How are globular clusters so old? Where did they come from, and how are they linked to galaxy formation? What makes them so globular, anyway? I discuss these questions and more in today’s Ask a Spaceman!

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EPISODE TRANSCRIPTION (AUTO-GENERATED)

What the heck are these things? They're weird. That's what they are. Most of them are relatively small, only a couple dozen pars across. And if you're not too hot on your Parex, think, say 50 light years across and smaller. But what they lack in size, they more than make up for in mass, they are truly massive. They typically contain hundreds of thousands of stars, sometimes millions of them. That means they are incredibly dense. On average, the distance between stars in these things is around one light year. Compare that to our nearest neighbor star Andromeda Proxima Centauri about four light years away. But in their cores, the average distance shrinks to a mere one third of a light year. That means stars are packed inside of them 1000 times more densely than our own stellar neighborhood. Imagine what the view must be like from inside of there, like the inside of a glittering jewel box and they're old, they have no new star formation.

They're all somewhere around 8 to 12 billion years old with little if any new stars forming in them. These are some of the if not the oldest intact structures in the universe that have not undergone distortions or evolution. They are simply there and have been for billions of years. These things are older than Galaxies. They're, they're galactic graveyards or at least galactic retirement communities. It's a very odd thing in our universe to not have new star formation. Typically, there's always new waves of, of gas and material, new turbulence to trigger a new fresh round of star formation, but not here. These things have persisted as they are since before the milky way itself formed. And they're generally round, which is itself a very useful piece of information because not many things in our universe are round planets around stars, around clusters of Galaxies round.

These are also round. This means that they are gravitationally bound. That means that all the hundreds of thousands or millions of stars that go into them are enough to keep it in a tight little ball, all the mutual gravitational interactions and attractions of all those stars keep it together. If these were not gravitationally bound, they would have all sorts of weird shapes, but they don't have all sorts of weird shapes. So they're gravitationally bound. That's another intriguing include, this ancient structure has been stable for all these billions of years, almost as old as the universe itself. And the fact that they're round gives us their name, their name comes from the Latin Globulus, which means small sphere. They're the globular clusters, we know of about 150 of them in the milky way. We suspect there are around 200 in total.

We can't see them all. Uh because our view of a good chunk of the space around the milky way is blocked by the milky way itself. However, we're not alone, we're not the only galaxy with globular clusters, all Galaxies that we are capable of observing globular clusters around them. We see globular clusters, the Andromeda triangulum, even little dwarf Galaxies have globular clusters. We've noticed an interesting pattern. Uh smaller Galaxies have proportionally fewer globular clusters. Little dwarf Galaxies might just have a dozen big Galaxies like us have uh one or 200 even bigger Galaxies have even more. Where did these globular clusters come from? And what did they want with us? But you can see them with the naked eye. That's, there's, there's a few of them, a few of them that you can see with the naked eye, which is really, really weird to contemplate uh like Omega centauri. It has been recorded since oldie times. There are almanacs star almanacs, thousands of years old, recording it as a star but to the naked eye, it just looks like any other star.

Ok, fuzzy bright dot in the sky. Once you look at it with a telescope and you can, it's a massive globular cluster weighing in at a whopping 4 million solar masses with a distance of 17,000 light years from us. It's one of the most distant objects you can see with the naked eye, the most distant, of course is the Andromeda galaxy itself. But this is not an episode on the Andromeda galaxy. It's a living fossil omega centauri and its 100 and 50 friends are astronomical selic camps, whatever they are, globular clusters are remnants from the formation of Galaxies themselves. Something happened, something happened when Galaxies were first beginning to form to trigger the formation of a galaxy of globular clusters. And I swear in this episode, if I accidentally say galaxy cluster and instead of globular cluster, I apologize in advance, take a drink or bite the cheese.

