Ep. 581: Other Kinds of Novae

Posted on Oct 13, 2020 in podcast, Science, Stars, Stellar Evolution | 0 comments


Don’t ever accuse us of not comprehensively covering every kind of exploding star. This week we gather up all the leftover ways that stars partially or fully explode, or don’t. Probably. Enjoy.

Download MP3 | Show Notes | Transcript

Show Notes

VIDEO: What is a Nova? How Does It Compare to a Supernova? (Fraser Cain)

Ep. 579: White and Black Dwarfs

Ep. 14: We’re all Made of Supernovae

Eta Carinae (NASA)

New Clues About Eta Carinae – The Star That Wouldn’t Die (SciTechDaily)

The Homunculus Nebula (Cosmos Magazine)

What’s A Kilonova? You’re Looking At It! (Universe Today)

Binary Star (Swinburne University)

VIDEO: Image Sequence of V838 Monocerotis Epochs (Hubble Space Telescope)

Light Echoes From a Red Supergiant (NASA)

Palomar Testbed Interferometer (PTI)

Finding the Helium Flash (astrobites)

These Two Stars Might Merge in an Explosion Visible From Earth This Century (Discover Magazine)

Gamma-ray Bursts (NASA)

PODCAST: Sakurai’s Object (Stardate)

White Dwarf Stars (NASA)

V603 Aquilae (Wikipedia)

The Supernova That Wasn’t (Science Magazine)

Type IIn Supernova (Swinburne University)

Novae (2012 edition) (AAVSO)

Shedding New Light on Luminous Blue Variable Stars (NERSC)

Caught in the act: UW astronomers find a rare supernova ‘impostor’ in a nearby galaxy (University of Washington)

P Cygni (AAVSO)

You Know About A Supernova. What About An “Unnova”?

Vera Rubin Observatory (LSST)

Download MP3 | Show Notes | Transcript

Transcript

Transcriptions provided by GMR Transcription Services

Fraser:                         Astronomy Cast, Episode 581: Other Kinds of Novae. Welcome to 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 is Dr. Pamela Gay, a Senior Scientist for the Planetary Science Institute, and the Director of Cosmo Quest. Hey, Pamela, how you doing?

Dr. Gay:                      I’m doing well. How are you doing?

Fraser:                         Good. The word “science” just comes up. You’re a senior scientist working for a science institute. Science, science, science.

Dr. Gay:                      It’s kind of the way I live my life.

Fraser:                         It’s nice to have people focused on science.

Dr. Gay:                      It is. It is. And happy Canadian Thanksgiving.

Fraser:                         Oh, yeah. It’s this weekend. Yeah. We’re not that big about Thanksgiving around here. In fact, my sister calls it No Thanksgiving. We keep it pretty low-key. But Chloe is gonna be coming back from university, for Thanksgiving, and we’ll be hanging out, so there’s that.

Dr. Gay:                      That is excellent.

Fraser:                         Yeah. But trying to organize a Thanksgiving dinner is very complicated around this house, so. We just tend to make something super yummy.

Dr. Gay:                      That’s all you need.

Fraser:                         Yeah, exactly.

Dr. Gay:                      Just food.

Fraser:                         But it’s also, as I mentioned, year after year it’s very civilized. Although I think this year, like nobody is going to be traveling in the U.S. to attend Thanksgivings, so I think you don’t have to worry. But maybe in future years, once the pandemic has wrapped up, then consider adopting Canadian Thanksgiving as your date, because it’s just air travel, traveling in general, is a lot easier in October than in November.

Dr. Gay:                      This is true, and we’re starting to get fall leaves. So, it’s more beautiful.

Fraser:                         Yeah, the leaves are great. Yeah, in November, they’ve all fallen. It’s gross.

Dr. Gay:                      Yeah.

Fraser:                         All right, so, don’t ever accuse us of not comprehensively covering every kind of exploding star. This week, we gather up all the leftover ways that stars partially or fully explode, or don’t, probably. Enjoy.  

