Another update episode, this time we look at what’s new and changed in the research of black holes. And it’s here that we find a lot of substantial new discoveries in the field, so much has been discovered since we first covered black holes a decade ago.
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What is a black hole?
Black Holes Explained – From Birth to Death – YouTube
Black Holes: Facts, Theory & Definition
Flavors of black holes
What are Gravitational Waves? | LIGO Lab | Caltech
Gravitational-Wave Detector Catches Lightest Black Hole Smashup Yet
Astronomers May Finally Have the First Picture of a Black Hole
Gaia data: This 3D Color Map of 1.7 Billion Stars in the Milky Way Is the Best Ever Made
Podcast Transcription provided by GMR Transcription
Fraser: Astronomy Cast, Episode 489. Black Holes Update. 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. My name is Fraser Cain. I’m the publisher of Universe Today. With me, as always, Dr. Pamela Gay, the Director of Technology and Citizen Science at the Astronomical Society of the Pacific and the Director of CosmoQuest. Hey Pamela, how are you doing?
Pamela: I’m doing well. How are you doing, Fraser?
Fraser: Great! Just mentioning in the pre-show the weather has just turned. My tulip game is strong. It’s like 26 degrees yesterday. It just feels like all the happiness has returned to the world.
Pamela: We’re still down around 24, 25 but there is the most amazing flowers out there. I’m gonna be posting pictures on Instagram so stay tuned. And I may actually up my Flickr game because I need some place to store all the photos I’ve been taking.
Fraser: Well, now Flickr’s been bought, you know.
Pamela: I know. I know. It’s now SmugMug. But –
Fraser: But Stewart Butterfield, founder of Flickr, posted on Twitter that he approved of the purchase. So, I think – a lot of astrophotographers still use Flickr as one of the best places to be able to store all your photos. And it looks like they’re under some good hands now as opposed to adrift under the Yahoo mantle. So I think it’s a good move. So, go ahead. I permit you to post your pictures.
Pamela: I shall do this with or without your permission. But I appreciate it anyways.
Fraser: I figured you might. Alright. Another update episode. This time we look at what’s new and changed in the research of black holes. And it’s here that we find a lot of substantial new discoveries in the field. So much has been discovered since we first covered black holes – ready for this? – a decade ago. More.
Pamela: It’s been more than a decade.
Fraser: Because it was one of the first – again, right – when we started up Astronomy Cast we went after all the low hanging fruit. So, our first episodes were like Extra Solar Planets: Why isn’t Pluto a Planet, Black Holes, Dark Matter, Dark Energy. Here we are 489 episodes into the show and it’s time to come back around and take another look at black holes.
Pamela: All the things I enthusiastically said were true that we now know aren’t.
Fraser: I want to get to the things that you were skeptical about that actually are. So, we’re gonna be able to see both sides of this. But, please proceed. Where would you like to start on what’s new about black holes – do we have to even say what a black hole is? Do you think we can just skip right past that part?
Pamela: We probably need to explain it because we do have new listeners now and then.
Fraser: Okay. Briefly, yeah. What’s a black hole?
Pamela: A black hole is any object that has smushed so much mass into such a small volume that you can get close enough to it that your escape velocity exceeds the speed of light. So the earth is person hole. I can’t jump up and escape our planet from the surface of the planet. Now, if you stuck me a couple thousand miles above the surface and somehow I was able to jump off of a non-existent surface up there, I could escape and the earth would no longer be a human hole. That sounded wrong. We’re going to move on –
Fraser: Classic Pamela.
Pamela: – but – but with a black hole if you took our sun and you squished it down to three centimeters and attempted to stand on its surface you would, first of all be squished to death, and second of all you could not jump off even if you were a photon of light because the sun would have become a black hole at three centimeters in radius.
Pamela: So, all it means is escape velocity greater than the speed of light.
Fraser: Right. And actually at that point no velocity will take you away from a black hole. Even if you could go faster than the speed of light all roads lead into the singularity.
Pamela: But you can’t go faster than the speed of light.
Fraser: Of course not. Yeah. And even if you could it wouldn’t help you. So. But, yes, you can’t go faster than the speed of light.
Pamela: It might help you.
Fraser: But people always sort of imagine black holes as these – you know, the name is terrible, right?
