Astronomy Cast

New instruments bring new mysteries, and when James Webb came on line it uncovered a collection of strange, compact, bright objects shifted deeply into the red end of the spectrum. These were dubbed “Little red dots” or LRDs. And the astronomical community continues to puzzle over what they are. When JWST first peered into the distant past, it discovered the early universe had a rash of little red dots. Their existence just 450 million years after the big bang meant either galaxies were forming way faster than anyone predicted, or something unimagined had been found. 

Show Notes

• Excitement and anxiety astronomers feel when new telescopes like JWST come online.
• James Webb’s discovery of mysterious “Little Red Dots” — compact, bright, redshifted objects from the early universe.
• Possible explanations: active galactic nuclei (AGN), dust-enshrouded galaxies, or direct-collapse black holes.
• Debate over black hole growth limits, primordial black holes, and the Eddington limit.
• Theories on early galaxy and star formation, and what “Little Red Dots” reveal about cosmic dawn.

Transcript

Fraser Cain: 

AstronomyCast, Episode 769 Little Red Dots. Welcome to AstronomyCast, our weekly facts-based journey through the Cosmos, where we help you understand not only what we know, but how we know what we know. I’m Fraser Cain, I’m the Publisher of the 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 are you doing?

Dr. Pamela Gay: 

I am doing well. It is the most glorious of falls here in Southern Illinois. The leaves are falling, the sun is out, it is hoody weather, I am happy, and Halloween is almost upon us.

Fraser Cain: 

So are you ready to feel a little bit self-conscious?

Dr. Pamela Gay: 

I guess.

Fraser Cain: 

So I was interviewing one of my patrons, and the patron said, hey did you ever notice that when Pamela says, I am doing very well, I’m like, no, you just did it. So apparently every time I ask you how you’re doing, you say, I, I am doing very well.

Dr. Pamela Gay: 

That’s all right, I have habits.

Fraser Cain: 

Yeah, yeah, crazy, like you just like, by the numbers, you did exactly on, so I had to bring it up. But the funny thing is that when we started doing this podcast, you actually asked me how I was doing, and I had to give the answer every time, and I hated it, and people made compilations of you asking me how I was doing, and me answering, and it all just sounding exactly the same, and so at some point I just flipped it on you, and you never complained. And so now, um, you, you are the one who I ask how you’re doing, and you always answer, and we’ve, you’ve never gone, wait a minute, why am I always the one who has to say how, how I’m doing?

Dr. Pamela Gay: 

So I think the switch happened, uh, not too different in time from when we switched how we were doing so many other things. This show has evolved.

Fraser Cain: 

Yes.

Dr. Pamela Gay: 

We went from just the two of us on Skype, uh, with both of us recording locally to Google Hangouts, to Google Hangouts on air, with you producing and going through myriad different technologies to, at some point it switched to me producing, and there’s been so many evolutions in there. We’ve also changed how, how we do the opening music, um, yeah, yeah. We’re almost to 800 episodes.

Fraser Cain: 

Yeah, you do, you do this many times, and you just, you fall into ruts and patterns, and so I’m like, I’m just glad people aren’t sick of us yet, that this is still what they want to listen to. So thank you for listening to us, and I know for some of you, uh, hearing us fall into these standard patterns of speech is your comfy place. This is, this is what you want to hear.

This makes you feel like the world is going to be all right, and for those of you who are annoyed, I’m sorry, that’s a Canadian sorry, nothing we can do. Um, new instruments bring new mysteries, and when James Webb came online, it uncovered a collection of strange, compact, bright objects shifted deeply into the red end of the spectrum. These are dubbed Little Red Dots, or LRDs, and the astronomical community continues to puzzle over what they are.

All right, take us back to the history, actually, before we do this, I want to talk like philosophically about your experience watching new telescopes come online, and what it’s like to be an astronomer knowing that the presents are about to be opened up.

Dr. Pamela Gay: 

So there is a Schrodinger’s box of emotion when new telescopes are coming online, and I feel it very deeply because of my experiences with my dissertation where an X-ray satellite I was planning to use failed on launch, and the Javier Burley telescope that I was planning to use to take hundreds of spectra allowed me to take 18 spectra. I’m very much aware that the telescope can fail spectacularly, or, or, if you’re very lucky, which I am not, it will be the best thing ever, and amazing new things will be discovered. And so this dichotomy of emotions as you wait for enough data to come in to fully understand what’s going to be possible is, I don’t even know how to articulate the internal dichotomy of I’m trying not to get excited, I’m trying not to get excited, I want to be excited, this could be everything, or we just wasted billions of dollars.

Fraser Cain: 

Yeah, it was funny, when the next generation telescopes came out from the European Southern Observatory with a very large telescope, we got the announcement of a whole bunch of new planets, new dwarf planets, and that was due to just the capability of that instrument, that more powerful instrument, new things happen. Some of the things, you get answers to the old questions, the whole point of building the telescope is to answer a bunch of questions. But the part that I enjoy even more is the new questions that pop up.

And so with James Webb, we knew there were going to be new questions, and there have been a bunch, but this is the one that I think has sort of best encapsulated what James Webb was supposed to do. And I think the answer is becoming more interesting over time as we’re seeing this. So let’s go back and sort of talk about this discovery of these little red dots for the first time.

Dr. Pamela Gay: 

So JWST took a series of different images to showcase what it’s capable of. These images included really pretty nebula that filled the entire field, and also background fields that included galaxies that were doing gravitational lensing, and just blank fields. Blank fields by which I mean not a gravitationally lensing galaxy cluster there, nothing is blank in astronomy.

And in those images that allowed us to see the background universe, there were these small red luminous objects that when you took their light and figured out roughly what red shifts they’re at, what time in the universe they’re shining their light at is from, and you adjusted their spectra, their rainbow of light back to what it would look like where they are, their rest frame magnitudes. They became these objects that were super bright in rest frame reds that had all sorts of hydrogen bomber light emissions and seemed to be hot and massive and they looked kind of like AGNs and they were just confusing. And so everyone just sort of went, but there aren’t supposed to be galaxies then, and then started doing research.

Fraser Cain: 

Right. And it turns out galaxies have been found then and even earlier, like we’re seeing galaxies at times that are less than 300 million years after the Big Bang. We’re seeing super massive black holes at times when the universe is less than half a billion years old.

We’re seeing spiral galaxies when the universe is less than a billion years old. The universe was surprisingly evolved, but these little red dots, okay, so those features you mentioned, they’re very bright, they’re very compact, they’re now shifted into the red. What kind of light were they giving off back in the day?

Dr. Pamela Gay: 

They were the moral equivalent. So James Webb Space Telescope is seeing them in colors our eyeballs can’t even see. If you take that light, shift it to what our eyeballs can see, they would literally be red galaxies, but they’re not galaxies.

Fraser Cain: 

Right. Well, we don’t know what they are yet.

Dr. Pamela Gay: 

Probably. So to give some context, if one of these was plopped into our local group, they’d be roughly the same brightness as the Triangulum Galaxy and 3% of Triangulum or 2% the size of the Milky Way Galaxy. So these are very bright, very small from what we can see.

Fraser Cain: 

And very red. Yeah. Yeah.

All right. So now think like an astronomer detective and put down all the pieces of evidence that we have so far that try to direct us towards what it is that these things might be.

Dr. Pamela Gay: 

So we’re seeing a bunch of different emission lines, which you get when you have a bright source shining its light through cooler gas that then has atoms that get excited into higher energy states. Nothing stays excited. And when they drop down to their lower energy level, they emit light.

Fraser Cain: 

Give us a sense of something that astronomers look at that has that phenomenon.

Dr. Pamela Gay: 

So, when you have cooler gas around very hot stars, you see emission lines. Active galactic nuclei quintessentially have this pattern of lines. It’s these scenarios where you have something very hot giving off light surrounded by gas and dust that allows you to have this kind of emission line.

Fraser Cain: 

Okay. So we’ve got an active galactic nuclei. Like.

