Ep. 519: Transients: What They Are and Why They Matter

Astronomers have found that sometimes the Universe changes. Things move, things explode, things get brighter or dimmer. In fact, knowing this has helped astronomers discover some very important aspects of the Universe. Today we begin a two part series on Transients and their role in astronomy.

This is part one of a double episode.
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Show Notes

What is a Transient?

Local (within solar system) transients – comets, asteroids & Kuiper Belt Objects

Surveying the Solar System – LSST

New Horizons

Finding transient objects by using comparison photos, missing light, etc.

Transient lunar phenomena

History of Transient Lunar phenomena

Clyde Tombaugh’s discovery of Pluto

Dark Spots and flashes on Jupiter


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Fraser: Astronomy Cast, Episode 519: Transients. Part One. Welcome to Astronomy Cast, our weekly fact-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, a senior scientist for the Planetary Science Institute, and the director of CosmoQuest. Hey, Pamela, how are you doing?

Dr. Gay: I’m doing well, how are you doing, Fraser?

Fraser: Great. I think that – I think this is the last time that we will be promoting this, but I wanna make sure that we do this, which is to let everybody know about the All-Stars party, which is gonna be happening in June, this summer, with me, Pamela, Paul Sutter, Skylias, John Michael Godier, awesome telescopes. We’re gonna be hanging out in the desert near Joshua Tree, and we’re just gonna be your personal guides to the night sky.

So, if this sounds like something that you wanna do, staying in a really, cool hotel, eating fancy food, and working with telescopes every evening. So, if this is something that you want to do –

Dr. Gay: I will hug a tree. I will hug a tree.

Fraser: You will hug a – Yeah, I can’t wait to see the Joshua trees. So, go to www.astrotours.co, and search for the All-Stars trip, and check it out. And I think this is the last time that you will hear about this. Then we will have received all of the – The reservations will be cut off at the end of this month, so –

Dr. Gay: And if you can’t make it in June, we fully understand. But we’re taking turns doing awesome trips. And the reason we’re doing a back-to-back episode today, is because someone is getting ready to go to Costa Rica.
Fraser: That’s right. I will be – Well, after the 3rd, Paul Sutter and I will be in Costa Rica, with again, 30 of our closest friends, going to see volcanoes and cloud forests, and of course, all of the wildlife, bugs, birds, many different kinds of monkeys. It’s gonna be awesome.

So, this is just another example of one of these AstroTours. So, if that one you can’t do, if you can’t do the All Stars, go and check out AstroTours. We’ve got trips going to Iceland. You’re doing the American Southwest, and more all the time.

Dr. Gay: And you are all welcome to join me in August, for the American Southwest.

Fraser: All right. Let’s get into it. Astronomers have found that sometimes, the universe changes. Things move, things explode, things get brighter, dimmer. In fact, knowing this has helped astronomers discover some very important aspects of the universe.

Today, we begin a two-part series on transients and the role in astronomy. All right, Pamela. Transients. So, what’s the plan here? We’re gonna do inside the Solar System, this week, and then next week, we’re gonna cover outside the Solar System. And it will probably be two episodes; I don’t know. Outside the Solar System almost feels like a 3rd episode. So, it may turn into three.

And you’re like, “What should I do?” But I wonder if it’s like, Solar System, and then Milky Way, and then, extragalactic transients. But who knows?

Dr. Gay: Okay. We’ll sort as we go.

Fraser: Yeah. Yeah. Always, our multi-part series have no fixed ending. So, what is a transient?

Dr. Gay: Put most simplistically, it is something in your astronomical data that is there one moment and gone the next, where one moment might be several minutes to several days, but not very long at all.

Fraser: And this a – it really feels like we’re moving into this new age of astronomy. Up until this point, everything took so long. You’d go out and you would take your telescope, and you would set it up, and you would take a picture, and you would get the galaxy or the star, or whatever it is you were taking a picture of, and you would study it.
But the reality is that the sky is changing from night to night. Things are getting brighter. Things are getting dimmer, things are exploding, things are crashing into each other, things are disappearing.