Every time I do globular clusters are old. Think of that age 8 to 12 billion years old. OK. Our milky way galaxy is like what nine maybe 10 billion years old it began forming before that, of course. And so all the the the these globular clusters, all 100 and 50 of them that surround us had to form around the same time when our galaxy was first getting going. But then our galaxy continued to evolve. It developed spiral arms, it had new generations of stars like the sun. And in that time, the globular clusters haven't changed. That is weird. That is not the norm for our universe. Things are dynamic. We live in an active living universe, new stars forming Galaxies crashing into each other black holes going off galaxy clusters rearranging their inside it it, we live in a dynamic cosmos, but the globular clusters refuse to get along with the picture.

OK. It's like going to grandma's house and she's still got the radio on and doesn't own a dishwasher. Maybe doesn't even have running water. OK. Globular clusters are the grandma's houses of the universe as perhaps the worst metaphor I've ever come up with on this show. I do find new ways to top myself. There is no universally accepted explanation for the origins of globular clusters. Let that sink in. We learn about these things in Astro 1010 Yeah. Yeah. Yeah. You got your, your nebulae, your stars. This is presumably your professor, your nebulae, your stars, you got your open clusters and you got your globular clusters, the globular clusters have lots of stars. They're pretty old and they're round. OK. Moving on these things, these objects that we've been observing for millennia without realizing it. And then a few centuries while realizing it come from the ancient universe, these are time capsules and yet we don't know exactly how they form.

Here's one massive massive, massive clue as to their origins and what they are and how to classify them. They have no dark matter, they have no dark matter, at least in the present day they may have in the past, but they don't today. And we can tell that through the motions of the stars everywhere you go on galactic scales when you start trying to weigh up how much a galaxy weighs and you say, OK. OK. OK. I add up all the sources of light, this gives me a mass and then you look at the dynamics of how stars are moving inside the galaxy and you're like, wait a minute, there's way more stuff that's not emitting light, something's wrong. This is dark matter. But when we add up all the light from a globular cluster and we look at the dynamics of how the stars are moving and see if all the mouse mass that we can see can account for it being stable and globular it matches globular clusters have little to no dark matter inside of them. This means that they are most definitely not Galaxies.

I mean, it, it's hard to define exactly what a galaxy is. And if your definition of galaxy is too loose where you say uh galaxy is a bunch of stars held together by gravity, then then that would automatically include the globular clusters. And you might be tempted to think the globular clusters are just very, very tiny Galaxies like we have giant Galaxies, regular Galaxies, dwarf Galaxies, maybe these are the dwarf of the dwarf Galaxies, sub dwarf Galaxies. But what allows us to say that globular clusters are not Galaxies is the fact that they do not have dark matter and Galaxies do have dark matter. The milky way is like 80% dark matter. It's better to think of Galaxies as pools of dark matter rather than anything else. So we're able to cleanly separate globular clusters from the category of Galaxies because there is a very, very striking difference between them. And so that is a clue, it tells us that globular clusters do not form in the same way as Galaxies.

They do not evolve in the same way as Galaxies. They do not have the same kinds of interactions as Galaxies. They are not just tiny Galaxies. There's something else, there appears to be two kinds of globular cluster populations. There's a kind of sort of two kinds, one kind are relatively younger and an emphasis here on the word, relatively leaning more towards the 8 to 10 billion year old range, which is still super old but younger than others amongst the globular clusters. These younger ones also tend to have more metals and metals is the worst word in jargon we've encountered before on this show, it's the astronomy word for literally anything but hydrogen and helium. I agree it's stupid and doesn't make any sense. But I'm sorry, I can't do anything about it. So the younger globular clusters tend tend to have more enrichment.

I like that word better. They they, they tend to have a little bit more variety, a little bit more chemical stuff in them. So therefore these kinds of globular clusters must have formed after there had been some sort of enrichment of whatever they came from why? Because all the stars, roughly speaking in a globular cluster are pretty much the same age. Sometimes with globular clusters, we do see stars uh two different populations of stars uh suggesting some other formation or, or rounds of star formation. But whatever the case, it was over and done with billions and billions and billions of years ago. Before I continue, I want to let you know that this show is brought to you by the wonderful folks at Better Help. That's better help dot com. I I know a lot of you listen to this show as a form of therapy, a as a way of of escaping the world and, and just going among the stars on this wonderful journey. Uh I am a big advocate for therapy.

