This is our second episode in a two part series where we look at Transients in astronomy. In last week’s episode, we talked about things that change here in our own Solar System. Now we’ll talk about everything else in the Milky Way and beyond.In this episode we mentioned donations and tours. Click to learn more!
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Fraser: Astronomy Cast, Episode 520: Transients, Part 2. Welcome to Astronomy Cast, your 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 CosmoQuest. Hey, Pamela, how you doing?
Pamela: I’m doing well. How are you doing, Fraser?
Fraser: Great. For the people who are listening to this show, it’s been a week. For us, it’s been about five minutes. That’s because –
Pamela: Not even that long.
Fraser: Not even since we wrapped up recording our last show to when we’re doing this show because I’m gonna be gone next week, and so, in fact, probably when you’re listening to this, I will be in Costa Rica with Dr. Paul Sutter will be as part of our Astro Tour as we explore the jungles, and beaches, and mountains, and volcanoes, and the dark skies of Costa Rica. And if you wanna be a part of any of these kinds of trips, go to astrotours.co. Pamela’s gonna be taking you to the American Southwest. It’s possible that I’m gonna be going back to Iceland at some point, so there’s a lot of really interesting trips coming up, so check it out.
Pamela: And I hope to see you in Tucson and Vegas.
Fraser: Excellent. All right, this is our second episode in a two-part series where we look at transients in astronomy. Last week’s episode, we talked about the things that change here in our solar system. Now, we’ll talk about everything else in the Milky Way and beyond. All right, Pamela, when last we saw our heroes, we were talking about asteroids, and comets, and things crashing into the moon, and things crashing into Jupiter, and potential extrasolar comets blasting through the solar system. But this idea of transients, the fact that the universe changes, extends out way beyond the solar system – all the way out.
Pamela: It is true, it is true, and the techniques that we use to observe transient phenomena don’t really change, but how they move through our images, how they vary in our images does change from inside our solar system to outside our solar system. Let’s face it, things inside our solar system, it’s a lot easier to see them moving, and so the fact that an asteroid is flying by the earth, that makes it a transient phenomenon. Now, as we look out at the sky, the fact that Barnard’s Star is creeping across the sky at a measurable rate does not make it a transient object. That just makes it an object that has a large motion. Now –
Fraser: Well, and that’s astrometry. I mean, that’s a whole other super fascinating science, and thanks to Gaia, we now know where a billion of these stars are moving, but yeah, it’s things that move, or flash, or change more rapidly than that ponderous movement of Barnard’s Star.
Pamela: And it’s the kind of thing that a lot of people don’t realize is going on. One of my favorite moments of “Oh. Oh, dear, that doesn’t work the way you think it does” was reading a Keats poem where he referenced wanting a love as constant as the stars. And stars are anything but constant, and –
Fraser: Oh, Keats.
Pamela: – they sometimes explode, and I don’t want that kind of love, thank you very much. Please keep mine as constant as a rock.
Fraser: Sometimes they change in brightness, and sometimes they explode just like real relationships. And they’re all moving.
Pamela: Yeah, yeah, but it doesn’t mean you want that.
Fraser: And eventually, they end up in a black hole.
Pamela: So, Keats was so wrong in what he thought he was gonna get, and in considering that, let’s consider all the things that go flash, flare, and flicker in the night, and one of the things that came up in our last episode was how our own sun likes to do things like fling coronal mass ejections out at us. And it turns out that while our sun from our perspective does throw some pretty amazing temper tantrums that lead to some pretty spectacular aurora borealis, it turns out that it has nothing on some of the other stars scattered out among our galaxy. There are young protostars that put out flare events that are 10,000 times more energetic.
Fraser: Hundred thousand times. Some of the flare stars can go 100,000 times brighter than the most powerful flares that our sun throws out. Tiny, little red dwarf stars. It’s crazy.
Pamela: And there are some stars that are known to do this more often than others. They get classified as flare stars. But this is definitely one of those cases of having to get lucky when doing your research because there is a chance, a non-zero chance that every single time you go to the telescope and look at a star, it’s just gonna be going, “Hi, I’m a star.” And what you really are interested in is that moment it goes, “Hi, I’m going to destroy my entire solar system with a flare right now.”
