What Lies Beneath The Outer Layers Of A Star?

FLORA LICHTMAN: This is Science Friday. I’m Flora Lichtman. Later this hour, looking at the FDA’S push to reduce artificial food dyes. And the problem of pests and whether they even are a problem. But first, you might think of a star as just a mass of incandescent gas.

[THEY MIGHT BE GIANTS, “WHY DOES THE SUN SHINE”] The sun is a mass of incandescent gas, a gigantic nuclear furnace. Where hydrogen is built into helium at a temperature of millions of degrees. Yo ho, it’s hot!

Yo ho, it is hot. But a new study suggests stars aren’t just a big ol’ 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 blooming onion. Astronomers 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 Doctor Steve Schulze. They’re a research associate at Northwestern University’s CIERA– that’s the Center for Interdisciplinary Exploration and Research in Astrophysics. Steve, welcome to Science Friday.

STEVE SCHULZE: Hi, Flora. Nice to meet you.

FLORA LICHTMAN: Nice to meet you, too. So I’ve been trying to come up with the right analogy. How off base is a giant blooming onion?

STEVE SCHULZE: This is absolutely perfect, perfect description. So–

FLORA LICHTMAN: I think you’re being kind, but go ahead.

STEVE SCHULZE: Yeah. So essentially a 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.

FLORA LICHTMAN: So how unusual or unlikely was it to even capture this kind of dramatic moment in this star’s life?

STEVE SCHULZE: 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 awestruck. And the paper just came out and it’s like reliving all of the emotions again from– since discovery.

FLORA LICHTMAN: Wait, was it an emotional moment when you first got the data?

STEVE SCHULZE: I wouldn’t say emotional, but it’s like, wow, what have you just discovered?

FLORA LICHTMAN: Did you know what you had when you saw it?

STEVE SCHULZE: 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 ejecta. So for instance, how much hydrogen is there, how much helium is there, et cetera. But in this particular case, we immediately realized this is something that we have never observed before, but we didn’t 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 that these stars could exist and that they could actually also explode as a supernova.

FLORA LICHTMAN: OK. So when you say stripped, what do you mean?

STEVE SCHULZE: So we have our cosmic onion and stars can lose their outer shells, for instance, through stellar winds. They could also have eruptions because of some instabilities in their very cores. And this process of losing material through winds, eruptions, or interaction with a companion star– this is what we call stripping. And the stripping can not only lead to losing a small amount of mass, it can actually lead to losing shells of this cosmic onion.

FLORA LICHTMAN: OK, so it’s lost its outer– it’s lost its outer shell. Its outer– it’s the onion skin.

STEVE SCHULZE: Exactly. And not just one, but it can also lose several of those. The outermost shell is hydrogen. Then there comes helium, then comes carbon oxygen, then magnesium, neon and oxygen, oxygen silicon, and eventually the iron core.

So when we usually observe explosions from stars that were stripped, usually they have lost their 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 could also explode.

FLORA LICHTMAN: 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.

STEVE SCHULZE: 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.

FLORA LICHTMAN: And what is that inner core that you’re looking at?

STEVE SCHULZE: OK, 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.

FLORA LICHTMAN: Well, sounds nice to be right. First of all,

[LAUGHTER]

Is there anything about the finding that is rewriting what we thought we knew about stars?

STEVE SCHULZE: Well, 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 asked 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.

FLORA LICHTMAN: Are the theoretical astrophysicists happy about this or annoyed?

[LAUGHTER]

STEVE SCHULZE: Well, I think they would be very happy because now they can play with their models. They can see, OK, how can we tune them to match these observations. Maybe also be like, OK, well, maybe this type of star fits, or maybe none of the existing models fits them. We need something completely new. So it will be– I think it is very exciting for theoretical astrophysicists.

FLORA LICHTMAN: How do you that the outer layers got stripped away instead of just got fused into the heavier elements that you see?

STEVE SCHULZE: OK. So when a star is born, it is this huge ball of gas. And at 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 helium, 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.

FLORA LICHTMAN: Got it.

STEVE SCHULZE: 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.

FLORA LICHTMAN: Well, I was going to ask that. I mean, how long does it take for a star to die or to explode like this?

STEVE SCHULZE: This is a very good question. The evolution of stars is very, very complex, and it depends on various parameters. That 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 it 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 were so huge that the photons that live in the very cores, they fuse together and produce electron positron pairs.

And because the photons, they are stabilizing the star against the gravitational collapse, this means then that if there are less photons, then the star contracts a little bit. And this could lead to some explosive nuclear fusion and– which can liberate a lot of energy. And we think that the star could have experienced this pair-instability a few times.

And what could happen 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 pair-instability, 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. And it, it took a few thousand years for it to start to contract again, start nuclear fusion again. Then it experienced another of those pair-instability episodes, lost more material, contracts again, experiences this again. And with each of these new pulses, it sheds more material. And eventually it also loses some of this silicon-rich material.

And the exact timescales are very uncertain because this depends on a 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 2021-YFJ and why it did what it did.

FLORA LICHTMAN: I love that. What comes next for you?

STEVE SCHULZE: 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 2021-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– 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.

FLORA LICHTMAN: This– the missing link of supernova. I love that. I think that’s the perfect place to land. Thanks, Steve.

STEVE SCHULZE: Thank you, Flora.

FLORA LICHTMAN: Doctor Steve Schulze, a Research Associate at Northwestern University’s CIERA, the Center for Interdisciplinary Exploration and Research in Astrophysics.

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