Supernova Remnants Should Fade Away. So Why are Some Flashing?

[00:00:00]If you're a fan of astronomy, and I know you are, then you're probably familiar with the supernova remnant. This is this beautiful glowing blob of hot gas and dust that is left over when a star that is much more massive than the sun detonates and it leaves behind this debris. Famous examples, the Crab Nebula, the Veil Nebula, Supernova 1987a. These are all different stages of what happens with a supernova remnant, but generally the process is the same.

[00:00:31]The star explodes, it can outshine the rest of its galaxy for a little while and then it fades away over the weeks and months and eventually when you get to the years, the actual bright part of the supernova is gone. You're left with the remnant. But a team of astronomers have found this very strange thing, which is that there are these supernova remnants which are old. They happened a long time ago and yet they are still changing in brightness sometimes by a factor of 100. Imagine if you were observing the crab nebula one day and then a few weeks later you observed it again and now it's 100 times as bright.

[00:01:07]We do not see that in our telescopes and yet this is exactly what astronomers are seeing and what is causing this bizarre behavior. So my guest today is Dr. Roy Kilgart. He's a professor of the practice in astronomy at Wesleyan University and he is part of the team that has been using the Chandra X-ray Observatory to observe these strange pulsating supernova remnants. Now I talked with Roy about this discovery and the underlying mechanism give you a hint it probably has something to do with black holes and how this sort of influences other kinds of observations that are out there. And so I think you'll you'll find this one story very interesting, but the sort of the wider conversation that we have is also very interesting. So enjoy this interview with Dr. Roy Kilgart. Roy, you were examining the galaxy M83 with the Chandra Space Space Telescope and you discovered something strange going on with some supernova there. What did you find?

[00:02:08]Yeah. So, um, supernovi, I think probably most most people who are watching this will know, uh, are the remnants of of dead stars. They've exploded. Uh, and the remnants that are left behind are, uh, you know, they're they're exciting objects, but they're not necessarily dynamic objects. They tend to not change on very short time scales. They tend to gradually fade on the time scale of of decades to centuries.

[00:02:33]uh and uh when we were looking at these X-ray observations of supernovas in M83, we found uh a number of them that are variable uh in their brightness um on time scales ranging from sort of within a single Chandra observation of hours to many years spanning between observations >> and and give people a sense of of like what is the normal bog standard supernova behavior that people would expect >> uh For these types of observations, people would expect for them to be faint uh uh sort of lower energy in their X-ray emission uh and very boring. Uh they shouldn't they shouldn't change at all. We should be able to observe them uh from the beginning of X-ray astronomy in the 1970s all the way to the present day. And they basically shouldn't change on that time scale. But you get this like instant brightening when the when the star explodes that where it's, you know, it can it can outshine the rest of the galaxy combined or, you know, it's they're bright.

[00:03:35]>> Right. And then >> and they'll do that for for you weeks >> and then they fade away to kind of obscurity.

[00:03:41]>> And there could be other things going on inside them and that we see things in the Milky Way like the the pulsar inside the crap nebula which is a very dynamic object but it's also much less luminous than these things that we're seeing in other galaxies.

[00:03:54]>> But in the the range that you see is a few weeks here or there, maybe a couple of months if it's a little stranger. But in general, you're going to get this very standard light curve coming out of this supernova supernova over a range.

[00:04:10]So they they're extremely bright and then they fade away over the next weeks and months and then they're hard to spot unless they're relatively close, which hasn't happened since 1987, >> right?

[00:04:23]>> Yeah. Okay. So, so then how did these objects that you were observing differ from what is the sort of standard playbook?

[00:04:33]>> Yeah. So, um we we ran tests of variability on on all of the sources, all of the X-ray sources in in M83 and then we crossorrelate them with cataloges of other types of objects and that's how these showed up as being flagged as supernova remnants that were identified uh in opt with optical and radio telescopes. Um and uh they immediately showed up as having a 100% probability of variability and so that struck us as odd and we dove into them further to try to figure out what they were and that was sort of a two-year long rabbit hole that we went down right and give me a sense of how much variability. So you know I've got a small telescope and I have observed say the Veil Nebula and and there are a few other supernova remnants. You can look at the crab nebula is a great example. I don't see variation, right? I see the same thing that people saw 50 years ago and probably it looks different from what how it looked a thousand years ago, but >> it is not definitely um fading away. Uh sort of is not sort of increasing in brightness again. So how much variability if I was like pointing my telescope at the Crab Nebula over the course of some series of nights, how different would the brightness be? Uh, for the most extreme ones, it's sort of a factor of a hundred in brightness.

