Ep. 497: Update on Globular Clusters

Is it globular clusters or is it globular clusters? It doesn’t matter, they’re awesome and we’re here to update you on them.
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Show Notes

Globular Clusters
Fraser calls them “globe-ular” clusters, and Pamela calls them “glob-ular” clusters. Both are correct.
Where are the Intermediate black holes?
Shapley–Sawyer Concentration Class
Dark globular cluster
Metallicity – the population of heavier elements as metals and to the proportions of these elements.
NGC 2808 contains three distinct generations of stars.
Djorgovski 1’s stars contain hydrogen and helium, but not much else.
Two populations of globular clusters, which became known as Oosterhoff groups:
type I are referred to as “metal-rich” (for example, Terzan 7)
Type 2 have lightly longer period of RR Lyrae variable stars, but are “metal-poor” (for example, ESO 280-SC06)
Exotic classes of stars, such as blue stragglers, millisecond pulsars and low-mass X-ray binaries, are much more common in globular clusters.


Podcast Transcription provided by GMR Transcription
Fraser: Astronomy Cast, Episode 497: Globular Cluster 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. I’m Fraser Cain, 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 you doing?
Pamela: I’m doing well. How are you doing?
Fraser: Great. This is our penultimate episode before we go on our summer break. Just a reminder, so that we’ve got this episode, and then we’ve got next week’s episode of which the topic is I have no idea.
Pamela: Dwarf galaxy.
Fraser: Dwarf galaxies. Right on. Exactly. So, we’re gonna talk about dwarf galaxies next week, and then we will be going on our summer hiatus. We’re not going anywhere. We’re gonna be producing a mountain of content, but we will be not producing Astronomy Cast in the weekly space hangout until we return in September.
Pamela: And in September, you are all invited to join us for a 500th episode.
Fraser: 500.
Pamela: 500. The Weekly Space Hangout Crew who wrote what I’m about to read, the Weekly Space Hangout Crew is inviting everyone to come to Edwardsville, Illinois – it’s where I live – the weekend of September 15th and 16th for a weekend of Astronomy related festivities, culminating in the live recording of the 500th episode of Astronomy Cast at the beautiful Wildey Theatre in Edwardsville almost 12 years to the day since Fraser and I released this show. People who come will be able to meet the two of us along with people from the Weekly Space Hangout Crew and the CosmoQuest family.
In addition to the live recording of the 500th episode, anyone who attends is going to be able to attend a special live Q&A episode of the Guide to Space. And some point during the weekend, Dr. Morgan Rehnberg is going to science with us too. We’re gonna rent a Brew Pub for Saturday, so all activities will be in a Brew Pub Saturday day. In the evening, weather permitting, we’re gonna do a Star Party. Sunday, we move into the theater. So, to register and for more information, head over to astronomycast.com. Go to the Trips pull-down menu and click on AC500 Weekend.
Preregistration is now open and requires a non-refundable $50.00 deposit. Everyone who preregisters, even if you have to cancel, will receive a goody bag full of memorabilia from the weekend. You may pay the full $200.00 registration fee now, or you have the option of paying $50.00 now and paying the balance of $150.00 anytime between now and August 15th. So, come see me in Edwardsville. Come see Eddie, come see Fraser. And Morgan will have a random rogue Wild Dr. Morgan sighting.
Fraser: Right on. Alright, so is it “glob-ular” clusters or is it “globe-ular” clusters? It doesn’t matter. They’re awesome, and we’re here to update you on them. So, let’s just get this out of the way. For the longest time, I called them “globe-ular” clusters. Pamela called them “glob-ular” clusters. There was a schism war in the Astronomy Cast-verse. Many lives were lost, harsh words were used, and David Dickenson, he brokered the peace. He went out and found an official way to say the word, and both are acceptable.
Pamela: It’s true. Just like both our pronunciations of Uranus are acceptable.
Fraser: All three: “UR-anus”, “Ur-Anus”, and “Auranus”. That’s apparently –
Pamela: I never use that one.
