The star that astronomers call S301 isn’t your average ball of fiery plasma. For one thing, this newfound star is travelling at more than 8 per cent of the speed of light. Blink, and you’ll miss it. But there is an even more striking thing about it: the simple fact that it shouldn’t exist.

S301 isn’t an isolated oddity, either; it is the fastest-known member of a superhighway of stars frenetically circling the black hole at the centre of our galaxy. The extreme gravity in this region explains why the stars are zooming around so rapidly and why their very existence is unlikely. Black holes usually rip apart the clouds of gas and dust from which stars form.

But the fact that these stars do exist – and that we can now observe them – is extremely handy. The centre of our Milky Way galaxy is so bright and so crowded that, for decades, we could barely discern anything about what was going on there. Today, thanks to S301 and its ilk, that is changing fast. “It’s such a beautiful astronomical system,” says astronomer Stefan Gillessen at the Max Planck Institute for Extraterrestrial Physics in Germany. “It’s a present that’s in front of our door, and we just have to keep watching it, and we will learn so much.”

And there is plenty to learn. The stars could reveal hidden properties of the black hole at the centre of our galaxy, expose what lurks around it and even test gravity in one of the universe’s most extreme places.

Earth is positioned in one of the Milky Way’s spiral arms, some 26,500 light years from the middle of the galaxy. From where we are in the sleepy galactic suburbs, trying to look into the galactic centre is like squinting into the beam of a searchlight. The region is so crowded, and thus shines so brightly, that for most of astronomy’s history, there was little hope of picking out individual objects. “This region is a complete mess… it’s the waste bin of the universe,” says Albert Zijlstra, an astrophysicist at the University of Manchester in the UK. “Pretty much anything that is interesting is dumped in this region of space because of the black hole’s gravity – everything falls in there – so it’s very chaotic and hard to understand.”

That isn’t the only problem. Earth’s atmosphere blurs our view, making tiny objects especially difficult to resolve. And the whole region is also shrouded in huge clouds of dust and gas. The central black hole, called Sagittarius A*, wasn’t discovered until 1974, just a few decades after astronomers concluded that its neighbourhood was, in fact, the centre of the Milky Way.

Only recently have astronomers been able to see through this mess well enough to pick out individual stars. “Think about a little glow worm next to the headlight of a car – if you don’t have very, very good eyesight, you might miss the glow worm,” says Gillessen. “What you need is the resolving power of a very big machine.” In other words, a very big telescope.

There is just one snag: we don’t have it, at least not yet. A conventional telescope would need a mirror that is some 30 to 40 metres across to pick out these ultra-close stars, and while several such observatories are being built, astronomers aren’t keen to wait around. “We are very impatient people,” says Gillessen. Instead, they have found clever workarounds to get a look at the stars near the galactic centre.

Peering into the glare

The first breakthrough was adaptive optics. Instead of relying on a rigid mirror, the technique uses one that can subtly change shape hundreds or thousands of times a second. Those tiny adjustments cancel out the blurring caused by Earth’s turbulent atmosphere, dramatically sharpening the view.

In the late 1990s, two groups – one led by Reinhard Genzel at the Max Planck Institute for Extraterrestrial Physics, the other led by Andrea Ghez at the University of California, Los Angeles – used adaptive optics to pick out the very first stars spotted near the galactic centre. These stars were moving at around 2000 kilometres per second and passing within a light month of Sagittarius A*. “It seemed like magic, the resolution they were getting,” says Zijlstra of those early observations.

Then, in 2002, Genzel’s team found a star that would dominate the field for years, called S0-2. At its closest point to Sagittarius A*, it comes within just 17 light hours and reaches around 7650 km/s, almost 3 per cent the speed of light. By tracking its orbit, Genzel and Ghez could begin to weigh whatever sat at the centre of the galaxy. It had to be enormously massive and extraordinarily compact – compelling evidence that Sagittarius A* was, in fact, a black hole.

The next leap came in 2018, when S0-2 made another close pass. This time, the researchers used interferometry, a technique that combines light gathered by many telescopes to create the effect of one, much larger observatory. That let them track the star’s orbit with unprecedented precision and pin down the mass of Sagittarius A*. The work earned Genzel and Ghez a Nobel prize in 2020. But that was just the beginning.

Orbits in disarray

The stars orbiting our galaxy’s behemoth black hole are puzzling in their own right. As astronomers have watched the galactic centre more closely, they have found stars that fall into three distinct groups. There are the S stars, which come perilously close to the black hole; the clockwise-disc stars, which follow more orderly orbits further out; and the off-disc stars, whose paths are more chaotic and sometimes appear to run in the opposite direction to the others.

The S stars pose a particular puzzle, part of which has to do with how surprisingly young they are. These stars can’t have begun their lives where they are now because there wouldn’t have been enough gas and dust from which they could have formed. But, equally, they come from a family of stars that lives for only about 50 million years, far too little time to have migrated gradually and peacefully into the galactic centre. “Their lifetime is shorter than the time between when the dinosaurs went extinct and now; these stars are young,” says Gillessen. So how did they get there?

One leading hypothesis is that they started out as binary systems – two stars orbiting one another – in more distant orbits. Then, if they drifted a little too close, Sagittarius A* could have torn the pairs apart, flinging one star out of the galaxy at extreme speeds as a hypervelocity star, while the S star got dragged into its perilously tight orbit. This would have sped up the process, allowing plenty of time for the S stars to get where they are now.