All right, Pamela. So, I’m not gonna lie, I did absolutely zero preparation for this week’s episode, because I have no idea what’s left. In fact, as we were prepping, I’m like, “Okay, so, what’s left?” And then, you then proceeded to rattle off a whole bunch of ways that stars can explode, that I had no idea had even had names. But… what?

Dr. Gay:                      These are minimal explode-y moments in their life.

Fraser:                         But you got a star, and the star gets brighter. It could be an explosion, it might not be.

Dr. Gay:                      And this is where we have to go back to: What does the word “nova” mean? It’s a new star. And so –

Fraser:                         Nova. New star.

Dr. Gay:                      Yeah. So, any time a star decides, “Hey, I may not have been visibly bright before, but look at me now,” and pops into our sky, that’s a nova. And so far, we’ve discussed where you have a compact object – white dwarf, neutron star, whatever – that is stealing matter from a companion, and periodically flares up in brightness, due to usually some sort of an explode-y activity.

We’ve talked many times in the past about supernovae, where an entire star decides, “I’m gonna kaboom; either my outer atmosphere, or all of myself,” and leave behind something or nothing, depending on the star’s mass and whim. But there’s other really cool stuff out there, and the other cool stuff out there has produced some of the most beautiful didn’t-actually-explode remnants for Hubble and other massive telescopes to point at.

Fraser:                          So, let’s run through a couple of examples, I guess, of stars that did something interesting, but not in a traditional variable star sense – because we’ve talked about them plenty of times, – but weird variable stars, right?

Dr. Gay:                       Right. So, here we have systems like – I think the most famous, maybe – Eta Carinae.

Fraser:                          Mm-hmm. Explode already.

Dr. Gay:                       Right. Well, or not. Here we have this star that wasn’t particularly noticeable until in the 1830s it decided to suddenly do the opposite of Betelgeuse, and it became brighter than Rigel.

Fraser:                          It’s about one of the brightest stars in the sky.

Dr. Gay:                       It became the second brightest star in the sky for awhile.

Fraser:                          Right. Right. Just after Sirius.

Dr. Gay:                       It’s had multiple episodes of getting brighter, getting fainter; and it’s thought that this homunculus nebula that is around it – it looks like an hourglass with an exploding waistline. I don’t know how else to describe it.

Fraser:                          That’s fine, yeah.

Dr. Gay:                       So, this hourglass of material around it is thought to be material that was given off during its great eruption in the 1800s, and its subsequent faintening occurred when all of this material cooled and coalesced into dust, and that dust hid the two stars we now know, that are lurking down in the center of this system.

Fraser:                          So, we’ve talked about this in the past, but what is Eta Carinae?

Dr. Gay:                       It is two advanced stars, one of which was probably well over 100 solar masses when it got its start. It’s subsequently, as the homunculus shows, lost a lot of that mass. Its companion is another luminous blue variable star. Another massive star, tens of solar masses in size. These two stars are orbiting each other in just over five years.

And as they orbit one another, we see changes in brightness, we see over time, right now, these two objects are getting brighter, and brighter, and brighter. It’s unclear if this is intrinsic to the stars, or this is the homunculus spreading out and getting thinner, and the star’s showing through better. We don’t know.

Fraser:                          And so, do we know what caused that flare-up the first time around, back in the 1800s?

Dr. Gay:                       So, that great eruption in the 1820s, the lesser eruption in the 1890s: We’re not entirely sure, but it was probably some sort of an ignition in the atmosphere, or an interaction of the winds of the two stars that ignited stuff.

Fraser:                          And so, does the fact that there are two stars interacting with each other – is that part of the special sauce?

Dr. Gay:                       Yes.

Fraser:                          Right.

Dr. Gay:                       The binary nature of this system, it’s not entirely – we can’t see through all the dust. We can’t see into the center. We’re trying. This thing has been observed in every wavelength that we have observatories to observe it at. The best we’ve been able to figure is it has a 5.54 period variability; that the winds of these two stars are shocking each other. Something about this combination of the shocks, two young stars, creates a colliding wind zone, and things periodically go boom in the night.