And it’s these moments that are time-based, that are really, hard to catch. And it’s only with our modern technology, now, that we’re starting to really get our handle on how much the universe is changing.

Dr. Gay: And –

Fraser: So, how much is the universe changing?

Dr. Gay: Well, we don’t fully know the answer to that, and we won’t until they finish building the Large Synoptic Survey Telescope. But right now, from what we can tell from all the different surveys out there, from Pan-STARRS, to the lost ComPADRE surveys, to the Palomar, and now, the Zwicky Transient surveys, it seems like there’s a whole lot of flicker, flaring moving and bust in the move that is going on out there, where we never know quite what is going to interrupt our field of view, no matter where it happens to be pointed.

And we started to realize this as soon as we started doing surveys. SOHO was I think, one of the first big moments of, “Oh, we can use this thing, a solar orbiter – a solar observer, rather, to find things other than its intended target. We know where the sun is; we don’t need to find the sun.

But the unexpected thing that came out of SOHO is it is chewing out sungrazing comet to sun-plunging comet, one after another, after another. And these comets that are in most cases diving towards their death, these are transient objects of the here today, never coming back tomorrow variety.

Fraser: I mean, I would classify that into things crashing into other things.

Dr. Gay: Yes.

Fraser: But just this example – Even this idea of having a field of view. Anyone who has ever done any astrophotography at all, you set up your telescope, you do some 300 second exposure of the sky. And what do you know, there’s a line moving all the way through your photograph that you weren’t intending to take a picture of, but it turns out you took a picture of a satellite, or you took a picture of an airplane.

And when you think about those first asteroids that were ever discovered, they were discovered because a thing was moving in through their field of view.

Dr. Gay: I think the earliest discoveries of transient objects can all be described as someone looking in their eyepiece, or looking out the output of their camera, CCD, and saying, “What the expletive!” Because they were seeing something they absolutely did not expect. And transient phenomena can be that asteroid that you don’t expect to see creeping through your field of view.

It can be Uranus or Neptune passing behind Jupiter, when you don’t know they’re there. It can be so many, different kinds of things, and this is where the study of transient objects causes us to do a series.

So, when we go specifically, looking for transient objects here in our Solar System, we have three basic strategies. The first is the “I’m just gonna take a image and see what I find.” So, this is where we have with observing the sun, we find comets. With doing studies of background galaxies, we may discover asteroids. So, that I’m taking a picture of one thing, and I discover something locally, as an accident. That’s one unintentional strategy.

What happens more often is you have people that either purposely survey along a specific area of the sky, night, after night, after night, looking for things that change or move. Or, even better, they start tracking their telescope, at a rate similar to the rate that they expect either asteroids or Kuiper belt objects to be tracking through their field.

This causes background stars to be straight lines across your image. And if you’ve got the speed right, causes those asteroids or Kuiper belt objects to be still points of light in your field.

Fraser: Yeah. If you’ve ever seen – People have done that. If you want to take a picture of a comet, there’s sort of two ways to go about it. One, is to take a long exposure of a comet, and then the comet becomes this blur that moves across your field off view. The other way is you track the comet, and as you said, the stars turn into these blurs across your field of view because the comet is moving.
So, I think if we were to classify the kinds of transient things that are happening here in the Solar System, we’ve got things that are moving. And those are as you mentioned, the comets, the asteroids. So, what is the technique that astronomers will use to find these things that are moving?

Dr. Gay: Well, it all comes down to survey images and a technique called photometry. This is where you simply measure the light across your image, looking for those places on your image that have more light, or less light, in the case of, well, you have an asteroid blocking a star that was there previously.

And by looking for these changes in light in registered images, images that have been lined up one with another, you can see, “Well, all this background stuff, it’s not moving.” So, the registered images, you don’t see a change in the non-transient stuff. And those transient objects, they’re the things that are either changing in brightness or changing in position.

Either way, you’re given place on your image has a change from one image to another. And one of the easiest ways to make this visible is you register two images. You line them up on the stationary objects. You adjust them so that those stationary objects are more, or less, the same brightness. Subtract one image from the other, so that your stationary objects get subtracted away.