But that leaves another puzzle. The S stars have highly elliptical orbits, swooping close to Sagittarius A* and then flying much further away. This leaves a striking shortage of stars in close, circular orbits – the so-called zone of avoidance. Indeed, that is part of a bigger mystery: how did three distinct groups of stars come to coexist in such different orbits so close to Sagittarius A*? Explanations have been proposed for each population of stars, but until recently, none of them has been able to account for all three at once.

Xiaochen Zheng, a researcher at the Beijing Planetarium in China, and her colleagues have proposed a bold solution. Perhaps there is another massive object lurking nearby – a smaller black hole, hundreds to a thousand times the mass of the sun, or a dense cluster of stars – orbiting close to Sagittarius A*.

This mysterious object could have thrown stars that were born from the same disc of debris into their current orbits, and may even explain the zone of avoidance. “This is the first unified model that attempts to concurrently explain all three populations rather than treating each in isolation,” says Zheng.

It remains unproven, but the team already has sight of an object that could have done the deed, a cluster of stars called IRS-13E that may have a black hole of the right mass at its centre. Like so many of the ideas about the middle of our galaxy, it will take more time staring at the whole region to see if it works out.

Unravelling the black hole

If looking towards the galactic centre is like squinting into the sun with bare eyes, trying to study Sagittarius A* itself is a bit like trying to understand the sun’s insides. We can see the matter around it and, in 2022, we even took a picture of its shadow with the Event Horizon Telescope – but the black hole itself is, by definition, invisible.

Fortunately, black holes are simple beasts. Aside from an electric charge, which they aren’t expected to hold on to for long, they have just two properties that matter: mass and spin. Thanks to the work of Genzel and Ghez, we know the mass of Sagittarius A*. Its spin, though – that is, literally the rate at which it spins – remains elusive.

“Black holes are notoriously hard to pin down,” says astrophysicist Ziri Younsi at University College London. “We can just about get their mass, but the spin is much harder. But just in the last 10 or 15 years, it’s crazy how far things have come.” We can roughly measure the spin of more distant black holes by measuring the matter falling into them or by the gravitational waves produced when they merge, but these routes are either too imprecise or just plain impossible for our own black hole.

The trick that works is to watch a star complete enough of its orbit to reveal the black hole’s influence. S0-2 takes 16 years to circle Sagittarius A*. “Once you have a complete orbit of S0-2 or one of these really close-in stars, you can now ask, how does gravity work near a supermassive black hole?” says Ghez. “There’s an imprint of the laws of gravity on these orbits that you can start to measure.”

One such imprint is called “precession”. As the black hole spins, it drags the very fabric of space-time with it. If our understanding of space-time is correct, this should slightly nudge the orbits of nearby stars so that they don’t retrace the same path each time around. Measure that shift precisely enough and, in principle, you can infer the black hole’s spin. “The dynamics is really simple and beautiful,” says Gillessen. “Then you can test your gravitational theory in the most direct possible way.”

Those tests may have just gotten easier. In 2023, Gillessen and his colleagues spotted S301 using the Very Large Telescope in Chile. It is a star that comes 10 times closer to Sagittarius A* than S0-2, passing just about 1.8 billion km from the black hole at its closest approach. It completes an orbit in just 8.7 years, making it a far more sensitive probe of the black hole’s gravity – and its spin. Gillessen thinks that could cut the wait for a measurement – it may be as little as a decade before we have it, he says.

Of course, it won’t be as simple as just watching a single star. “You have to put 30 years of data [together] accurately and precisely to measure these effects – it’s really easy to go wrong,” says Ghez. There are plenty of places for errors to creep in. “We spend a lot of time and effort worrying about that and thinking about how [those things would] rear their ugly heads.”

But once we measure Sagittarius A*’s spin, it will open an unprecedented window into the details of our galaxy’s behaviour. One long-standing mystery is why our black hole is so much quieter than others of its size. “The mass accretion rate of Sagittarius A* is ridiculously low. If you scaled Sagittarius A* to a human mass, it would be the equivalent of eating a grain of rice per year,” says Younsi. Something appears to be stopping mass near the black hole from actually falling in, and the black hole’s rotation may be part of the answer.

The missing jet

Then there is the mystery of its missing jet. Many supermassive black holes fire enormous beams of energy out into space, and observations of other galaxies tell us that the size of these jets may be linked to how fast the black holes spin. One possibility, then, is that Sagittarius A* simply isn’t spinning fast enough to power such a cosmic fountain.

“Almost every other galaxy in the observable universe hosts these prodigious jets, and yet we don’t seem to have one at all – that’s quite mysterious,” says Younsi. “It speaks to the history of our galaxy and also our future, where we’re going, whether it could switch on one day.”

And behind all these questions lurks a still-bigger one: how does gravity behave in such an extreme place? Albert Einstein’s theory of general relativity has passed every test we have thrown at it so far, but there are signs that in the most extreme areas in the universe, it might fall apart. “Gravity is the most mysterious of the four forces of nature, and so the idea is that we can test our theories of physics and gravity in more and more extreme regions, and that could point us towards how to resolve some very big problems in physics,” says Tuan Do, an astrophysicist also at UCLA.

Those problems include what is going on at the centre of a black hole, and whether general relativity ultimately gives way to the effects of quantum mechanics at its edge. The S stars offer one of the few ways we have to probe that territory from a safe distance.

The field is moving quickly; S301 has already pushed astronomers closer to Sagittarius A* than ever before, and more discoveries may be waiting in its glare. “The centre of the galaxy just keeps getting interesting,” says Ghez. “The more we learn, the more questions there are to answer.”