Fraser: That’s apparently the Greek way to say it. So, either way, you can say “glob-ular”, you can say “globe-ular”, just remember that they’re amazing. So, before we give the update, let’s talk about what they are.
Pamela: So, these are stellar laboratories that allow us to understand star formation like nowhere else. They exist in the outskirts of most major galaxies and even some non-major galaxies, and most of the time, all of the stars in a “glu-bular”/“glob-ular” cluster formed out of –
Fraser: I love that I’ve even got you saying it now. It’s great.
Pamela: Most of these stars in this “glu-bular”/“glob-ular” cluster – which is just fun to say; it feels good – they all formed out of one blob of gas and dust, and because of this, they all have the exact same chemical formation. They all formed at roughly the same time, which means they’re all roughly the same age. So, when we look at a variety of different clusters in the sky, we are able to say, “Aha, this one over here that is this age shows us what stars at this age of all masses look like.” This other one over here that is a slightly different age allows us to see what all stars at that age look like.
And because different globular clusters were made out of slightly different blobs of gas and dust, we can also study how things evolved as a function of what their atomic content is, metallicity is how we refer to this is astronomy. This allows us to essentially check our simulations. You build a simulation of how a stellar population evolves, set your metallicity, and then find a globular cluster to match it to in the sky.
This is one of the places where I started my work as a researcher studying how RR Lyrae stars evolve in globular clusters, and they remain one of the most interesting to study, and both misunderstood and understood objects. I love the fact that these are both some of the best understood and worst understood objects simultaneously. They’re just awesome.
Fraser: Yeah, and I think science says they’re super fun to look at. I’m pretty sure that that science has remained unchanged since we first did our episode of them pretty close back to the beginning of the show.
Pamela: And the science that they’re fun to look at is psychology.
Fraser: Well, the great thing about them is that you can see them even with a pair of binoculars. It was –
Pamela: Yeah, they’re the quintessential [inaudible] [00:06:40].
Fraser: Yeah, yeah. And I bought a pair of binoculars, and sort of was teaching my wife the night sky, and she now can find M-13, she can find the globular cluster in Hercules every time, no problem.
Pamela: The one in Hercules is easy. That one’s awesome.
Fraser: It’s got these great waypoints to be able to find it.
Pamela: Now, while the fact that they’re a stellar laboratory that allows us to understand star formation and evolution as a function of age, mass, and composition, beyond that, they’re greatly baffling. And in reviewing the press releases and reviewing the new literature that has come out in the past five years on these objects, the thing that gets me the most is all the times we’ve gone forward, gone back, gone forward, gone back on the exact same ideas.
Here, I am talking about do they or don’t they have black holes in their core, and it seems like every major science conference that involves stars, there’s a new press release saying one or the other, and I think the result is because they refuse to have angry, active black holes and because they aren’t dust rich like great galaxies are, it’s gonna be awhile before we can answer that one for certain. And just the fact that they’re refusing to give up these details is kind of amazing.
Fraser: Yeah, and so you’re talking about those intermediate mass black holes, those in between ones between the stellar mass black holes that have five to 50 times the mass of the sun and the super massive black holes that have millions of times the mass of the sun. Where are the ones with thousands of times the mass of the sun? You’d think it’d be the places where there were once lots of really big stars, and they smurshed into each other, and merged into something that was halfway in between these stellar and super massive. And you’d think, again, you would be seeing mini quasars –
Pamela: Or something.
Fraser: – or stars going in funny orbits the way they found the ones at the core of the Milky Way, and yet they haven’t found any of those data.
Pamela: Things conclusively. There’s been a few papers saying we have modeled the proportion of highly elliptical objects, and it is consistent with there being an intermediate black hole located in the center, and this is where you get the press releases of we have –
Fraser: We found this intermediate mass black hole, yeah.
Pamela: Like, no, you found fast-moving stars. And this starts to reflect on updates in our understanding of black holes and galaxies. It was already understood when we started doing this show that the super massive black holes in the centers of galaxies have a mass that is proportional to the size of this spheroid of stars in the center of the galaxy. And we brought this up a few episodes back in regards to the fact that we’re now finding that these perfectly flat spiral galaxies that don’t have a central bulge may not have a central super massive black hole.