[00:05:54][laughter] >> So, so I would look at the crab one night. Yeah. Yeah. I'd look at the crab nebula one night, you know, and it's faint and there it is and I do a long exposure and then another while I do it, but now it's 100 times as bright and it is like the brightest thing in the sky.

[00:06:08]Yeah, that's amazing.

[00:06:11]>> Like over what kind of time period will it make this oscillation? Uh most of them it's more like a factor of 10, but some of them can are are pretty extreme.

[00:06:19]That's still a lot. And um it varies from source to source. Uh some of them are are kind of flickering within their observations on uh time scales of I I guess hours is the the time resolution we're sensitive to in those observations. Um and then with the rest of them, we don't really know. We're subject to the frequency with which they were observed with the telescope, which is somewhat arbitrary. This wasn't like a routine monitoring program of the galaxy. So we have observations that span a decade and a half or so, but a bunch of them are lumped together and then there are big gaps between them. Um so anything from hours to years u but but we don't have a good probe of all the different time scales in between that to say, >> right? And again, going back to my the images that I'm taking of the Crab Nebula, am I seeing it brighten in the core, or is the whole thing just getting brighter and fainter at these distances, we don't have the the spatial resolution to tell that the whole thing looks like a point source. Uh, but it of course isn't at at if the supernova remnant at least is not a point source at at these distances. M83 is uh uh like 12 million lighty years away, 13 million lighty years away. So, it's we don't have great spatial resolution. A single pixel in the camera is like tens of light years across. So there's not a lot of resolution uh there.

[00:07:40]>> And do you have any sense of their age?

[00:07:43]>> Um none of Well, there I think there was one of them in M83 that was uh associated with a historical supernova that was in the 1950s. None of the rest of them are associated with historical supernovi, but we've only been really identifying supernovas in other galaxies for about a century. So um older than a century but um uh the X-ray bright lifetimes of supernova remnants is at hundreds of years probably at that distance. So sometime in the last few hundred years uh is the the best guess which is not very long astronomically speaking.

[00:08:24]>> Right. Right. And so speaking of best guesses, what do you think's going on?

[00:08:30]>> Yeah. Yeah. So, we have kind of two thoughts for things that that are that could explain this type type of variability. Um, and one is the idea of fallback accretion, which would be material that was blown off of the the star in the supernova explosion falling back onto the compact remnant, the former core of the star that's now either a neutron star or a black hole.

[00:08:52]Um, and so it can kind of fall back and and form a dis of material around around the compact object that would then accrete onto it. Um, that is possible.

[00:09:03]Um, and that would happen relatively fast. That fits the time scales very well. We don't have great examples of that process locally happening that we know of. Um and so that's sort of more conceptual than practical until or unless we can find a good local analog of such a system. Uh there are there are some things but they don't quite match the observational properties of this thing. But that's a reasonable possibility that it fits the age of the systems well.

[00:09:32]>> And and sorry, so would this be like a one time event? Like I'm imagining sort of the supernov Nova goes off. You've got all of the material that was maybe shed by the star before it actually detonated as a supernova and then this new shock wave goes through that and heats it up and you get more brightness.

[00:09:51]But then is it all just falling back into the center in kind of blobs and chunks or is it sort of like this, you know, it expands and then it all comes down together at at once? Would we expect this to this variability to fade away in I don't know a couple of years when the when the infalling is complete?

[00:10:10]>> Um I so the infall could form a stable disc of material around the compact object. there will be some built-in rotation even if it's slight in the in in the initial ejecta from the supernova and that will kind of retain its angular momentum as it as it comes back down and that will form a spinning disc but you don't you you're correct you don't have a resupply so that will eventually run out >> but you've got like some kind of accretion disc that is just >> yeah but that could give us a reasonable kind of decades long lifespan for these things um decades to centuries kind of lifespan uh And so that fits the the time scale. Okay.

[00:10:48]Um it doesn't really fit the dynamic changes very well though. And so the other possibility and the one we we currently favor u is that there is a a baby uh x-ray binary in the system where you have the compact remnant of the dead star and a still living star in a binary orbit around uh with one another. And uh the supernova was not so violent that you kind of blew the binary apart. Um and then the the compact object can kind of siphon material from its still living companion star uh and form an accretion disc around it. And that fits the dynamics. X-ray binaries are very common in galaxies. Uh we have, you know, a hundred or so very luminous X-ray binaries in in our own Milky Way. When we look at other nearby galaxies, we see about the same number based on the the stellar mass and star formation rate in those galaxies. And so that fits uh the dynamics of these systems very well. It doesn't fit the time scale very well. We think the time scale for forming an X-ray binary uh after the supernova explosion is longer than the X-ray luminous lifetime of the supernova remnant that we think we see uh these things embedded in.