Hold on. Active galactic nuclei, case closed. Right?

No. Okay. So why can’t you just say it’s an active galactic nuclei?

Dr. Pamela Gay: 

So there’s other colors of light. And while we see emission lines and this V-shape in their spectrum that reminds us of active galactic nuclei, active galactic nuclei have other things like hot corona of gas around them that give off x-rays. And as hard as we look, we’re not finding x-ray emission from these little red dots.

It’s just not there. And by the way, I just need to warn people, do not Google little red dots without adding the word cosmology or JWST. Cosmology.

Fraser Cain: 

You’re just going to get rashes. You’re going to get rashes. Yeah.

Yeah. Yeah. And perhaps a suggestion that you go see your doctor. Yeah. Right. And this is the key, which is that you can take an active galactic nuclei, an actively feeding supermassive black hole. It’s feeding so heavily.

There’s so much gas coming in that material is piling around it. You’ve got this shroud of gas and dust around this central core. And yet because the accretion disk heats up, it’s giving off x-ray radiation.

Astronomers have looked at each of these objects with the Chandra X-ray Observatory, for example, and they’re not seeing the kind of x-ray emissions that you would expect coming from an active galactic nuclei. So active galactic nuclei off the table, case closed.

Dr. Pamela Gay: 

So that means we have to come up with something else. And like I said, this is something, these are something, we’ve found over 350 of them now, that they exist in this window of time from about 300 million years after the Big Bang to about 1.2 billion years after the Big Bang. So this is a point in time where the diversity of elements in our universe wasn’t that great, when things were still in the process of forming.

And there’s a lot of problems that we want to solve with this era of the universe. And so people then go, I wonder if little red dots can solve this problem we have. And so the kinds of solutions that folks have looked at…

Fraser Cain: 

Well, what are the problems? Sorry, but you mentioned there’s a bunch of problems we’re trying to solve. What are those problems that we know of at the early universe?

Dr. Pamela Gay: 

So how do you get supermassive black holes that early? How big could stars get if they weren’t able to cool through metal lines? It turns out that heavy atoms provide ways for stars to do star things, and you can’t get massive stars if you have heavy elements in the stars.

So we have these two different problems that we’re trying to solve. We also have the, well, what if there were primordial black holes? What if things were happening with high turbulence in ways that we don’t think about in the early universe?

Fraser Cain: 

How did the dark ages end?

Dr. Pamela Gay: 

Right. And how did the dark ages end is something you can’t really solve with little red dots because it requires a lot of ultraviolet light. So the first thing we realized was, well, these don’t solve that.

And then the next question became, but what if they have a whole bunch of star formation? And then we started doing the weight. They’re like 3% the size of Triangulum, which is pretty small.

So where do you hide that much dust to enshroud star forming regions to hide the ultraviolet light? So that doesn’t work.

Fraser Cain: 

Well, hold on. So just like that line of thinking, right, is essentially you have stars compacted about as closely as you would see at the center of the Milky Way or in a globular cluster, and also with large amounts of dust and material all around them, like a super compact stellar nebula, like nothing we’ve seen in our nearby universe.

Dr. Pamela Gay: 

And one of the other problems that you run into with little red dots is they’re what’s called a super editing luminosity. This means that they aren’t in equilibrium between the amount of light going out and the amount of mass falling in. And so they don’t fit stars.

It’s this weird combination of things. So now we have to start saying, all right, so AGN is really, really close. It almost works.

We’re just missing the hot corona and some of the other details in the stellar spectra of a galaxy and the spectra of a little red dot don’t quite match. So is there a way to have essentially a naked AGN, something that doesn’t have that galaxy and that galactic halo around it? And that was the next place that people started looking.

Fraser Cain: 

And that would be weird. Yes. Right?

So you were talking about how instead of the traditional idea of a supermassive black hole at the heart of a galaxy, where it is maybe, I mean, right now the Milky Way’s black hole is only 10%. No, sorry. The Milky Way’s black hole is only 1%, maybe a 10th of a percent, I think, of the entire mass of the Milky Way.

Back in the early universe, they were more like 10%. So they were more dense, but still the black hole wasn’t the dominant mass in the Milky Way. It was the stars and the gas, the dust and the dark matter.

So, but in this case, we’re saying, well, maybe it was just a supermassive black hole and nothing else.

Dr. Pamela Gay: 

Well, a whole bunch of gas and dust.

Fraser Cain: 

Right.

Dr. Pamela Gay: 

Yeah. And so the idea here becomes, and people are getting at this from multiple different directions. So I’ve seen papers that are like, okay, so let’s just imagine for a moment you could get a supermassive population three star that like collapses down and then all the density of material falls in towards it.

Can we create through turbulent infall of material a situation where you have a rapidly growing into a supermassive star with turbulent infalling material? I’ve also seen, well, what if you start with primordial black holes and you have turbulent infall of material? Right.

However you get the black hole, the idea is you have this supermassive black hole in a universe that it’s able to be a seed for gas to flow inward. That material is turbulent and because of the turbulence, it’s able to give off momentum in ways that we don’t normally think about when we watch things like toilets flushing and bathtubs draining. Turbulence allows this chaotic buildup of material around that supermassive black hole and all that material is now glowing like the disc of a AGN.

And in this scenario, which don’t let us name things, astronomers should never be allowed to name things.

Fraser Cain: 

And yet they keep doing it.

Dr. Pamela Gay: 

Yeah, yeah. We need to like ask our kids, our spouses, our friends, just, yeah. I’ve seen two names pop up consistently.

One is quasistars where the idea is you don’t have, well, let me give both names. One name is quasistars and the other name, please don’t use this, please drop it forever, but you’re going to see it out there, is black hole stars, which is just leading people into a world of misunderstanding.

Fraser Cain: 

Isn’t a black hole a star?

Dr. Pamela Gay: 

No.

Fraser Cain: 

Gas?

Dr. Pamela Gay: 

So it’s a remnant.

Fraser Cain: 

It’s a remnant. Sure. Fine.

Dr. Pamela Gay: 

So stars, by definition, have nuclear reactions going on in their course.

Fraser Cain: 

What about a white dwarf?

Dr. Pamela Gay: 

A white dwarf?

Fraser Cain: 

Star. White dwarf star. What about a neutron star?

Dr. Pamela Gay: 

Yeah, so those are stellar remnants that have terrible names. Yeah, I know. That’s all I’m saying.

Fraser Cain: 

That’s all I’m saying. It’s just like this, this is a barn with a lot of horses that are already running free.

Dr. Pamela Gay: 

I know. I know. I’m just trying to keep the last horse in the barn.

Fraser Cain: 

You are wasting your time.

Dr. Pamela Gay: 

I usually do.

Fraser Cain: 

Black hole stars. That’s what’s for dinner. That’s what we’re going to talk about now.

So I want to just sort of go back and sort of re-describe what you’re saying here, which is that, you know, there’s these two mechanisms, both of which are, which most people don’t think work. One is that you have a giant cloud of gas and dust and something sets it off and the whole thing just turns into a big black hole. That’s not supposed to happen.

That as you heat up, as the black hole gets hotter and hotter, as the accretion just builds up around it, then the radiation starts to pour out of it and infalling material is prevented from happening. This is the Eddington Limit. And yet black holes have been seen beating the Eddington Limit.

And so maybe it is possible. And we have seen examples where perhaps black holes have formed directly. This idea of direct collapse.

You don’t need to go through star just, you know, accretion material until it’s fat. This other idea, as you say, primordial black holes, these might’ve formed in just ripples of space-time moments after the Big Bang. And then they got a headstart.

Yeah. You can have a black hole with, uh, 5 million times the mass of the sun early on in the universe. If it started at 4 million times the mass of the sun, right at the Big Bang and just kept on feeding from there and could be an explanation for dark matter.

So, you know, everything comes together nicely, except that we, we don’t see evidence really of, of either of these things, of direct collapse black holes or of, of primordial black holes.

