Hi, good morning, good evening, everybody, and welcome to SETI Live. My name is Simon Steele. I'm Deputy Director of the Carl Sagan Center at the SETI Institute, located in sunny Mountain View, California. And I have the pleasure of talking today with Max Herbler. Max, welcome. Good evening. You're based across the world at the Max Planck Institute in Heidelberg, I believe. Yes, that's correct. Thank you very much. Hi, welcome.
Tell us a little bit about the Max Planck Institute as a center for astronomy. So the Max Planck Institute is part of the Max Planck Society and the Max Planck Society has I think more than seventy institutes spread all over Germany that focus on very different types of basic research. But the Max Planck Institute for Astronomy located in Heidelberg focuses on observational and theoretical astronomy and astrophysics and does research in several of these directions.
Okay. How many astronomers and scientists do you have there approximately? I think we are now more than three hundred. So it's quite a big center for astronomy. Must be one of the biggest ones in the world, I imagine, at that size. So that's quite a quite a dynamic environment, probably. Yes. And in Heidelberg, there's also the university. So there's lots of interaction with the astronomy institutes of the university as well. Right. Cool. Thank you. So welcome, everyone.
We're going to be talking about black holes, not the big ones and not the small ones, but the ones in the middle, medium sized black holes and why they're very important and why this discovery is very important. Do tell us where you're tuning in from and we'll give you a shout out. And also, if you do have any questions, please put them into the comments and we'll get around to your questions a little bit later on.
First of all, Max, tell us, you know, Everyone's familiar with black holes, although they are very, very weird things. We've known about small black holes for a while. Supermassive black holes have been in the news. What's special about medium sized black holes? Where does this fit in apart from the mass to the others? So as you say, we have observed black holes in basically two different, very separate categories. We have stellar mass black holes.
where we think they form by the death of massive stars, and they have masses of up to a hundred solar masses. We can observe them as X-ray binaries, so we see sources with a strong X-ray emission, or more recently, we have now also observed them in large numbers by gravitational wave events, which has been a very exciting development.
And then on the very other end of the mass range, a thousand times more massive, we have supermassive black holes, which are typically found in the centers of galaxies. and they weighed at least a hundred thousand solar masses. But between a hundred and a hundred thousand, so in a very large mass range, there have only been very few debated detections. And so this is the elusive intermediate mass range.
Okay. And you mentioned a couple of ways you can find stellar mass black holes, X-ray binaries and gravitational waves. it's not easy to find black holes unless they're affecting their environment. I mean, by their very nature, black holes are invisible by themselves. How, you know, we'll talk a little bit more, how do you use stars, as we'll talk about Omega Centauri in a moment, how do you use stars to detect something that's invisible, which is what your work was?
so so as you say we we never see the the black hole directly but the only um possibility we have to to learn more about it is to look at everything that's around it and that is influenced by the strong gravity of um of any black hole and so if you have stars in the vicinity of um of the black hole these stars um show typically faster motion than if there would be no black hole.
And there are situations in which we can observe the stars orbiting the black hole, just like the Earth, for example, would orbit around the Sun. It's a more massive object. It's interesting. It's the same physics of orbits. As you say, the planets go around the Sun, the Moon goes around the Earth. Stars go around black holes. It's all the same laws, thanks originally to Johannes Kepler. And you can tell a lot about what you're orbiting by how the orbits happen, even if you can't see it.
So we're going to tuck in now to the object itself, Omicron Centauri. I want to call out some of the people who are tuning in. We've got Nicole from Boise, Idaho. Julia from Germany. We have people contacting us from rainy Scotland, Amarillo, Texas, Birkenhead in England, and Louisiana. Thank you very much for letting us know. A couple of Italy as well. Thank you for tuning in. Max, let's talk a little bit about Omega Centauri. And this is a globular cluster.