Now, if the size of the black hole is proportional to the size of the spheroid, and globular clusters are nothing but spheroid and in many ways follow the relationship of physical properties you see with spheroids in galaxies, well, all of this mapped out to maybe globular clusters are nothing more than the completely stripped bare former core of former baby galaxies that our Milky Way ate with verve. And so, you imagine this situation where you have a baby galaxy, it has a baby spheroid, and as these baby galaxies merge to form our Milky Way, their nuclei get flung off and become this suite of globular clusters.
But if that was the case, [inaudible] [00:10:48] intermediate black holes. And when we first started recording the show, we didn’t know where super massive black holes came from.
Fraser: We knew fairly certainly that quasar super massive black holes are the actively feeding centers of galaxies, but the question was was it sort of a top-down, bottom-up formation process? And now we know with quite a high degree of certainty which one of those it is, that it’s bottom-up, not top-down. But is that the uncertainty that you’re talking about?
Pamela: So, the question was in this particular instance, did the super massive black holes come from the merger of a whole bunch of super massive stars that form smaller black holes that became bigger black holes, that became even bigger black holes, or is there something about the way galaxies form that intrinsically forms super massive black holes?
And what we’ve learned is that the rare, original super massive galaxies that formed – these are the first forming massive galaxies in our universe – co-formed in models that include turbulence the super massive black hole, the galaxy, and the stars all at once, that these turbulent inflow processes of this massive collapse of gas, this chaotic collapse that include turbulent processes would’ve been capable of generating inflows of material that allowed a super massive black hole to co-form with the galaxy.
Fraser: Right. But that’s not what this episode is about.
Pamela: No, and this is where the other model that appears to be true for the majority of galaxies like our own is that you took a bunch of baby galaxies – baby galaxies formed early on – and over time, they merged in different ways, and whatever was in the heart of these baby galaxies, that would grow to form the super massive black holes that we now see in systems like our own Milky Way. And if that is a true story, and if globular clusters are the flung-free remnants that were once the cores of these baby galaxies, they should have intermediate black holes, or we need models that strip them of their black holes, but otherwise keep them intact.
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Fraser: So, I’ve got an update that came out today. There was one I wanna talk about shortly, which is sort of redating the ages of clusters, but the one that came out today – and I forget where the research came from; it’s sort of in my queue of news that I’ll be sharing in Twitter today – is that some researchers have done some supercomputer simulations based on the weird chemical signatures that are in the stars inside globular clusters – “globe-ular clusters”, now you got me saying it.
And one explanation for why they have really strange chemicals in them is that there were once super massive stars inside these globular clusters that detonated shortly after they formed that could’ve had potentially tens of thousands of times the mass of the sun, and they formed quickly, and then just exploded inside the cluster and sort of sprayed that material, seating all of the other stars with heavier elements, which is really a fascinating concept and the kind of thing that I wish we had the telescopes that could see these clusters back when they were forming.
Pamela: And this is one of the real struggles is trying to explain how “glu-bular”/“glob-ular” clusters formed. And we have a few cases where Alma the Great finding things early on telescope [inaudible] [00:16:45] a large millimeter array –
Fraser: That’s its technical term.
Pamela: That’s why I’m excited. It keeps finding things [inaudible] –
Fraser: Alma is the greatest.
Pamela: And it has recently identified some tight, high-pressure pockets of gas in colliding galaxies that appear to be the potential of future homes of globular clusters. So, the idea is that rather than these being the cores of past galaxies, instead, in certain merger circumstances, you have the leftover gas and dust from the system, the two merging galaxies that through the process of the merger ends up bound up with really high pressure.
And with these massive millions of solar mass pockets of high-pressure gas, these systems have the potential to form the clusters that we see today that are highly compact and are single epoch of star formation because under these high-pressure environments, everything is like, “Gonna collapse now,” and there’s just nothing leftover at the end.