And where you have a black spot in the resulting image, that’s were the subtraction led to a bright object then subtracted out, and the subtracted image had the thing in it. And where you have a bright spot, that’s where the object you subtracted from had something in it.

And what’s cool is with slow-moving objects is that you’ll actually see a bright spot and a dark spot, side by side, in these registered and subtracted images.

Fraser: And for a lot of astronomy, people are always like, “What’s the point?” Right? “Why will any of this matter to me?” But in this one field, for example, this technique is used to find the potentially, dangerous asteroids that will crash into the earth. So –

Dr. Gay: It’s also used to find the potentially, interesting objects that will be visited by the New Horizons spacecraft. And that’s the cool thing about this is it’s a technique that you can use with your backyard work, as well as a technique that gets used by some of the most advanced spacecraft in our Solar System.

Fraser: Now, we just recently actually, I think about a week ago, had an interesting opportunity. I don’t know if you were following this, that an asteroid was gonna pass in front of the brightest star in the sky. And this is another kind of transient that we see here in the Solar System.

Dr. Gay: Right. So, this was an occultation of the star, Sirius, by a asteroid with a extraordinarily hard to pronounce name, and I’m not going to embarrass myself. It will be in the show notes.

And there was a line across the world, spanning north to south, that if you were located somewhere along this line, you had a chance, depending on the unknown geometry of the asteroid, to see the brightest star in the sky, wink out for a small moment. Exactly how long that moment is depends on the geometry of the asteroid.

If we have a dog-bone shaped asteroid flying through space, such that its long waves is the waves that goes across the star, it will blink out for a longer period of time. If instead, we have that same dog bone, but it’s aligned so that its long access is perpendicular at a right angle to its motion, its skinny bit will instead pass over the star. And you’ll see that blocking out of the light for a significantly shorter moment.

Now, depending exactly where you are along that line of potential occultation on the planet earth, you’ll have it slightly different lineup between you, the asteroid, and the background star, Sirius. And so by combining measurements of how long the star appeared to disappear, for observers all along that line, we can actually get that really, cool mapping of the shape of the asteroid, the silhouette of the asteroid against the sky, at that particular moment in time.

Fraser: And some of the most fascinating discoveries about asteroids, and comets, and Kuiper belt objects in the Solar System, have come from these occultations, these changing in brightness.

I’m gonna give you some examples, but I’m sure there’s some others that you know of as well, things like, finding moons around asteroids, things like, finding rings around asteroids, discovering moons around planets. That it’s these discovering atmospheres around Pluto, things like that.
It’s these occultations, where as you see this change in brightness, where you have this moment to make these additional discoveries. And in a future episode, we’ll be able to talk a lot more about occultations and some of the really, amazing discoveries out there. But just this technique gives so much.

Dr. Gay: It makes the invisible, visible. It allowed us to measure new aspects of the thickness of Saturn’s rings when they were edge on, and occulted the star, Mira. All these different ways that light can get blocked, that gives us new information. And we’re so used to thinking of, “Well, how do we study things by measuring the light coming from them?” This is a case of, “How do we measure things by how they prevent light from getting to us?”

And variations in light also can be an extremely powerful tool for doing science. One of the other things that we do with asteroids is we don’t just measure where they are in an image; we measure how bright they appear in an image. And variations in the brightness of these bodies, actually allow us to get at “How are they rotating? How are they tumbling? How are they shaped?” And it puts limits on our understanding.

We can’t get a complete understanding on an asteroid’s shape by how it reflects light. But if we see one moment it’s significantly brighter, and the next, it’s significantly fainter, that gives us the sense of either, we’re seeing it on the big flat side, or now, we’re seeing it on the small, tiny side.

Or maybe, one side is significantly more reflective than the other. That’s not generally the case. The probabilities tell us that we’re seeing it an asymmetrical object tumbling through the sky.

Fraser: So, one interesting mystery that has transient baked right into the name, transient lunar phenomena – Phenomena, phenomena, phenomena – was considered like, a legend for a while there –

Dr. Gay: Yeah.

Fraser: – Astronomers were seeing bright flashes that were coming off of the moon. And for a while, amateur astronomers were seeing them, and other astronomers weren’t believing it.