Fraser: Because it’s like, “hole.” And even when they show these simulations of what a black hole looks like and you see this sort of curved space time going down to the singularity your brain thinks, “Oh, it’s like a thing that you can jump into and then you would emerge in – I don’t know – some kind of library that would allow you to see your whole lifetime and communicate with…” Anyway I’m not gonna –
Pamela: I loved that movie up until that part. And then I raged.
Fraser: That’s the part where Kip Thorn just sort of went, “I am done with this movie.”
Pamela: Yeah, something like that. So, in our solar system were we to suddenly squish the sun, via alien means that we do not have, down to only being three centimeters in radius and somehow keeping it that size. Then, while it would no longer be emitting light in a way that is as useful for plants and humanity on our world, our planet would just keep on orbiting the way it’s orbiting right now because orbits depend on the mass of the object, not the size of the object, as long as you’re above their surface. So, black holes don’t go around eating things. That’s just not a thing they do. They wait for things to stumble into them. They’re kind of the alligators of the cosmos.
Fraser: Yeah. And people always ask when is the black hole, the middle of the Milky Way, going to gobble up everything in the Milky Way and the answer is it’s not going to.
Fraser: And if you – and as you said, you replace the sun in the solar system with a black hole of the same mass and the planets will gladly orbit around it for billions of years and everything will be fine. No problem.
Pamela: It will be very cold. There will be problems.
Fraser: It will just be cold. Yeah. Yeah, because black holes make terrible stars. So, fine. And there are a couple of varieties of black holes, a couple of flavors.
Pamela: Yes. So, we always imagine that there’d be this nice friendly continuum where you have the stellar mass black holes which do exist – they do exist. These are what happens when your everyday, overly large star that was either born that way or it ate it’s companion – we don’t care which – decides to go supernova and leave behind so much mass that a neutron star cannot form because the neutrons cannot push each other far enough apart.
So, we start with your everyday 30 mass star that undergoes a certain amount of mass loss and by the end of its life dies as a black hole. Or you a take a white dwarf neutron star and you radically eat its companion – mergers – all these different things – however you do it, you can form a stellar mass black hole. And, as far as we know, these things go up to tens of times the mass of the sun – maybe a hundred or so times the mass of the sun – these suckers are found.
Then we have also identified – and this is where I was so wrong the last time we did this episode – we have identified that some, but not all galaxies, have in their core supermassive black holes that start to get to be tens of thousands of times the mass of the sun.
Pamela: Yeah. The exact mass is directly proportional to how big the bulgy center spheroid of stars is. So, in our own Milky Way, if you look at it from the side, it has this large spheroid in the center. That spheroid is related to the velocity of the stars in that spheroid and it is also related to the size of the supermassive black hole in the center. And we do not know which came first, the velocity of the spheroid or the black hole. And they probably formed via chaotic processes at about the same time.
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Okay, great. So now everyone’s caught up. And then now we’re gonna move into the ‘what do we know that’s new’ and a lot of this is the search for other kinds of black holes as well, right?
Pamela: And the realization of, “Oh expletive, it turns out not all large galaxies actually have black holes in the center.”
Fraser: Wait, so how were you wrong? Had you said that they did have them?
Pamela: We thought – when we recorded the last one of these episodes a decade ago – we thought that all large galaxies, and maybe even all galaxies – although the dwarf spheroidals were baffling us – all large galaxies had supermassive black holes in the center. This is what we thought. We were wrong. It turns out that the super flat spirally ones that do not have a bulge, do not have this spheroid – the flat ones like M101, the pinwheel galaxy, appear to have absolutely no black hole in their center.
Fraser: So, where do they go?
Fraser: Well, I guess the question is do we know why there are galaxies that don’t seem to have a supermassive black hole in the middle of them?
Pamela: They seem to have figured out how to form in a much less chaotic way. This means that if the normal hierarchical way of forming galaxies where you take a couple baby galaxies and you throw them at each other just right and they become a bigger galaxy and then you throw in a few more baby galaxies and you end up with a bigger galaxy – it appears that if you do this just right, maybe – we’re not sure. This is what we’re guessing – you can end up slowly and carefully building a perfectly flat galaxy that doesn’t have a bulge and doesn’t have a supermassive black hole in the center.
Fraser: And the other thing is that they can have collisions with other galaxies and the black holes can get kicked out.