[00:12:07]So that's mysterious. Either these are really rare examples of fallback accretion that we don't see locally or they're a much more common type of object, but that's formed faster than we think they should form uh dynamically after the supernova explosion.

[00:12:22]Well, there there are more possibilities, but those are kind of the two leading ones.

[00:12:26]>> Yeah. Yeah. I mean, like like I'm thinking about say I don't know, Signis X1, right? Which was like the first black hole that was ever really discovered. And that's a classic X-ray binary where you've got this black hole and a star and the star is going around the only way you know about the black hole is just because of this material that it is it is feeding on. So what kind you know when you think about something like say that what is the the length of time like you're saying that that process takes a long time for that for that that feeding to begin long after the supernova remnant has kind of cleared out and faded away to the back you know blown away by the interstellar winds.

[00:13:05]>> Yeah that's right. So, uh, if the if the objects are too close together, then when the supernova happens, you will disrupt the binary probably, and so the the companion star is no longer a companion, but one of the things will get kicked out of the system or both.

[00:13:19]Um, and you don't have a binary anymore.

[00:13:21]Uh, so you need them to be a little farther apart to form the binary in the first place. But, you know, gravity is very stable. And if neither one of these things is losing uh losing mass or losing momentum, then they're going to stay the same distance they are apart from one another uh for a really long period of time unless something changes.

[00:13:43]Um, and the thing that can change is the other star can evolve. It can it can lose mass as it's aging. Um, or uh it can be it can be close enough that it is losing material to the compact object.

[00:13:56]Um it's very hard to find theorists who are willing to place really good constraints on how quickly that process happens of those objects uh coming together that closely because it's a very complicated problem to solve. But u estimates are kind of in the tens to hundreds of thousands of years time scale uh which is again astronomically short and it's shorter than the lifetimes of the stars by quite a bit but much longer than the supernova remnant will stay visible to us before it fades away into obscurity.

[00:14:30]>> It does mildly overlap. I mean I think about I mean that example I gave earlier, right? You've got the Crab Nebula. It's around a thousand years old. We've got Tao supernova, you know, we've got the 1987 one. So, we can see what different supernova are. And I think about some of these really old ones like the Veil Nebula. It is on the order of of many thousands of years old.

[00:14:51]And it is very large and diffuse and fading away. And only with a long exposure on your telescope do you even get it.

[00:14:58]That that that sort of sets that upper boundary, right? Like 10,000 years and these things are are hard to spot at that point.

[00:15:05]>> That's right.

[00:15:06]>> Yeah. And but this you're saying no, we're we're looking at say 10 times as long for these things to finally get into a place where you can start to get that accretion going off. Okay. So you've ruled you've you've ruled out your two ideas. What's what do you got left? Come on. I want to hear the I want to hear the crazy one that happens to you know.

[00:15:24]>> So I I don't think we've ruled out either of those ideas. I think they're both viable but but have some have some challenges with them. Um the the I think the the most out there idea that's not sort of breaking physics in any way um is that maybe the things that look like supernova remnants aren't supernova remnants. U maybe the the X-ray source is one of these compact objects. It is an X-ray binary.

[00:15:50]Uh but instead uh the X-ray binary is accreing a lot of material and producing jets uh the way kind of super massive black holes often do. uh and those jets are interacting with the material in interstellar space around them and sort of blowing a bubble around the object.

[00:16:07]And this isn't crazy. We have uh a couple of examples of these in the Milky Way. The most famous one is an object called SS 433 probably. Um and even SIG X1 that you mentioned uh it doesn't quite have a bubble around it, but it does have a shocked nebula around it that you can see uh in in certain certain wavelengths of light. um you wouldn't see it in a nearby galaxy, but there are examples that we certainly would. So that is a possibility.

[00:16:34]Um I think the the there well there are a couple problems with that. One is that uh I guess the people who made the cataloges of super supernova remnants might not be too happy about me telling them a bunch of their supernova remnants are not that. Um uh but also there are a lot of these things in M83. Um, and that would be uh somewhat surprising to find a large number of these uh kind of X-ray binaries blowing bubbles around themselves in a single galaxy. Um, but it's allowed there. We have examples of them.

