Instant Genius - The hunt for the first stars in the Universe

Episode Date: October 12, 2025

Stars feel like a fundamental feature of the Universe – as essential as planets, galaxies and space itself. But since we know the Universe had a beginning (the Big Bang), there must also have been a first star. Before it, there was only darkness; after it, the cosmos as we know it began to take shape. Exactly what those first stars were like – and how they transformed everything that followed – remains one of astronomy’s great mysteries. It’s a mystery that astrophysicist Dr Emma Chapman has dedicated much of her career to solving. In this episode, Emma joins us to talk about her book First Light: Switching on the Stars at the Dawn of Time, recently updated to include discoveries from groundbreaking telescopes like the James Webb Space Telescope. Learn more about your ad choices. Visit podcastchoices.com/adchoices

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delivering digital precision with analogue warmth. So you can experience exceptional sound at home. Music just as the artist intended. Visit name audio.com to love. learn more. Hello and welcome to Instant Genius, the bite-sized masterclass in podcast form. Each week, you'll hear from world-leading scientists and experts talking about the most fascinating ideas in science and technology today. I'm Tom Howarth, trends editor at BBC Science Focus. Stars feel like a fundamental feature of the universe, as essential as planets,

galaxies, and space itself. But since we know the universe had a beginning, the Big Bang, there must have also been a first star. Before it, there was only darkness. After it, the cosmos, as we know it, began to take shape. Exactly what those first stars were like, and how they transformed everything that followed, remains one of astronomy's great mysteries. It's a mystery that astrophysicist Dr Emma Chapman

has dedicated much of her career to solving. In this episode, Emma joins us to talk about her book, First Light, switching on the stars at the dawn of time, which has recently been updated to include, ground-breaking discoveries from the latest generation of observatories, including the James Webb Space Telescope. So Emma, welcome to Instant Genius. Thank you. Thank you so much for having me. So it feels a bit stupid to ask because most of us probably intuitively know what a star is. But for those who aren't sure what physically we mean when we call something a star, what makes a star a star?

So firstly, it's not a stupid question, and that's how you get anywhere in research is by asking a series of silly questions that you think nobody else, or rather that everybody else knows the answer to, and it turns out that, you know, nobody does. And I've had that question asked to me at a public science lecture before, and I think it's because we kind of just assume we know what stars are, because it's one of the first words we learn, twinkle, twinkle, little star. But it's not twinkle, twinkle, little star, you know, how I wonder what you are. It doesn't go on to give you the explanation that a star. is a large collection, a massive collection of gas. It can be hydrogen, it can be helium, lithium, lots of different gaseous elements, and they will come together, they will get attracted to each other, all these atoms under gravity, and eventually you'll get so much mass in this big cloud of gas that it will collapse, and there will be enough pressure, enough heat right at the core of this cloud that this very lucky cloud of gas will ignite, will turn on,

will brighten up the sky as the fusion, nuclear fusion starts to take place. And you get heat, you get light, you get the birth of a star. So your book is all about the hunt for the very first stars in the universe. And then to that obviously implies that there was a time before that where there were no stars at all. And I'm guessing that it was quite a dark place to be. Could you maybe take us through that early universe from the Big Bang at the beginning to those first spark? of starlight. What was happening then and what was it like? Yeah, sure. So the idea of having a first star, a first of anything, the prerequisite is that there's a beginning and we're very

secure as astronomers that there was a beginning to the universe, the Big Bang as you've alluded to. So at the start of the universe, you have space time expanding out and it's a really hot, violent process. So you don't have a universe suddenly expanding after the Big Bang and ready-made stars popping out, ready-made planets, ready-made galaxies, not at all. Nothing can survive it in terms of large kind of structure. And so in the very early universe, you've got the most simple atoms. So you've got hydrogen and helium, or the vast majority of a garnish of lithium, if you will, but pretty much as hydrogen and helium. And very quickly, this hot, bright soup, if you will, primordial soup of gas, it will start out kind of very hot and almost like a glowing red.