Fraser:                          Right. Now, of all of the stars that we know of – we’ve talked about this in the past – Betelgeuse is actually not as advanced as Eta Carinae.  Eta Carinae really is that star that could go any moment, now. But would both go? Would one go? Would one be thrown out into the universe? What would happen?

Dr. Gay:                       We don’t know these things. This is one of those scenarios of: You have two stars that are each large enough to go supernovae. We know there are cases where the shock from one star triggers the other star to do things. It’s now, in fact, thought that kilonovae are due to the shockwave from the one star going supernova, accelerating the rotation of a companion star that gives out material through its magnetic field. Binary stars are capable of really complex, cool stuff, and are also computationally very difficult to model.

Fraser:                          Right.

Dr. Gay:                       So, rather than accept that we actually understand exactly what’s gonna happen, I’m gonna go with: Anything is possible; it is most probable that only one star will go supernova. The other one will be ejected. It’ll take some damage. It’s unclear if this will trigger additional mass loss; if this will cause it to spin up; if this will impale mass onto it that increases its mass, and causes something weird; in all likelihood, it will get blasted. It will move out of the way. It will continue on its life in a new location.

Fraser:                          Right. Yeah. Now traveling through the Milky Way. Okay, so, that’s Eta Carinae. Again, I think we should expect it to brighten up again significantly again at some point, and then dim again, and then brighten up, and release more material into this cool nebula. But another star that we’ve seen a similar kind of effect – you have mentioned this before – is V838 Mon, right?

Dr. Gay:                       I love this object. This is an object that back in 2002 suddenly came into our visible existence. Prior to that, if you had dug deep enough into the glass plates, you could sort of say there’s a star there. Initially, we totally screwed up how far away we thought the star was, and it was initially thought to be an F dwarf. No, it was actually probably an A or a B star.

So, here we’re talking about a hot, bright star. And the reason we initially screwed up the distance is because this sucker was so big, that it was assumed that in order to get something that big it had to be initially faint and nearby, so that it would appear big in the sky because it was nearby, physically expanding.

Fraser:                          So, it actually turns out it’s just really big.

Dr. Gay:                       So, you start out with a star that is like Jupiter’s orbit kind of size.

Fraser:                          So, that’s kind of Betelgeuse-sized.

Dr. Gay:                       Yeah. Yeah. You then ask it to flare. So, with V838 Mon, you have this star that – it gave off a flash of light that had a width to that flash. And the way to think about this is: If you turn a laser on and off rapidly, you are actually creating that sci-fi beam that travels through space, but it doesn’t move slow enough you can see it like in movies.

The reality is you’re creating something that has a set beginning and end. It’s moving through space, and it’s illuminating parts of space as it goes; and what we do is we can watch clouds of gas and material have different parts of the cloud illuminated as this shell of light moves through the material. With V838 Mon, we had a star that had undergone significant mass loss. It had blown material all around it. It does have a companion. These things tend to have companions.

And as that shell of light moves through space, you have this increasing-in-size illuminated set of gas clouds. There were two different flashes of two different colors, which in some ways makes this even more interesting. And the question is: Exactly what happened? We don’t entirely know why does something like this occur.

Fraser:                         You don’t know this, but when you were talking, I was showing to the people who were watching the show live the animation of the V838 Mon and just watching this, just this incredible structure. Like, you’ve got this bright red star in the middle. That’s the star, right?

Dr. Gay:                      Yeah.

Fraser:                         And then you’ve got just this surrounding material that really looks like a dusty, exploding, expanding sphere of material. It really is one of the most stunning observations that humanity has ever made of something like this. Because not only are you seeing something with this level of resolution, you’re seeing it over time. When normally we’re just like, “Oh, yeah, there’s Andromeda. Oh, there’s the Orion Nebula.” But here’s something changing so quickly, just over the course of a few decades.

Dr. Gay:                      And the time at which this happened couldn’t have been better, in some ways. Because the time that it occurred – the Palomar Testbed Interferometer was up and being used, so they were able to measure its size on the sky at 1.83 milliarcseconds. And that angular size they measured on the sky? By figuring out where it actually is located in the galaxy, they were able to calculate the physical size of this object, which is how we know it was the orbit-of-Jupiter-sized. And that’s just kind of awesome.

