One phrase I find myself saying nearly every day is: ‘at the centre of every galaxy there’s a supermassive black hole’. I say it so casually. It’s a throwaway remark; like the sky is blue, the Earth is round, or Taylor Swift is the greatest lyricist of my generation.⁸⁰ I take for granted that it’s a fact humanity knows. But rewind just fifty years and that phrase would have been met with disbelief and perhaps even a guffaw or two from a fellow physicist. That change in attitude didn’t happen overnight. It took decades; another reminder that scientific theories don’t just spring out of the ground fully formed; they take time.⁸¹ Scientists are working with a few tiny scrap pieces of a jigsaw puzzle that didn’t come with a picture on the lid, and so they have no map of what they’re working towards. As more pieces of evidence are collected, the big picture starts to take shape; pieces that didn’t look like they were connected at the beginning turn out to fit together and an accepted theory emerges.

The very first piece of the supermassive black hole jigsaw puzzle came in 1909. A chap named Edward Fath was observing ‘spiral nebulae’ at the Lick Observatory, just outside San Jose, California.⁸² Back then, ‘nebula’ was the term used to describe anything that didn’t look like a star in the sky. It encompassed all the dusty, fuzzy things in the sky (the word ‘nebula’ is Latin and literally means ‘mist’ or ‘cloud’). Back in 1909, the size of the Universe was thought to be as big as the Milky Way – the most distant thing known was a star a hundred thousand light years or so away on its edge. All the nebulae were therefore also thought to be inside the Milky Way; either they were places where new stars were being born out of giant gas clouds, or they were the remnants of a star that had gone supernova and dispersed all its outer layers back into space.

Splitting the light from a nebula through one of Fraunhofer’s spectrographs reveals the unique fingerprint of light for that object. By doing this, you can tell what the nebula you’re looking at is made of. But unlike what we see for stars, where there are gaps of specific colours (i.e. specific wavelengths), for gas clouds like nebulae there are instead extra bright patches where we’d usually see those gaps. Instead of absorption of light by different elements, we’re seeing emission (like Kirchoff and Bunsen saw when they burnt sulphur).

As we learnt in Chapter 5, Niels Bohr explained how every electron orbits at a very specific distance from the nucleus, crucially, with only a limited number of special positions where an electron can orbit to keep the atom happy and stable. The orbit position of the electron tells us about how much energy it has to keep it in that orbit. This means the electrons around atoms in specific orbits have very specific amounts of energy. If you give an electron more energy though, perhaps by shining ultraviolet light on it from a nearby star, you can cause an electron to jump to a new position, giving it enough energy to jump to the next stable orbit (this is the absorption that happens in stars; with enough energy the electron escapes the atom entirely and becomes ionised). We say that the electron is in an ‘excited state’; like a teenager on their first caffeine high.

Electrons aren’t supposed to be in excited states in atoms though, because like teenagers, they like to be at the lowest energy possible. So, as soon as they can, the electrons lose energy to drop back to their original orbit. It’s always the exact same amount of energy that the electron loses, because remember, there are only certain positions where the electron can orbit the atom to be happy and stable. That energy is lost as light. Since the same amount of energy is lost each time, the same wavelength of light is emitted, and so the same colour is always given off. So hydrogen gives off a lot of light at a specific wavelength of 656.28 nanometres, which is a deep red colour. When we split the light from a large cloud of glowing hydrogen gas through a prism into its rainbow of colours and trace the amount of light received of each colour, we get a huge peak of red-coloured light at 656.28 nm, which resembles a stalactite in shape.

Spotting the stalactite peaks of different colours on the traces from spectrographs that indicate when a specific element is present is key to understanding what kind of nebula you’re observing. If there’s lots of hydrogen then it’s likely that you’re looking at a nebula where new stars are being born, or if there’s oxygen, carbon and nitrogen colours then you’re looking at a nebula where a star has died: the supernova poop.

Anyway, back to our chap Fath in 1909. He was on the hunt for signatures of either supernova poop or of pure hydrogen gas in the light coming from a different type of nebula – the ‘spiral nebulae’. What he found was that the spiral nebulae didn’t fall into either one of those categories; instead they looked like the traces seen when observing clusters of stars with both the signatures of hydrogen and the heavier elements (and also some absorption of light too). What Fath had observed, but didn’t realise at the time, were galaxies; islands in the Universe made of billions of stars. Just like our own Milky Way. This was the first of many results that contributed to the jigsaw puzzle for the size of our Universe. It was only after the work of scientists like Henrietta Leavitt, Heber Curtis and Edwin Hubble throughout the first two decades of the twentieth century that the distance to these ‘spiral nebulae’ could be measured. It was then that the scientific community finally realised that the Universe was far larger than they ever considered before: the Milky Way was no longer the only kid on the block.