Dr. Pamela Gay: 

So, so I’m not ready to say that these aren’t direct collapse black hole systems. Um, I’m just not ready to say we’ve disproven that there’s enough papers that are able to fit the pieces together. They’re able to model what the spectrum would look like and it matches.

And because the densities in the early universe were so different, it seems that for this moment in time, it may have been possible. And this idea that you have collapse coming in from multiple directions with a disc that is the bulk of the source of light, but it’s heating everything around it. It seems to fit the lack of variability in light that we’re seeing.

That’s another one of the differences is AGN flicker. It’s kind of awesome. It allows us to map out the sizes of accretion discs through echo mapping.

Um, the, the size of the disc and the size of the variations in time have to match. And, and so we don’t see any of that variability, but if you have a cloud of material all the way around that desk, you’re not going to be able to see into the core of the desk and see that variability. So there’s that enshrouding nature that’s getting answered.

The spectrum seems to match. And if this is the only case where these things would be observed, they’re rare enough that it could explain massive galaxies early on and everything else forms through hierarchical clustering, but it’s too early in the story. We’re going to have to come back to this in five years, but you need to know these exist.

Fraser Cain: 

Right, right. And there’s a couple of issues. One is where is what comes next?

So, you know, because James Webb and the other telescopes are time machines, they let us see little snapshots of the universe at different times. And so we’re seeing all of these little red dots at different ages, but within the first billion years of the universe, but you would expect to see objects that are now the next phase of whatever those started out as. And this has not been very well documented.

There’s a couple of papers where people are starting to propose that some objects that are actually giving off a lot of bluer light are a transition that maybe you’re getting this switch on of the visible light, the ultraviolet light, and you’re getting sort of turning into whatever comes next. You know, we’re seeing their baby Pokemon version with James Webb, and then hopefully we’re trying to find the larger versions. We’re looking for the Eevees.

Yeah. So that’s one sort of angle that we can watch is to look for the kinds of things that they turn into and that it gives you more information about what’s still there. Another paper that I was looking at that I found really interesting is that people simulated how big stars, how big that first generation of stars should be.

And the expectation is that they should be gigantic, but they simulated, in fact, they should be tearing off into small stars, just like normal-ish sized stars, and then exploding as well. And so maybe you just got a whole bunch of black holes clustered together that are in the process of coming together to maybe just like a ton of stars. And maybe, like, here’s my hope, is that these are globular clusters.

That maybe whatever it is, like right around the very beginning, they formed giant collections of stars, and we see them to this day. That would be really exciting to me. So yeah, it’s a wonderful mystery.

And hopefully, almost certainly, we will come back in a couple of years and go, okay, here’s what they are. And right now, it’s all in play.

Dr. Pamela Gay: 

And the wild thing is there’s so many different options. Like me, I’m kind of expecting the little red dots shut off as extremely dusty, extremely enshrouded disks or clouds of stars begin to light up, and they just haven’t produced the ultraviolet light yet necessary to clear out and make things visible. But we don’t know.

Fraser Cain: 

Yeah, we don’t know.

Dr. Pamela Gay: 

That’s the amazing part.

Fraser Cain: 

Yeah. And so you ask anybody, what I think is, you know, astronomers are going to have what they think is their most likely outcome. But if you showed them a piece of evidence that just proved that, they’d be like, yep, okay, that’s out.

They’re going to hold their ideas very loosely right now, because there’s still so much science to be done, and it’s an exciting mystery. So I look forward to that episode where we’re like, okay, here’s the boring explanation for what it is.

Dr. Pamela Gay: 

Just please call them quasi stars and not black hole stars. That’s the only thing I ask.

Fraser Cain: 

Good luck with that. I know. Thanks, Pebble.

Dr. Pamela Gay: 

Thank you, Fraser. And thank you to all of our patrons. You truly make this show possible.

This show wouldn’t exist without the amazing support of so many over on patreon.com slash astronomycast. This week, we would like to thank in particular Andrew Stevenson, Antisor, Arno DeGroot, Astro Bob, Bob Kale, Boogie Nat, Smansky, Daniel Schechter, David, David Rosetta, Dr. Whoa, Don Mundus, Dr. Jeff Collins, Elliot Walker, Father Prax, Frodo Tanenbaugh, Jeff McDonald, James Roger, Jim Schooler, J-O, Jonathan Poe, Kenneth Ryan, Kimberly Rake, Labrat Matt, Larry Dotz, Marco Yarasi, Mark Schneider, MHW1961 Super Symmetrical, Michael Hartford, Michael Prashada, Michael Regan, Nyla, Papa Hot Dog, Paul D. Disney, Paul Jarman, Philip Walker, Randall, RJ Basque, Robert Hundle, Robert Pelasma, Ron Thorson, Sam Brooks and his mom, Shersom, Semjan Torfason, Zeggy Kemmler, Steven Rutley, Thomas Gazzetta, Tiffany Rogers, Van Ruckman, Wanderer M101, Will Hamilton.

Thank you all so very much. That pause is where Rich is now going to drop in the previously recorded names.

Fraser Cain: 

I like this where we don’t do this live. This is great.

Dr. Pamela Gay: 

Oh, I know.

Fraser Cain: 

All right. All right. Thank you, Pamela. Thanks, everybody. And we will see you all next week.

Dr. Pamela Gay: 

Goodbye, everyone.

Live Show

So it’s been decades since we’ve seen a bright comet in the sky. And actually there was a pair — Hale-Bopp and Hyakutake. And then, silence! And unmet promises by the Universe to give us a bright comet. Comets are unpredictable, and they arrive precisely when they intend to. Is it time again for a bright comet? If you asked us in January if 2025 was going to have any outstanding comets would fly through the Solar System, we would have (and we did) say “no.” And we were wrong. Comets are fickle, unpredictable, and like to do exactly what we didn’t predict.

Show Notes

• What makes a great comet
• Why predictions are hard
• Historical highs & hopes
• Hyakutake & Hale-Bopp set the bar
• Halley’s best (1910) and next return in 2061
• Current/near-term watchlist (Fall 2025):
• C/2025 A6 (Lemon)
• 2025 R2 (SWAN)
• Big units: 2014 UN271 (Bernardinelli–Bernstein)

Transcript

Fraser Cain: 

This is a test. 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, I’m the publisher of Universe Today.

With me as always is Dr. Pamela Gay, a senior scientist for the Planetary Science Institute and the director of Cosmic Quest. Hey Pamela, how are you doing?

Dr. Pamela Gay: 

I am doing well. Our sights are still a bit on the struggle bus with the onslaught of scrapers, but I seem to have things running slow, but not crashing. And I’ll take slow and not crashing and continue to work on it.

So thank you everyone for your patience. Yeah, this was not what AI ever talks about doing to people.

Fraser Cain: 

Where are my Terminators? I would prefer Terminators over them slowly degrading our website service. Yeah, at least the Terminators are honest with you, as opposed to sneakily pretending to be Chrome browsers that are just browsing your website thousands of times a second.

So we did something unusual this week over on my YouTube channel. We released, instead of doing our normal news roundup with Space Bytes, we did a roundup of news about Comet 3i Atlas. And it was a very different episode, something unlike we’ve never done before, which was that I went very carefully through every single paper that has come out about 3i Atlas over the last three months.

Dr. Pamela Gay: 

There’s some doozies in there.

Fraser Cain: 

Yeah. The ones that I felt were scientifically valid, all of them in archive. Okay, fair.

Dr. Pamela Gay: 

Thank you.

Fraser Cain: 

And then we connected it all together into a video, but it’s just reference after reference. You could see us going through the paper, quotes. It was footnoted.

It was like an academic journal of its own. Normally, I’m very much fast and loose about the news. In this case, I was saying the names of the principal investigator.

We were providing a link to the paper on archive. We were showing it. We brought receipts for this episode.

Dr. Pamela Gay: 

That’s so important.