[00:17:10]>> Is there anything special about M83? Is it a starburst galaxy or >> um M83? So, that's a great question. Uh, M83 has uh it's a very high star formation rate galaxy. uh it has a a really intense uh star forming region in its nucleus. So its global star formation rate isn't actually all that high, but where it's forming stars, it's doing it very very intensely. But this was actually the first question we asked ourselves was is M83 unique? Uh and the answer is no, it's not. Um, uh, one of my, uh, former students, summer student, uh, Zoe Hyland, who's, uh, not a co-author on this paper, but is writing the follow-up paper, uh, right now, um, looked at these in in the galaxy M51, uh, which is the Whirlpool Galaxy. It's a canonical example of a nearby interacting galaxy, and it is very high star formation rate. Uh, if you kind of scale it for stellar mass compared with the Milky Way, its star formation rate is close to 100 times what what ours is.

[00:18:08]skill, you know, kind pound-forpound.

[00:18:11]Um, and so we look for these things uh in M51, which also has a really nice catalog of supernova remnants produced by many of the same people who made the catalog in M83. And it also has a bunch of them. There's about two dozen of them in M83, and there's about two dozen of them uh in M51 as well. Uh, and so it seems like just from from that piece that there's some correlation with star formation, which makes sense because supernova supernovas are correlated with star formation as well. You get them from high mass stars that live for these brief moments of time. Um, and so we also looked at uh M101, which is another nearby galaxy, but has a much lower star formation rate and also a good catalog of super diverments. and it has zero to one of these things. It has one, but it's not a great example. It's variableish, but kind of right on the edge of you could maybe uh dismiss its variability as as a statistical fluke. It's not a high significance detection of variability. Uh so, uh that's kind of where where we were at the time of uh when we when we had our press release about this. Um, and over the last few weeks, I've had a current one of my undergraduate students looking at kind of everything that's been observed with Chandra over the last couple of decades.

[00:19:33]And um, there aren't as many galaxies that have been observed as many times to look for that nice long baseline of variability, but we've found a few more of these things in a handful of other galaxies. Uh, and we're kind of now starting to wrap our brains around what this means. Uh, and if we can say something more definitive than seems like they're associated with star formation, which is all we can currently say.

[00:19:58]>> Right. Right. But, but I mean supernova are also associated with star formation.

[00:20:03]>> So, so having supernova remnants in places where there's actively star forming is not a surprise. What would you use something like Reuben to try and help you double check?

[00:20:19]Oh. Um, >> because you've got this great survey that's coming out, you know, every night they're finding loads and loads of new supernova across the universe. Is there is there some pattern in there that you could cross compare with?

[00:20:32]>> We would need to be able to do good X-ray followup >> of those super heavy that Reuben is detecting. And um we don't currently have an X-ray telescope that can do that because while Chandra is still a functioning X-ray telescope thankfully um uh these objects are kind of at the very red end of the X-ray spectrum.

[00:20:56]They're the lowest energy X-ray uh objects and over its lifetime uh Chandra has been losing sensitivity in exactly the place where we need it most for these observations. So, while it can still do amazing science for a whole suite of other stuff, for this very specific problem, it can't quite do it uh anymore. Um, but that's okay because we think the time scale for this is probably, you know, we need the supernova to be done doing its business anyway, uh, and for the system to settle down. So, we need the time scale to for follow-up to be at least a few years, if not more. So this is actually a project that's well primed for whatever the next big X-ray telescope we build uh is going to be uh because that will probably be kind of a decade or more from now time scale. So it's an exciting prospect for the future uh but not anything we can currently do with it other than start making our list and thinking about what we're going to do a decade or so from now.

[00:21:54]>> Right. And and hoping for that next generation X-ray telescope that was requested in the last decal survey.

[00:22:00]>> Yeah.

[00:22:01]>> Or two decal surveys ago. Right. I'm I'm cautiously optimistic, I would say.

[00:22:06]>> Okay. Good. Good. Um, I'd like to talk just about X-ray binaries in in general as as a as an object because, you know, I my audience is quite fascinated with this idea like this line between a neutron star and a black hole. Um, do you always know which one you're looking at in some in some system?

[00:22:29]you're like, "Okay, that's obviously a neutron star or that's obviously a black hole." Or is it actually hard to tell?

