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Chapter 1: What is Sagittarius A* and why is it significant?
At the centre of the Milky Way lies a dark beast, millions of times more massive than the Sun, and everything in our galaxy rotates around it in a gentle dance. This is Sagittarius A star. For decades, astronomers have tracked the orbits of its closest stars, and every observation has pointed towards the same conclusion. It is almost certainly a supermassive black hole. But
Even with the publication of the iconic image that came from the Event Horizon Telescope in 2022, it's important to remember that this hasn't yet been proven.
And now, fresh research is rewriting this familiar story.
Sagittarius A-star may not be a black hole at all, but a compact core of dark matter. One that spreads through and beyond the galaxy into a vast spherical halo. This model not only challenges the consensus on the nature of Sagittarius A star, but also tackles one of the biggest unanswered questions out there. What is dark matter? Perhaps it's been at the very heart of our galaxy all along.
I'm Alex McColgan and you're watching Astrum. Join me as we explore a radical take on Sagittarius A star, which could overturn everything we thought we knew about the very core of our galaxy. A theory that suggests it's a different beast entirely, and that it connects and could even solve two of the biggest mysteries in modern physics.
Around 27,000 light years from Earth, in the very centre of our galaxy, is Sagittarius A-star, a massive and extremely compact object shrouded in dense clouds of interstellar dust that, by its very nature, we cannot observe directly.
The first clues of its existence were found in the 1930s, when Carl Jansky detected an unusual radio signal coming from the direction of the Sagittarius constellation. At the time, no one knew what it was. Black holes were still considered mere mathematical curiosities, and hypotheses about what it might be buzzed around the scientific community.
Could it be clouds of star clusters, or perhaps remnants from a supernova? It wasn't until 1974 that it was identified as a single compact object by Bruce Ballack and Robert L. Brown. Brown later named it Sagittarius A-star.
The theoretical groundwork for the existence of black holes had been laid in the intervening decades, and it didn't take long before two and two were put together, and speculation grew that Sagittarius A-star was likely one of gargantuan size. As observations improved in the 1990s, astronomers were able to see stars orbiting an apparently empty region of space at astonishing speeds.
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Chapter 2: What evidence supports the black hole theory of Sagittarius A*?
The path of these stars, termed S-stars, became remarkably well mapped. They are the closest known stars to Sagittarius A star, and move at speeds of up to 24,000 km a second, which is around 8% the speed of light. One of them, S2, completes a full orbit in just under 16 years. For context, our Sun takes more than 200 million years to complete its orbit of the galaxy.
By tracking the orbital motion of these S-stars, astronomers can calculate the size of whatever it is that is sitting at the heart of the Milky Way. In fact, the researchers who did this won the Nobel Prize in Physics in 2020.
Just like many modern-day astronomical endeavours, this discovery was a truly international effort. Scientists from all over the world made up the teams who did these calculations, and to do so would have required them to communicate despite their differing native languages. And that's where today's sponsor Babbel comes in. It's one of the top language learning apps in the world.
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Chapter 3: What new research challenges the black hole hypothesis?
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For now though, let's head back to the centre of the galaxy, where two independent teams, led by Reinhard Genzel at the Max Planck Institute and Andrea Ghez at UCLA, found that Sagittarius A star has a mass of around 4.3 million suns, and it's confined to a region only 23.5 to 25 million kilometers across, which is small enough to fit within the orbit of Venus.
Their Nobel Prize was shared with Roger Penrose, who provided the mathematical proof that black holes are a direct consequence of Albert Einstein's theory of relativity. It seemed that Sagittarius A-star's status as a supermassive black hole was cemented.
But detail here is important, and the official citation on Gensel and Gess's Nobel Prize reads, for the discovery of a supermassive compact object, at the centre of our galaxy. And whilst the consensus is that Sagittarius A star is a supermassive black hole, it has not actually been proven, and scientists are still testing alternative ideas.
In fact, it's never been the only possible explanation, and other theories have been around since the first detection of Sagittarius A star's radio signal. but the discourse moved to the fringes as other possibilities, such as a star cluster, were eliminated as the S-star data built over the decades.
But some ideas remain, including boson star models, which propose that Sagittarius A consists of bosonic particles Gravastars, dark matter cores surrounded by a shell of high energy matter, and compact cores which are made entirely of dark matter. To understand why these ideas have persevered in the literature, we need to consider what we don't know about Sagittarius A star.
And it's here we find one of the greatest unsolved problems in astrophysics. How do supermassive black holes form? Well, here's the simple truth, we don't know. Stellar and intermediate mass black holes, which range from a few times the mass of our Sun up to 100,000 solar masses, form when a massive star uses up its nuclear fuel and collapses in on itself.
Under certain conditions, including the starting mass of the star relative to the remaining core, these violent deaths leave behind a black hole. This process is very well understood. and the signatures of these black holes are well observed. Some astronomers estimate that there may be as many as 1 billion stellar black holes in the Milky Way alone.