Fraser Cain: 

Yeah, because there’s so much AI slop out there. I think if you want a very science-based reference about 3i Atlas, definitely check out the episode that we did this week over on our YouTube channel, the Universe Today YouTube channel. But it’s also sent out in my newsletter.

We have it on the podcast in all the places, but I hope people really enjoyed that. It took me about four times as long as it normally takes me to do a Space Bytes because it was so carefully, meticulously researched. It felt very weird to do the episode.

I thought it was just going to be a disaster, but then Anton, our editor, he just nailed it. It’s a beautiful video, great graphics, lots of really cool stuff, and also really meaty scientifically. Definitely check that out.

I’m really proud of what we did.

Dr. Pamela Gay: 

Yeah. Some of the stuff you do is must watch. That one is getting added to the list of things that I must watch.

Fraser Cain: 

So it’s been decades since we’ve seen a bright comet in the sky. Actually, there was a pair, Hale-Bopp and Hyakutake, and then silence and unmet promises by the Universe to give us a bright comet. Comets are unpredictable, and they arrive precisely when they intend to.

Is it time again for a bright comet? We’ll talk about it in a second, but it’s time for a break, and we’re back. So, Pamela, is it time?

Are we due? Is this it? Is this our moment?

Dr. Pamela Gay: 

I don’t think we’re going to get a Hyakutake or Hale-Bopp, and this makes me sad because those two were my company at the end of undergrad, the beginning of graduate school. Yeah.

Fraser Cain: 

I mean, it’s sad that it’s been so long, 26 years?

Dr. Pamela Gay: 

Yeah.

Fraser Cain: 

27 years? There are people watching this episode, people listening to this podcast, that were not born when there was a bright comet in the sky.

Dr. Pamela Gay: 

Yeah.

Fraser Cain: 

Yeah, but what about McNaught? What about Comet Swan? What about Comet Lemon?

Those are nothing. Those are garbage. Those are meaningless.

You don’t know what a bright comet looks like when you stand outside, and it has a tail that stretches for 20 degrees in the sky, that you can see it as the only object when the Sun has gone down, that it just is there night after night like a familiar companion. You go to dark skies, and you just marvel at it. You take pictures of it with a pocket camera, and it still looks amazing.

Dr. Pamela Gay: 

Yeah.

Fraser Cain: 

The universe has just been, for a quarter century, nothing. I’m outraged.

Dr. Pamela Gay: 

I know. I know. Seriously, folks, unless you saw it, it’s impossible to imagine.

I was a baby grad student for Hale-Bopp, and the first night, I was using the McDonald Observatory 30-inch, which is kind of nestled into the side of the mountain. The person who was teaching me how to use it, Philip McQueen, he was like, just hold on until 5 a.m., because I had to drive all day, then I was going to observe with him all night. It was like being awake for 24 hours required for your education.

I was just melting into the desk. At 5 a.m., he’s like, okay, go outside and look. I opened the door from the dome, and straight in front of me, taking up a good 30 degrees of the sky, is Hale-Bopp.

Up to the left are the McDonald Observatory, 107-inch and 82-inch. They’re reflecting off of them the entire horizon of thunderstorms that are off in the distance. Then straight overhead, like you were in a planetarium, was the Milky Way.

It was this moment of, if you showed this in a planetarium, people wouldn’t think it was real. We saw that for months.

Fraser Cain: 

Months. Then we saw it again. I think it’s really important.

Pamela’s describing 30 degrees in the sky. Take your hand out at arm’s length, put your fingers out as far as they’re going to go, measure the bottom of your thumb to the top of your finger, and it’s more than that. That’s 25 degrees.

It’s bigger than that. Imagine that. Hold your arm out and imagine seeing a comet, bright comet, on the horizon, high up in the sky, that is just monstrous.

The problem is that we have to get pictures. People take amazing pictures with their telescopes, but that’s not what you see with your eyes. When Hakutake and Hale-Bopp were here, that’s what you saw with your eyes.

You didn’t need a camera. A telescope was of no benefit because it didn’t fit. It was so big.

It’s madness. Anytime people are like, yeah, but what about this comet? It is unacceptable.

The universe can do better. It knows it, and I accept nothing less than the best.

Dr. Pamela Gay: 

We need a comet 1729 P1. This was a comet that reached magnitude negative three. It is the brightest comet ever on record, although there are arguments that the Great Comet of 1882 might be even brighter.

Fraser Cain: 

You could read your newspaper to a comet that bright.

Dr. Pamela Gay: 

Yeah, and what’s amazing is because these are related to how close we were to them, there’s always that every single time we pass through the tail of a comet when there’s a shower, there’s this moment of, ah, but the comet could have just gone through and wouldn’t that be amazing? And this is what I want, but we don’t have any periodic comments that are due to do that anytime soon.

Fraser Cain: 

All right. So let’s set this up then. What is it going to take?

What are the confluence of factors that make a comet great versus one that is not great?

Dr. Pamela Gay: 

So a truly great comet has to have a number of different things. It has to be near the sun and the earth at about the same time, but not so close to the sun that we can’t see it. So you want this angle between looking from the earth to the comet to the sun, you want the comet earth sun angle to be greater than 20.

Fraser Cain: 

Right. And ideally you want the comet on the other side of the earth from the sun and close.

Dr. Pamela Gay: 

Yeah. Yeah. That is absolutely ideal.

But at least 30 degrees away. So greater so that it’s behind you.

Fraser Cain: 

And the, and the problem with that story is, is that comets get exciting. Comets get big tails when they’re close to the sun. It, and think about how hard it is to see mercury, right?

Mercury is close to this is always interior to us. We’re always having to look close to the sun to be able to see mercury. Very few people have even ever seen mercury because it’s close to the sun.

And so the comments are usually at their best when they’re close to the sun.

Dr. Pamela Gay: 

Right. Right. And so close to the sun, close to the earth at the same time, very important because you also want the object to have a lot of volatiles on its surface that can get exposed to the sun, form a bright coma, form the tails that we see both the ion tail and the tail of stuff and things.

So we’ve been seeing lately comments that have a lot of carbon in them and they become this amazing shade of green as they get ionized. And, and so the colors that we see are due to the composition. So having, it’s not a requirement, but it’s a bonus feature if they can have an interesting set of atoms in them.

Fraser Cain: 

Right.

Dr. Pamela Gay: 

And then you just want the sucker to be big, a little tiny comment that is like 90% volatiles and well-placed isn’t going to be able to put on as large a show as a significantly bigger object that may not have as many volatiles, but has a lot more surface area for those volatiles to be getting active on.

Fraser Cain: 

Give us a sense. What is a small comment versus a big comment? Like the size of the nucleus?

Dr. Pamela Gay: 

I mean, like the biggest comments can be measured in kilometers across. Generally they’re, they’re not going to be that big, but they can get that big.

Fraser Cain: 

Right. So hundreds of meters, two kilometers across. I think the largest comment that we’ve seen, we actually stopped fairly recently was in the tens of kilometers across.

It wasn’t very close. Unfortunately, even three eye Atlas is an interstellar object. It’s five and a half kilometers across.

Like it’s a big object.

Dr. Pamela Gay: 

Yeah. So 2014, I’m going to say this wrong. I, I am so sorry.

Comet 2014 UN 271 Bernard Nenilly Bernstein was a hundred kilometers, 62 miles across. And it is the biggest we know of so far, but we’ve only had the ability to measure their diameters very recently. So, so there’s that factor working against us.

Fraser Cain: 

All right. We’re going to talk about this some more, but it’s time for another break and we’re back. All right.

So we know that we want the comment to come close to the earth, but be on the opposite side of the sun. Ideally we want it to be large so that it’s going to be able to give off a lot of volatiles, be able to, uh, fire material out into space, the bigger, the coma, the longer the tail, the brighter it’s going to be. So then when comments are first discovered, there’s a lot of unpredictability.

Like, like we’ve done this quite a bit where a new comment has been discovered. And then we talk about whether or not it’s going to be great. Why can’t we be more certain?

