What Lies Beneath The Outer Layers Of A Star?

Astronomers found a supernova whose lighter outer layers had been stripped away, revealing an inner shell rich in silicon and sulfur.

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• 1:03Flora Lichtman?

  Hey, I'm Flora Lichtman and you're listening to Science Friday.

• 1:09Flora Lichtman?

  Today on the podcast, astronomers caught a glimpse inside an exploding star and found some surprises.

• 1:17Speaker 02

  We immediately realized this is something that we have never observed before, but we didn't know what we actually did observe.

• 1:30Flora Lichtman?

  You might think of a star as just a mass of incandescent gas.

• 1:47Flora Lichtman?

  Yoho, 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.

• 2:08Flora Lichtman?

  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 Dr. Steve Schultz. 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.

• 2:32Speaker 02

  Hi, Flora. Nice to meet you.

• 2:34Flora Lichtman?

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

• 2:42Speaker 02

  This is absolutely perfect, perfect description.

• 2:46Flora Lichtman?

  I think you're being kind, but go ahead.

• 2:48Speaker 02

  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 onion looks like. So stars, at the end of their lives, they're huge cosmic onions.

• 3:06Flora Lichtman?

  OK, so how unusual or unlikely was it to even capture this kind of dramatic moment in this star's life?

• 3:15Speaker 02

  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 since discovery.

• 3:44Flora Lichtman?

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

• 3:47Speaker 02

  I wouldn't say emotional, but it's like, wow, what have you just discovered?

• 3:55Flora Lichtman?

  Did you know what you had when you saw it?

• 3:58Speaker 02

  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 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.

• 4:34Flora Lichtman?

  Okay, so when you say stripped, what do you mean?

• 4:36Speaker 02

  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 cannot only lead to losing a small amount of mass, it can actually lead to losing shells of this cosmic onion.

• 5:04Flora Lichtman?

  Okay, so it's lost its outer, it's lost its outer shell, its outer, it's the onion skin?

• 5:09Speaker 02

  Exactly, and not just one, but can can also lose several of those. The outermost shell is hydrogen, then there comes helium, then comes carbon oxygen, then magnesium, neon, and and oxygen, oxygen silicon, and eventually the iron 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 that lost even more shells and that could also explode.

• 5:53Flora Lichtman?

  So I, 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.

• 6:03Speaker 02

  Right. For us, it was surprising, and 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.

• 6:14Flora Lichtman?

  And what is that inner core that you're looking at?

• 6:17Speaker 02

  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.

• 6:42Flora Lichtman?

  Well, it sounds nice to be right, you know, first of all.

• 6:46Speaker 02

  Yes.

• 6:48Flora Lichtman?

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

• 6:54Speaker 02

  Hmm. 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.

• 7:30Flora Lichtman?

  Are the theoretical astrophysicists happy about this or like annoyed?

• 7:36Speaker 02

  Uh, 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 astrophysicists.

• 8:03Flora Lichtman?

  How do you know that the outer, the outer layers got stripped away instead of just got fused into the heavier elements that you see?

• 8:11Speaker 02

  Okay. So when a 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 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.

• 8:43Flora Lichtman?

  Got it.

• 8:44Speaker 02

  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.

• 8:55Flora Lichtman?

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

• 9:01Speaker 02

  Hmm, right. This is a very good question. Um, the evolution of stars is very, very complex and it depends on various parameters. There can be uh maybe some abrupt changes that can lead to uh huge changes in the evolution of a star. Uh, in this particular case, we have some ideas of uh how this progenitor star could have looked like or how it could have evolved, but we are uh we are not absolutely certain. Our leading hypothesis is that the progenitor star was it was a very massive star when it started to to um to explode.

• 9:41Speaker 02

  And um we think that it was so massive that the temperatures and the densities in the core was so huge that uh 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 uh the gravitational collapse. This means then that if there are less photons, then uh the star contracts a little bit. And this could lead to uh some explosive nuclear fusion and which can liberate a lot of energy. And uh we think that the star could have experienced this uh kind of pair instability a few times.

• 10:26Speaker 02

  And uh what could happen then then is that you have this very massive star. Uh, we think it was around sixty solar masses shortly before it it died. It experienced uh its first parent stability, lost about nineteen solar masses. So it's a lot of material and at the same time, it also um 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, um, so it was a very fluffy object. Um, and it, it took a few thousand years for the star to contract again, um, start

• 11:08Speaker 02

  nuclear fusion again, then it experienced another of those, uh, pan stability episodes, lost more material, contracts again, experiences this again. And with each of these new pulses, it hits more material and eventually it also loses uh, some of this uh silicon uh rich material. And the exact time scales uh are very uncertain because this uh depends on a lot of, lot of different factors and um. Uh, so maybe this is something that happened uh over the time span of a few thousand years. May- maybe it could have also taken a bit longer. The exact time scales uh of this process in this ob- uh, in this object is unknown.

• 11:52Speaker 02

  And this is something that uh theoretical astrophysicists need to investigate in great detail. And this is very important to understand what is twenty one Y F J and why it did what it did.

• 12:04Flora Lichtman?

  Hmm, I love that. What comes next for you?

• 12:08Speaker 02

  Okay, so the next thing for us is able to detect more objects. One object is not enough. We want to uh find similar objects and then by studying more objects of the same type, we can get a better idea of uh, of their core properties and also what was the most likely scenario for, for, for, for this type of explosion. The other thing is that um, the supernova twenty twenty Y F J is such an extreme object and uh the properties are so starkly different from the other supernova classes that we know, that we actually think there could be also uh some, uh, there could be other classes of supernovae that we haven't detected yet.

• 12:52Speaker 02

  So, um, there might be now uh something like a.

• 12:57Speaker 02

  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.

• 13:10Flora Lichtman?

  The missing link of supernova. I love that. I think that's the perfect place to land. Thanks, Steve.

• 13:16Speaker 02

  Thank you, Flora.

• 13:17Flora Lichtman?

  Dr. Steve Schulze, a research associate at Northwestern University's Sierra, the Center for Interdisciplinary Exploration and Research in Astrophysics.

• 13:30Flora Lichtman?

  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.

• 13:41Flora Lichtman?

  Today's episode was produced by Charles Burquist. I'm Flora Lichtman. Thanks for listening.