[00:22:33]>> It's it can be very hard to tell. Uh and the the only definitive way uh that well, I guess there's kind of two definitive ways you can know. uh one is that you can measure uh the the mass of the companion star very well and then measure the properties of the binary orbit very well such it such that you can use Newton's laws to figure out the mass of the thing that you can't see and if it puts it comfortably into the acceptable mass range for neutron stars or or black holes then you know what it is so you can measure it dynamically that's that's one way um the other way is uh if you see variability from the object that is on a time scale that's so short that if it were happening in a black hole, it would be within the event horizon of the black hole. Then that's impossible. And so, you know, the thing is a neutron star. So, like millisecond pulsars can't ever be black holes because the pulsation time scale uh that we see from those things would happen within the event horizon of a black hole if the thing was a black hole.

[00:23:40]>> Right. Okay. That's interesting. But but for a lot of the ones that are farther away, if they're not giving off that that sort of regular radio waves, you kind of have to and that's why you call them X-ray binaries. It's funny because you you don't hear them as like binary neutron stars, binary black holes. It's this general classification.

[00:23:58]>> Yeah. And you know, we confuse the matter even further because we talk about them as being either low mass X-ray binaries or high mass X-ray binaries. But the mass in that case is the star. It's not the compact object.

[00:24:09]Uh because the star is the thing you can see. So I know if it's a blue star, it's a high mass star. If it's a red star, it's a low mass star. It's a piece of cake, right? It's >> right. But that has nothing to do with the compact at all. You can have all combinations of things.

[00:24:23]Um so so one thing that's that's quite interesting to me is is how astronomers are finding that these these compact objects you know are are almost are very similar in behavior to the more super massive versions that you you know you get very similar behavior on a micro quazar you know a tiny little black hole that's feeding on on material. It produces jets. It it has an accretion disc. It's g it's sending off you know as it feeds it's sending out X-ray X-ray radiation in a way that is really directly applicable to the super massive black holes and I know you've been also doing a lot of work on on super massive black holes and active ones does it give you kind of like a mini lab to to study some of those ideas for the for the larger ones that are six billion layers away >> yeah welcome to my TED talk [laughter] >> okay I stumbled into your TED talk >> this is this is a one of the main questions I've been interested in for my whole career is what are the physical properties that scale across kind of eight or nine orders of magnitude in mass. Um, and for some things we see uh really direct correlation and and things that scale beautifully well and for others we just don't know. And uh the problem is that you will often see similar behaviors in these things that are you know 10 solar masses versus 10 million solar masses. Um but we have scarce examples of things with masses in between to know that if to know that these things that we see that are similar are really kind of scaling across that whole mass range. So it would be lovely if we could find, you know, a 10 solar mass black hole and then a 100 solar mass black hole and then a thousand solar mass black hole and then all the way up uh that are otherwise identical and uh and see what really scales across those uh those mass changes >> and and so which which elements do seem like they scale?

[00:26:20]Um there there is a relation between the temperature of the dis of material around the black hole and its mass and that scales beautifully across uh all of these orders of magnitude. Um it seems likely that what's causing the launching of the jets in the systems uh must also scale across these uh huge uh ranges in mass. But the physical scales involved are so much different that it's it's a little hard to understand how that works. So those are the the obvious things. Um but the the the physical models that we use to model them are are very similar otherwise. It's a disa of hot material sometimes have some clumpy cloudy other stuff around them and jets and it's they're very very physically similar.

[00:27:07]>> And so you made some you mentioned something that I've never heard before.

[00:27:10]This this correlation between the mass of the black hole and the temperature coming from the region around it. Does does that give you a way to estimate the mass of a black hole by measuring the temperature?

[00:27:20]>> It does, assuming that the material extends all the way to the innermost stable orbit around the black hole. It doesn't have to. The material can be further out than that. Um but uh it will tend to fill into that most stable orbit uh unless unless something physically changes in the system that prevents it from doing that. So yeah, there is a direct correlation between that inner disc temperature and the mass of the compact object. So uh we we can in some systems if we can get place a very nice temperature constraint on that inner edge uh place a limit on the mass of the black hole but it's only a limit because the material could be further out. Uh but it's a lower limit for that reason is if it's if the the the a bigger black hole has material that's farther and farther away from it. So, this kind of a a maybe counterintuitive thing, I suppose, but the more massive the black hole, the cooler the inner edge of the accretion disc because it's farther away.

[00:28:19]>> Right. Right. And I mean, isn't there a kind of It depends on the direction the black hole is spinning and the direction that the accretion disc is spinning.

[00:28:27]>> It does depend on those things, too.

[00:28:28]That's true.