But as the universe expands, everything cools down a little bit. And so if you were there, you would slowly see everything start to fade. So this soup would start to fade away into invisible wavelengths to your eyes. And so the universe enters into the dark ages. And the dark ages are exactly what it says on the tin. It's a period of We don't know. This is the best subject to give any kind of interview on, by the way, because I don't even have to remember numbers because we're that uncertain. I could be like, well, you know, dark ages lasted until 80 million years, 50 million years. We don't know. That's the beauty. So tens of millions of years and to our eyes, nothing's happening at all. But if we had some different spectacles on that could, for example, see dark matter, then, underneath the structure of the universe, you've got a scaffolding, a 3D web of dark matter,

extending all across the universe. And the gas, this hydrogen and helium, it's being attracted onto this scaffolding. So it's copying it basically. It's imprinting itself all along this scaffolding and this web. And at the densest parts where you've drawn in all of this hydrogen and helium, it's at these parts that you'll get enough mass in one place that these very lucky clouds of gas will start nuclear fusion and you'll get the first stars coming to life, brightening up the skies like fireflies in the night and ending the dark ages and starting what we call the cosmic dawn. And we think that happens around 80 million years, let's say, after the Big Bang. But there's a big uncertainty on that number.

Yeah, so as you pointed out a few times there, there's a lot we don't know about this period in the universe. What do we know concretely about these first stars, I guess, particularly any major differences between stars like our sun that we see in the universe today? Yeah, there's a lot of differences between the first stars and any of the stars around us that we see today. And I think that's why we have so much resilience in our field for searching for them, because the first academic paper that came out saying, where are the first stars was 1970. So you really have to kind of want to find these things. And the reason we're looking is because they are as distinct as like, you know,

looking for the dinosaurs, the fossils of dinosaurs compared to the birds around us today. Anybody that's like, oh, birds are dinosaur? No, you know, it's just not the same as finding a T-Rex. And so the first stars were, first of all, very massive. Now, the reason we know this, having not even ever seen one, is because the laws of physics don't change across the universe, either through space or through time. So the laws of physics that govern how massive a star can get, how long it will live, these apply just as much 100 million years after the Big Bang as they do now. The difference is the chemistry. So very early in the universe, as I've said several times, you've just got hydrogen and helium. Nothing more complicated than that. No atoms with, you know, more protons, more neutrons to create carbon nitrogen, for example, just can't survive in this very chaotic universe. They're just too easily breakable. So the chemistry means that these stars can't cool down, or rather these clouds of gas,

can't cool down as efficiently. And that means that more mass, more gas can't cool down as efficiently. And that means that more mass, pile on to these clouds before you get the kind of ignition of a star. So the first stars, we think on average, we're around 100 times the mass of our sun. Now, on its own, that's not necessarily an astonishing number, believe it or not, because we can find stars around us in the Milky Way that are tens to almost, you know, almost 100 times the mass of our sun. Differences, they're very rare. They're the tail end of a mass of a matter of. distribution, which peaks in the Milky Way, kind of below the mass of the sun, these kind of of red dwarfs. Whereas in the early universe, we think that the average star was about a hundred

times the mass of our sun. And we think they were quite fluffy. So we think they were less, kind of less dense, so a bit fluffier. Because they're more massive, you've got a more rapid rate of fusion. And so you get more heat, you get a higher temperature. And with that, you're that, that means they look bluer. So they're white-hot, you've heard that phrase. And so these first stars were not only very massive, but they were also very blue, very hot. And they were the engines to create all of the wonderful heavy elements around us that we needed to create planets and galaxies. And so within their cores, they're fusing hydrogen into helium. And then in turn, they'll go into lithium, beryllium, boron, carbon, nitrogen, and they'll go up and up and up,