Fraser:                         So, what do we think is going on? So, it doesn’t have a binary companion in the same way Eta Carinae does, right?

Dr. Gay:                      No, and this is where it’s this crazy combination that we can’t fully understand, of –

Fraser:                         Yeah, get used to that idea, that we don’t understand anything about this.

Dr. Gay:                      And so, there are guesses. We don’t know what any of these are necessarily true, but it’s a massive supergiant that flared, and in all likelihood, it was some sort of a ignition; a helium flash, or some other kind of “And-now-we’re-going-to-burn-new-things,” and these brief, transitory “And-now-the-star-ignites-and-it-does-new-things” can lead to these transitory events.

Fraser:                         The thing that I love is this idea that it briefly became like one of the brightest stars in the Milky Way, but it also briefly became one of the largest stars in the Milky Way. Which is such a weird idea to think about, just the fact that a star could go to whatever size it normally has, and then suddenly it goes up to 1,500 times the size of the Sun, out past the orbit of Jupiter as you mentioned.

That is bigger than Betelgeuse. It’s gigantic. It’s nearing as big as stars can possibly be, and it did it on our watch. People were watching it, and this star just bloated out orders of magnitude in size, released immense amounts of energy, but didn’t explode. Just a thing it did.

Dr. Gay:                      Well, and here’s the thing. If it was some kind of a thermal pulse; if it was that “And-now-we-shall-burn-new-things,” we know, theoretically, these things are supposed to happen and not destroy the stars. Otherwise, we don’t get heavy elements. And so, to catch one of these so-brief transitory events, where the flashes of light are given off and the star is still there, tells us that: Okay, so this is what it looks like, maybe, when these things that we’ve been saying hand-wavey have to be happening, actually happen.

Now there is one less accepted – but still worth mentioning, because it’s just a cool idea – possibility of what occurred, and this is a merge burst. This is the idea that what we now see as a double system was once upon a time a triple system, and two of the stars decided, “And now we shall be one.” And the flash associated with that merger – this isn’t the canonical idea. This is not – but it’s worth, because we don’t know. Sometimes it’s worth just saying, “Hey, there’s also this other possibility,” but it’s probably something ignited thermal pulse.

Fraser:                          But it is kinda safe to say if you have something really unusual and really energetic happen, then binary stars, multiple stars, can be to blame.  

Dr. Gay:                       Yes. Yes.

Fraser:                          That’s where astronomers start, is they go, “Okay, we’ve got some mayhem going on.” Just a regular star, all on its own, doesn’t get up to that kind of mischief. But you throw multiple stars into the mix, and mayhem ensues.

Dr. Gay:                       Well, and that’s actually a new thing to be doing. For the longest time, we were always trying to figure out how a singleton could do all the crazy stuff we see, and this is where the whole idea with gamma-ray bursts and the associated novae was decades of people trying to figure out, “Well, what if you spin it this way? What if you do this? What if you do this other thing?” and now we’re pretty sure it’s just two stars, and the physics became both harder and easier. Easier to understand, harder to do, because more dynamics.

Fraser:                          Yeah. Yeah. Okay, all right. So, we’ve got two not-novas but some of the most energetic events that astronomers have ever seen, here in the Milky Way.  

Dr. Gay:                       And these all boil down to, basically, we have an object in the process of evolving; it’s not blowing itself apart, but it’s blowing itself into brightness in a momentary stage. And Sakurai’s Object is another one of these “Hi-I’m-not-gonna-actually-behave” kind of objects, and in this case –

Fraser:                          I’ve never even heard of this object.

Dr. Gay:                       It’s this –

Fraser:                          How do you spell it?

Dr. Gay:                       S-A-K-U-R-A-I. It’s also known, unpoetically, as V4334 Sagittarii.

Fraser:                          Yeah. This is totally, totally new to me.