But Sagittarius A star is a completely different category. At 4.3 million solar masses, it is far too large to have formed from the collapse of any known star. There simply are no stars massive enough to collapse and form a black hole of this scale. What's more, by definition, we... cannot observe black holes directly.
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Chapter 4: What is dark matter and how might it relate to Sagittarius A*?
Their existence is only inferred by the effects they have in their surroundings. And this leaves the door ajar for alternative theories, and an elegant one hit the headlines in February 2026. Valentina Crespi at the Institute of Astrophysics La Plata in Buenos Aires studies dark matter at galactic scales.
She and her collaborators statistically compared an alternative black hole model, one where Sagittarius A star is composed of a dense dark matter core, to the observational data of S2 and 5 known G objects. G objects are a unique class of objects discovered in the early 2000s They behave like stars, but look like gas, visibly changing shape as they move closer to Sagittarius A star
distorting under the influence of its intense gravity. It's thought they could be compact dust clouds, or stars cloaked in a thick shroud of gas and dust. This strange, shifting morphology would need to be replicated by any models challenging the supermassive black hole consensus, and that's exactly what Crespi set out to do. Her aim was to determine whether a dark matter core
could reproduce the behaviour of S2 and the G objects to the same level of precision as the black hole model. Published in the monthly notices of the Royal Astronomical Society in February 2026, the results were remarkable. A dense core of dark matter in this model could reproduce the orbits, with less than 1% difference to the black hole model.
What this means is that you cannot tell the difference between a dark matter core and a black hole with the observational data tested. Both have the same gravitational effects on S2 and the G objects. Even more remarkable is that this model has implications far beyond the nature of Sagittarius A star.
Crespi and her international collaborators propose that it solves another huge mystery in physics, what dark matter is made of.
To understand how significant this is, we first need to take a small detour and unpack what we currently know about dark matter.
I'll cover it in summary here, but for a deeper dive please check out my other video about the recent possible observation of dark matter. Dark matter was first proposed by Fritz Zwicky in the 1930s, after he observed that galaxies in the Coma Cluster were moving too fast for known physics to explain. with relative speeds of more than 2,000 km a second.
These galaxies should have flung themselves apart if the gravity holding them together was proportional to the matter he could see. Something else was adding mass to the system, and he termed it dark matter. Later observations saw something similar in the rotation of galaxies. Stars in their outer regions orbit at speeds that cannot be explained by visible mass alone.
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Chapter 5: How have historical observations shaped our understanding of Sagittarius A*?
It's an idea that was first put forward by astrophysicists Ruffini and Bonazzola in 1969, and it's been fizzling in the background ever since. Fermions are subatomic particles, including electrons and quarks, with half-integer spin and are basic building blocks of matter. Of crucial importance for us is that they obey the Pauli exclusion principle.
no two identical fermions can occupy the same quantum state. In other words, they cannot be in the exact same location in space with the same energy and the same spin at the same time. This is a fundamental rule of quantum mechanics, and a key reason why matter doesn't collapse in on itself, and why we can't walk through solid walls. This is a property they share with the proposed dark fermions,
They are posited to be elementary particles too, but they only interact with the rest of the universe gravitationally, not electromagnetically, making them dark. Because of the Pauli exclusion principle, dark fermions cannot be infinitely squeezed together. They push back and resist collapse, so they could coalesce and, under the right conditions, build up huge amounts of internal pressure.
This results in an ultra-dense, stable object which, in theory, could reach masses similar to a supermassive black hole. Proponents of this model, including the group leader at La Plata, Dr. Carlos Arguelles suggests that a core of fermionic dark matter would be so dense that it would, in every observable sense, be indistinguishable from a supermassive black hole.
But unlike black holes, such cores would not form a singularity, nor have an event horizon. And that's not all. Fermionic dark matter would extend beyond a compact core, naturally forming a distinctive structure that diffused through the galaxy and beyond to form a halo. Put most simply, it's proposed that Sagittarius A star and the dark matter halo are not two separate objects.
They are two parts of the same continuous structure made from fermionic dark matter. This is something no other Sagittarius A star hypothesis does. In every other scenario, the compact object and the dark matter halo are two distinct structures. This is the only theory that unifies what we see in the galactic centre with the hypothesised dark matter structure of the galaxy. It's
incredibly elegant, and rather convincing I personally think. Now, as we've seen, one of the key pieces of evidence for Dark Matter's existence is how galaxies and the stars within them rotate. Something is adding mass and influencing their speed. ESA's Gaia mission is bringing extraordinary detail to this picture.
Its aim is to accurately measure the motion of one billion stars as they orbit the centre of the galaxy, from its inner regions to the outer disc. And the published data in 2022 brought an intriguing twist to this tale, with particular importance to the thermionic dark matter model.
Gaia revealed something totally unexpected at the outer edges of the galaxy, a slowdown in the rotation of its outer arms called Keplerian decline. This presented a problem for the standard picture of our galaxy. Most dark matter models cannot reproduce this observation. But thermionic dark matter, thanks to our old friend the Pauli exclusion principle,
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