Dr. Gay: Right.

Fraser: But now we know, this is a very real thing.

Dr. Gay: And I first learned about this when I was a graduate student, working out at McDonald Observatory. A astronomer from another university, Arlin Crotts from Columbia, he would come out and use McDonald Observatory for his research. And one of the projects he was doing was looking for bright flashes on the dark part of the moon. So, you look at a crescent moon, you point your telescope at the part not being illuminated by sunlight, and you look for flashes in that shadowed region.

And I remember talking to him about a paper he had out at the Lunar and Planetary Conference one year, and at the time, it was unclear. Are these outgassing events that are somehow giving off light? Are these collisions? Are these just flukes in the data? And he was certain they were real but wasn’t certain what they were.

Well, now we know that rocks are still falling out of the sky and hitting things scattered throughout our Solar System. We have seen new craters arrive on Mars. And recently, we saw what we expect create a new crater hit the moon. During the last lunar eclipse, just a few weeks ago, there was a global, phenomena, where half the world had the opportunity to look up during that eclipse, and see a bright slash occur along the edge of the moon. And that bright slash is thought to be a impact that was imaged by multiple telescopes, seen by multiple eyeballs.

And this is the first time known to have happened, since a whole group of monks a couple of hundred years ago, were all looking at the moon together and saw something like this. So, to go from an experience shared by a group of theologians living together in Europe to something shared by half a planet, is pretty, extraordinary.

Fraser: Yeah, I was live streaming the entire lunar eclipse. So, in theory, I’ve got a picture of that. I haven’t gone back through my data yet, but it’s in there somewhere, I would assume.

Dr. Gay: And this was definitely one of those moments where you find out just how careful one observer you are because the fact that there are some people who were able to just be standing there watching through their telescope –

Fraser: Yeah, to see it.

Dr. Gay: Yeah. That’s amazing acuity. I am much more of a “Let’s digitally compare things and figure out what moved.” And one way to think about this is Pluto when it was discovered by Clyde Tombaugh was actually just a transient object moving through the background stars. And that blink comparison method that Clyde Tombaugh used to find that thing that didn’t stay put across multiple images, he was looking for a transient object, and he found it. And throughout his life, it managed to stay a planet.

Fraser: Right. For his life anyway, but not after that. So, what are some other examples, some other transient phenomena that astronomers are looking for here, in the Solar System?

Dr. Gay: Well, we don’t often think of it this way, but coronal mass ejection and other solar flares are transient phenomena. We usually think of solar flares, stellar flares, as being transients for astronomers to observe, but it turns out there are solar observers interested in these things, as well. And if there are transient to the astronomer looking at a far-off star, they’re a transient phenomena for people looking at our nearby star.

So, we have sungrazing comets. We have regular comets observed different ways, sungrazers. We’re using things like SOHO, SDO, looking to see them plunge. And through these dedicated observers looking at the sun, we have the sun transient phenomena itself, moving outwards. We have asteroids in all their different orbits. We have Kuiper belt objects, the icy objects in the outer Solar System. We have all the random, we’re not sure if they’re asteroids, comets, or something in between, in between.

And then we have the really, weird stuff like, ‘Oumuamua. It’s not aliens. It’s never aliens until it’s aliens. But ‘Oumuamua was a transient phenomena, that like a sungrazer, is never coming back. But it didn’t commit death into the sun; it just left our Solar System all together. It noped us and said goodbye and flew away. So, that is yet another transient phenomena. So basically, at the end of the day, transients are all of the rocks, ice, debris, moving around. And it’s also the things they create. It’s also the things it creates. It’s stars they block out.

And the one thing we haven’t talked about is another form of transient phenomena is the dark stop spots we sometimes see on Jupiter, those bright flashes of light we sometimes see on Jupiter when it gets hit. Now, do you remember where you were back the summer of 1994 with Shoemaker-Levy/9?

Fraser: Ah… I was… Yeah, I was in Vancouver, working at a software company. So, I do remember, sort of watching that as it all – or was it ‘94 or was it ‘97?

Dr. Gay: It was ’94 –

Fraser: It was ’94, okay, yeah.

