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«Astronomy Cast Episode 68: Globular Clusters Fraser Cain: This week, we’re going to study some of the most ancient objects in the entire universe: ...»

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Astronomy Cast Episode 68:

Globular Clusters

Fraser Cain: This week, we’re going to study some of the most ancient objects in the

entire universe: globular clusters. These relics of the early universe contain

hundreds of thousands of stars held together by their mutual gravity. In fact,

when I get a telescope out to show my friends and family, the great globular

cluster in Hercules is one of the first things I’ll point out. It just looks like a

fuzzy ball through a telescope, but in my mind I can see all the stars.

Let’s just talk a bit about globular clusters. What are they?

Dr. Pamela Gay: They are, at the most simplistic level, they’re collections of 10 thousand to hundreds of thousands of stars gravitationally bound together that formed in some cases 12 billion years ago. They’re out, orbiting on the edges of our galaxy, and on the edges of most of the galaxies we observe out there.

Fraser: How many are we going to find in a typical galaxy like the Milky Way?

Pamela: It’s all a function of how big the galaxy they’re attached to is. Our own galaxy seems to have well over a hundred different globular clusters. We’re finding new ones every day as satellites like the Spitzer Infrared Observatory peer through the dust and gas and are able to find new globular clusters in places we hadn’t been able to look before.

They basically form a spherical distribution all the way around our galaxy, orbiting in some cases in two different directions. There’s two different populations. They’re old, metal-poor, and everywhere we look. They’re the ancient stewards of our galaxy.

Fraser: Okay. When we talk about ancient, how ancient are they?

Pamela: One of the great mysteries for a long time was, we looked at them and they seemed to be older than the universe. It turned out we had miscalculated how old the universe was and we had miscalculated how old the stars were. Around the year 2000, once we got everything put together, it began to show up that our universe is 13.7 billion years old and these clusters of stars are 12 billion years old.

Fraser: I love that. I love that up until the year 2000, astronomers knew there were stars that were older than the estimates of the age of the universe, and that bugged them, but they were able to just kind of deal with it – “yeah, we have our estimate for the age of the universe wrong, and we probably have our estimate for the age of the stars wrong, but for now this is the best we can do.” [laughter] 1 I think that’s great.

Pamela: I’ve had this moment at the chalkboard before. You start off at the upper-left- hand corner of three chalkboards and you start deriving equations, and you keep going and going and get to the very end and look at the number on the board and it doesn’t match the number you calculated in the quiet of the privacy of your own office. You know somewhere on those three chalkboards, there was a mistake. And you don’t know where.

Now, at the chalkboard, I can usually go through and my students are more than willing to help me find where I dropped the one-half or squared something that should’ve been cubed. But when making calculations of the age of globular clusters, you’re not talking about three chalkboards of calculations. You’re talking about thousands of lines of computer code going through and trying to calculate stellar evolution models, saying, “a star spends this long on the main sequence doing these things” In all those thousands of lines of code, in all of the mathematics that go into the simulations to write those thousands of lines of code, there are so may places where our approximations might not be right, or where we might be missing a term in our calculations. It took us a long time to figure out what was going on and to get computers powerful enough that we didn’t have to make as many approximations.

Then, when it came to measuring the age of the universe, it was an observational challenge that was pretty much unsettled until the WMAP results came in. There, we just had to build the bigger, better microwave telescope.

Fraser: Okay, fine. So they’re not older than the universe. That’s still plenty old. What kind of forces came together to build these globular clusters in the first place?

Pamela: A large, dense, glob of stuff all by its lonesome settled into forming dense, rich stars. Over time, the stars segregated themselves by mass.

Fraser: Why did they form all these different stars and not just one big, supermassive black hole?

Pamela: As the cloud of material collapses, it ends up fragmenting. It turns out that you don’t generally have one nice, completely smooth cloud of gas. Rather, you have a cloud of gas with a few knots in it. Those individual knots, those individual places that are a little bit more dense than other locations, as the entire cloud collapses those little knots end up collecting gas to themselves, hogging it and forming individual stars out of this large clump of gas and dust.

–  –  –

Fraser: Do they form as separate clumps as the galaxy is forming, almost like planets inside a solar system might form around a star? Or did they form as kind of mini-galaxies and get absorbed into galaxies through collisions later on?

Pamela: One of the large mysteries we’re trying to sort out is why we have globular clusters with very specific geometries and star distributions that are roughly the same size as dwarf galaxies. What is it that made one clump of dust and gas form a globular cluster, and another clump of dust and gas form a dwarf galaxy?

We’re still working to figure that out.

We think part of it might be globular clusters form in the halos of pre-existing giant galaxies. Dwarf spheroidal galaxies tend to form in isolation all by themselves. Some how, the kinematics involved ends up with two different things forming. Part of this might be the dark matter involved. Globular clusters don’t have the same dark matter halos associated with them that you get with little tiny dwarf galaxies. If you take a dark matter halo and throw a globular cluster’s worth of mass inside of it, you can get a dwarf galaxy.

If instead you just take a clump of dust and gas and embed it inside the much larger dark matter halo of a giant galaxy like the milky way, then you seem to get globular clusters.

Fraser: I didn’t realise that the amount of matter in a globular cluster could be the same amount as in a dwarf galaxy. That’s quite interesting.

Pamela: It’s one of those weird things. This is only true for the smallest of the dwarf galaxies and the largest of the globular clusters.

Fraser: What about composition? What kinds of stars are they? You called them metalpoor – why’s that?