Pamela: And those are the scientifically interesting moments. And so, the trick is how do you catch all those moments?
Fraser: Well, you just asked yourself my next question. How do you catch all of those moments?
Pamela: Well, in the future, it’s gonna be with the Large Synoptic Survey Telescope, at least for the part of the sky that it can see. For now, we have a variety of different surveys scattered all over the world looking at different parts of the sky that are night after night working to observe as much of the visible sky as they can and comparing the measurements of brightness from one night to the next to identify those things that have outbursted in their own unique way. So, we have flare stars. That’s one way. Those are often young stars, hot stars, energetic stars. B-type stars like to flare in some cases.
And all of these hyperenergetic young stars, angry stars, they aren’t at least blowing themselves apart. We also have cases where we don’t know which star it’s going to be, we don’t know when it’s going to be, but let’s face it, supernovae are another kind of transient event. They are that star that explodes once rather than flaring over and over and over.
Fraser: I did an episode on habitability around red dwarf stars, and we looked into these flares that happen. And, yeah, the 100,000 times the amount of radiation would cook any life, and these planets are much closer to their star than we have here, so you can’t comprehend the amount of energy that is being dished out to one of these planets and that some of these stars flare every day. It’s crazy how different these places are than the sun, but as you said, there are other ones that will flare. Now, you went straight to things exploding, but there’s an in-between stage, so –
Pamela: Well, there’s lots on in-between stages.
Fraser: Lots of in-between stages. So, why don’t we talk about novae first, and then we’ll go full supernovae next.
Pamela: So, we have basically the life and death of stars is one of change. Baby stars like to go flare a lot, middle-aged stars pulsate. That’s not so much as a transient event. It is a change in brightness.
Fraser: I guess we should talk about mere variables.
Pamela: So, we have all the pulsating variables, the Cepheids, the RR Lyraes, the Mira variables. We have the random –
Fraser: Actually, you know what, I’m gonna stop you because what’s actually happening with some of those different kinds of variable stars is mind blowing. If you could actually see them up close, what’s actually happening to these stars is just tremendous. So, instead of just naming them like it’s some kind of library catalog, could you just take a second and talk about – I mean, we did a whole episode on those variable stars, but just to give people the sense of some of the most dramatic versions of this.
Pamela: So, stars are very carefully balanced between gravity trying to collapse them in and light trying to push out and expand them. And a star like our own sun is for the most part nice and completely stable in radius. It has various harmonics built up on its surface, various soundwaves are moving through us out our atmosphere, and so it has these little small-scale pulsations from place to place. No big deal. It’s a star. It’s doing a star-like thing.
Well, at certain points in stars’ evolution, as they very gradually change in density, temperature, and radius, they can hit a point where as the light goes out, the temperature is such that instead of pushing outwards on the star that light starts getting absorbed into the gas in the outer layers of the star, and so the star’s like, “Ooh, not getting supported as much as I’d like. Gonna collapse now.” And as it collapses, it gets brighter, it gets brighter, it gets brighter, and that energy that is stored in the atmosphere also gets given off, and all of this gives the star an extra push expanding it outward.
And so, a star will hit a resonance point where it is collapsing down and pulsating out, driven by energy getting stored in the gas and its outer atmosphere. This sets up long-term pulsations that can last millions of years and elder stars, the RR Lyraes I mentioned, are small ones that pulsate over a period of hours. Cepheids are larger ones that pulsate over periods of days or tens of days. And when I say pulsate, I mean they’re actually changing radically in radius, in color, in brightness such that we can see stars in other galaxies undergoing these kinds of pulsations.
Fraser: Just amazing. Can you imagine what it would be like to be on a planet around one of these stars and seeing the outer envelope of this star expand over the course of hours to a dramatically larger size and then again over the course of hours shrinking back down into a much smaller size and changing in brightness? It’s one of those things – people always ask me, “What’s a thing that you wish you could see up close?” and I wouldn’t wanna be too close, but I would love to see one of these variable stars doing its thing from a safe distance.
Pamela: I know. I haven’t done it before, and I should’ve. I now really wanna work out the change in energy received by a planet that is on average at the center of a habitable zone for one of these stars and see does it go in and out of the habitable zone at the extremes. This is math I can do, and I now want to do.