Fraser: I mean, they’re such a fascinating object. I mean, this tight ball of stars that they’re, in some cases, in strange orbits around the galaxy itself.
Pamela: We have stolen some.
Fraser: Right, we’ve stolen some. But they’re these treasures that you steal from other galaxies and add them like some kind of snail that’s –
Pamela: Trading card?
Fraser: Yeah, that’s adding gems to its shell, or like a crab that does this. Anyway –
Pamela: Crows. Crows pick-up the shiny things and add them to their nest.
Fraser: Crows do this, but there are crabs that add shiny things to their shell.
Pamela: I did not know that.
Fraser: Clearly, this analogy’s falling apart. But they’re clearly so special, right? And astronomers have just been wondering where do they come from. And so, one of the things that’s very interesting as well is that maybe astronomers have started to change what the age is, and I think we talked about this in the stars episode, so maybe go back to that, but that kinda changes everything, you know?
Pamela: Well, and in reviewing things for this episode, the thing that got me in addition to the do they/don’t they have intermediate black holes was the are they/aren’t they old or young, by which I mean are they in their 60s or are they in their 100s, if you compare them to a human lifespan. They’re old, but are they ancient? And here, you see a variety of different people that are running different computer simulations and using their different stellar models to synthesize what a stellar population will look like after a certain number of eons, millennia, millions, billions of years.
You get the majority of the groups have been coming out saying, “We’re looking at nine to 13 billion years,” “We’re looking at 10 to 12 billion years.” The error bar has changed; they stay old. But we recently had this new discovery that it sounds like you’re prepped to talk about saying that globular clusters may be four billion years younger than we had previously thought.
Fraser: Yeah, so not just older than the universe itself, but significantly younger than the universe, which again, is very handy when you realize that the objects that you thought were older than the universe that they’re inside of is not the case. So, this is actually fairly recent. It came out about a month ago, and it’s a paper called Reevaluating Old Stellar Populations.
And the gist of it is just that they sort of took into account binary star evolution within globular clusters and just tried to track how the binary stars were moving around each other, and when they did that as their calculation, they came to that they’re about nine billion years old, not 13 billion, which is still old, but significantly younger, and as I said, conveniently younger than the universe itself.
Pamela: And the reason that Fraser keeps bringing that up is it was only about the time that we started this show that we finally came to terms with the fact that we finally had stellar models, and we finally had values of the Hubble constant, and we finally had data from it would’ve been the WMAP mission at that point, Wilkinson Microwave Anisotropy Probe. We finally had all of this data lining up to make our universe more in the 13 billion, which was new and awesome.
Fraser: That was on our watch. Before we started Astronomy Cast, how old was the universe? Somewhere between 13 and 18 billion years old.
Pamela: And there were even people that were pushing it as young as 10. So, on our watch, they figured out how old the universe is.
Fraser: To within plus or minus 100 million years, which is great.
Pamela: And then, they finally figured out stellar evolution models well enough to be able to say that these clusters are certainly less than the 18 billion we had for a while, and probably down around 12 billion, which is consistent now with the age of the universe. It’s kind of amazing that just these mysteries that existed when I was a grad student are things that were never a mystery for today’s post-docs, and that makes me feel old.
Fraser: Let’s talk about a kind of star that’s really interesting – blue stragglers, and they’re in “glob-ular”/“globe-ular” clusters.
Pamela: And this goes to the uniqueness of how stellar interactions occur in these clusters. These are the densest star regions we’ve got, and the cores of these clusters are so dense that we can’t even measure the motions of stars when they’re down in the core, which is part of the enigma of the intermediate mass black holes. And because these are three-dimensional spheres, no matter what angle we’re looking at them, they’re still gonna have too high a density of stars that you can’t see all the way down into the center.
Now, with this dense population, one of the more interesting papers I came across was the idea that the reason that we’re not finding planets is because star-star interactions tear planets out of solar systems and fling them willy-nilly. And the other side of that is that star interactions allow existing binaries to be torn apart and new binaries to be formed, so these stars are square-dancing. And if you look at long-term models of globular clusters, they’re pulsing like a heart as these different dosey-doe interactions of swing your partner round and round essentially lead to things getting flung on highly elliptical orbits, and then getting recaptured into different orbits or perturbed into different orbits over time.