Dr. Gay:                       It’s the cutest little tiny red nebula. It’s 10th magnitude, so you can get images of it in a reasonable telescope. And in this case – it’s this cute little red planetary nebula that doesn’t have a white dwarf in its center, which is what planetary nebulae are supposed to have in their center. And it’s thought that this star was happily on its way, cooling down the white dwarf, cooling branch, and then had this moment of, “Nope, wait, I’m not actually done here, folks.”

Fraser:                          Right. One last gasp.

Dr. Gay:                       One last gasp, and it underwent a helium flash. So, you have the remnants of a star that has stopped undergoing thermonuclear action. It has collapsed down to a degenerate white dwarf. It’s radiating away light. It was like 100,000 degrees kelvin. And as it got dense enough during this collapse process, it was like, “Oh, wait. Hold on a moment.” And it became a red giant again in 1919, because why not? Why not?

Fraser:                          Yeah. Right. So, and, have we seen other examples of objects doing this kind of thing?

Dr. Gay:                       There’s been a number of similar objects we catch, but they haven’t been close enough to create as cute a little red planetary nebula to look at. And that’s the thing with all of these. It is –

Fraser:                          So [inaudible] [0:22:16] –

Dr. Gay:                       Yes, we will get one nearby really cool example happening while we have modern telescopes pointed at it.

Fraser:                          Right.

Dr. Gay:                       And then we’ll see all these other examples that are either far enough away that we only catch the light curve, but not the nebulosity. And we look at these and we’re like, that looks like the same kind of thing. So, for instance, with Sakurai’s Object, there was a similar late thermal pulse from V605 Aquilae. We see these similar objects, but they often – we don’t get the same. There’s a bunch of false novae known, a bunch of false supernovae known. These are luminous blue variables that temporarily flare up in brightness. They sometimes get mistaken for Type IIn supernovae, but they turn out to not actually be bright enough to be an actual supernova, and then there’s still a star left behind.

Fraser:                          Right, they’re still there. Like, that’s –

Dr. Gay:                       They’re still there.

Fraser:                          That’s the thing you’re really expecting from a Type Ia supernova, or any of these, is to be gone.

Dr. Gay:                       Not gone.

Fraser:                          Right.

Dr. Gay:                       No, no, not a neutron star. Not a black hole. Just still a star.

Fraser:                          Still a star, yeah. Gone, or –

Dr. Gay:                       Hanging out.

Fraser:                          Yeah, or no compact object remaining. No, no, it just goes back to being a star. We talk about this idea of the novae, where you’ve got this material that is falling on to a white dwarf, and piling up, and then it explodes with this thermonuclear explosion, and goes through the process again. And yet, it sounds like some of these events can rival the amount of energy released during a nova, and nearing the amount of energy released during a supernova, and yet they withstand it, and come back for more.

Dr. Gay:                       And we end up just calling them all sorts of different things. We call the Sakurai Object, the V605 Aquilae Object. We call it a slow novae.

Fraser:                          I love that. I love that name. A slow novae.

Dr. Gay:                       And then, we call luminous blue variables that are doing Eta Carinae-like things but we don’t see the homunculus nebula around them: We call those either imposter supernovae, or false supernovae. And then there’s another class of misbehaving object called P cygni objects. These are objects that have massive amounts of mass loss.

They actually have a different shape to their stellar profiles. When you do spectroscopy, the lines are a different kind of profile. Because of these massive lens, these are hyper giant, luminous blue variables. And they, too, periodically decide, “We’re going to get much brighter,” and this is thought that their eruptions may be mass transfer events.

So, here, instead of having the same kind of mass transfer flare-up that you get with material being stolen from a companion star and pulled onto a white dwarf, or pulled into an accretion disc around a neutron star, here you have the P cygni is transferring a massive amount of mass onto some sort of a smaller, say, 3 to 6 solar mass companion star. Stars are mean to each other, and this leads to fascinating variability, and we call a lot of this stuff novae when they get super bright and appear as new stars in the night.

Fraser:                         And before we started, I mentioned one additional kind of object that you weren’t familiar with the name, which is the unnovae.