Pamela: Oh, man, you’re giving me ideas.
Fraser: Nice. There you go. And then it’ll turn into a science fiction story. Now, I was about to get you talking about novae, and then I side-turned into variable stars. Let’s go back to novae now, which are the next level of flashes of brightness.
Pamela: So, there are a whole variety of different novae that recur at varying levels. There are some novae that we see that repeat like clockwork. These are ones where you have a situation with a white dwarf star, a star roughly the mass of the sun that is compressed down to roughly the size of the moon and is no longer generating its own light through nuclear processes. It’s just radiating away the heat that it has stored from when it was a star. And if one of these is in a binary system, and it decays to get close enough to its neighbor, it can become a cannibal and begin stripping material off of that neighbor.
And as that material spirals in towards the white dwarf forming an accretion disc, that disc of material can periodically get large enough in mass that it just blows itself apart. So, in this case, you have recurrent novae that are driven by, well, cannibalism of one star off of another. We also have cases where it’s unclear do these things recur consistently or not and various levels of do they recur consistently or not where you have, again, a white dwarf cannibalizing its neighbor where it pulls down the material, and the material on its surface goes kaboom, clearing off the surface, clearing out the accretion disc.
It’s a little bit more violent, and each of these kinds of situations has distinct spectra, has distinct patterns. You can actually look at some of these stars from night to night and see the variations in brightness as that hot disc of material moves around and is viewed at different viewing angles. These are fascinating systems that change in brightness because they are binary stars, because there is this hot disc giving off its own light, because this disc may explode occasionally, and because stuff that lands on the surface of the star may explode occasionally. There is a lot of explodiness going on in this system that finds all sorts of different ways to have transient light in the sky.
Fraser: And actually, we just did an article on Universe Today again about a nova that’s been going off every year like clockwork for millions of years. So, it’s this process where it’s pulling this material off of a companion star, accreting it onto the surface, it builds up to a certain size, it detonates like a bomb off the surface of the white dwarf, and then the whole process starts again. It clears out the material, and then it starts again. And it’s been building this gigantic gas – this sort of shell of dust and gas around the star from millions of these nova explosions. But for each one of these white dwarves, there will be the final explosion.
Pamela: So, that material funneling down onto the surface of the star doesn’t always get politely cleared off through explosions. Sometimes it just gradually builds up, and builds up, and builds up until the total mass of the white dwarf exceeds essentially the supporting power of the electron degenerate pressure that is holding the star together. So, in these systems, it’s not like pushing out and gravity pushing in that is supporting the star so that it doesn’t collapse down into a black hole. Instead, it’s all of the electrons in the atoms of the white dwarf that are going, “Don’t come near me,” and –
Fraser: “It’s my space.”
Pamela: Yeah. It’s the electron degeneracy pressure of all of these electrons trying to abide by the Pauli exclusion principle and all maintaining their own energy levels, spin up, spin down, not gonna break any rules. Well, at a certain point, they just don’t have the capacity to push one another apart, and through this electron degeneracy pressure support the star any longer. And when that pressure, when that electron degeneracy pressure gets overwhelmed, everything goes kaboom, and this is when you get a Type Ia supernova.
Fraser: And this is just one example of a supernova. There’s the ones where you have a big, young hot star – and of course, we’ve done whole episodes of this – and they detonate, or they are left with a black hole, or they’re left with a neutron star, and each one tells a story that astronomers can use to understand what came before. And as we mentioned as well, they’re used as standard candles to help understand the distance of things in the universe.
Pamela: And these two different objects, supernova and asteroid discussed in the last episode, are really the two main justifications for all these global surveys that are going on of the sky. We’re trying really hard to understand what is the future of our universe. Will we die by fire, or ice, or great rip, and I don’t even know how to consider that. And that future evolution of space-time is going to be defined by understanding the rate at which our universe has been expanding since the earliest days, which we see through supernovae.
Now, the other thing is how is the earth going to live and die, and hopefully death by rock is not our definitive future. And the way we prevent ourselves by going the way of the dinosaurs is by finding any rock that is potentially going to hit us. So, when we go surveying for transient objects, when we build all of these different telescopes designed to find transient after transient after transient, our justifications are measuring the future of the universe and preventing death of the planet Earth. But all this other science is like free extras, so I have science for you, I have science for you, I have science for me in the form of all these Cepheid variables.