With this mind-blowing realisation, one of Fath’s other observations went largely ignored: one of the traces from the ‘nebulae’ that Fath had observed looked different again from all the others. It had signatures of hydrogen, oxygen and nitrogen but they were much stronger and brighter than had ever been seen before, as if there was an extra source of energy causing them to glow. So not only had Fath observed galaxies without knowing it, he had also unwittingly observed the gas glowing as it spiralled around what we would one day call a supermassive black hole. Of course, it would be decades before anybody recognised what Fath had really observed. This is often dubbed an unknown known – the things that we’ve observed or done the experiment for, but have missed the meaning behind. It fascinates me to think about all the experiments that have been done in the past few decades that have likely already revealed something extraordinary, but we still don’t have the knowledge to understand what else they might be telling us. Or perhaps, even more likely in the era of data science and ‘big data’, information that’s buried somewhere on a computer archive that has been missed by human eyes.

Similarly, Fath’s strange observation of a galaxy with a very different trace of light was largely forgotten while astronomers and astrophysicists alike got distracted by what were considered the ‘bigger questions’ for a few decades. After determining that the Milky Way wasn’t the entire Universe in 1920, their focus turned to how the Universe began. This continued for much of the interwar years, eventually leading to the development of the Big Bang Theory for how the Universe has evolved and expanded over the past 13.8 billion years. A worthy pursuit, but perhaps delaying our knowledge of black holes for a few decades. It wasn’t until 1943 that American astronomer Carl Seyfert finally picked up Fath’s observations and once again observed six galaxies with similar-looking traces of light. What he noticed was that the emission of light from hydrogen gas in these galaxies wasn’t a sharp peak – instead it was smeared out into something that looked less like a stalactite and more bell-shaped.

Seyfert guessed that this smearing was due to Doppler shift; the light was being stretched and squashed as it moved both away from and towards us. If the glowing hydrogen gas in a galaxy is orbiting something, then some of the gas will be moving towards you and the light emitted will be squished to a shorter wavelength than was first emitted by the electrons jumping orbits, and some of the gas will be moving away from you and the light emitted will be stretched out to a longer wavelength. This is what turns our nice stalactite shape into a broadened-out bell shape. But here’s where it gets really clever – the amount of broadening is related to how fast the hydrogen gas is moving. And if you know how fast the gas is moving, then you can work out how massive the thing is that it’s orbiting.⁸³

The Doppler shifts that Seyfert measured for his six galaxies were huge. Unprecedentedly large. At this point you might think people would have started to realise that there had to be a massive object somewhere in these galaxies in order to create this kind of smeared-out trace of light. But again, people didn’t yet have all the knowledge needed to understand what Seyfert had observed. It would be another twenty years (with the work of Stephen Hawking and Roger Penrose in the late 1960s) before theoretical physicists even began to take the idea of black holes seriously.

Seyfert’s work wasn’t the only new result found in the post-war era. During the Second World War, the need to pick up faint radio signals from afar resulted in huge leaps forward in radio technology. After the war, those antennae were turned towards the sky, and many observatories with telescopes detecting radio waves were set up around the world, from Manchester⁸⁴ and Cambridge (where Hewish and Bell Burnell were discovering pulsars) in the UK, to the outskirts of Sydney, Australia. Instead of being used to pick up radio signals on Earth, bigger and bigger antennae were built to pick up even fainter radio signals from space, and radio astronomy was born.

It was the efforts of radio astronomers in cataloguing the new objects they were detecting in the sky that gave us a fair few more pieces of the jigsaw puzzle. First, one of the strongest radio signals in the sky was detected coming from the direction of the constellation known as Sagittarius. The father of radio astronomy himself, Karl Jansky, had detected radio emission coming from the direction of Sagittarius way back in 1931, but it fell to two Australian astronomers, Jack Piddington and Harry Minnett, working a radio telescope in Potts Hill, Sydney in 1951, to resolve that radio emission to a bright point in the direction of the centre of the Milky Way (astronomers had already agreed that the direction of the centre of our galaxy, the Milky Way, was in the constellation of Sagittarius since more stars could be seen in that direction – like looking towards the city centre and seeing more lights than if you look out towards the suburbs⁸⁵). The second thing they found was a large number of radio-emitting objects scattered across the sky in all directions that didn’t coincide with any object that had been seen by visible light. That made people wonder whether the objects producing these radio waves were so far away that the visible light from them was simply too faint to see with the optical telescopes available at the time.

Along with radio astronomy, X-ray astronomy was also on the rise after the Second World War, literally, with the use of balloons and rockets. Giacconi had discovered Scorpius X-1, as we learnt in Chapter 7, and Iosif Shklovsky explained Scorpius X-1 through accretion of material around black holes (and neutron stars) a bit heavier than the Sun found in our own Milky Way. But as X-ray astronomy gained in popularity, people started to spot other X-ray sources peppered across the sky that were incredibly faint, and yet incredibly energetic. To explain the incredibly energetic X-rays seen from these very faint, unknown sources (dubbed ‘quasars’ – quasi-stellar objects), you would need accretion around an unfathomably large object. It was British astrophysicist Donald Lynden-Bell⁸⁶ who, in 1969, first proposed the idea that the huge amounts of energy coming from quasars could be explained by accretion onto an incredibly large collapsed object (much larger than the one powering Scorpius X-1 in the Milky Way), and suggested that all galaxy centres could have collapsed in this way. He even suggested our own galaxy, the Milky Way, could be a ‘dead quasar’ (i.e. a collapsed object that was no longer accreting material).