Dr. Gay: It was the summer between my sophomore and junior year of college.

Fraser: Yeah, and I do remember sort of everything leading up to that, and I was interested in astronomy. I wasn’t doing it as a career yet, for another couple of years. But yeah, no, absolutely.

Dr. Gay: And this was an event that you could see through a backyard telescope, those dark spots arising on Jupiter. And at the time, we thought that was a once every 500-years kind of event. And what’s been amazing is to watch in recent years as the price of digital cameras has come down. As more and more people have bought CCDs, have set up robotic telescopes, it seems like almost yearly now, someone catches something hitting Jupiter and giving it a big black spot.

Fraser: Yeah. We did an article, probably about six months ago, about how people are still discovering objects smashing into Jupiter and releasing a flash.

In our remaining time, I just wanted to – I think what’s great about transients, almost whenever there is a new field of astronomy that really opens up there’s this space for amateurs who are dedicated to be able to really make some serious contributions into this science. So, if people are wanting to be a part of this, what’s a kind of set up, and where could they maybe get involved in some of the projects for helping us here in the Solar System?

Dr. Gay: So, for the Solar System, the best thing you can do is find out about how to join the Minor Planet Center. It’s an organization that is run by the Astronomical Union and based out of the Harvard Smithsonian Center for Astrophysics. And the Minor Planet Center will start out by saying, “Okay, go out, observe these known objects, and report back what you see. Give us your brightness measurements, your position measurements. And build up trust. And once you build up that trust, we’ll give you and observer code and set you free to go out and begin discovering asteroids.”

And what gets me is there are so many unmeasured rocks out there, that amateur astronomers are continuing to find new asteroids on a regular basis. It’s getting harder, because there are so many automated large surveys out there. But there’s still space for everyone to make their own discoveries. So, start with the Minor Planet Center, and then stake out your own corner of, well, right ascension and declination. And make it your own. Get to know what’s supposed to be there and find all those interlopers sneaking into your field.

Fraser: And you don’t need that powerful or expensive a telescope. You probably want a photometer. You probably want a nice CCD to be able to do some photometry. But even a 6”, an 8” telescope, what it is, is it’s about practice. And it is the unthrilling work of night after night, observing a chunk of sky, at the greatest detail you can, and comparing that with what’s supposed to be there, and finding out if there’s anything new.

So, this is – We’re really, serious. If this interests you, and even if you’re an amateur, and you’ve always wanted to be an astronomer for $1000.00, you can do real science and provide a real benefit to the astronomical community. You can discover asteroids. One of the great things about asteroids is that if you discover them, you get to name them.

Dr. Gay: He has a –

Fraser: I have an asteroid –

Dr. Gay: – asteroid named after –

Fraser: – named after me.

Dr. Gay: I do not.

Fraser: Yeah, I know. So, somebody, discover an asteroid and name it after Pamela. That’s only fair.

Dr. Gay: Please.

All right. Well, Pamela, this was great. Next week – next episode we’ll be talking about what is outside the Solar System, as the universe changes.

Thanks a lot.

Dr. Gay: Thank you. Goodbye everyone. Wait! Stop!

Fraser: Oh!

We have to read names, we have to read names. All of you on Patreon, I love you. I don’t mean to almost forgetting you. And I’m now going to thank a group of you.

So, our special thanks go out to Brian Cagle, Helge Bjorkhaug, Bill Hamilton, Greg Thorwald, Richard Rivera, Jos Cunningham, Silvan Wespi, Jeff Collins, Marek Vydareny, and John Drake.

And I will be reading more names at the end of the next episode. And if you want to hear your name said with joy and emphasis, on happiness and thanks by me, check out www.patreon.com/astronomycast. And thank you, all of you that make our show possible.

Fraser: Thanks, everybody.

Dr. Gay: Bye.

Female Speaker: Thank you for listening to Astronomy Cast, a non-profit 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 info@astronomycast.com, 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 1900 UTC. Our intro music was provided by David Joseph Wesley. The outro music is by Travis Searle, and the show was edited by Susie Murph.

[End of Audio]

Duration: 30 minutes

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