Pamela: That’s true, but if they’d had collisions with other galaxies we wouldn’t expect to see the amazing structure that we’re seeing. So, M101, for instance, the Pinwheel galaxy, is this grand design spiral with extraordinarily open, well-formed arms. It has extensive amounts of star formation going on. This is a dusty system that you wouldn’t expect to see this kind of grand design spiral without any bulge, without any warp to the – well, we aren’t in a plane to be able to see if there’s a warp – you wouldn’t expect to see this kind of structure if there’d been significant collisions in the past. You’d expect to see some sort of a puffing out.
Our own Milky Way galaxy, part of the reason we have this, what we call a thick disk, this thickness to the disk of our galaxy, is from collisions that have added velocities, added energy that puffed up the disk.
Fraser: So, I guess errata update to our previous show, most galaxies have supermassive black holes in the heart of them.
Pamela: Most large galaxies and, as far as we know, all galaxies that have a spheroid structure have massive to supermassive black holes in their center.
Fraser: Okay. Alright, what else is new?
Pamela: So, when I start talking about black holes I said we have stellar mass and I said we have supermassive black holes. But there’s this whole range of masses between a hundredish solar masses and tens of thousands to millions of solar masses. And in that hundred to one million solar mass range this is what we call intermediate mass black holes. They are hard to find, but we’re starting to find them.
Fraser: But it’s even fairly indirectly, right? Like the place that they’ve been looking for these intermediate mass black holes is in – man, you guys have wrecked my brain – globular clusters.
Pamela: Come on. Say it right for us.
Fraser: Globular clusters. So that they have now – they haven’t necessarily seen them directly, but they’ve teased out mathematically that this intermediate mass black holes have to be in there.
Pamela: And they’re starting to find indications they may exist in really interesting places. So, it looks like they might exist in globular clusters. As you were saying, it looks like they may exist in some of the smaller mass baby galaxies. So, when you’re out there looking around, if you look at, for instance, a spiral galaxy NGC4395 it looks like it has a ten thousand mass black hole in its center.
So this is where I corrected my earlier to statement to say massive but not supermassive are being found in the centers of galaxies. And this whole idea that globular clusters which don’t really fit anything – they’re kind of their own weird creatures – that they could also somehow be related to this spheroid mass function for black holes is kind of awesome.
Fraser: And so I would say if we went back and had this conversation about intermediate mass black holes ten years ago, we would have less evidence for them then. There would have been no observations, no data indicating that they’re there. And now we have slight data that they’re there but no real smoking gun observations that they exist yet.
Pamela: No. But the gravitational wave science is starting to give us tantalizing results.
Fraser: And this is – as I was mentioning this is where I think the science has proceeded a lot faster than I think you had ever expected that it would.
Pamela: It’s true.
Fraser: I’m ready for you to recant.
Pamela: Oh, no. I’m not recanting.
Fraser: Okay. Alright, well then – eventually there will be a time when you will recant.
Pamela: I’m gonna maintain that the first ten years of LIGO funding was too early.
Pamela: I have strong opinions.
Fraser: Okay. Okay. But let’s talk about the amazing exciting news of black holes colliding together.
Pamela: Some of the gravitational waves that have so far been detected, thanks to the great expansions that have recently occurred to the global network of gravitational wave detectors, they’re finding things that look like they can be explained through the collision of intermediate mass black holes. So this could be that smoking gun for perhaps primordial black holes, black holes that formed, basically, in that period of time between moment zero and the release of the cosmic microwave background – which kind of sounds like you’re releasing the hounds. So that’s now how I’m gonna imagine the photons.
Fraser: And so obviously LIGO, with the gravitational waves, they’ve – I think they’ve detected five black hole collisions at the time we’re talking about right now –
Fraser: The masses of these black holes are in the dozens of solar masses. I think the biggest is in the 30 range. So these are not intermediate mass black holes and this is really just a limitation of what LIGO can do. It’s not that those black holes aren’t out there. It’s just that LIGO and other instruments can’t detect them yet.
Pamela: And people do want to argue, every time we start finding black holes that are 30 solar masses and these multiple tens that maybe, maybe we can start to say that this definitely says that they do form this big. So, this is where it’s tantalizing evidence of larger black holes and a way to detect them in the future.
Fraser: And so is 30 times the mass of the sun, say, is that a surprising mass for a black hole that could come from a regular star collapsing in on itself?
