Time for another update, this time we’re going to look at what’s new with supernovae. And once again, we’ve got good news, lots of new stuff to report.
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Supernova in Wikipedia
What is a supernova?
What Is a Supernova?
Supernova: Exploding Stars – YouTube
Classifying Supernovae | astrobites
Type Ia -Presents a singly ionized silicon (Si II) line at 615.0 nm (nanometers), near peak light
Type Ib/c -Weak or no silicon absorption feature
Type II-P/L/N – Type II spectrum throughout
Type IIb– Spectrum changes to become like Type Ib
Hubble Sees the Fireball from a “Kilonova”
You Know About A Supernova. What About An “Unnova”?
Podcast Transcription provided by GMR Transcription
Fraser: Astronomy Cast, Episode 490: Supernova Update. Welcome to the Astronomy Cast, our weekly facts-based journey through the cosmos, where we help you understand not only what we know, but how we know what we know. I’m Fraser Cain, publisher of Universe Today. With me, as always, Dr. Pamela Gay, the Director of Technology, Citizen Science at the Astronomical Society of the Pacific and the Director of CosmoQuest. Hey, Pamela, how you doing?
Pamela: I am doing well. May the 4th be with you.
Fraser: That’s when we’re recording. And as we mentioned, I don’t observe May the 4th. I need a special Star Trek day for me and likeminded individuals, so if anyone knows what day that should be, that would be great. Time for another update. This week we’re going to look at what’s new with supernovae. And once again, we’ve got good news, lots of new stuff to report. Now, before we even start, the one big update that has not happened is we have not had a close, bright supernova since 1987.
Pamela: This is true.
Fraser: Another decade has gone by, and we – and that wasn’t even in our own galaxy.
Pamela: And so, here’s the thing, guys. There is statistically, supposed to be a supernova on average every hundred years in every galaxy like ours. And we haven’t had one in way more than a hundred years. And so, the question is, are they on the other side of the galactic nucleus [inaudible] [00:01:38] gas and dust, is our galaxy going nah-nah-nah-nah-nah-nah, you’re not allowed to have one, which is my personal belief, or are we just going to catch up by the rule of averages and have six all at once?
Fraser: Now, I’ve heard they happen maybe even more often than that. But there is a big chunk of the galaxy that we can’t actually see. So, it could very well be that our supernova went off, and we didn’t see it thanks to the stupid galaxy in the way.
Pamela: This is true. This is sad. The best hope is maybe we’ll get better at watching them maybe with neutrino detectors. We have detected some gamma ray bursts with the neutrino detectors. I think there have been a few supernovae that have been seen this way. So, maybe as we get better and pinpointing what direction supernova stuff comes done, maybe if we get unlucky – well, lucky because they’d be far away – the direction that GRBs come from, if we have a GRB in our galaxy, we could see it just in other wavelengths of light, in other kinds of detections like neutrinos, which is a particle instead of light.
But yeah, bad times, if you’re a supernova explorer who wants to study the individual chemical components, which is what we use nearby supernovae for.
Fraser: But I mean, there have been a lot of supernovae. Stars are exploding across the entire universe all the time, and fortunately, they’re bright enough that we get a chance to see it, even though it’s so distant. What new and interesting things have we learned since we first covered supernovae? I’m not sure when, but I’m sure it was right at the beginning because again, we were going after all the low hanging fruit. So, what have we learned?
Pamela: Well, one of the biggest and kind of frustrating things that we’ve learned is – last time we talked about this, I talked about how we generally use Type 1a supernovae as standard candles. We assume that all of them are at the exact same luminosity. This means, if you’re a Type 1a supernova, you start with this amount of material, you go kaboom because of degeneracy pressure and things like that, which would be an entire episode. We did one. And because you’re going kaboom with the exact same mass of fuel basically – and it’s that basically that gets us in trouble every single time – same size detonation, same size explosion, same amount light given off, standard candle.
Fraser: So, before you go deeper down this rabbit hole, I just want to give people the quick – for when you say Type 1a supernova and just went on – that are all the same – just, can you give a brief explanation of what is the orbital mechanics? What makes that Type 1 supernova go off with roughly the same energy every time?
Pamela: So, you have – and I’m looking for props on my desk because that’s a thing I do. I have a red giant star. I am now looking for a white dwarf.
Fraser: Podcast listeners, I apologize. I will try to explain what Pamela’s going to be doing here.
Pamela: So, you have an itty-bitty little tiny star called a white dwarf that is roughly the size of the Earth’s moon. And it is orbiting nearby a regular everyday star. It could be a red giant. It could be a main sequence star. It could be any kind of everyday star. And our white dwarf star has so much gravity and is so tiny, that it’s able to snuggle up right next to our everyday kind of star, and as it snuggles up really close, its gravity is like, I’m going to nom-nom-nom-nom-nom-nom on that super giant, on that main sequence, on whatever that companion star is. It gets eaten.