Dr. Pamela Gay: 

So it turns out that when comments are struggling to stay in one piece, they get super interesting on the sky. So one of the things that can temporarily allow a comment to get super bright, super interesting is just when that sucker completely falls apart, which sometimes comments choose to do. We, we had one back in 2013 that opted to completely fall apart on Thanksgiving day.

So, uh, when you fall apart, you have many different pieces, all producing a lot of volatiles and very temporarily, uh, you can end up with a nice, cool, bright thing, but very temporarily. Uh, there’s also the issue that we don’t know when we first spot a comment, what its size is and what its composition is necessarily. And we make approximations based on how bright it appears.

And if you have a really big comment that has a lot of dark organic molecules on its surface, it will appear smaller than it actually is. If you have a little tiny comment that is super, super shiny, um, or asteroid in the case of the target for Hayabusa2, uh, if you have a small object that appears super shiny, you’re going to assume it’s much bigger. And there’s also the issue of sometimes we just don’t know that a comment is coming.

So when you and I make our predictions every December of what to look forward to the following year, we don’t know about comments that have never been seen before that are going to decide to show up. So, um, we can misidentify how interesting they’re going to be. They can decide, Hey, I’m just going to completely fall apart over here and become super bright.

Um, and then there’s all the ones we just didn’t know were coming.

Fraser Cain: 

And I think the most compelling ones are the ones that are most uncertain. They’re the ones that are going to do a close flyby of the sun. Um, you know, that trajectory you want is it falls down into the inner solar system, just scrapes past the sun and then goes into a trajectory that puts it on, as we mentioned on the opposite side of the earth from the sun, or at least 30 degrees away from the sun.

So you can see it in the, in the sky. The problem is the closer you get to the sun, the more dangerous a path you’re taking. And the sun will tear these comments apart in with its title forces into just a, uh, just a pile of dust.

And, and so you have to sort of thread that needle. You want to get really close to the sun so that you have a lot of volatile elements come off of it, but you don’t want to get so close that you get completely torn apart. And so there’s been examples where comments we’re watching for the comment to complete its, its flight past the sun, where if it survives, then it’s going to be one of the best comments we’ve ever seen.

It’s going to be super bright comment of the century. And we wait and we wait and we wait and no comment comes out the other side of comes around the back of the sun. It was destroyed.

It’s gone. It’s just, it’s, it’s, it’s smashed into little pieces. And then all the volatiles are blown away and maybe a debris cloud comes out the other side, but nothing that’s going to put on that big sky show.

It’s got to hold together and be able to make that journey. And so that’s one of the biggest unknowns is will it survive? It’s it’s travel around the sun.

Dr. Pamela Gay: 

And one of the best apparitions that, uh, many of us heard about either from great grandparents or from music, it turns out was the 1910 apparition of Haley’s comment, uh, that particular year, all the right things occurred. And just after the comment went past earth’s orbit, passing through earth’s orbit, that’s how we get the meteor shower from it. Um, we basically flew through the tail.

And so if you can time it just right, so that it crosses the earth’s orbit as you are heading towards that crossing point. So it ends up behind you in the solar system. Um, you can get an amazing meteor shower that leads to really good music getting written.

Um, the stars fall over Alabama is the song I’m thinking of as well as an amazing comment to view at night. And that double whammy of shooting stars and comment to view is, is just the kind of thing. I for one dream of eventually seeing when some comment we don’t know about currently decides to grace our solar system with the passage.

Fraser Cain: 

All right. We’re going to take another break and we’re back. So you mentioned Halley’s and I wanted to sort of go into that a bit.

So, you know, we have now three classifications of comments. We have the interstellar comments of which we have seen three, maybe three.

Dr. Pamela Gay: 

Yeah.

Fraser Cain: 

Well, I mean, was a moment, a comment was an asteroid.

Dr. Pamela Gay: 

Well, okay. That’s fair. We’ve seen three interstellar objects, right?

Yeah.

Fraser Cain: 

Boris Borisov was very much comment three. Alice is acting very much like a comment, but they come randomly at high velocity. They take whatever journey they’re going to take through the solar system and then they’re gone.

And so far they have not been very bright, hard to spot. Yeah. You have the long period comments, the ones that are entering the inner solar system for the first time in their entire experience that they’ve been out there for 4.5 billion years. They finally through some gravitational interaction, make the long journey. They take a million years to fall down into the inner solar system, zip through, and then they head back out.

Dr. Pamela Gay: 

Yeah.

Fraser Cain: 

Random. We, we have no way to predict them, but we do have the periodic comments, the ones that are, that come on a regular basis. Comet Halley is the most famous of these.

Dr. Pamela Gay: 

Right.

Fraser Cain: 

So when can we expect a good, you know, can we know when there’s going to be good Halley’s comments and bad Halley’s comments to a certain degree?

Dr. Pamela Gay: 

I, a lot of us who were tiny children expecting to see a massive comment because so much went into educating children across at least America. I don’t know what you experienced in the Canadian. Yeah.

You had the same thing. It was just like all of this all of this prep and like kind of math. Yeah.

Yeah. I, I was not impressed and where it gets frustrating is at a certain point, the really cool stuff is also associated with stuff like landslides and disruptions, and we don’t know when those are going to occur. So like Rosetta got some amazing closeup images of Sherry Gary having outbursts related to disruptions of the comment.

And so we, we can run software and you can do this to a certain degree in Stellarium into the future and figure out when will we have passages of the earth right before or after a comment has crossed its orbit. And there’s, there’s none anytime soon. It’s kind of sad.

But at the same time, we keep finding comments that are the non-periodic ones. So currently the periodic ones, we’re going to get some, we’re not going to get anything like Yakutaki or Hale-Bopp from any of the periodic ones anytime soon.

Fraser Cain: 

I mean, our next Haley’s is 61.

Dr. Pamela Gay: 

Yeah.

Fraser Cain: 

20, 2061, we’ll get another flyby. But, but as you said, we probably won’t get what we, what we got in 1910 where we went through the tail.

Dr. Pamela Gay: 

Yeah.

Fraser Cain: 

Where it was very bright. It was, you know, it was a, it was a, it was like things worked out really well for that one. In this case, it’s going to be, I don’t know what the, what the kinematics are for this upcoming version, but, but in general, like there are a bunch of them, but they, but very few come even relatively close to the sky.

You can, you know, every year we get a couple that you can see in the telescope that are periodic comments. They come every 20 years, every thousand years. But it’s the, really, it’s the, the ones that have come from the Oort cloud that are the random, and they can be amazing or they can be mediocre.

Dr. Pamela Gay: 

And, and this is where, when we’re recording this in the fall of 2025, we’re looking at the surprise apparition of comets, Lemon, Swan, and then there’s a bunch of others that are just telescopic ones. But 2025 A6 Lemon is predicted to get maybe as bright as magnitude three by the end of October. Yep.

So I’m looking forward to being able to see that with my unaided eye.

Fraser Cain: 

Yeah, that you can see with your eye. And it’s been a while since we could even see a comet with our eyes. This one, and it’s also well-placed.

Dr. Pamela Gay: 

Yes.

Fraser Cain: 

So, and Swan as well. Swan is really well-placed.

Dr. Pamela Gay: 

2025 R2 Swan.

Fraser Cain: 

Yeah. So Comet Swan is, is up in, is going to sort of pass up through Hercules. So it’s a very high, very easy comet to spot.

Lemon is going to stay very close to the horizon.

Dr. Pamela Gay: 

Yeah.

Fraser Cain: 

So you’re going to need a good view to the West to be able to see it. Both should be visible near the end of October and into early November. You just, you kind of need to know where to look and have some, have some clear skies, but really a pair of binoculars and a small telescope will get you to this place where you can see just a hint, but you should feel angry while you’re looking through the binoculars at these comets and going like really universe, this is the best you can do.

Dr. Pamela Gay: 

Fine. Yeah. Yeah.

