object passed in front of a star – its gravity bending and focusing the star's light – and we hope to use this to uncover the existence of otherwise invisible black holes.

What are black holes?

A black hole is one of the things that can be created when a star runs out of fuel. Not all stars will leave behind a black hole. What is left behind depends on how massive the star is. Less massive stars, like our Sun, leave behind white dwarfs. Stars that are about ten times the mass of the Sun leave behind neutron stars. And the biggest stars, 20 times the mass of the Sun or more, leave behind black holes.

In all three cases, the object left behind is much more dense than the star that it formed from. A white dwarf has a mass roughly the same as the Sun (not all of the original star's material makes it into the remnant, so in every case what's left is less massive than the original star) and is about the size of the Earth. A neutron star is more massive, up to about twice the mass of the Sun and about 10 kilometres across. Black holes don't have an upper mass limit, but most of them should be about 10 solar masses with a radius of about 30 kilometres.

Black holes are strange objects. The radius mentioned above isn't a physical surface, it's just the distance from the centre where the escape velocity (how fast you'd need to travel to get away) equals the speed of light, and this is called the Schwarzschild Radius or event horizon. That means black holes are truly black because their gravity is so strong that light gets trapped inside that radius. Physics can't fully explain what actually happens inside a black hole. General relativity gives the nonsensical answer that the black hole's contents are compressed into an infinitely dense point with zero volume. Quantum physics doesn't have a full explanation either, yet.

While their insides are a mystery, physics can explain what happens just outside black holes – even if what happens there is extreme. Black holes can pull in material from nearby stars in a process called "accretion", whipping it up to super-hot temperatures – so hot that it shines brightly in X-rays. When they're not accreting, they're more or less completely invisible, but they do leave a few clues that can help us to find them.

The Galaxy's hidden black holes

Many stars aren't alone. Often, they are paired up in what are called "binary" systems ("binary" means there are two of them) with two stars orbiting each other. Sometimes they are in even bigger groups, all bound together by gravity.

Most stars which are massive enough to create black holes are in binary systems. But when a star collapses to form a black hole, its outer layers explode in a powerful supernova. So what happens to the other star during the explosion?

You would think the force of the supernova explosion would either destroy the other star, or at least shove it away from the new black hole so that both of them would end up on their own. Sometimes both of those things can happen, but often gravity is stronger than the explosion so the other star not only survives, but the star and the new black hole actually end up orbiting each other.

It's in systems like this – binaries containing a star and a black hole – that we think most of the Galaxy's black holes should be. Some systems like this are known (about 70 of them), but almost all of them were only discovered because the black hole happens to be close enough to the star to accrete, so we are able to see the obvious X-ray signature that tells us the black hole is there. The problem is, most black holes aren't accreting most of the time. A couple of quiet black holes have been found thanks to the precise measurements from the Gaia mission – but there should be more than 10 million of them in total! That's a lot of black holes that are missing, hidden from us because they don't emit any light.

Self-lensing: a method to locate hidden black holes

The black holes we're looking for might be invisible, but there are still a few tricks we can use to look for them. The black holes don't emit any light, but they still affect their surroundings through their gravity. What we're looking for in this project is an effect the black hole's gravity can have on its companion star's light, called gravitational lensing.

If the star and the black hole happen to be lined up so that the black hole occasionally passes between the star and us, then the black hole's gravity can bend the star's light, acting like a magnifying glass and making the star look brighter.

Figure 1: The light from a star is magnified when the black hole passes in front of it. This creates a distinctive peak in the lightcurve.

And that's what we're looking for! We have millions of measurements of stars so we can see if their brightness suddenly changes. But we need your help – the microlensing events are going to be very rare (since everything has to be lined up just right), and there are other things that can make stars briefly look brighter (such as flares and pulsations). We need your help to find the black holes amongst everything else!

How you can help

All we need you to do is look at some simple graphs, like the one below, and tell us if you see anything that looks like distinctive peak-like shape. These graphs are called light curves, and they show how bright a particular star was each time it was measured. We're looking for times when a star suddenly gets brighter for a while and then goes back to normal.

When you spot something that looks like this, just highlight it on the graph. Don't worry about making a mistake and don't worry about being wrong! We're going to show each one to several people and we'll put your answers together, so any mistakes will average out in the final results.

Once we've got a final set of results, we will use that to come up with a short list of candidates that we can investigate further. There are other kinds of measurements we can take of each star to try to confirm if there is a black hole there (for example, we can very precisely measure if the star is moving). These measurements take a lot of time, which means they're expensive to do and we can't do them for every star – that's why having a good shortlist to start from is such a big help!

You might wonder why we need you to do this, instead of getting a computer to do it. The short answer is that computers are good at some things and humans are good at other things. By combining the work of humans and computers we can do a better search than we could with either one on its own. We can train computers to find all the most obvious examples, but we think people – you! – will be better at finding the harder to spot ones. What would be very hard for a computer should be pretty easy for you.

Where our data comes from

The measurements we're working with at the moment come from the space-based Transiting Exoplanet Survey Satellite, or TESS. The TESS satellite has been in orbit since 2018 on a mission to find planets outside our solar system (known as exoplanets) by looking for small decreases in brightness as the planets pass in front of their stars. This is very similar to what we're looking for, which makes the TESS data ideal for our work.

Before starting on the TESS data, we did a similar search using data from a ground-based survey called SuperWASP. Like TESS, SuperWASP was also an exoplanet search which ran from 2004 until 2016. Results from our SuperWASP search are still being analysed and should be published soon.