[00:28:29]>> Yeah. And so, you can get a gap depending a different gap depending on those. And often I know you measure the direction the black hole is spinning by measuring the gap. And so um >> yeah and even measuring the black hole spin can be pretty hard when you get to these objects that are quite a bit farther away. We can for some but not for a ton of them.

[00:28:47]>> Yeah. Yeah. I know I know astronomers are still arguing about the direction of the even the super massive black hole at the heart of the Milky Way. What what's its inclination compared to to us?

[00:28:58]>> Yeah. For for studying Yeah. for studying that black hole uh in more detail. It would be good if it were feeding more actively. That would not be good for any other reason, but it'd be great for studying. [laughter] >> Right. Right. Yeah. The last your last research, your final your final research. Um so one thing that really fascinates my audience is this dividing line between neutron stars and and black holes that you know both are continuing to create material and there is an upper limit to the mass that a neutron star can collect.

[00:29:32]>> Do we think and you know space is big we've seen plenty of things happen out there. Do we think we're seeing what happens when a neutron star collects more material than is sort of managed by the polyexclusion principle that these things are going into some other form?

[00:29:51]>> Yeah, I I wonder if we would know if we saw it when it happened uh in an X-ray binary. Um, I think we would need we need very very good observations of a nearby one to to know if we actually witnessed uh one cross that line because I don't think >> it would be >> like not a supernova like it wouldn't be dead obvious.

[00:30:13]>> I don't think so. I think it would be more subtle. Um >> I think it would depend on what observational properties like what are what are the the properties you have been able to to observe from the thing while it's a neutron star. Um, but there's so much we don't know about what's going on in the interior of neutron stars that it becomes a very difficult problem to to solve. We don't know if neutron stars, you know, further collapse and become quark stars, which has been one of the the proposed things as a stage before a black hole. We don't know if that happens. We don't know uh if if all if neutron stars are really these kind of 99% balls of neutrons that have thin crusts of of normal material or if there if it's more like the onion layer uh that we see in you know living stars uh especially living high mass stars where you have multiple different layers of of of material inside from different cycles of fusion. Um, it could be that a similar thing goes on where the very beginning is or the very center of the thing is some weird super fluid that's surrounded by a quark star layer that's surrounded by a neutron layer that's surrounded by another crust. Um, we don't know. Um, the way that people try to place constraints on this now is by measuring the radius the radi of multiple neutron stars. uh because if it is kind of ball of neutrons all the way down, we can make a measurement of what the diameter ought to be based on its mass. And if it's smaller than that, then you know something else is going on its in its interior. Uh there's been this little little telescope bolted to the space station for a decade or so now or called NICER and its whole job in life has been to measure the uh the radi of neutron stars and place constraints on on this problem. Um, and it hasn't answered the the question fully yet, but it certainly is pointing to the solution being more complicated than ball of neutrons all the way down.

[00:32:10]>> Right. Right. Like but but it could be neutrons at the center, something that's more akin to a white dwarf on the surface and maybe something more dense in the in the center. Not clearly way too small and so it has to be >> There have been none that have been way too small yet. Yeah, that's correct.

[00:32:29]>> Yeah. Yeah. Okay. Okay. But still, I mean, I we sort of think about this tipping point for white dwarfs when they're pulling material off some companion star and then they detonate in an instant as a type 1a supernova.

[00:32:41]>> And you would think that the a similar energetic event would happen with a neutron star, but it's interesting that it's that it could be a more subtle shift.

[00:32:51]Yeah, it's well, you know, a neutron star is not much larger than what the event horizon of an equivalently masked black hole would be.

[00:33:03]And so if the sort of transition from compact object made of real stuff to black hole is instantaneous, there's not a lot of stuff that's outside the the event hor effect effective event horizon, if you will, that could get away. Uh, so I think you're not allowed to have too dynamic an explosion. Um, but that's that's a bit outside my area of expertise, I think.

[00:33:29]>> Right. [laughter] Yeah. Yeah. I mean, it it's such a fascinating idea that that you have this object that is already just defying the our comprehension at the density of it and then it at at some moment it just kind of goes, you know, it sort of goes in one more notch.

[00:33:46]>> Yeah. and yet um and then and then eventually makes that final transition into into a black hole as it continues to feed. But it, you know, if you're saying you can't tell the difference between a neutron star and a black hole, then it does not surpris you can't tell the difference between a neutron star and something that's in between in a black hole. Like this is a this is a tricky observation and so more data needed. Well, um Roy, what are you obsessed with right now?