Fraser: Because I remember way back to our supernova – and we should totally do an update on supernovae – when I think about our supernova show that the biggest ones, they just detonate entirely.
Fraser: You only get those black holes from this middle range of sizes. So, is 30 starting to kind of break what we thought was possible?
Pamela: Yeah. And the way it’s breaking it is we know that there’s these massive stars, like the pistol star, that form out of large, large, large amounts of material, but then they undergo huge amounts of mass loss. They’re blowing these high power winds. They’re spewing matter out as they do this. As they go through their life they’re gonna just keep shedding mass, keep shedding mass, keep shedding mass.
When they go supernova the part that becomes the black hole is what’s left after you explode out most of your atmosphere and after you’ve gone through this few million years of massive amounts of mass loss. So the question is – and we’re still trying to come to terms with mass loss rates – the question is how much mass can be retained by the end of a star’s life. And there are a few people out there that are arguing that it’s gonna be a number under ten. So, this starts to be well, what other ways do you make these big black holes?
Fraser: And you could collide black holes together obviously.
Fraser: Smoosh two black holes together and you get a bigger black hole And then smoosh more together and you get an even bigger black hole. And so it’s not like – what was surprising was the millions of times the mass of the sun for a while there people couldn’t understand how you can get a black hole that big that quickly.
Fraser: And this is one of the other mysteries that we had talked about – I know it was sort of mid-way through our run – this idea of which comes first, the supermassive black holes or the galaxies around them. Because the two do seem to be kind of locked together in some way. And now, it really kind of looks like that mystery has been mostly solved. That it is the bottom-up process as these galaxies come together, not the top-down way. So –
Pamela: It’s looking like both actually. So there is evidence that the earliest black holes – not black holes. Well, them too – the earliest massive elliptical galaxies may have formed through a chaotic collapse process where it was turbulence in the collapsing cloud of material that allowed both sufficient cooling to allow that kind of a collapse to take place and also allowed the generation of the supermassive black hole.
So, if you’re trying to figure out how do we end up with massive elliptical galaxies in the first few million years of the universe and how do you end up with all the star forming galaxies that we see today, you need to have two different answers. And this is where it looks like everyday galaxies like our Milky Way are this bottom-up – let’s just slam things together gently, conserving angular momentum and such things as that, but let’s also allow within these dark matter halos to have this turbulent, fabulous collapse down to form massive ellipticals that died young, died fast. James Webb, should it ever launch, will allow us to hopefully observe these.
Fraser: Yeah. So we talked about one class of missing black holes that we’re starting to see some evidence. The other one that was theorized is this idea of primordial black holes.
Fraser: Any evidence for that?
Pamela: So, this is where it comes down to what are we seeing with the gravitational waves. If you look at a list of all the black holes that we’re like, “Yes, that is a black hole” with a couple of weird exceptions that we’re not sure about the data on yet, it appears that all of them that are bigger than 15 solar masses are detected through gravitational waves.
And if, by looking around in huge amounts of detail we’re finding things that are seven solar masses, ten solar masses, four solar masses. But with gravitational waves we’re finding 36, 31. There seems to be this segregation in what’s being found. So, it could be that these 30, 36 solar mass black holes that we’re finding through gravitational waves may be the smallest of the intermediate mass black holes that may have been primordial black holes left over from prior to the release of the cosmic microwave background.
Fraser: But what about the little guys? Like the ones with say, the mass of a house or the mass or an asteroid?
Pamela: Yeah, we can’t find those yet.
Fraser: And this has been theorized to be one of the explanations for dark matter, except now nobody believes that.
Pamela: Right. So the problem with trying to blame tiny baby, like under the mass of the sun, black holes on dark matter – or blaming dark matter on them – is if Hawkins was right, then they should be evaporating. And all the baby, baby, microscopic, tiny, tiny, tiny ones should have gone poof early on.
Fraser: And the ones that are a little bigger should be going poof right now. We should see some kind of background radiation from black holes evaporating and we don’t see that. So –
Pamela: We’ve never seen the right color of poof.
Fraser: Right. So if they are out there they are more massive, but less massive than black holes. Still a fascinating thing to be looking for. Okay, you know what would be great? An actual photograph of the event horizon of a supermassive black hole. How’s that coming?
Pamela: We’re working on it.