And in the process of consuming its partner, through a process referred to as stellar cannibalism, the white dwarf gets more massive, and more massive, and more massive. And as this occurs, the protons inside of it, the atomic nuclei inside of it – because they’re a mix of hydrogen, helium, and carbon. Different starts have different dominant atoms – doesn’t matter. As it gets more massive, the gravity trying to collapse it eventually overwhelms – the force of all the little protons going, don’t come near me – which is the electromagnetic force.
And when that electromagnetic force gets overrun, when the electron degeneracy pressure off all the electrons going, don’t come near me, when that gets overrun, it tries to collapse. And depending on the situation, how we see it can vary, but whatever happens, it should give off the exact same amount of light as it goes kaboom. Now, it gives off the exact same amount of light. We see different light curves. The difference in how we see it is how quickly we catch it. Sometimes, we catch it during the brightening phase, which is awesome, more science occurs.
Sometimes, we catch it during the downside of this brightening and fading, and we’re sad because we don’t know what its maximum brightness was, and that’s what we use for research. But wherever we catch it, in this brightening and fading, it is always the explosion of basically the exact same mass because that’s the mass at which protons and the electrons are like, dude, I can’t support you anymore against gravity.
Fraser: And so, now, the problem is, is that we found out that it’s not always exactly the same mass.
Pamela: Well, it’s not so much that it’s not always the exact same mass as it’s not always the exact same brightness, which is worse. So, the theory-theory was you’re exploding the same mass, it should go kaboom with the same luminosity. But it turns out, just like if you’re looking at a hundred watt red light bulb vs. a hundred watt blue light bulb, they do not seem to illuminate your room the same way. If you have a explosion that has more stuff that isn’t hydrogen and helium in it – you start adding in carbon. You start adding in iron. You start adding in titanium. You start adding in these entire rest of the periodic table that is not hydrogen and helium.
And that change in content, which astronomers – because we’re lazy, refer to metallicity, and hydrogen is X, helium is Y, and everything else on the periodic table is Z. If you increase that Z value, it turns out that it looks like the supernovae are dimmer, which means not standard candles, which means, as we look back in time and the amount of that Z, that amount of that metals that has had a chance to form goes down, which means how supernovae that we use as standard candles to measure the expansion rate of the universe, how they behave changes with distance.
Luckily, we have measured the acceleration of the universe’s expansion through multiple methods, so we’re good. That’s still there. But now, we’re confused about this one specific – very specific kind of supernova.
Fraser: But can you, I guess, observe more of them, use other standard candles to double check the distances? And once you look at the chemical signature of the supernova, can that give you a better sense of how inherently bright it was? Is there a way to get back to a really nice standard candle with a Type 1a supernova, or is that measurement tool worthless?
Pamela: Well, it’s not that the measurement tool is worthless. It’s that we need more local supernovae that come from different pockets of metallicity. We can only really measure the distance to a supernova precisely for nearby supernovae that are in galaxies that have other standard candles. So, they have to have the Cepheid variable stars that we’ve talked about so much. They have to have, as a secondary deterministic – certain stars, when they die, they poof off their outer atmospheres as planetary nebula. We can sort of use that as a standard candle.
We can sort of use – and by sort of, I mean more error. We can sort of use some old red stars like Mira variables as standard candle. So, we have all of these high error standard candles, but we really need those high accuracy Cepheids in the same galaxy as the supernova. And as you said, things aren’t exploding a lot nearby.
Fraser: And that’s the problem. If you’re looking for a thing that happens once in a hundred billion, you have to wait a long time.
Pamela: Well, and the other problem is volume. When we look out, we see this little tiny sphere of stuff that’s nearby. Now, if I double the size of that sphere, I have done way more than double the volume because the volume goes as the cube. And so, as we look further and further out, we’re looking at enormous volumes of space. And as the volume that we’re looking at goes up, and up, and up, there’s a higher chance of seeing these things. Unfortunately, it’s a higher chance of seeing them in places that we can’t see the Cepheids.
Fraser: So, that is a downer.
Fraser: But it feels like the kind of thing that new surveys, things like the Large Synoptic Survey Telescope is going to find a lot more supernovae, and it’s going to just dump mountains more data back into the hands of astronomers, and who knows when they come back around with a much more accurate yardstick and learned a lot about stars at the same time. What else is new about supernovae – because I’ve got one.
Pamela: What one do you have?
Fraser: We’ve seen two neutron stars collide with each other and –
Fraser: Kilonova, and turn into a – I guess it’s not quite a supernova.
Pamela: It’s a kilonava.