And fundamentally, even the Kepler telescope with its planet hunting capacity was a transient searching telescope where it was looking for the changes in brightness due to planets passing in front of the stars.
Fraser: And that’s where I was gonna go next, right? In the previous episode, we talked about this idea that we can observe stars and watch occultations as an asteroid passes in front. The universe is a really big place, and things pass in front of other things all the time in ways that are really amazing and teaches a bunch of really interesting stuff about the universe. So, can you talk just a bit about the transit method of transients?
Pamela: So, in this case, we have a distant system that has its own planets, its own asteroid belt, its own disc of material. We’ve seen examples of all of these kinds of phenomena, and that distant solar system stuff passes in front of the star that it calls its home star and blocks out some of that star’s light.
And we can learn about what is doing the blocking by how that transit occurs, and we can, by measuring the star’s motion in and out of the plane of the galaxy, by measuring how it gets Doppler shifted is the scientific way of saying it, we can measure how much mass is in that material, and we can start to get really cool and detailed measurements of distant solar systems that start to give us more detailed pictures of how normal or abnormal is our own solar system, and it’s looking like we’re pretty weird.
Fraser: Right. We thought we were normal. We’re not normal.
Fraser: But you mentioned when we were talking about, for example, when you watch an asteroid pass in front of a star and dim the star depending on where you are on earth, you can learn different characteristics about the asteroid. You can learn whether it has a moon, whether it has a ring, whether it has an atmosphere. There are all these really interesting things, so what can you learn about the object that is passing in front of the star and what happens when a star – actually, we’ll get to that in a second, but when stars pass in front of other stars.
Pamela: So, with planets’ discs passing in front of their home stars, the two basic pieces of information that you can get at if you have a nice, discreet planet is what is the planet’s radius, what is its distance from its home star, and what is its mass. Now, for these three things to all happen, you need to have a planet that eclipses its star, passes right in front of it, allowing us to measure how long it takes to get in front of the star, how long it takes to get out from in front of the star. That gets us its radius. We need to be able to see this multiple times. That gets us its period of motion.
And if we can measure the Doppler shifting that’s going on, how that planet is in turn using its gravity to move the star that it’s going around, that will give us its mass. So, we can get pretty detailed understandings. Now, one of the other weird things that happens is there are examples out there of discs of material going around stars that get involved in this whole eclipsing act, leading some really weird long-term eclipses. Now, these kinds of events don’t allow us to measure the mass in that disc, but they do allow us to say, “Huh, yeah, other stars have dust discs,” and even that is pretty cool science.
Fraser: And then sort of to take that to the next level, right, you can have this situation where the two stars are perfectly lined up so that one acts like a lens to the star that’s behind it, and that star that is lensing, you can then learn a tremendous amount about that star or the one that’s behind it.
Pamela: And this is gravitational microlensing. And so, what we’re looking at – and this was done a lot by the survey projects, MACHO and OGLE, that looked at the large and small Magellanic Clouds and towards the inner spheroid of our own galaxy and looked to see do any of those background stars suddenly appear to get brighter because a foreground object has passed in front of it, and that foreground object’s gravity bends light that would otherwise never get to our telescope into our telescope – well, into the telescope of everyone in our general direction.
So, that gravitational bending that gets extra light to us, the observer, that’s the gravitational lens part of it. And it tells us where that otherwise hidden material is and has actually allowed us to see stars that have planets that potentially have moons in places that we don’t otherwise have the capacity to be looking for planets. It allows us to go looking for, well, are there rogue Jupiters wandering another solar system, wandering our galaxy. The answer appears to be no. What is the number of rogue black holes out there? Can you use black holes to explain dark matter? The answer appears to be no.
So, these searches for gravitationally lensed nearby stars that are lensing background things in either the galactic sphere, spheroid in the center, or in the Magellanic Clouds, these events help us find the invisible material that is in our galaxy and help us realize just what is the diversity of stuff that we don’t otherwise get to see. It’s gravity making the invisible visible.