[00:34:10]>> Wow. We've touched on a lot of my obsessions, but I'll give you one that's related to the topic that you brought up about uh what the uh where this tipping point is between when something becomes a neutron star and a black hole. A question that I've been obsessed with for my entire professional career uh but that continues to obsess me uh nonetheless is uh what is the upper limit to the mass of a black hole that we can make from a single parent star?

[00:34:40]And we know that you form things like the black hole in Signis X1 from a single parent star. And we know that that thing is around 20 times the mass of our sun, which means the star was presumably quite a bit more massive than that before it died. Uh but what's the upper mass limit to that? Uh we don't really know what the upper mass limit to to a star is, but we have kind of some constraints that we place on the modern universe. We think when the universe was mostly hydrogen, you can make them a little a little more massive. So maybe in the early universe, you can make them a little bigger, but what is the the the upper mass limit? Um, and the reason why I'm I'm obsessed with this, I guess there's two reasons. One is that uh we still don't know how you form the super massive black holes that are at the centers of galaxies. Although maybe we're starting to understand that picture a little bit better thanks to the James Webb telescope. Uh but uh also it gets back to that question of how uh physical phenomena scale over many orders of magnitude in mass and the scarcity of examples of black holes that have masses in this in between range.

[00:35:50]And when I was a a baby scientist, I was fortunate enough to work on an object in the galaxy M82, which is our local starburst galaxy. And it's the brightest X-ray source in M82. And so it gets the clever name M82X1 because that's how we do. And uh it at at when we first started looking at this thing 25 or so years ago, it looked like its mass is around four or 500 solar masses. And that's a weird range uh because you can't we think we can't form that from a single parent star. And uh over two and a half decades of observing nearby galaxies, we have zero more good examples of objects in that mass range. U and all of the other constraints we have on black holes that are approaching but not quite at that mass range come from gravitational wave observations where we're starting to get up to a couple of hundred uh solar masses, but not too many examples. And so uh yeah, how how massive can you make a black hole from a single star uh sets the limits on uh on black hole mergers that we see with uh gravitational wave telescopes and it helps inform uh the scenarios for formation of super massive black holes in the early universe. So that's a thing that I'm really obsessed with. Hm. Do do you do you have a sense of of where you think this lies? Because I I mean I know what you mean about this idea of of super massive black holes. I mean thanks to web we are seeing these quite massive super massive black holes early on in the universe things that are approaching a billion solar masses within the first say billion years of the history of the universe.

[00:37:38]Who ordered that?

[00:37:39]>> Right. Um and and so then you and if you're just like saying, well, you had a star and it went supernova and then it found another star and they merged and then another one and then another one and it's also been feeding all this time. It doesn't get you to that kind of mass regime. And so now you got to go, well maybe bigger stars make bigger black holes. What if one has a thousand or 10,000 or 100,000 times the mass of the sun? Well, can you have a star that big? Apparently probably not. So then do you can you have a direct collapse black hole? Can a black hole feed faster than we thought?

[00:38:10]>> That's right. That's right. Those are exactly the questions. Yes. Can you form them through direct collapse? Can a can a star get to keep more of its mass when it when it dies uh than we think it ought to if it explodes as a supernova?

[00:38:22]And then how quickly can you grow them on reasonable time scales? And and what constraints can we place on this in the local universe where they're much easier to study than the things that we're seeing with with James Webb that are at the farthest reaches of the universe.

[00:38:36]And and like obviously I've been obsessing about this question as well.

[00:38:38]Do you feel like an answer is coming?

[00:38:44]>> I I don't know. Um I I think uh the direct the question of direct collapse black holes uh I think an answer to that question is coming thanks to observations with James Web. And I'm thinking in particular about a thing I know that that you've talked about before, little red dots. Um, and but there but there was another example that that happened uh where some astronomers are claiming that they had seen essentially a region of a galaxy merger that was too dense and had to be the result of a of a direct collapse black hole. That was a couple of months back.

[00:39:26]>> Yeah. There have been a handful of examples of of po possible direct collapse black holes. There have also been uh a couple of uh examples of highmass stars in relatively nearby galaxies that have disappeared without any obvious supernova remnant associated with them. And those are great examples of kind of the smallcale uh uh potential direct collapse black holes. U so we have compelling evidence that that's happening and I think that's the direction where we'll see more progress in the very very near future. I think uh we need an upgrade to the existing gravitational wave telescopes that is probably going to happen in in the reasonable future to place constraints on it from the gravitational wave uh the black hole merger side of things. Um and then on the observational that side of things in the nearby universe I think we need a next generation X-ray telescope to answer that question.