Fraser: I mean, it’s one of the methods that had been theorized was a gamma ray burst, and now we’ve actually seen this, and detected obviously in both of the gravitational waves and in the visible light spectrum, which is huge. And I think the most interesting thing is the elements that they saw formed in that collision. And that has implications for supernovae.
Pamela: What is super cool is – this was a GRB discovery of two colliding neutron stars. And my wedding ring is made out of white gold and stones. And it turns out that this particular ring, all of the atoms in it that aren’t strengtheners, so all of the atoms in it that are actually gold atoms came, we think, from kilonovae explosions, where you have two neutron stars merging into one object. We have this massive – all of space ripples with a gravity wave. We have this ripple moving through space and massive amounts of nuclear fusion going on. And that nuclear fusion has one of its end products – gold. And we didn’t know this for sure before.
Fraser: Well, and that’s the thing. Again, if we had this show – and if you go back to previous shows, you’ve heard us have this conversation many times, where did the heavier elements comes from? And your answer would have been supernova – core collapsed supernovae. And now, the possibility is that some or maybe even all of it comes from colliding neutron stars, and some or none of it comes from these core collapsed supernovae. So, that’s huge.
Pamela: Just to back up, supernovae come in many formats. We have the small dense object, like a white dwarf – like a white dwarf next to a star. The white dwarf eats the star, goes kablooey. We have neutron stars that eat things off of their colleague. We have blackholes that eat things off their colleagues. So, there’s lots of cannibalism going on in the universe. We have the collision of neutron stars with white dwarfs or blackholes. We have the collisions of neutron stars with neutron stars. We have the collisions of blackholes with blackholes. I don’t know if that technically goes supernova, but it certainly creates gravitational waves.
Now, in addition to that, we have massive stars that don’t need to eat anything. They are happy to explode on their own. Because when they stop generating enough light in their core – it turns out stars are supported by light. Right now, if you’re in a room where light is shining on you, sunlight, lamplight, florescent lights, all of those photons are actually exerting a pressure on your body that’s so small, you can’t notice it. Now, The Planetary Society, they’ve launched light sails, and they’ve been able to maneuver spacecraft using the light from the sun.
Now, with supernovae, what happens is, you have a star that is supporting itself by generating photons, generating light in its core. And the outer atmosphere of the star is going okay, I would like to collapse, and then visit the core, but photons are holding me up. I’m good up here. Now, when the nuclear reactions end in that core, those photons stop going out, and the atmosphere of the star is like, oh, expletive, collapse. And during that collapse, all sorts of nuclear reactions occur because you have all these atoms smashing into one another, very short time periods.
And it’s that nuclear reaction caused by core collapse, the word you used, of these giant stars, that also produce a lot of heavy elements, and this massive shockwave of light moving out from all these nuclear reactions that get triggered during the collapse process.
Fraser: So, that’s a thing we learned. What else have we learned about supernovae?
Pamela: That some of them just plain weirdos.
Fraser: Weirder than a star with many times the mass of our sun, exploding inward at a significant portion of the speed of light, and releasing more energy than potentially the rest of their galaxy combined, spewing heavy elements out into space? Weirder than that?
Fraser: Tell me how. Explain how.
Pamela: So, every family has to have that weird uncle, that weird cousin, that weird aunt. We’ve all got them, and it turns out, even supernovae families have them as well.
Fraser: If you don’t, is it you?
Fraser: Yeah, alright. So, the weird uncle of supernovae…
Pamela: So, there was a supernova that was observed, and everyone measured it. It was about 50 years ago. That’s a thing. They’ve been going off. They’ve been studied for thousands of years. We can actually use ancient Chinese records to find out when different supernova remnants that we see in the sky were formed. So, there’s a record of one going off 50 years ago. And then, along comes Las Cumbres Observatory, which is doing an amazing survey. They pick up supernovae on a regular basis. They also pick up asteroids, comets, all sorts of cool stuff.
And a grad student – because it’s always a grad student – was going through the latest data, picked up what looked to be a supernova, and it was the same object. So, it appears that we can have these objects that periodically go off, but don’t completely explode. They just think about exploding. We think they’re a supernova. Turns out, we’re wrong. And they surprise us later by finishing the job.
Fraser: Wow. But did it finish the job on the second go around, or…
Pamela: We think so. Now, what’s cool is this is basically the exact same thing that Eta Carinae is going through – well, not the exact same things. It’s very similar.
Fraser: Because it flared up a while ago.
Pamela: So, the way stars are named is, in general but not always, the most bright star according to human eyeballs in a given constellation is alpha – next one’s beta, next one’s gamma, keep going through the Greek alphabet, which means Eta Carinae, which is the brightest star in Carina, was not the brightest star by a few stars when it was named. For a while, it was the brightest star in the entire sky. And then, it faded away. And it has flared up periodically. And it is one of the coolest objects, and one of the ones most likely to go supernova in the southern skies.