and just create a kind of a, what would you say, a storage yard of materials to create the stuff we see around us today. I love the idea of big fluffy stars. And I guess we should say that stars are finite, they live and then they die. And I guess by nature of these early stars being so massive, it means that they lived for a very short amount of time, which presents a challenge to us, right? Yeah, precisely. So this is the bit that I try and avoid to say right at the start because I like people to get excited about them before I tell them the bad news,

which is that if you can try and imagine two cars, I'm really bad at cars, so I really need to come up with a better analogy. But let's say a Fiat 500, I think that's a small car, compared to a massive Range Rover, huge thing, massive engine, big gas-guzzling thing. Now, the Rangerover's got a much bigger tank like our first stars. Our first stars are much more massive.

They've got loads of hydrogen to fuse. shouldn't be a problem. A little Nissan, whatever I said, small car has a small amount of fuel, but it's really fuel efficient. And so whereas our sun, for example, one times the mass of the sun, obviously, because it's the sun, whereas our son, for example, will live around nine and a half, 10 billion years, it's really fuel efficient, whereas the first stars, even though they have much more mass, much more hydrogen to fuse through. They do it at such a pace that they will go into these big supernovae about one million years, two million years, you know, a few million years after their birth. Now, that's really bad news for anybody that wants to find them as they are because they were

born, let's do some simple math. They were born, let's say, 100 million years after the Big Bang. It's been 13.7 and change billion years. They're long gone. But, luckily there's two pathways that might mean that we haven't given up yet. I mean, I'm talking to you, which means I haven't given up this career yet. So clearly there's a reason that we're still looking. One is that I was careful to say the average first star was 100 solar masses, 100 times the mass of our sun, whereas actually increasingly, and this has really changed even in the last five years actually.

Increasingly, we think there might have been a kind of a tail end of much smaller sibling stars. So maybe, let's say, 80% the mass of our sun even. And they could have survived. So that lifetime would extend up to today. And there were people looking for them. They're called stellar archaeologist, coolest job title in the field. And the other way is that astronomers get to look back in time. So that's what we do every day.

We use telescopes to gather light that's been traveling to us for 13 and a half billion years. And so we're gathering the light from the first stars, even though they're long dead. Peak pollination season, and my business is scaling fast. To keep the nectar flowing, I need a phone plan with top priority data speed. That's why I chose GoogleFi Wireless. My connections stay strong even when the hive is buzzing. Plus, unlimited plans started $35 a month. Now that's a deal that doesn't stay.

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Visit focal powered by name.com for more information. So we obviously talked about some of the stuff we know about them. What are the big sort of questions that we're currently trying to answer in astronomy and astrophysics right now? There are so many questions it's actually very difficult to know where to start. It's easier to fill one minute of this podcast as we have earlier with what we do know. And then the rest of it is what we don't. We don't know how many there were. We don't know when they were born.

and therefore when they died, we don't know whether it happened all across the universe. So let's say 10 years ago, we were kind of assuming that everything was happening the same across all of the universe. Increasingly in simulations we're seeing pockets of pristine universe extending for much, much longer so that you could actually get first stars being born much, much later into the universe, which would kind of give, let's say, James Webb Space Telescope or Lofar, a really good chance of being able to capture the signs of them. The other items that were kind of on my list of wanting to know are what's the neighbourhood like? So we talk about first stars a lot,

but it wasn't just stars back then. There were the first black holes, for example. And especially with the findings from James Webb Space Telescope in the last, what, three years, that has transformed my vision of this pristine dark ages turning into kind of star, star, star, star, all across fireflies in the night is what I've always imagined. Now I'm seeing this bustling, diverse neighbourhood where you might even have direct collapse black holes where these massive gas clouds get so huge that they might collapse right through the star stage straight into a black hole, explaining a lot of the mysteries that we don't know about the black holes at the center of our spiral galaxies like the Milky Way around us today. So yeah, I think it's mostly about finding out what the populations were like, what was the neighborhood like, how long did they live, did they do it the same everywhere?