Dr. Gay:                      Yes. Yes, because they’re not new stars.

Fraser:                         They’re not new stars, no. They’re the opposite. They’re where a star should have produced some kind of bright flash, be it a supernova or a novae, and it just didn’t; where a star just disappears. And they’re really tricky to find because you have to find a star and then notice that that star isn’t there anymore, and so, astronomers are starting to find evidence of this.

And we had a recent story about this, and the thought is that – well, up until this point, everyone thought that, okay, all giant stars like Eta Carinae, when they die, all that material falls inward; piles up; you get the supernova; you get the bounce off of the core; and you get the supernova in the enormous release of energy.

But in some cases, some percentage of these stars: As the black hole is starting to form in the middle of the star, it’s able to gobble up the material so quickly that you don’t get that bounce. It all just goes in, and the star just implodes, and it’s gone. And there’s starting to be more and more evidence that these things exist, so.

Dr. Gay:                      And this is why we need the LSST to be finished, and the only thing between us and it is COVID and a little bit of civil unrest, and hopefully both these things will end quickly. We wish the best for the Chilean nation and the best for the entire global population, because we want to do science.

Fraser:                         Yeah, I can’t wait for that telescope to come online. Thank you, Pamela. Do you have some names for us this week?

Dr. Gay:                      I do. As always, our show is supported through the generous contributions of all of our patrons. And now, more than ever, we’re grateful for what you’re able to give. We know that COVID is making it hard, especially for those of you that work in small businesses, or aren’t able to work from home. We see this reflected in changes to what you’re giving each month, and for those of you still out there: Thank you for allowing us to do everything we do. You allow us to pay our servers, to pay Richard, to pay Allie, to keep everything going, have Beth doing social media. Thank you.

And right now more than ever, I would like to thank Jordan Young, Burco Roland, Burry Gowen, Ramji Enamuthu, Andrew Palestro, Jeffrey David Mercini, David Truog, Brian Cagle, TheGiantNothing, Dan Litman, Laura Kittleson, William Robert Palesmo, Jos Cunningham, Paul Jarman, Les Howard, Emily Patterson, Adam, Agnes Brown, InfinitesimalRippleinSpacetime, @lovescience, Gordon Dewis, Bill Hamilton, Richard Riviera, Joshua Pierson, Frank Tippin, Jake Mudge, and Alexis. Thank you, thank you for supporting us.

Fraser:                         I just want to follow on what you said. We have dedicated our lives, most of our working careers, to educating people about space and astronomy. To sharing the love. We don’t get paid for this, but the money goes to the people that help get this podcast out, and to increase the reach, and to make sure the servers stay up, and to make sure that we can let everybody know that we’re working, and doing the editing, and so on.

And it feels so gratifying that we can provide salaries to people to be able to do this work while we are doing the thing that we love, which is to communicate and teach space and astronomy. And so, we really couldn’t do this without you. And for those of you who are on the fence: that if you want to contribute – if what you love is more space knowledge and outreach out there in the world – Pamela and I, I hope, have demonstrated that we’re in this for the long-haul, and we’ll continue to do this for as long as we can, and your support matters.

.

Dr. Gay:                      And one thing that I’m super proud to be able to say – and this audience didn’t get to hear it over the summer, is – starting August 1, we were able to offer healthcare to anyone on our team who wanted healthcare.

Fraser:                         That’s amazing.

Dr. Gay:                      So, here in the United States, during COVID times, to be able to say to part-time staff, “We got you.”

Fraser:                         Right. Oh, yeah. That’s incredible.

Dr. Gay:                      We got them, because you have us.

Fraser:                         Wonderful. All right, thank you everybody. Thanks Pamela, and we’ll see you all next week.

Dr. Gay:                      Bye-bye.

Astronomy Cast is a joint product of Universe Today and the Planetary Science Institute. Astronomy Cast is released under a Creative Commons Attribution license. So, love it, share it, and re-mix it. But please credit it to our hosts, Fraser Cain and Dr. Pamela Gay. You can get more information on today’s show topic on our website, astronomycast.com.

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