Fraser: And the one thing as we start to wind down this episode is we’re very familiar with the sorts of things that we can see, and all the things that we’ve talked about today are all about using phenomena, that essentially visible light, the kinds of things that you would see in your normal telescope, but there’s a whole bunch of other transient phenomena that exist in other wavelengths. Things in the radio waves, things in the x-rays. So, could we just talk briefly about that, that once you expand what you’re looking at, another whole level of interesting changing objects show up?
Pamela: So, this is where you start to get magnetars, neutron stars with extremely strong magnetic fields that periodically rearrange their magnetic fields and give off massive amounts of gamma ray radiation. This is where you start to be able to look at distant, actively-feeding galaxies like the BL Lac object and see flickering and flaring in the core of a distant galaxy. This is where you start to get, well, gravitational waves are a transient event, just not one you’re gonna see using the photometry technique that we’ve largely talked about today.
Gamma ray bursts in all their forms are a transient phenomenon, in some cases linked to massive supernova that have magnetic fields called hypernova, in some cases linked to the crashing together of a pair of neutron stars where we also get gravitational waves. All these different things are showing that our universe is not a constant place. It is not that unchanging place that it was thought of to be by the ancients, and unless you’re a crazy person, you certainly don’t want a love as constant as the stars.
Fraser: I love this idea that for the longest time, astronomers would pick a thing, and they would look at it, and then they would study it, and then they would do some research. And then we moved to this idea of surveys, things like the Sloan Digital Sky Survey where you would try to just take a picture of everything so that everybody could have a picture of everything that was in the entire sky.
But that’s not enough. We are now moving to this time where we have to be able to see everything that’s in the sky all the time. So, it’s the difference between a photograph and a video that we are now moving to an age of astronomy that is the video version of where old astronomy was the photograph, but it’s even weirder than that.
Pamela: Well, this is what we always wanted. You have to –
Fraser: I know it’s what we always wanted. Now we’re finally getting it, right?
Pamela: Right, we have the technology.
Fraser: And the final step is to in addition look across all of the spectra at the same time, so you’re not just recording a video of the entire universe in purely visible, but you’re gonna wanna do that in gamma rays, and in x-rays, and in radio waves, across all the different spectrums to see every single thing that the universe was doing when we weren’t looking.
Pamela: And one of the biggest frustrations is while we have the capability with optical telescopes to take images of a large chunk of the sky at once, while we have that capacity with infrared telescopes and some ultraviolet, we do not have the capacity to take wide-angle, high-resolution images in all these other colors of the rainbow. And the electromagnetic spectrum makes it hard for us to do large-scale detailed surveys of a lot of the sky, but we’re gonna get there. We’re gonna get there.
Fraser: Well, that’s it.
Fraser: And so, we are entering the golden age of transient astronomy, and as you can see, there’s a lot left for us to be able to do, so lots of work to be done into the future. Well, that was great. Thank you so much, Pamela.
Pamela: Thank you, Fraser.
Fraser: Oh, wait. You should say some names.
Pamela: Wait, we have to read names. Yes, thank you. We’re gonna remember.
Fraser: I remembered.
Pamela: So, we love our Patreons dearly, and we’re just learning a new way to end this episode. And I’d like to end this episode by thanking a few more of our fabulous Patreon donations. This week, I would like to thank Nate Dutweiller, Joe Wilkinson, Raymond Buzinski, James Platt, Jonathan Tronson, Philip Walker, Elod Avron, Paul D. Disney, anti-user, and Scott Bieber. If you too would like to hear your name read by me in an overly enthusiastic and grateful tone of voice, just support us on Patreon. Thank you so much for making this show possible. We wouldn’t be here without you.
Fraser: Thank you, everyone. We’ll see you next week.
Announcer: Thank you for listening to Astronomy Cast, a nonprofit resource provided by the Planetary Science Institute, Fraser Cain, and Dr. Pamela Gay. You can find show notes and transcripts for every episode at Astronomy Cast. You can email us at firstname.lastname@example.org, tweet us @AstronomyCast, like us on Facebook, and watch us on YouTube. We record our show live on YouTube every Friday at 3:00 p.m. Eastern, 12:00 p.m. Pacific, or 19:00 UTC. Our intro music was provided by David Joseph Wesley, the outro music is by Travis Seale, and the show was edited by Susie Murph.
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