Can you say a little bit about what globular clusters are? Maybe we'll bring up an image of Omega Centauri to have a look at this thing.
so um in in the universe we can observe star clusters those are accumulations of stars and globular star clusters typically contain very large numbers of stars so the lighter ones hundreds of thousands and the more massive ones such as omega centauri can contain many millions of stars these stars are gravitationally bound and They typically form a spherical cloud of stars, so to speak, and all these stars are moving.
What is also interesting about globular clusters is that these are typically some of the oldest stellar systems that we can observe in the universe. So the stars in globular clusters are typically more than ten billion years old and that's are very interested in them because they can also allow us to study the early history of the universe. So these are, this is Omega Centauri. It's, it's a sort of, is this a Hubble image we have here?
Yes. It's a combination of several Hubble images and different colors that has been assembled. Right. And so we are looking, we're looking at the heart of this, this gigantic ball of stars of, of million stars. Swarming around, sort of like bees swarming around a honeypot type of thing. Where do you find globular clusters? Are there any particular location you'd find these things in the sky? So global clusters are typically scattered around the halo of a galaxy.
So, um, they're not, not concentrated in, um, in the discs, which are, um, most stars, but they are rather, um, in, in the area around it. And for example, the Milky Way has around a hundred and fifty globular clusters. But actually, Omega Centauri is the most massive one that we have observed in the Milky Way. And it's a little bit unusual with respect to other globular clusters. So the stars show a relatively wide range in ages and also chemical composition.
That's why Nowadays, many astronomers believe that Omega Centauri is not just a regular globular cluster, but that it actually might be the accreted core of a dwarf galaxy that has been dissolved, disrupted by the Milky Way through gravitational interaction. Right. We're going to come back to that very exciting point, because if you look at images of globular clusters, you do a Google image search and you can't tell them apart, really.
I mean, they all look, you know, there's something Is this, you know, M-thirteen? Is this, you know, the other globular clusters? They look pretty similar. And yet there's a there seems to be a distinction between Omega Centauri and all of the other globular clusters swarming around the Milky Way. That's that's really cool. So I have to, you know, somewhere at the center in this globular cluster, there may or may not be a black hole.
Tell us about the process of beginning that quest and what you had to do to sort of start observing and looking for this thing that's basically invisible at the center of this swarm of ten million stars. So astronomers have suspected that there is an intermediate mass black hole in the center of Omega Centauri since quite a while now. The first papers claiming this came out already in two thousand and eight.
But there has always been a controversial debate because there's also other explanations for the observed signatures that were used in these early works. And my team and my supervisors at the Max Planck Institute for Astronomy, Nadine Neumeier and Anil Seth at the University of Utah, They saw an opportunity because in the Hubble Space Telescope archive there was a large number of images that were taken of the center of Omega Centauri and these images were actually not yet properly analyzed.
So I took up this task and I downloaded hundreds of images from the Hubble Space Telescope archive. All these data are actually freely available. And then I went through the task of measuring the positions of the stars in these images very precisely. This was a long process because it was a large number of images and each image contains hundreds of thousands of stars. So it took a while to go through all of that.
But the ultimate goal was to create a catalog that contains all the stars that we can observe in this field of Omega Centauri and characterized the motion of each of these individual stars. And at the end of the process, we were left with this very nice catalog that contains one point four million stars with their motion. Okay. You measured the motions of one point four million stars, which seems are you still sane after this process? How do you how do you do that?
Obviously, you would want to automate that as much as possible. And you you you dove into the images, which are which are, you know, obviously Hubble images. First of all, how do you measure the velocities and how do you automate it enough to measure one point four million within a single lifetime? So to measure velocities, and in that case, we call these velocities proper motion. That's the projected motion of stars on the sphere of the sky.
We, in the easiest case, would just need to compare two measured position for these stars. So we know where the star is, for example, back in two thousand and two when we took our first images. And then in two thousand twenty three, when we took the last images that we analyzed and within these twenty one years, the star will have moved by a very tiny amount, typically only the fraction of a pixel in these images.
And in our case, we were actually lucky because we not only had two measurements in the center of Omega Centauri, but about hundreds of them. And fortunately, we don't have to do this manually anymore, like in the photographic plate area. But we have computer programs that can both detect the stars in the individual images and then also very precisely determine where exactly the center of the stars lie in each image.