It's lots to find out. You mentioned the James Webb Space Telescope there, and I think one of its major discoveries has been that galaxies seem to, to have been formed much earlier in the universe than we thought. Could you maybe explain what that discovery was and how it ties into this investigation into the first stars? Absolutely. So James Webb Space Telescope has flawed as all. It's been exhausting, if I'm honest. It's been three years where we've just had to constantly update our simulations and understanding.

I took a few weeks off over the summer to look after the children. I honestly came back and the entire landscape had changed. So what James Webb Space Telescope has really managed to do for us is it's managed to dig right back into that era of the cosmic dawn. And what we're very surprised about is that we were kind of saying, as a rule of thumb, we think the first stars were born, let's say, 400 million years after the Big Bang. Let's say, a couple of hundred million years after Big Bang. James Webb Space Telescope is routinely finding galaxies at 800 million years. is 700, 600, 500 million years after the Big Bang that are way too big for us to be able to explain them. And the black holes within them, way too large. They shouldn't have been able to get that large.

We can work out how much gas a black hole can accrete. So that means how much gas can fall onto a black hole and kind of make it more massive. We can work out that rate. And it just doesn't quite work out in terms of the mathematics. And so we've had to work out ways to jumpstart black hole growth. That's what James Webb Space Telescope's really done for us. We've realised that we either need the first stars to form earlier so that they die earlier, so that they can start growing into black holes earlier, or increasingly we need a different mechanism where, for example, we have these gigantic gas clouds directly collapsing into black holes that are far more massive than just a single, single first star. Obviously, astronomers, once you've got one amazing new piece of kit, i.e. the James Verde

Space Telescope out the door, you're always looking at what the next big project is. So we've got James Webb and it's giving us a lot more questions probably than answers at the moment. But what other new telescopes are we hoping to help us explore this cosmic dawn period? It's funny because I honestly see James Webb Space Telescope as my side hustle. It's kind of, you know, the telescope I check up on now and there to get a bit of pocket money in terms of research. No, I'm a radio astronomer and that's, I think, the real gateway into the cosmic dawn. Because when we're investigating the early universe, what we do is we're gathering light, which has been, we can think of it as stretched. It's a little bit of a reach

to describe it like that. But if light is emitted, let's say, in the really bright UV, like the first stars, for example, billions and millions of years ago, by the time it reaches us, it will be much longer wavelengths. So as a rule of thumb, if we're looking further back in time, we need to look at longer wavelengths of light. So down from the infrared through the visible, down through the microwaves and into the radio. So the radio wavelengths are able to dig back further than the James Webb Space Telescope right into that cosmic dawn and actually see the gas that's collapsing and forming those first stars. So we can even dig back into the dark ages. We don't need stars to have formed. We don't need that visible light or the infrared light that James Webb is picking up

to be able to see what's going on. So the radio telescopes that I use, are Lofar, which are in the Netherlands. They look like kind of little metal bow ties, we can almost imagine it as like the TV antennas that you've got on your house, but just on the ground and 3,000 of them spread across Europe. I've lost track of how many, to be honest. And they all talk to each other, and they all look at radio wavelengths, and they're able to kind of dig back. But the real big one is the square kilometre array. And that has already been start a construction in the Western Australian Desert. Merchston does it. And so what that's going to be is about 130,000 what we call Christmas tree

antennas. They are about, I don't know, they're about my height. So let's say five foot six, five foot seven. And they look like Christmas trees and we decorate them accordingly in our offices, of course. We always have like, you know, competitions, put the baubles on the Ska tree. And that's going to be 130,000 of them. And what they're going to be able to do when they start observations in a couple of years is take a pick. of that gas over large sections of the sky and tune the telescope so that it's following what's going on in the universe over a period of about one billion years. So dark ages, cosmic dawn, and through an era after that where the first galaxies really start to kind of

change the universe and get rid of all that gas. And we'll be able to build up a home movie of the first steps of our universe. And all those firsts, all those firsts that you want to capture, just like your kid, you know, first teeth, first. first steps and everything like that. So we'll be looking for the first stars, the first black holes, the first supernovae, all of that. And so do you think that we'll actually, within the next few years, be able to directly kind of detect the first stars?