[00:40:23]>> Right. But I but something like Ver Rubin which just says you know someone pours through Ver Rubin says show me all the stars that disappear.

[00:40:30]>> Yeah you'll you for sure will be able to place some interesting constraints on >> on direct collapse. Will it tell you about black holes? I don't know right because you're not necessarily going to see a neutron star in those cases either. Uh and and it and it doesn't necessarily answer the question of can a can you have a star turn into a black hole, which is sort of a different question is can you have a a cloud of gas with a billion times the mass of a star turned into a black hole.

[00:40:58]>> That's right.

[00:41:00]>> Different creature. So, uh but maybe there's some common elements in there.

[00:41:04]Yeah. It's it's a it's such a fascinating question because I think people were really hoping that web was just going to go like here's your answer and no. Turns out It's the reality is >> turns out it made the problem worse.

[00:41:15][laughter] >> Yes. Which is in the end the you know that's what keeps astronomers. So I think that's all fine.

[00:41:22]>> That's great. It's we get excited when there are questions that we don't know the answer to. And it's not just because that keeps us in business. It's because that's where where our obsessions lie.

[00:41:29]Uh >> yeah. Yeah. Exactly. Yeah. Well, it was fascinating conversation. Roy. Thank you so much for taking the time to chat with me and good luck with your research.

[00:41:38]>> Thank you.

[00:41:38]>> Well, I hope you enjoyed this interview with Dr. Roy Kilgard. As always, I went into more detail and on an additional topic with Roy. And this was about his fascination, his infuriation with light pollution. And he has been working very hard to tackle light pollution on the campus where he works and really the city that he is in and the state that he is in. And that there's, you know, there are great reasons to reduce your light pollution, that you can save energy, that you can protect migrating species, that you can make a place that is more energy efficient and yet also brighter in the places that you need it. Point the lights at the ground, not up in the sky. And of course, that also helps out astronomers. And if you've ever been to a city like Tucson where they take this very seriously, it is night and day, pardon the pun. And so Roy gives some real practical examples on on how you can help fight this battle of both improving your own house and just get involved in your local community to try and reduce the amount of light pollution that's going on. I'm going to make this longer version of the interview available over on Patreon. As always, these longer editions are completely free. There's no ads. You can just go watch it right now. Uh and to reward you, we've got that additional content.

[00:42:55]I'll put a link down in the show notes.

[00:42:56]I'm going to give you some final thoughts, but first I'd like to thank our patrons. Thanks to Abe Kingston, Andrea Pretty, Brian Bod, Kerwin, Chuck Hawkins, Commander Beak, Durkfinga, David Gilton, David Mats, Thor Reading, and Math for Toddlers, Evan.Pro, James Clark, Jerry Matter, Jim Burke, Marcel Smiths, Michael, NASA, Ocean Guy, Nordspace, OnePan Animals.org. Please follow my nephew at Vbrook694, Ren Kaidu, Richard Williams, Shawn Sergeant, Steven Fland, Money, Team49, Telescopes Canada, Vlad Jepplin, and Wolfgang Clots who support us at the Master of the Universe level and all our patrons. All your support means the universe to us.

[00:43:31]It's funny this conversation, it wasn't really to the until the end where I think the real obsessions started to uh reveal themselves. And this is just this idea of how super massive black holes get so massive. so quickly. It's just like it's absolutely an an infuriating, beautiful, wonderful, crazy mystery that is just compounding year after year that you know we see the stellar mass black holes, we see the super massive black holes.

[00:44:01]How does the super massive black hole grow so quickly in relation to the galaxy that is around it? And it was hoped that James Webb would answer this question that you would get to this place be like, "Okay, we can see the building blocks of these small galaxies that are coming together to form these larger galaxies. There is your answer."

[00:44:20]And and that the super massive black holes of the small galaxies are feeding in. But no, we've got galaxies with super massive black holes that are billion times the mass of the sun seen early on in the history of the universe.

[00:44:33]And so whatever mechanism is going on, it is surprising and yet is an entirely new field of inquiry. And I think it's the the message that you get is that the universe is not only weirder than we suppose that it is weirder than we can suppose to use the famous quote and that and that the universe is not obligated to play by our rules. the universe does what it does and that it's our job to just kick back, to trust in the process, to observe, to see what we see and enjoy the satisfaction of the mysteries that unfold. And every time a new mystery comes along, you should be grateful, glad. There's more to learn, more to discover, and it should keep your curiosity going for the for the rest of our days. All right, we'll see you next