And, and even though Hale-Bopp is a periodic one, it has an orbit of hundreds of years. So, uh, yeah, we’re not going to get to see it again.

Fraser Cain: 

Right. In our lifetime.

Dr. Pamela Gay: 

Right.

Fraser Cain: 

Unless we get a robot bodies. Yeah.

Dr. Pamela Gay: 

Yeah.

Fraser Cain: 

But who knows? Right. You could be listening to this episode I’ll be here from now.

And there was the three great comments that happened in December.

Dr. Pamela Gay: 

Exactly. Exactly.

Fraser Cain: 

Right. The three great comments that happened in May and we’re all still talking about them. And that one time that the comet passed right through the atmosphere of the earth, that was scary is, is a thing that can happen.

Dr. Pamela Gay: 

Yeah. Yeah. And, and here’s the thing.

We suspect that perhaps the Tunguska event was, uh, a object that had a large volatile load. So it ended up having an air burst. Um, I’m not saying it’s a comment.

I’m not saying it’s an asteroid. I don’t think that debate will ever be solved, but it had enough volatiles in it to have this amazing air burst that flattened trees in spectacular ways. Um, there, there’s this, this hope that we can get a non-destructive really bright kind of event in our lifetime.

So I’m not saying please hit the earth. I’m saying please blow up in our atmosphere, uh, in places where there are monitoring cameras like they have out in Australia, but not a lot of life forms.

Fraser Cain: 

Are you sure you’re not a supervillain? No. Okay.

Dr. Pamela Gay: 

Um, it’s, it’s entirely possible. I’m also team. Please hit the moon for IR4.

Fraser Cain: 

Right. Yeah. But, but if YR4 hits the moon, causes debris that could take out a bunch of worth it maybe for the, for the mother of all meteor storms.

Dr. Pamela Gay: 

Well, and, and it’s like a 50 meter asteroid. So it will create a roughly one kilometer crater, which is right at the edge of what a human eye can, can make out. It’ll appear as a point source.

And I now need to figure out what will the phase of the moon be at the time of impact? Like, do we have enough information to know that? And if we do, then comes the question of if it’s in the shadow, can we see it?

And I’m actually working out the math for this. I meant to finish doing it yesterday, but we went down a neutron star rabbit hole. Um, so, so patrons, the $5 a month and higher level, we have a new, that takes math series that you can join me for.

Um, so yeah, I can consider doing that. Um, but comets, I mean, the real thing that we just haven’t hit on nearly enough is the more research we do, the more we’re realizing that things like landslides and other disruptions on the surface of these objects, cause them to form comas, cause them to periodically grow tails. It is one of these things where comments being not as structurally sound as one might hope for causes them to put on amazing light shows.

And if they’re big enough and the disruption is small enough, we get to enjoy the light show and have a tail later to see because the comment doesn’t fall apart completely.

Fraser Cain: 

Yeah. So, uh, you know, now we’ve reminded the universe what it’s, what it’s possible, what it’s capable of. It knows how it’s falling short of our expectations and we hope no demand demand demand, uh, better comments.

So how’s that coming universe? Thanks, Pamela.

Dr. Pamela Gay: 

Thank you, Fraser. And thank you to all the patrons who support this show. This show wouldn’t exist without the amazing support of so many of you over on patreon.com slash astronomy cast. This week, we would like to thank in particular, a pronounceable name, Alex rain, Astro Zets, Bart Flaherty, Bresnik, Brian Cagle, Cami Rassian, Cody Rose, Cooper, Daniel loosely, David Gates, Dwight ilk, evil milky, Frank Stewart, Hal McKinney, John Baptiste, Lamar, Jim of Everett, Joe Holstein, John Drake, John M. John days, Justin Proctor, Justin S. Christian Magerholt, Kinsaia Penflanko, Les Howard, Lou Zealand, Lona Spencer, Mark Phillips, Matt Rucker, Matthew Horstman, Matthias Hayden, Michael Wichman, Mike Heisey, Nick Boyd, Olga, Pauline Middlelink, Ruben McCarthy, Ryan Amory, Scott Bieber, Sean Matz, Sergei Monolov, Slug, Stephen Veidt, Stephen Miller, TC Starboy, the mysterious Mark, Time Lord Iroh, Travis C. Porco, Will Field, William Andrews. Thank you all so very much. I read your names pre-recorded.

Rich will insert them. And yeah, that’s a thing I do now.

Fraser Cain: 

Right. All right. Thanks, everybody.

Thanks, Pamela. And we’ll see you next week.

Dr. Pamela Gay: 

Bye-bye, Fraser. Bye-bye, everyone.

Live Show

You can only describe a black hole by its mass and its spin. And maybe it’s charge. But allow us to propose a new criteria: the personal experience. Some black holes have seen things… Experienced the laws of physics at their most extreme. And today we’ll tell their stories. The more of the sky we observe, the more bizarre situations we find black holes in. Let’s explore!

Show Notes

• AI space content quality problem
• Star–black hole interaction behind SN 2023ZXD
• How BH–core mergers can trigger supernovae
• Accretion beyond the Eddington limit (e.g., LID568, ~40×)
• Mass gap challenges (e.g., GW231123 intermediate-mass BHs)
• Recoil kicks/ejected BHs (e.g., GW190412, ~50 km/s)
• BH size extremes: IGR J17091 to ~36-billion-M☉ candidate
• Primordial black holes: formation & detection prospects
• Early-universe BH/galaxy formation: mergers + direct collapse
• Science evolves: new data reshapes theories

Transcript

Fraser Cain: 

Welcome to Astronomy Cast, our weekly facts-based journey through the cosmos, where we help you understand not only what we know, but how we know what we know. I’m Fraser Cain, I’m the publisher of Universe Today. With me as always is Dr. Pamela Gay, a senior scientist for the Planetary Science Institute and the director of Cosmic Quest. Hey, Pamela, how are you doing?

Dr. Pamela Gay: 

I am doing well. I’m trying very hard not to burst out laughing at all the stuff our audience members are sticking in the chat, because right before we went live, we were talking about AI slop, and some of it is so bad, you can laugh or cry, and I usually do both. Both is a good answer.

Fraser Cain: 

Yeah, the recommendation that I make is go open up Chrome, open up a new incognito window, go to YouTube, do a search for whatever space topic is interesting to you, comets, space, James Webb, whatever, and you will see the sloppiest slop that ever slopped. And it is just, this is what YouTube is now showing to people when they come to their website, and not more legitimate science. So, what are you going to do?

You can only describe a black hole by its mass and its spin, and maybe its charge, but allow us to propose a new criteria, their personal experience. Some black holes have seen things, experienced the laws of physics at their most extreme, and today, will tell their stories. All right, you’ve got a bunch of stories queued up that are extreme black holes experiencing extreme experiences.

What’s your first one?

Dr. Pamela Gay: 

So, this is my so far favorite story of the year. A young 30 solar mass star was hanging out with its more massive previously sibling and made the mistake of consuming its sibling, which was a black hole at this point.

Fraser Cain: 

Right. So, what was the sort of series of events that led up to this, I guess, unfortunate mistake?

Dr. Pamela Gay: 

Right. So, the two stars were born with very different masses. The one started its life with about 30 solar masses.

The other one was significantly larger. We’re not sure how much larger because mass loss is a thing. But however big it started out, it ended up making a 10 solar mass black hole.

So, we have a binary system with a 10 solar mass black hole and a 30 solar mass regular everyday star. And due to drag forces, they were getting closer and closer to one another. And the 30 solar mass star made the unfortunate mistake of consuming that 10 solar mass black hole, which led to the black hole rapidly feeding on the contents of the star, the star changing in brightness over time.

And ultimately, the core of the star and the black hole together triggering one heck of a weird supernova. This object is Supernova 2023 ZXD. When they looked at its light curve, its light curve didn’t behave over time the way you would expect.

They went back through archival data, saw the system had been brightening over several years. And the research paper that came out of this is a spectacular example of, could it be this? Probably not because of this.