And then the challenge was to set this up for such a large amount of data and then also to cross match the different images. So if we had measured a star in one image, we wanted to check whether we have measured it maybe in other images, which is required to measure its motion. And so this was the principle process.
right so i mean it's it is amazing you know you've got this object which i think is about eighteen thousand light years away um which is a long way away uh and so you're managing to resolve the center of this this ball of stars and actually measure um incredibly small changes in the positions. The resolution is good enough. What sort of velocities are we talking about, just in order of magnitude, for these stars moving at the center of Omega Centauri?
So the typical star in Omega Centauri will have a velocity of around twenty kilometers per second. Okay, okay.
so as i compare to the the earth's orbit around the sun how how that's a it's sort of similarish velocities isn't it i think i think it's not not very far off numbers but i think also for example uh rockets leaving from from earth have also velocities of that order of magnitude okay okay So you've got one point four million stars and some of these are very close to what you're looking for, which is this invisible black hole.
Can we dive into the center of Omega Centauri now and talk about the observations you made that actually confirmed the discovery? So, yes, you can maybe get from that image. It's not very straightforward to know exactly where to look because the stellar density in Omega Centauri varies rather slowly in its center. And so it's not perfectly clear where the center actually is. And this has been a challenge for other astronomers who tried a similar analysis in the past.
There have been a few works that took great care of analyzing where actually the center of the cluster is. And so we just looked at the velocity distribution of the stars more or less around that best center estimate. but we looked at a rather larger area. And then we looked at the velocity distribution and searched whether we find something unusual. And maybe here I have to explain the concept of escape velocity, so to speak, a speed limit in omega Centauri.
If you reach a velocity that is high enough to escape the gravitational force, potential of Omega Centauri, a star would just leave the globular cluster and never come back. Something similar exists also on the Earth. So if you have a rocket that should leave the Earth, you also have to reach a velocity higher than the escape velocity that you have on the center of the Earth. And the escape velocity for Omega Centauri is around sixty two kilometers per second.
And so we searched for stars that had a velocity above this escape velocity value. And there we found something quite exciting, which was that we had seven stars moving faster than this escape velocity, all concentrated in a very small area around the suspected center of the cluster. And so the question was, What can keep these very fast-moving stars there in the center? Because actually, we would just expect them to fly out of the center and the globular cluster and be gone forever.
So the fact they're still there moving faster than the escape velocity of this star cluster means something's holding them there. And so if we can look at another picture right at the core of the cluster. So we zoom in here to right at the center. And some of these stars, as you say, are moving. They're orbiting incredibly quickly. But I love this picture.
It's similar in some ways to the picture of Sagittarius A star at the center of the Milky Way, where high velocity stars were seen orbiting a black hole. But there's nothing there. I just love the fact, you know, that there it is, but it isn't. It's invisible. And yet its location has been sort of betrayed by the motions of these stars. That's amazing. So are the stars that you measured visible in this image here?
Yes. So I don't know if I can point here, but if you look at the black circle in the center, And you look towards the right, maybe it's three or four o'clock. Then you see a faintish orange star basically touching the circle. This is the center most star. So the star which we think is closest to the black hole. And it's also the fastest moving one. So it has a velocity of one hundred thirteen kilometers per second, which is almost twice the expected escape velocity of the global cluster.
Yeah. And so then you can just use basic orbits to measure the mass. Yeah, so we, in the galactic center around the black hole Sagittarius A star, astronomers have now observed full orbits for the star. So they could really track the motion around their full orbit, which for the center-most star took, I think, seventeen years or so.
and we are not quite there yet in omega centauri so we saw that these stars are moving very fast and we think that they need to be bound by a black hole to to keep them there in the center but we have not yet observed any full orbits or any accelerated motion and this is actually expected because we think that the black hole is more than a factor of a hundred um lower in mass than the massive black hole in the center of the Milky Way.