Fraser: I’ve got another one for you that’s new and weird. This is another class of supernova that have turned up, and they’re failed supernova. Have you looked into these at all?
Fraser: They’re also called unnova.
Pamela: I didn’t know that part. That’s awesome.
Fraser: That’s the term, unnovae. And so, the gist is often, they’re red supergiant stars, and they give all the indications of going through the first stages of a supernova, but then they fail. And what it looks like is the inward pressure, and that sort of need to turn into a blackhole, just instead of them blasting out all that material, they just completely implode, and just disappear.
Pamela: That’s awesome.
Fraser: It’s totally crazy. There’s only been a couple of them – have been found so far within the last ten years. But still, it’s sort of this really interesting potential path that some of the big stars – and so, there may be more supernova out there that we just don’t see because they just go – and they’re gone, which I think is just amazing.
Pamela: That is cool. That is very cool. I did not know that name.
Fraser: Unnovae, yeah.
Pamela: I’m going to be laughing about that for a long time. That is the one time astronomers could be trusted to name something well. So, did you hear, we finally found a survivor from a supernova explosion.
Fraser: That was just a couple of weeks ago, yeah. That’s amazing.
Pamela: So, when we do these update shows, sometimes the updates were yesterday or last week because science is constantly evolving. The reason we do science is not because we know everything. It’s because we want to know everything, and we don’t yet. So, we keep looking, and we keep finding new stuff. And in this case, supernova, 2001ig, which tells us when it was found, those letters correspond to which half of which month, and which one in that half of the month.
So, back in 2001, there was a supernova that went off about 40 million years ago, in a galaxy named NGC, for New General Catalogue, even though it was started over a hundred years ago – NGC 7424. Bad name, cool object. So, this particular galaxy had this supernova go off, and over time, the brightness for the supernova fades, the gas, dust, shockwave that’s pushing through space around it expands. And as it expanded, Hubble went back to look to see what it could see. And it found this little survivor going hi, I’m still here. I wasn’t eaten. And that’s just kind of cool.
We’ve theorized that there are supernovae, core collapse supernovae in binary systems. And this is the first time that we have evidence of this, yes, we were right.
Fraser: Well, and the super fascinating part about this is that the supernova went off, and it plastered the companion star with material. And so, they’re able to study the companion star, and look the elements that are in the companion star and get a sense for what was in the other star as well. So, it’s a really amazing discovery, and the kinds of things that astronomers have been really hoping to see for a long time.
Pamela: And so, now the question is becoming, can we get better at going back, now that we have all these amazing surveys? Sloan Digital Sky Survey, Gaia, oh my gosh. I think it’s safe to say, we are both in love with Gaia. Go back, listen to the episode if you hadn’t. It’s an amazing technological marvel of data product – duct – product – that word.
Fraser: We haven’t done an episode on Gaia, have we?
Pamela: Yeah, no, we did one on it when we were talking about all the big survey missions that were coming up.
Fraser: But we didn’t specifically – yes, we did, 365, never mind. We did it.
Pamela: So, with all of these surveys, we’re going to have the ability – and this is new, or at least new since the Sloan Digital Sky Survey really – to go back and see image, after image, after image of the same fields, taken with the same high-quality detectors, that aren’t glass plates because we used to have to go crawling through Harvard to find this stuff in their glass plate library. We’re going to be able to go through all of this digital material and go, I found a supernova. I’m going to find its progenitor, and this ability to match thing that exploded with thing that was going to explode before it exploded. This is going to open the door to so much new understanding.
Fraser: There are so many of these surveys, as you said. And I’ll do another callback to the Large Synoptic Survey Telescope, which is going to be – it’s going to find so much material in the night sky. But even as – things like Gaia, even things like TESS are going to potentially turn up some of these objects. So, the more just surveys that are going on all the time, the more of this kind of material we’re going to start picking up. And it’s funny how I think astronomy is going to be less and less about picking some target and observing it very carefully, and more and more about crawling through these gigantic databases to come up with insights.
Pamela: And as a database programmer, I am so happy about this. This is my kind of science. So, I’m stupidly pleased with the direction that we’re going in.
Fraser: Have you got any more interesting supernova news?
Pamela: I think we hit all the key big changes to the field, and I’m hoping Adam Riess will force us to do another update in a year or so, as we figure out what’s wrong, as we’ve talked about before, with calculating age not in the local universe vs. using the early universe. But we’ll be here, recording Astronomy Cast when we find the answers.
Fraser: That sounds great. Thanks, Pamela.
Pamela: Thank you, Fraser.
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Duration: 27 minutes