It was the Hubble Space Telescope, launched in 1990, that eventually detected visible light from these X-ray and radio sources dotting the sky, confirming that they were indeed incredibly distant galaxies. These incredible distances meant that the X-rays and radio waves were even brighter than first thought, far too bright to be caused by accretion onto a black hole just a few times more massive than the Sun. In fact, when they corrected for those immense distances, astronomers found they were even brighter than the very faint X-rays observed coming from the centre of the Milky Way. The logical conclusion was that there must not only be accretion onto an incredibly massive object occurring in these distant galaxies, but also in our own. Since we couldn’t see any such object towards the centre of the Milky Way, these objects eventually got dubbed MDOs – Massive Dark Objects – in part because people were incredulous over the idea of a black hole so large, so supermassive, you might say.

During the 1990s, interest in what was going on in the centre of the Milky Way spiked. The problem was that seeing to the centre of the Milky Way is extremely frustrating because there’s lots of dust and stars in the way, blocking the view. All hope was not lost, however: this was infrared astronomy’s time to shine. Infrared light has a longer wavelength than visible light, meaning it easily passes around much smaller dust particles and allows us to see through to the centre of the galaxy. This technology kick-started a decade-long experiment to observe the positions of the stars in the very centre of the Milky Way, led by American astrophysicist Andrea Ghez at the University of California, Los Angeles and using the Keck telescopes on Mauna Kea in Hawai‘i.⁸⁷ Ghez and her team recorded how the positions of the stars changed in order to determine their precise orbits around the centre. This is the same thing we do when we spot asteroids in the Solar System; we observe how their position changes night on night, and then use that to figure out their orbit around the Sun. By studying the orbits of the stars in the centre of the Milky Way we can also determine how massive the object is that they’re orbiting. We’ve now even seen one star complete an entire orbit around the centre in just sixteen years at a speed of over 11 million miles per hour. Compare that with the 250 million years it takes the Sun to orbit around the centre at ‘only’ 450,000 miles per hour.

In 2002, the results of Ghez’s project were published, and astronomers finally knew how massive the dark object at the centre of our galaxy was: four million times the mass of the Sun. It is found in an area sixteen times the distance between the Earth and the Sun (to put that into context: Uranus orbits at nineteen times the distance between the Earth and the Sun⁸⁸). For something to be so big in such a relatively small space and invisible to all wavelengths of light, there was only one thing it could possibly be – a supermassive black hole.⁸⁹ Proving this won Andrea Ghez the Nobel Prize in Physics in 2020, shared with German astrophysicist Reinhard Genzel, who had the first crack at using the orbits of stars to study the object at the centre of the Milky Way, and with British mathematician Roger Penrose for his work with Stephen Hawking in the 1960s showing that black holes were inevitable in nature.

Accretion of gas onto a supermassive black hole explains all the X-ray and radio observations astronomers puzzled over during the twentieth century. The supermassive black holes at the centre of distant galaxies were so massive that the superheated gas spiralling around them was unbelievably hot, and so gave off unbelievably energetic X-rays. Heating gas to these extreme temperatures means that even the very atoms themselves separate into their constituent particles, so that the electrons are no longer bound in orbits around the nucleus. This means you have charged particles moving through space, which, when they move through a magnetic field, give off radio waves. This accretion onto a supermassive black hole, the idea used to explain all these observations and complete this scientific jigsaw puzzle, has eventually become known as ‘active galactic nuclei unification theory’. To me, it once again represents one of the most misunderstood concepts about black holes; they’re not ‘black’, they are the brightest things in the entire Universe. Completely unmissable, blazingly bright mountains of matter.

We’re now lucky enough to even have an image of that superheated material spiralling around a black hole in the famous ‘orange donut’ picture: this is the very first image ever taken of a black hole, specifically the one in the centre of the nearby galaxy Messier 87. The orange light shown in the picture shows the radio waves detected from the disk of material spiralling around the black hole. There’s an ominous shadow cast on this orange glow from the black hole, from which no light can escape. Compare that shadow of black in the centre to the dark on the outskirts of the image. No light can reach us from the inside because it is one of the Universe’s heaviest, densest objects, surrounded by hot, furious activity. Whereas on the outside, there is no light because it is the quietest, coldest, emptiest place in the very same Universe. I get chills every time I look at it.

The first ever image of a black hole, taken in radio waves by the Event Horizon Telescope in 2019, of Messier 87*.