Fraser: Well, and these blue stragglers, they’re weird pot-blue stars that are inside globular clusters, and people think –
Pamela: Nowhere else that we’ve seen.
Fraser: And nowhere else that we’ve seen, and the question is you talked about how these clusters are used as a way of telling time because all the stars formed around the same time, and then all of the big ones died as supernovae, and so you should only have stars like our sun and cooler remaining, and yet there are these occasional stars. And the thought is that they are from stars colliding into each other. They’re buzzing around.
Pamela: I would go with merger because collision implies they came flying from opposite sides of a globular cluster like a couple in love, and bash together, and –
Fraser: But the binary star merger that they just got closer and closer, and then they merged into each other, and this new star got a new lease on life.
Pamela: It combined the atmospheres of the two systems. It now had extra hydrogen falling into its center, and there are horizontal branch stars that going, “Hi, I’m blue. Watch me. I’m bright.” And these we started to have hints of their identity back when I was in graduate school in the early 2000s, and the models have gotten better and better, and the fact that we can now say that you have new binaries forming and old binaries getting torn apart, and these close interactions make it probable that you’re going to get extraordinarily close black holes. This is all kind of awesome.
Fraser: Do you have any more updates on globular clusters?
Pamela: I think it’s worth mentioning that we’re now starting to map them out using pulsars because people are like, “Okay, we need better resolution on this.” And because radio light can cut through gas and dust in our own Milky Way that may be preventing us from seeing the stars and clusters with enough detail to otherwise be able to measure them. The radio waves from pulsars – first of all, the super-fast pulsars that have been spun up by stealing material after their binary companion.
Again, binaries play a major role in these systems. They spun up pulsars, maybe spinning a thousand times a second, and this makes is super easy to measure their changes in position because as they move, their pulses get closer together or further apart, depending on which direction they’re going in. And so, we can measure the kinematics within a globular cluster by looking at how various pulsars are moving relative to one another within one of these systems, and this is just a new and fascinating way to map out a star cluster.
Fraser: Yeah, and pulsars are just this wonderful, natural way to be able to map all kinds of things in the Universe, find planets, navigate yourself within the galaxy, within the solar system. So, yeah, absolutely. What else have you got?
Pamela: Oh, man. There’s so much going on. Let me see what I haven’t hit on yet. So, I hit on the fact that we’re now looking at them, trying to figure out are these things leftover streams or not? I talked about the do they/don’t they of black holes. Then there’s interesting images of what may become globular clusters in the future that we’re seeing and galaxy-galaxy interactions. One of the more interesting discoveries that involved a black hole but not in the center is for the first time ever, we’ve been able to use the stellar interactions within a globular cluster to find a black hold that was eating nothing.
There was just a perfectly, every day, regular star, minding its own business that when astronomers looked at it, they were able to see that it was moving with huge velocities that varied over time. And what they figured out was every 28 minutes, the companion star would be orbited by the black hole, and that the separation between these two objects, a white dwarf and a stellar mass black hole, is only two and a half times the separation between the earth and the moon. And while that seems extraordinarily close, the thing about this is a white dwarf is only the size of the moon, and a neutron star has about the same diameter as Manhattan island.
So, you have these two tiny objects that do have, compared to their size, a vast emptiness between them. The white dwarf has nothing for that black hole. It’s a steal, and they’re just minding their own business, orbiting one another, and that’s awesome. And these compact objects that we’re finding may eventually cause [inaudible] [00:29:07] to have new black hole merger candidates that she can look at. This is a future spacecraft trio that will do works similar to what’s being done by the ground-based Ligo facilities. So, that’s cool.
Fraser: Well, I think we’ve reached sort of enough time, so save the rest for 10 years from now, and we’ll come back around and take another crack at it.
Pamela: We can do that.
Fraser: Perfect. Alright, thanks, Pamela.
Pamela: Thank you.
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Duration: 31 minutes

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