I personally see a therapist and you would be surprised if you don't currently see a therapist. How much they can really help you just navigate a difficult life, just like you see a doctor to help you with physical conditions. You should see a therapist. Better help dot com is a way to do that. That's convenient. It's affordable. Uh These are professional counselors that you can connect to online, a range of expertise worldwide. It really is an invaluable resource. Uh As a listener, you can get 10% off your first month by visiting our sponsor at better help dot com slash Spaceman. You can join 1 million people who have taken charge of their mental health. Again, that's better help he LP dot com slash spaceman in a normal evolving galaxy that has continued rounds of star formation. The stars are the ones creating the metals. They're fusing hydrogen and helium, turning it into heavier elements and then dying and spreading those elements throughout their environment.

So as Galaxies evolve, they get richer and richer and richer with metals, they get more and more enriched, but they have ongoing star formation. The globular clusters do not have ongoing star formation. So in order for them to have a lot of metals, they had to be born with a lot of metals, they didn't make them locally. These younger globular clusters, which is about a third of them tend to be closer to the central bulge of the milky way and tend to be in the same plane as the rest of the galactic disc. The other population, about two thirds of them are old as in old, old, more than 10 billion years old, they tend to be far from the center and they have all sorts of janky orbits. They're not all in one nice neat plane, all sorts of different angles. These guys hang out in what we call the halo, which is just the stuff surrounding the disc of the milky way and they tend to have fewer metals. They are not enriched since we have two distinct kinds of globular clusters, this suggests we have two different kinds of formation histories.

And I and nobody else categorize these different kinds of formation histories as the Innie and the Audi the in are the globular clusters that formed with the rest of the galaxy, along with the bulge, the bulge of the central bulge of the milky way is the oldest part of our galaxy. It is the part that formed first, it is the dense core that first congealed billions of years ago. And then it started to gravitationally attract more gas and dust that formed the larger galaxy, which is we live in that larger part of the galaxy. And because the in their stars look a lot like the stars in the bulge, the bulge has a lot of metals, has a lot of enrichment. And so because those globular clusters are aligned with the disc, tend to hang out near the bulge and look a lot like the bulge itself, just smaller versions of it.

We think it formed along with the earliest seeds of the milky way. The other two thirds, the older population are the Audis, they formed somewhere else probably as part of a dwarf galaxy. Because again, the stars we see in an old globular cluster look a lot like the stars we see in a dwarf galaxy, not a lot of metals, not a lot of enrichment, not a lot going on. Again, all this is relative, the inn are younger by a couple billion years, which is about when the Milky way started forming 8 to 10 billion years ago. While the Audis are older, they're 10 to 12 billion years old, no matter what, they're all super old. But the Audis are a little bit older and we think they formed somewhere else. And then the milky way that something else, like if you were part of a glob cluster, you formed with a dwarf galaxy, you were fine, you were living your life, you were having a great time and then boom in comes the milky way it eats you.

It destroys your parent galaxy in front of your very eyes and then forces you to stay in orbit for billions of years. What a fate that is. But the globular clusters are gravitationally bound, they stay strong. The dwarf Galaxies are big enough that they get torn apart by tidal forces by gravitational interactions. But because the globular clusters are small and compact and dense, they can survive. OK. So that's helpful to know some globular clusters were born with our galaxy and some were born earlier but came into our galaxy later. So how did they form? We don't know, seriously, we don't know since all the stars in a globular cluster are roughly the same age, they probably formed all at once and they probably all contributed to Patreon all at once. That's Patreon dot com slash PM stutter. Thank you so much for your contributions that keep the show going. I really do appreciate it. And so if you want to concoct a scenario for making globular clusters, you need to make, I don't know, 100,000 to a million starss at once, which is kind of challenging.