Could it be this? Probably not because of this. As they work through idea after idea.

And the thing that fit was star 8’s black hole, which is the opposite of what we’re used to. But at 30 solar masses, it was definitely the dominant player in the system.

Fraser Cain: 

Yeah. Yeah. I think our title on universe today, the story was star eats black hole and instantly regrets it.

Dr. Pamela Gay: 

Yeah. We had very similar for EVSN.

Fraser Cain: 

Yeah.

Dr. Pamela Gay: 

It’s my favorite story this year. Maybe my favorite in many years.

Fraser Cain: 

Yes. Just gloriously wrong. And so what do we think would have happened?

Like, you know, we, back in the day there was the star, the star with 30 times the mass of the sun. And then there was a much bigger star.

Dr. Pamela Gay: 

Yeah.

Fraser Cain: 

It died first because the more massive you are, the shorter your life.

Dr. Pamela Gay: 

Right.

Fraser Cain: 

Detonated as a supernova and left a black hole remnant. And now you’ve got this binary star system. And then the two got closer and closer and closer until the black hole went into the star.

Dr. Pamela Gay: 

Yeah, that is exactly what happened. And it all comes down to the fact that, yeah, the black hole was definitely nibbling on its companion as frictional forces brought them closer and closer together. But once it’s inside that atmosphere, it’s definitely a merger event.

And it’s definitely the blue star, which has gotten stirred up and bloated out a little bit, doing the consuming. And while that black hole inside the 30 mass star is going nom, nom, nom, nom, nom, nom on everything around it, it took time for the core and the black hole to merge and explode as a supernova.

Fraser Cain: 

And what caused the explosion? Like, why did it go supernova? And why did the black hole not just gobble up the star from within and disappear it?

Dr. Pamela Gay: 

It all comes down to the rates at which things can happen. So the 30 solar mass star, which was no longer 30 solar masses, to be clear, mass loss also did it in. It had a core that was already fairly evolved.

It was already on its way to going supernova at some point in its future. And the black hole simply accelerated the process. So the black hole itself couldn’t eat all of its surroundings fast enough to prevent a supernova.

What it could do is begin that core collapse process that prevented further nuclear reactions from going on. A star is carefully balanced between gravity trying to compress the entire situation, light pressure pushing out, trying to keep the star being a star. But the second you don’t have all the nuclear reactions you need going on in the core, the outer layers of the star are no longer supported by light and they’re going to collapse down.

As they collapse down, the higher densities that they get just because they’re now becoming a crumpled ball of star, those higher densities allow new nuclear reactions to go on, which explode out light, creating the supernova we see. So it’s this process of you kill what’s going on in the core, the light shuts off, everything collapses, generates new light that supernovas out the outermost layers while the core collapses to form whatever it forms in the end.

Fraser Cain: 

All right, what have you got next?

Dr. Pamela Gay: 

Oh man, there are so many different things to choose from and I’m just not going to get through nearly as many as I wanted. So I’m going to talk about LID568. This is a black hole in a dwarf galaxy that is feeding at rates a star should not be feeding at.

So there’s this thing called the Eddington limit and it is how we say black holes should be limited in what they should do. The idea is that as material tries to stream in, it gets hot, it gets dense, it generates light. This is a recurring theme in the universe.

That light then pushes out the material, preventing further infall, cutting off the feeding frenzy the black hole is experiencing.

Fraser Cain: 

Right, and this is the same kind of limit that we see with stars. We don’t see stars that have a billion times the mass of the sun. We see stars with about a hundred times the mass of the sun at the most and that’s because as they get bigger, hotter, more radiation, at a certain point they’re just blowing away any other material that’s going to try to fall into them and they just can’t get any bigger.

And black holes can do the same thing with the accretion disk that builds up around them.

Dr. Pamela Gay: 

But for reasons that my read-through of the discovery paper didn’t see an answer to, this thing is feeding at 40 times the Eddington limit.

Fraser Cain: 

Right.

Dr. Pamela Gay: 

And so suddenly we are discovering that black holes are capable, in some circumstances, of consuming material much faster and more thoroughly in a rapid, sudden epoch of growth than what we had expected. And researchers are actually thinking this particular star got all grown up in essentially one massive feeding frenzy that broke the limits we’d previously attempted to put on these things.

Fraser Cain: 

Yeah. There was a couple of interesting things about this story. So they said, using this idea of the Eddington limit, they said, well, if you sort of rolled this black hole back to the beginning of when it formed shortly after the beginning of the universe, it would have had to have formed from a star that was about 10,000 times the mass of the sun was one option.

The other option is that it was a direct collapse black hole, that it just went straight into from cloud of gas and dust to black hole and then grew from there. And those would help you explain a black hole that you saw with this kind of size. But the sort of really interesting idea is that there are ways that black holes can maybe cool themselves down, that they can actually allow inflows of material that gets around this Eddington limit by artificially cooling the regions around them as well.

So there’s a lot of, you know, obviously the universe can do things that are weirder than we had ever anticipated. And this just shows like your theories are great, but at a certain point, you know, reality has some lessons to explain to us.

Dr. Pamela Gay: 

And this idea that black holes can form at masses that a stellar mass star going straight into a black hole can’t explain is something that comes up over and over again. So another example of things behaving weirdly is a LIGO discovery GW231123, which is beautifully symmetric. This particular paper had 10 and a half pages of authors.

I just want to call out collaborations are amazing and sometimes eat a lot of papers. So don’t print those things out. This particular merger was between a 103 and a 137 solar mass black hole.

So this is two intermediate mass black holes merging together into a larger intermediate mass black hole. And the thing about this is both of the black holes that went into the merger fell in what was supposed to be a gap in black holes formed through stars. What happens is as a star gets bigger and bigger and bigger, it eventually reaches the point where the core completely combusts the entire system.

This pair instability means that at lower masses, sure, it collapses down, has a black hole in the core. At higher masses, it can overcome this. Sure, collapses down, becomes a black hole again.

But in this gap, when it tries to do that, the helium in the core is like, I’m not going to have anything to do with that. And it explodes completely. So here we have a system.

Fraser Cain: 

There’s like no remnant, no black hole, nothing.

Dr. Pamela Gay: 

There’s nothing, nothing left behind, all gone. And here we have a system with two black holes that both fall into the black hole mass gap. And the question becomes, were we completely wrong on the mass gap?

Is there another mechanism for forming black holes? Or is this a statistically improbable system that was able to go from two things that formed black holes that merged, two things that formed black holes that merged, and then those black holes merged to get this eventual outcome? And the statistics on how hard would it be to get that double black hole binary system that forms, so you start out with a system of four black holes, and then you ultimately end up with one.

How difficult is that? And it’s not entirely improbable, but it’s at least 25% possible and 75% improbable. So probably not.

We’re learning black holes do not follow the rules, people. They do not care.

Fraser Cain: 

And we talked about a similar story, I feel like within the last year, same situation. Although one of the black holes shouldn’t have been. The other one was fine, but one of them was in that mass gap.

And so the fact that we’ve seen now two of them, maybe more, I’m not sure exactly how many LIGO has seen, is showing that either this theory that black holes aren’t formed from this certain mass of black holes, or that there’s some other mechanism. And the one that’s really exciting, which would also explain the previous story, is that you have primordial black holes. So you have black holes that were formed early on in the universe of any mass, some that were the mass of an asteroid, some that were the mass of a billion times the mass of the sun.

And then you can have any mass you like be able to meet with each other and merge. And you’re not dependent on that process of stars forming bigger black holes, meeting other black holes, meeting other black holes, and eventually building up that chain of events to get the kind of collisions that we’re seeing.

Dr. Pamela Gay: 

And this is one of those things where, again, I think the answer is going to be both. That we’re going to realize that, yes, there were primordial black holes, and there’s no other really good way to explain all the stuff JWST is finding in the early universe. That we’re going to realize that, in many cases, you have giant blobs of material that collapse straight into a black hole thanks to turbulence and other cooling factors that are able to take place.