And for this reason, the orbital velocities are also much lower. And the time it would take for a whole revolution would be probably hundreds of years. So it's not unexpected that we have not seen that yet. But we hope that with a sufficiently long monitoring one, eventually you can see that these stars deviate from a straight line motion and that would then also directly reveal the mass of the black hole. Right. And of course, we measure black holes in solar masses.
Kilograms and tons don't really work for things this massive. So everything is compared to the mass of our sun, becomes the standard unit. And the supermassive black hole at the center of our galaxy is a couple of million solar masses. So this one came out, was it around eight thousand? So eight thousand is the lower limit on the mass. That would be the minimum mass required to hold on to all of the fast moving stars that we have observed.
We think that the actual mass might be actually a little bit higher, maybe twenty thousand. Some earlier works have suggested forty thousand, but this then depends on some of the assumptions that have been made in the mathematical models. And so I think what is important in our work is that we were finally able to provide a firm lower limit. So that somehow makes it clear that there is something massive, but we don't know the exact mass yet. Right. Okay. So we've got a few more people tuning in.
Thank you from California, from Connecticut, Illinois. Uruguay, welcome. There was Italy and Pakistan in the long list in the comments here. I'm going to take some of the questions now from you all. And the first one from Blob Rana. Welcome, Blob. And this goes back to something, Max, that you are thinking in theory what makes Omega Centauri different from other globular clusters, which is that what we're seeing here in Omega Sen is that it is a former galaxy in its own right.
so um i think this is nowadays actually uh widely accepted and maybe to give a little bit more context and so we we are trying to understand how our milky way our own galaxy has formed in the last few billion years and we think that it has experienced a series of mergers where smaller galaxies have been basically caught by the gravitational pull of the milky way And then they were orbiting around the Milky Way and due to the strong tidal forces introduced by our own galaxy,
these smaller galaxies get then completely disrupted. So all their stars get scattered around and start to form somehow a diffuse background of stars. But if you have a very dense region in such a small galaxy, such in its center, then this dense region can stay bound even though they are strong tidal forces. and can survive until present day. And we think that Omega Centauri is such an object. So it has lost its host galaxy, but it now remains as a witness to that merger event.
OK. So it's sort of like a mini spiral galaxy bulge is what you have left. And you lost the arms in some ways. Yeah. Interesting. We had a question from Mauro. What's the average distance between stars in Omega Centauri? I think it would be interesting if you were on a planet at the center of a globular cluster, what would the night sky look like? Yeah, I think that would be quite an amazing view and it would never really get dark.
So I think the the stellar density in the course of these global clusters is very high. And we have thousands of stars per cubic light year, while in our surroundings, we probably have less than one. So I'm not sure I can give the exactly right answer, but we are talking about way less than a light year. Yeah. But they're probably still all point sources, aren't they? I mean, you would it's it's because they are separate.
It would just be like a sort of jewel box all over your head at these very, very bright point sources, which would just be absolutely amazing. Yeah. Like, you know, a million planet Venus is. That would make a planetarium show. It would be pretty good. What else have we got? This is the first confirmed intermediate black hole. I know there's been possibilities of others. From Cristiano, how many intermediate black holes do you think are out there?
Where will these things crop up if you're looking for them? So the problem is that with one data point, it's still a little bit hard to make statistics. But I think we have now demonstrated that these things can exist in massive star clusters. And there's also a few other candidates of massive star clusters which may host an intermediate mass black hole.
so i think it should be possible to detect more of these in the in the future and then eventually we can estimate a fraction and and get concrete numbers but as somehow the long debate about the about the black hole in omega centauri has shown it's indeed not not very easy to to find them And for intermediate mass black holes, I think it's still not entirely known whether it's hard to find them just because they are rare or because of the conditions in which they can be found.
For example, the problem with intermediate mass black holes in star clusters or in globular clusters is that in these objects, there's only a very low gas fraction and there's also only very little dust. And so the black hole does not have material to accrete. And this is what we typically have found for either supermassive or stellar mass black holes. So material falling into the black hole, getting heated up and then starting to radiate.