You look at something like a milky way galaxy or even a dwarf galaxy and there's all sorts of stars, there are stars that are just getting going, there are stars that are just hanging on to their last legs. There's middle aged stars like our sun. But globular clusters, you have to form all the stars at roughly the same time, which is an amazing feat and you need to shut off star formation so that there isn't even more stars coming online. Think about this when our star was born, when the sun was born, the globular clusters in the milky way were already 5 to 7 billion years old. And there had been no new star formation in those globular clusters for all those billions of years when our sun first ignited. So there's two ways to view globular cluster formation. Remember you, we, we have the Inn and the Audi we think the Inn were born with the milky way and the Audis were born with somebody else and then we later ate with somebody else and that all that's left is the globular cluster.

So how do you actually generate stars like this in galaxy formation? One way is to view is to view and I'm gonna contradict myself and I know it's because I make this show up as I go that it is a mini galaxy or at least was a mini galaxy that you have a whole bunch of dark matter in the early universe. Dark matter is doing what it loves to do, which is glue itself together, make bigger and bigger things and it starts making a milky way and, you know, maybe some stars form and there's the bulge and it's super cool. But then there's this really dense pocket of dark matter way over here and it attracts its own gas and triggers star formation and then out pops 100,000 stars, boom, you've got a globular cluster but because it's so busy interacting with the milky way galaxy as it's forming all the dark matter strips away and you're just, just left with that globe of stars, maybe. But if this is the case, we should see field globular clusters which are globular clusters, not associated with any galaxy just hanging out in the middle of nowhere, which we don't really, we do see some but we think they, those have been ejected from some sort of galactic system and weren't just formed where we happen to see them.

But that could also be because we're not very good at being astronomers. And so we can't see these little globular clusters that are dotting the universe or that it could be that the conditions for this are only ripe when a galaxy is forming and there's tons of dark matter and there's big flows and, and, and all sorts of interesting physics. And that's the only time you can cook up a globular cluster. And by now in the modern day universe, it's just too late. Maybe. Or you can view a globular cluster as like any other star cluster. Just way more of it, star clusters form from the collapse of giant molecular clouds. You get a whole big cloud of dust collapses under its own weight, splinters off those splinters become pockets, the pockets become stars and boom, you can make 100,000 stars like that boom star cluster. So maybe if you just scale up that process, instead of getting 100 stars or 1000 stars, you get 100,000 stars or a million stars.

It's possible and that this could have only happened in the early galaxy when there was lots of gas to go around, lots of material, lots compact. There were lots of mergers. You can imagine a young milky way galaxy just starting to form. It suffers a massive collision with a neighbor. And then you have this giant wall of gas, crashing into another giant wall of gas and they smash together and out pop a million stars and that's your globular cluster. And it's hard to do that nowadays because things are less dense. We live in a bigger universe, things are thinned out and it's just not popular to do that anymore. Basically, these two scenarios, whether you view a globular cluster as a failed mini galaxy or as a superstar cluster hinge on whether you want to include dark matter in the process or not. And uh the truth is we don't know, we don't know which scenario works better. We don't know if it might be both.

We don't know in each of these scenarios, whether you include dark matter or not. Look at globular clusters as a failed mini galaxy or as a super successful super duper star cluster. It's really hard to form globular clusters. We don't have a lot of observations because we only have the globular clusters as they are today. We don't have observations of when G globular clusters were forming like with stars, we can see stars forming with galaxy mergers, we can see galaxy mergers happening. We don't get to see globular clusters popping up. And so it's hard to tell, it's hard to observation and constrain what's going on. But why do we care so much about globular clusters because lots of interesting things happen in globular clusters because they're such high density regions. We got all these stars crammed next to each other. And even though they're old, they can still get interesting just like a retirement community. It's there are lots of mergers, there are lots of collisions, there are lots of near misses.