Pamela: Stars come in a lot of different compositions. Our Sun tends to have, for a star, a lot of things like iron – a lot of heavier elements (like silicon). We can look at it’s spectrum and say, “look at all those rich titanium lines, those rich strontium lines in the spectrum of the star.” Instead, if I start looking at the elements found in the stars of a globular cluster, I’ll see a lot of those elements just aren’t present. These stars can have a hundred or even a thousand less metal than our Sun has in it. We call these stars metal-poor because compared to the Sun, they have only a percent or a fraction of a percent of the same number of heavy atoms in their atmosphere.

–  –  –

Pamela: 12 billion years ago, there just wasn’t that much heavy metal hanging out waiting to be eaten into the newly forming stars. One the really cool things about globular clusters is pretty much all of the stars in the globular cluster formed in one violent period of star formation.

When I look at a sample of a hundred different stars in say, M13 (the Hercules cluster you mentioned), all those stars are going to be basically the exact same age. They’re going to have formed out of the same cloud of material (so they have the same composition). The only thing that varies from star to star in these systems is their mass.

Fraser: That was going to be my next question. We learned early on that the heaviest stars burn their fuel quickly and then detonate at supernova, while the smaller – the Sun-sized stars and smaller can live on for billions and billions of years as main sequence or white dwarf stars. Is there some kind of mass limit where you just doesn’t see a certain size of star in those clusters anymore?

Pamela: That’s right. You look at these things and none of the large stars are left any longer. You’re down to stars smaller than the Sun hanging out on the main sequence. Then, you have remnants of the stars. You have whit dwarfs, neutron stars, all hanging out going, “hey! We used to be big!” these are stars that shed their mass, exploded as supernova and went through planetary nebula formation. Those planetary nebula have, in many cases, been largely destroyed just by the passing of time. Globular clusters are systems rich in ancient stars and stellar remnants – nothing young or big.

Fraser: Are there any forces that will take a globular cluster apart? They’ve been around for 12 billion years – there must be some really serious forces keeping them together.

Pamela: They’re one of the most tightly bound objects we know of (in terms of large populations of stars). Open clusters, in the disk of our own galaxy are much smaller – hundreds of stars in some cases. They get shredded by gravity over time. Globular clusters are tightly bound systems that are able to, in general, sustain orbiting our galaxy.

As we look around we do see instances of globular clusters that are elongated or a little bit mis-formed, that have gone through gravitational interactions with our galaxy or with other galaxies. That’s the cool thing: we can observe globular clusters around our galaxy, around some of the dwarf galaxies (the Fornax dwarf has its own globular clusters, we see them in the Large

–  –  –

Fraser: Why do astronomers find globular clusters so interesting? Do they use them as a tool for some of the science they’re working on?

Pamela: They’re laboratories. Because you can look at M3 and get several thousand stars made out of the same stuff, you can see “if I change this variable involving mass, I get this difference in outcome. If I create a binary system, I get this difference in outcome” We can use them to say “I’ve now controlled for age and composition, all I’m going to vary is whether a star is in a binary or not, and what the mass of that star is.” I can then see the outcome in the star’s evolution.

These things, while they’re all more metal-poor than our Sun (at least the ones around our own galaxy), they’re all slightly different ages. They’re ancient – but they’re slightly different versions of ancient. It’s sort of like going from a 70 year old to a 90 year old. They’re all grandparent-age, but there are differences between a 70 year old and a 90 year old biologically. With these systems, they’re all ancient, but there are differences in stellar evolution that we’re able to observe.

They’re one of the most fascinating tools for studying stellar evolution that we have, because you can see so many stars and control what you’re looking at so carefully.

Fraser: I guess with a hundred thousand (or more) stars in a cluster, you can see every single mass of star from the smallest white dwarf, or the smallest red dwarf, all the way up to the largest star that hasn’t died yet. I guess you can see, in some clusters that line falling off. In some cases, the bigger stars have died, and in other clusters they’re younger and the biggest stars haven’t died yet.

Pamela: Yeah, no. All the big stars are dead. That’s the funky thing about them: there are no big stars. You’re left looking at strictly solar-sized type stars and smaller in most cases.

Fraser: Do we see any clusters that are younger than this 12 billion years old? Do we see any that are just forming anywhere?

Pamela: Not locally, but as we look out at other galaxies, we do start to be able to see them around other galaxies, particularly in star-forming regions and in areas where galaxies still have chunks of basically, virgin gas waiting to get used. We did, starting in 2000, start to discover newly-forming globular clusters. That was kind of cool. Up until then, we had no clue where these buggers came from, we just knew they were out there. We didn’t know which came first: the galaxy or the globular cluster. Now we know that they form together.

–  –  –

Pamela: This is something we’re still trying to work and figure out. One of the problems is we can steal globular clusters from other galaxies when we eat them. It’s hard to sort out the naturally born, biological globular clusters (to use a bad analogy) and the adopted children.

Fraser: How would we tell the difference?

Pamela: That’s the problem. With the Milky Way Galaxy, we have these two populations of globular clusters. One is orbiting around the galaxy in the same direction the galaxy is rotating. The other population seems to either not be rotating relative to the Milky Way or it’s going in the wrong direction.

With these two different kinematic populations, we also find differences in the composition of the stars. One population has even fewer metals than the other population. Astronomers are left thinking this is probably because we ate another galaxy and stole its globular clusters, but there’s also the possibility that maybe one group of these systems just formed a little later on, a little further out. We’re not really sure.

We need to keep studying, and keep looking at other galaxies with highresolution images. If I watch a galaxy that’s just starting to form (and we’re just starting to find occasional examples of galaxies still forming today), how is it the globular clusters form with them?

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