And if there's just no material, then it's hard to create that signature. Yeah. Yeah. I think there's an estimate there's about a million plus solar mass black holes or sort of small black holes, stellar black holes, not solar mass black holes. in our Milky Way, but we only have confirmation of a dozen or two, simply because you need that material. You need something to affect the black hole to actually see it. And they're out there, but we can't see them, which is really amazing.
A question from Ron. And this actually, I want to expand on this. How does this cluster galaxy compare to our Milky Way galaxy in size? stars and dimensions. And maybe that ties in with one of the theories that the black hole size correlates with the galaxy size. Say a little bit about that. So I think we first have to be careful about how we call things. So Omega Centauri is not an entire galaxy anymore. It's just the core region of the galaxy.
But we estimate that Omega Centauri itself weights around four million solar masses and has around ten million stars. And the Milky Way, our galaxy, is many orders of magnitude bigger. So there we have hundreds of billions of stars and also the mass will be a factor of more than a thousand larger. And so this is also why a much lower black hole would be expected. So, yeah, I mean, these things are big, but compared to spiral galaxies like the Milky Way, they are small. They are smaller.
Yeah, yeah. Quick call out to Julia for the stars on Facebook. Thank you very much. Thank you very much for everybody watching today. Max, what's next for you? Are you going to keep your vision firmly fixed on Omega Centauri, or are you thinking of other things to study now? So I think we want to do two things. The first thing is we want to learn more about the black hole in Omega Centauri, because now we are fairly certain that it is there, but we don't know its exact mass yet, for example.
And so we are trying to get follow-up observations with different telescopes to even better characterize the motion of the stars in the center of Omega Centauri. other people and ourselves as well are trying to find accretion signatures of that black hole. As I said, the gas density is low, so those accretion signals are probably very weak and hard to detect. But now that we are sure that there is a black hole, it's worth it to look again, maybe longer and deeper observations.
So this is what we want to do for Omega Centauri and its black hole. And then we also want to explore other globular clusters in the Milky Way to see whether we maybe can find something in their centers and then understand better the statistics or the occurrence rate of these objects. What would be your telescope of choice to do more observations? What sort of instruments would you like to get onto this project? Not that we have any influence at all on that.
I think there's a few different telescopes that can actually help us with the observation. So for the accretion signal, it's probably good to look in the submillimeter wavelength range. So there we are trying with the ALMA radio telescope array. And then to better characterize the motion of the stars, we first want to measure the so-called line of sight velocity of the stars. So to see whether they move towards us or away from us.
And we cannot do this by simple imaging, but there we need a spectroscopy. And this gets very challenging if the stars are faint. And so they are the telescopes of choice are either the James Webb Space Telescope or also the very large telescope of the European Southern Observatory. It also has some very powerful spectrographs. Right. Yeah. And then on the longer run, when we would like to observe the curved motion of these stars, we need much better resolution.
So there we have to wait for the next generation of telescopes. Just for example, the extremely large telescope, which is currently under construction. Right. So this is as technology gets better and the new telescopes come online, you can really find out a lot more about these intermediate size black holes. I'm just going to end up with one last question. And this is from Zapfan. Where do you think these intermediate mass black holes come from? Is this a merger of small ones?
Is this just a big one growing? Or is this still a theorist's question? So I think there are still different theories out there.
And people are still exploring the possible formation mechanisms and now probably many billion years later we cannot see what exactly happened but some recent works suggested that in the centers of star clusters where you have a very high density of stars so many stars crammed together these stars start colliding and merging in a dynamical runaway process and so they form a very massive star that weights thousands of solar masses and then eventually collapses into an
intermediate mass black hole. So that's one possible formation chance. OK, but still still in in in a sort of a theoretical mode rather than observational one for these these objects. Max, thank you so much for joining us today. It was a pleasure. Thank you. Thank you, everybody, for watching from around the world. A reminder that the SETI Institute is a nonprofit research institution. So if you want to find out more about the Institute, please go to our website, www.seti.org.
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