So we expect in globular clusters. A lot of very interesting astrophysics that tend not to happen inside of normal Galaxies. Like you don't really wanna know what's going on inside of a, inside of a nursing home or a retirement community. You don't really want to know what all those old people with nothing to do lots of time on their hands are up to. We don't want to know there are black hole mergers, there are tidal disruption events, there are flares, there's stellar cannibalism, there's lots of cool astrophysics. There are very interesting laboratories for these kinds of mergers and trans events that we don't get to see in our normal universe. And because they're so bright because there's like a million stars and they tend to hang out outside of the plane of the galaxy. So we don't have to look through the galaxy to see and we can just look and boom there, they are 17,000 light years away and you can just see it with your naked eye. That's pretty handy as an astronomical laboratory that said, because of all those collisions, we do not expect a lot of planets.

If one of those stars was born with a population of planets, billions of years ago, it's almost certainly lost it by now because the stars are just too close. There are too many gravitational interactions, too many tweaks orbits can't be stable or have a very, very difficult time being stable. So far, we've found one planet in a globular cluster and that's around a pulsar and we think that planet came after the pulsar formed. It wasn't part of the original system we're interested in globular clusters because we're searching for intermediate mass black holes. These are the black holes in between the the normal stellar mass black ones and the super giant black ones. Uh The inter intermediate mass is kind of in the name we're interested in the globular clusters because like everything makes black holes and Galaxies make black holes and dwarf Galaxies make black holes do globular clusters make black holes. Maybe if they do, that'd be an important clue because that means they would perhaps evolve like Galaxies do involving a lot of dark matter, involving a lot of mass and then they lose the dark matter at some point in the future.

While the stars hang on. If they don't have black holes, then maybe they look more like giant star clusters because star clusters don't have black holes or big black holes. There have been some claims of detection, but so far, nothing inclusive. Their biggest role is using them as a clue to the formation of Galaxies. We don't understand the formation of globular clusters. We also don't fully understand the formation of Galaxies. Obviously, these are tied together because the globular clusters are leftovers from when Galaxies themselves formed. So they are obviously going to tell us something. We just don't know exactly what that something is. They were used 100 years ago. In a very cool way to pinpoint the location of the center of the Milky way. Because if you just start making star maps and star surveys, of course, we're gonna be roughly at the center because any direction you look, we're surrounded by stars up to a certain observational limit. So it looks like we're at the center of the universe. And before we even knew about things like Galaxies and universes, we didn't know where we were in relation to everything else, but some astronomers figured out uh huh the globular clusters, they tend to be on one side of the galaxy than the other.

They tend to have certain orbits. They're likely because they're big and they're moving, they're likely going to orbit the center of the galaxy, which was estimated to be tens of thousands of light years away from us. So that's cool. But honestly, it turns out there's not much to say about globular clusters because we don't know much about them. They exist, they're mysterious. They are fossils. We don't know where they came from. We don't know how they formed. We don't know the role they play in the formation and evolution of Galaxies and vice versa. So there isn't much to say, but that doesn't mean we can't love them too. Just not a lot. Thanks to Laura w on Facebook and at vital statistics on Twitter for the questions that led to today's episode. Thank you to my top Patreon. Contributors, Justin G. Chris L Barbeque Duncan, M Coy D, Justin Z Nate H Andrew F, Naia Aaron Scott M Rob H Lol Justin Lewis, M Paul G and John W for being my top patreon contributors. There are a lot more go to patreon dot com slash PM Sutter.

So you can join the Space Cadets and as always, leave a review if you can, I really do appreciate it. Buy a book if you want pm Sutter dot com slash book or just look up my name on Amazon or wherever you find books in your neighborhood. My books are probably there and hey, keep sending questions. Hashtag ask the spaceman, ask us spaceman at gmail dot com or the website, ask us spaceman dot com and I will see you next time for more complete knowledge of time and space.

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