It’s just like what we’ve found with galaxies. It used to be that people thought we were going to one day realize that it was either a hierarchical merging of smaller systems into progressively larger systems, or that it was clouds of gas that collapsed down into the size we see generally. And it turns out the answer is both.

That early in the universe, there were massive elliptical galaxies that formed, but there were also hierarchical growth that led to things like spiral galaxies we live in.

Fraser Cain: 

All right. What’s your next example?

Dr. Pamela Gay: 

So we’ve been talking a lot about massive black holes because, I mean, we all love massive black holes, but we’re starting to find small black holes. Now, neutron stars can’t, for questionable values of can’t, be larger than 2.25 solar masses because above that, the neutron degeneracy pressure can’t support the star and it collapses. Now, there’s always people who are moving that limit up or down by tuning different parameters in the system.

But around 2.25 solar masses is where the limit in the size of neutron stars is. And in looking at stellar mass black holes, there appears to be this gap where there’s like nothing around three or four solar masses. And it’s not entirely clear why there’s nothing in this particular mass range.

But we’re starting to find smaller systems. One of these is G3425. It has a mass that looks to be around 3.8 solar masses. It’s definitely beneath four solar masses. And it looks like part of the way it got that way was through having jets. So we need to figure out how do you take a truly massive object, have it collapse, leave behind only its core, and in the process, shed enough material that you end up with just the right size of a tiny core.

Fraser Cain: 

Wow. And so was this happening like as it was forming?

Dr. Pamela Gay: 

Yeah.

Fraser Cain: 

Huh. Very cool. I’ve got a story up here and let me know if this is in your list.

GW190412, the recoil event.

Dr. Pamela Gay: 

No, that one I don’t have anymore.

Fraser Cain: 

All right. All right.

Dr. Pamela Gay: 

The universe is big.

Fraser Cain: 

Yeah. So this is a black hole that was discovered in 2019 between two black holes. One was eight times the mass of the sun.

The other one was four times the mass of the sun. And this is one of the largest discrepancies in masses between two black holes. You had one that was double the mass of the other one.

And so the question that the astronomers were wondering was, would you get a recoil? Would you get a kick because you had this big difference between the black holes? And so they were able to sort of carefully examine the collision in the LIGO data and determined that, yes, indeed, after these black holes came together, because their masses were so different, it was kind of like a thruster on the side of the finished black hole that then gave it a kick at about 50 kilometers per second to the side.

So it was just because it was asymmetrical, as the black holes came together, then you got this kick, this recoil. And it happened inside a globular cluster. And so the recoil was enough to kick the black hole out of the globular cluster.

And so one of the big expectations is that the place to look for intermediate mass black holes is in these globular clusters, that you’ve got a lot of stars. They’re very close together. A lot of them were massive.

They died, became black holes, black holes merged, and so on. But they have found a few, but not as many as that they were expecting. And so now, maybe, over the course of enough time, a 50-kilometer-per-second kick is going to push your black hole right out of your cluster.

And so it might be that these black holes, they’re forming, if they’re not exactly equivalent masses, you’re going to get these kicks. These things are going to be spat out into the larger galaxy. And a lot of these globular clusters are outside of the plane of the galaxy.

And so they’re just spat out randomly into the universe.

Dr. Pamela Gay: 

Conservation of momentum is a bear. This is literally the black hole equivalent of what happens when, don’t do this with human beings, your AI-driven three-wheeler collides with your AI-driven SUV, and you end up going primarily in the direction of the SUV’s mass. Yeah.

Okay, that is impressive. So let’s look at the other extreme. This is my favorite discovery story of a black hole.

It wasn’t behaving badly. It was just behaving bigly, to use a word that I shouldn’t. Extremely, yeah.

Yeah. Hugely. Its distance is 5 billion light years.

Its mass is 36 billion solar masses. And it’s in the cosmic horseshoe galaxy. And what was happening is this really massive galaxy was getting used to study gravitationally lensed background objects.

And they were working to try and deconvolve what’s going on with all these gravitationally lensed things. And used the Hubble Space Telescope to do some very sophisticated observations of the core of the galaxy. And it was enough to be able to measure the motions of stars and gas down in the core of the galaxy and get the mass of the central black hole.

Normally, you can’t do this with any but the closest of the massive galaxies. But this black hole is so unbelievably large that even at 5 billion light years, they were able to use spectroscopy to measure velocities. And it’s just, yeah, it’s just awesome.

Accidental science is the best science sometimes.

Fraser Cain: 

Yeah. And I think a lot of people were listening to this episode. You’re familiar maybe with TON618, I think is the name of the black hole.

And that was largely considered the most massive black hole that’s ever been seen in that same kind of region. And now it looks like this new cosmic horseshoe is a serious contender, perhaps the winner of the most massive supermassive black hole in the universe that we’ve ever found.

Dr. Pamela Gay: 

It’s definitely the winner of the best discovery story for a massive black hole.

Fraser Cain: 

Right, right, right. But yeah, it is a contender for the most massive, the biggest. Check the Guinness Book of World Records shortly.

And that might be the black hole that’s in there. What else you got?

Dr. Pamela Gay: 

So that was really my list of favorites. Um, the star eating the black hole. I mean, there’s a few others out there.

Chandra found IGR J17091, which is a contender for being the smallest black hole. It’s somewhere between three and 10 solar masses looking closer to three than to 10, but they don’t have enough data to say for certain how tiny it is. And I just want to point out that we are now looking at things that are ranging from between IGR J17091 and G3425, which are both down around three solar masses.

And then the cosmic horseshoe galaxy central supermassive black hole, 36 billion solar masses. We’re looking at roughly a 10 billion solar mass factor, like multiplier of 10 billion between the smallest and the largest we’ve seen so far. And there’s absolutely no physical limit on how small or large a black hole can be.

The only limits are how do you form it? And, and this is where to get the small ones, we need to master mass loss to get the biggest ones. We need to master mass inflow and physics likes to break both of those things.

And so primordial gets us, okay, so what leftover small things are floating around our universe.

Fraser Cain: 

So speaking of that, um, I’ve got one last story then before we can close this out. And this is a prediction. So this idea of primordial black holes, we brought this up earlier.

One of the, if these things are out there, then it could have been formed at any mass and the, um, the smallest ones, the least massive ones will evaporate and have already evaporated. And so there is a sort of the smallest possible black hole in the universe is the one that is about to evaporate.

Dr. Pamela Gay: 

Yes.

Fraser Cain: 

And so people have, have done some calculations and that if this is happening, our modern neutrino observatories should, with about a 90% chance detect the presence of this last gasp. When a black hole evaporates, you get this sort of burst of high energy radiation neutrinos and that we now have detectors that are sensitive enough to find this. And so within about a decade, if, if this actually happens, we should detect the burst of a black hole evaporating.

And so that’ll tell us whether or not these primordial black holes exist.

Dr. Pamela Gay: 

And, and the only thing we have to remember is this could very well be like all the predictions of protons decaying, where we have these theories that say this thing should be happening and then we just never see it. And, and this is the reason we keep doing science is we are building a picture of a universe that when we create our science has to conform to everything we know is in the picture. It makes predictions about things that haven’t been seen yet, but until we see everything, which we’re never going to be able to do, we can’t fill in all the details and we’re going to periodically be wrong.

And that’s kind of awesome because that means there’s new physics waiting to be discovered.

Fraser Cain: 

I’m sure this will not be the last time we’ve done a story of an episode that is about this kind of thing that we’re going to find exoplanets, black holes, galaxies, things doing extreme things. Thanks, Pamela.

Dr. Pamela Gay: 

Thank you, Fraser. And thank you so much to everyone out there who is watching us live. And especially thanks to all of our patrons.

Thank you all so very much.

Fraser Cain: 

Sounds good. All right. Thanks, Pamela.

We’ll see you all next week.

Dr. Pamela Gay: 

Bye-bye.

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