Directly detecting the first stars, no. So to set people's expectations, if you're talking about one of these really massive stars, you cannot escape the fact that they will be long gone. They will be long gone. And even though they're very massive, they're so far away, if we're going to gather light that's kind of gone for this many billions of years, they're too faint. So J.D.R.C. is not going to be able to see them. Neither is any other telescope in the pipeline.

What we pick up instead is their effect on the environment. So like Tyradosaurus rex or any other dinosaurs, footprints in the clay, footprints in the mud, That's what we're picking up. And so by looking at the gas, we're seeing how those first stars affect that gas, how they create kind of bubbles of burning it away. And yes, and so we'll be able from that to be able to tell kind of how many first stars there were, how many black holes there were. Fundamentally, other than obviously being incredibly interesting,

why is it important that we kind of learn and investigate about this period in the universe's history? I'm always a bit stuck on that one. I, and it's because I'm lucky enough to have an insatiable curiosity. So if somebody says to me, we're going to build a giant radio telescope to look back in time and why don't we see how the first stars of first black holes created, I think, why not? It's certainly a nice distraction compared to what's going on closer around me. But on a more scientific basis, firstly, you're missing a huge amount of information. So JWST has gone some way to addressing this.

So let's say five years ago I would have been very comfortable saying we're missing information on the first billion years of our universe. That's equivalent in a human lifetime to missing everything from the first ultrasound right up to your kids' first day at school, for example. That's a very formative time. So if you're going to make any conclusions about later in life, what's going on later in life and you're missing all of that, you're probably going to be making some pretty shaky assumptions.

So we really need to fill in that timeline if we want to be comfortable as cosmologists of understanding the entire history of our universe. But the other reason is stellar astrophysics. And this is the part that kind of really fascinates me, actually, which is that these first stars, you can't create them now. The universe is too polluted. There's too much pollution from heavy elements. So all of these generations of stars have gone into supernovae,

They have ejected all of these nitrogen or oxygen, all of these lovely heavy elements that we need, but they've polluted the gas. So you can't produce one of these pristine first stars anymore. And the fusion processes that go on within that star are completely unique. And so there are people right now trying to recreate those kind of chemical reactions in labs, not creating a star. That would have been a little bit more headline news. But just kind of the individual interactions of atoms under that kind of.

of stress within a first star, for example. So in terms of fusion science, it's really important as well. And I suppose if these stars or the large clouds of gas were responsible potentially for forming some of the black holes that we see in the universe today, then studying that early period can give us a bit of a clue into the supermassive black holes that are at the centre of our galaxies and things like that. Yeah, absolutely. So, you know, it's the elephant in the room. It's the elephant in the galaxy, if you will, got a black hole at the centre of the Milky Way that's way too massive to exist. So that was Emma Chapman, a Royal Society Research Fellow based at the University of Nottingham.

If you'd like to learn more about the ideas we discussed today, why not check out her book, First Light, switching on the stars at the dawn of time. Thanks for listening to this episode of Instant Genius, brought to you by the team behind BBC Science Focus. If you like what you just heard, then please consider subscribing to Instant Genius on your preferred podcast platform. If you'd like to watch our presenters and guest speaking in person, then you can also check out our YouTube channel at ScienceFocus. The current issue of BBC Science Focus magazine is out now. Pick up a copy wherever you buy your favourite magazines or download us on your app store of choice. You can also find us on Apple News or online at sciencefocus.com.

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