Science Friday - What Lies Beneath The Outer Layers Of A Star?

Episode Date: August 27, 2025

You might think of a star as a mass of incandescent gas, a gigantic nuclear furnace where hydrogen is turned into helium at a temperature of millions of degrees. But researchers recently reported that they’d observed some of what lies beneath all that hydrogen and helium, at least inside one unusual supernova. The star, named supernova 2021yfj, had its outer layers stripped away, leaving behind a silicon- and sulfur-rich inner shell.Astrophysicist Steve Schulze joins Host Flora Lichtman to describe what the team spotted in the heart of a dying star.Guest: Dr. Steve Schulze is a research associate at Northwestern University’s Center for Interdisciplinary Exploration and Research in Astrophysics.Transcripts for each episode are available within 1-3 days at sciencefriday.com. Subscribe to this podcast. Plus, to stay updated on all things science, sign up for Science Friday's newsletters.

Hey, I'm Flor Lichtenen, and you're listening to Science Friday. Today in the podcast, astronomers caught a glimpse inside an exploding star and found some surprises. We immediately realized this is something that we have never observed before, but we didn't know what we actually did observe. You might think of a star as just a mass of incandescent gas. The sun is a mass of incandescent gas, a gigantic nuclear furnace. Yo-ho, it is hot, but a new study suggests stars aren't just a big old mass of gas. This week, researchers report that they've observed some of what lies beneath all of that hydrogen and helium, at least inside one unusual supernova. And they found layers, like a giant bloom-in-union.

Astronomer spotted this star in the act of exploding, and they were able to see, one of those inner layers after all that outer star stuff was stripped away. Joining me now to talk about it is study author Dr. Steve Schulze. They're a research associate at Northwestern University's Sierra. That's the Center for Interdisciplinary Exploration and Research in Astrophysics. Steve, welcome to Science Friday. Hi, Flora. Nice to meet you.

Nice to meet you, too. So I've been trying to come up with the right analogy. How off base is a giant bloom and onion? This is absolutely perfect description. I think you're being kind, but go ahead. Yeah, so essentially star starts as a huge ball of hydrogen. But then through nuclear fusion, it gets transformed into a structure of shells. And this is essentially how an onion looks like.

So stars at the end of their lives, they're huge cosmic onions. Okay, so how unusual or unlikely was it to even capture this? kind of dramatic moment in this star's life. So we detect exploding stars routinely every night. This particular object is something that we have never observed before, and we didn't even expect that something like this would exist. So when we started to see the data and grasp what we are actually holding in our hands, we were completely off-struck.

And the paper just came out, and it's like reliving all of the emotions again, since discovery. Wait, was it an emotional moment when you first got the data? I wouldn't say emotional, but it's like, wow, what have you just discovered? Did you know what you had when you saw it? No, not really. So when we detected the object, we tried to get a spectrum, because the spectrum tells us something about the composition of the supernova ejector.

So, for instance, how much hydrogen is there, how much helium is there, etc. But in this particular case, we immediately realize this is something that we have never observed before, but we didn't know what we actually did observe. What is unusual about this object is that we have never observed an explosion of such a highly stripped star. We didn't even know that these stars could exist and that they could actually also explode as a supernova. Okay, so when you say stripped, what do you mean? So we have our cosmic onion and stars can lose their outer shepherds. shells, for instance, through stellar winds,

they could also have eruptions because of some instabilities in their very course. And this process of losing material through winds, eruptions, or interaction with a companion star, this is what we call stripping. And the stripping cannot only lead to losing a small mode of mass. It can actually lead to losing shells of this cosmic onion. Okay, so it's lost its outer shell. It's outer, it's the onion skin. Exactly.

And not just one, but can also lose several of those. The autonomous shell is hydrogen, then there comes helium, then comes carbon oxygen, then magnesium, neon, and oxygen, oxygen, silicon, and eventually the ion core. So when we usually observe explosions from stars that were stripped, usually they have lost the hydrogen envelope, or maybe then exposed the hydrogen layer. They could have also lost the hydrogen shell,

and then we only see the carbon oxygen. But we have never observed stars that lost even more shells and that they could also explode. So here's what I understand. The missing shells get stripped off, not because of the supernova explosion, just because that can happen in a number of other ways. And then you have this stuff left behind.