Fraser: They took the picture almost a year ago. And people won’t stop nagging me.
Pamela: It’s not your fault.
Fraser: So, I’m gonna ask you when do we get to see a picture from the Event Horizon Telescope?
Pamela: Not my responsibility. I’m just happy the Gaia dumped data yesterday.
Fraser: That is mind-bending. So, let’s tell people what the Event Horizon Telescope is. And so maybe in a couple of years we can give that update.
Pamela: So, a whole lot of astronomers pooled a whole lot of resources to simultaneously try – try being the optimal word – to observe the event horizon of a black hole. When they did this they were using radio telescopes. This was very long baseline interferometry. You group together a whole bunch of radio dishes all over the world, different countries, different organizations, different kinds of telescopes. You very, very carefully record the time that you are observing or you are condemned forever to not be able to match up the images.
You then combine all the data. And you can only do this because radio wavelengths are sufficiently long that we can record them and actually track their modulation with the waves. So, you record all the data. You then send it all to one central place. You do what’s called fringe finding, which is aligning everything so that all of the telescopes are essentially observing the exact same incoming waves at the exact same time. And when you are done combining all of this data, and you have yelled at somebody because their clock was off, you then have what is the highest resolution possible image for modern technological capabilities.
Their goal was to look at Sgr A star. This is the black hole in the center of the Milky Way galaxy. We’re still waiting.
Fraser: One of the challenges that I think is so great was that they used a telescope in Antarctica to assist with taking the picture and they couldn’t even retrieve the data from the Antarctic telescopes until the Antarctic winter had ended. And so that was in mid-fall of 2017 was when they could even just get those – they could make the first flights to start carrying hard drives out of Antarctica. And so now it’s just a tremendous computational challenge to pull this together. And I think it’s important to sort of prepare people emotionally for what they’re gonna see.
Pamela: And it’s probably not gonna be very much.
Fraser: It’s gonna be a little blob that astronomers are going to be oohing and aahing over.
Pamela: I’m hoping it’s more exciting than that. So we’re at – as we record this – just past one year from when the observation was made. And I know when I worked on my master’s thesis – and admittedly I was one dumb student because as graduate students we all qualify at our beginnings as one dumb student – I know I re-reduced my data multiple times. And it was only at the end of two years that I was completely confident that everything was reduced in a completely consistent manner that could not be improved upon.
They’ve had their data one year and they have data coming from multiple countries, multiple facilities that all have different clocks – and I cannot stress how annoying it is when the clocks aren’t the same –
Fraser: Even spacecraft.
Pamela: Yeah. And so they have to master the data reduction process for all of these telescopes in a self-consistent way, then combine all of the data, do the fringe finding, the whole nine yards, and then decide that they like it enough to share it.
Fraser: Alright. Are there any new interesting discoveries in the field of black holes that you wanted – that you have dug up?
Pamela: For me the big one has just been that not all massive galaxies have black holes. I just can’t tell you enough how weird that is that there’s this relationship between is there a bulge, then there’s a black hole, is there not a bulge, well, how do you form that without the chaotic processes that lead to the kind of turbulence that give you a black hole.
Fraser: I think for me, having reported on all of this stuff over the last ten years as well, and probably we have contributed a thousand articles to Universe Today about black holes, and it’s just tiny, little, incremental, interesting discoveries. More evidence for intermediate mass black holes, seeing gas that’s about to fall into the supermassive black hole at the Milky Way, as you said, starting to find out that there aren’t supermassive black holes in other places, obviously all the collisions that were discovered with LIGO.
So it’s very much – I think we’re in this really mature stage, much more mature stage, of the black hole side, of being able to at least make observations of these things. Which – when you think about it, right, a think that absorbs all light and energy and matter that falls into it, it’s very hard and quite a technological, telescopic feat to be able to even see these things. So, just kudos to all of the astronomers that are making really great strides forward in just being able to observe these things when it’s such a tricky subject.
Pamela: And all the advances that are being made in gamma ray astronomy, in x-ray astronomy. It’s finding that high energy light that lets us find these super compact object. So kudos to all of you who are collecting so few photons that you can name them, that are helping us find black holes in the process.
Fraser: Yup. Alright. This was fun, Pamela. We’ll see you next week.
Pamela: Sounds good. See you later, Fraser.
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Duration: 31 minutes