Right. For us, it was surprising that the star could essentially lose almost all of its shells. And we could just see at the very core or the very heart of the star. And what is that inner core that you're looking at? Okay. So what we found is that in the inner core, shells exist, and we found that there is an oxygen silicon shell. There were predictions that stars should have this kind of structure, but it was never observed. So this discovery was very important to confirm our existing models of how stars should form, how they should evolve.

Well, it sounds nice to be right, you know, first of all. Is there anything about the finding that is rewriting what we thought we knew about stars? One of the things that we observed is actually extremely puzzling, and this is the presence of helium. This is an element that should have been consumed at a much earlier stage of the star's life. So there should be no helium left, but we found helium. And this is very puzzling for us. And this is also not expected by any model. And we ask several people who study or who develop models for stars and how stars evolve, how they could explode.

They all didn't expect that there should still be helium. So it's a huge mystery at the moment. Are the theoretical astrophysicists happy about this or like annoyed? I think they will be very happy because now they can play with their models. They can see, okay, how can we tune them to match these observations? Maybe also be like, okay, well, maybe this type of star fits or maybe none of the existing models fits, then we need something completely new. So it will be, I think it is very exciting for theoretical astrophysicist.

How do you know that the outer layers got stripped away instead of just got fused into the heavier elements that you see? Okay. So when the star is born, it is this huge ball of gas. And the end of its life, it has this onion structure. And each layer of these shells in the onion have a particular chemical composition. Hydrogen on the outside, then here you know, carbon oxygen and so on.

The material that is always in these shells, it cannot fuse further because in order to fuse elements you need high densities, you need high temperatures, and those conditions are not met in these shells. Got it.

So, since we do not observe these elements in the spectra that we obtained, means that the star must have lost those shells a very long time ago. Well, I was going to ask that. I mean, how long does it take for a star to die or to

explode like this? Right. This is a very good question. The evolution of stars is very, very complex, and it depends on various parameters. There can be maybe some abrupt changes that can lead to huge changes in the evolution of a star. In this particular case, we have some ideas of how this progenitor star could have looked like or how it could have evolved. But we are not absolutely certain. Our leading hypothesis is that the progenitor star was a very massive star when it started to explode. And we think that it was so massive that the temperatures and the densities in the core was so huge

that the photons that live in the very course, they fuse together and produce electron-positron pairs. and because the photons, they are stabilizing the star against the gravitational collapse, this means that if there are less photons, then the star contracts a little bit. And this could lead to some explosive nuclear fusion, which can liberate a lot of energy. And we think that the star could have experienced this kind of pair instability a few times, And what could happen then then is that you have this very massive star. We think it was around 60 solar masses shortly before it died.

It experienced its first panstability, lost about 19 solar masses. So it's a lot of material. And at the same time, it also expelled several of those shells. When the star ejected so much material, it was just, it was almost at its breaking point. So it almost exploded, but it didn't. So it was a very fluffy object. It took a few thousand years for the start to contract again, start nuclear fusion again, then it experienced another of those panstability episodes, lost more material,

contracts again, experiences this again, and with each of these new pulses, it shifts more material, and eventually it also loses some of this. silicon rich material. And the exact time scales are very uncertain because this depends on a lot of, lot of different factors. So maybe this is something that happened over the time span of a few thousand years. Maybe it could have also taken a bit longer. The exact timescales of this process in this object is unknown.

And this is something that theoretical astrophysicists, need to investigate in great detail. And this is very important to understand what is 21 YFJ and why it did what it did. Hmm, I love that. What comes next for you? Okay, so the next thing for us is A to detect more objects. One object is not enough. We want to find similar objects, and then by studying more objects of the same type, we can get a better idea of their core properties. And also, what was the most likely scenario

for this type of explosion. The other thing is that the supernova 22 and YFJ is such an extreme object and the properties are so starkly different from the other

supernova classes that we know that we actually think there could be also some, there could be other classes of supernovae that we haven't detected yet. So there might be now something like a gold rush moment where we all try to find the missing link between the known supernova classes and this new supernova class that 2021 YFJ showed us.

The missing link of supernova. I love that. I think that's the perfect place to land. Thanks, Steve. Thank you, Flora. Dr. Steve Schulze, a research associate at Northwestern University's Sierra, the Center for Interdisciplinary Exploration. and research in astrophysics.

Thanks for listening. Don't forget to rate and review us. Wherever you listen, it really does help us get the word out and get the show in front of new listeners. Today's episode was produced by Charles Bergquist. I'm Flora Lichtman. Thanks for listening.