Anna Frebel is an astronomer and astrophysicist at MIT. Please support this podcast by checking out our sponsors:
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OUTLINE:
0:00 - Introduction
1:02 - First elements
8:11 - Milky Way
11:47 - Alien worlds
14:52 - Protogalaxies
20:05 - Black holes
25:03 - Stellar archeology
34:18 - Oldest stars
42:08 - Metal-poor stars
57:41 - Neutron capture
1:02:37 - Neutron stars
1:08:06 - Dwarf galaxies
1:12:46 - Star observation
1:41:03 - James Webb Space Telescope
1:46:53 - Future of space observation
1:50:02 - Age of the universe
2:03:10 - Most beautiful idea in astronomy
2:06:59 - Advice for young people
2:15:53 - Meaning of life

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I would run outside and just lay on the ground under the Southern Milky Way, beautiful, right up there.

And I would just lay there like the snow angel and just kind of let my thoughts sort of pass through my brain.

And this is when I personally have the feeling that I'm a part of it, I belong here, rather than feeling kind of small.

Yes, I'm small, but there are many other small things and lots of small things make 1 big whole.

The following is a conversation with Anna Frabel, an astrophysicist at MIT, studying the oldest stars in the Milky Way galaxy in order to understand the chemical and physical conditions of the early universe and how from that, our galaxy formed and evolved to what it is today, the place we humans call home.

To support it, please check out our sponsors in the description.

And now, dear friends, here's Anna for Bell.

What did the formation of the Milky Way galaxy look like?

Or maybe we wanna start even before that.

What did the formation of the universe look like?

Well, we scientists believe there was the Big Bang, some big beginning.

But what is important for my work, and I think that's what we're going to talk about, is what kind of elements were present at that time.

So the Big Bang left a universe behind that was made of just hydrogen and helium and tiny little sprinkles of lithium.

As it turns out, it's actually quite hard to make stars or any structure from that.

And so the very first stars that formed prior to any galaxies were very massive stars, big stars, a hundred times the mass of the Sun, and they were made from just hydrogen and helium.

So big stars explode pretty fast after a few million years only.

And in their explosions, they provided the first heavier elements to the universe because in their cores, all stars fuse lighter elements like hydrogen and helium into heavier ones, and then that goes all the way up to iron, and then all that material gets ejected in these massive supernova explosions.

And that marked a really, really important transition in the universe because after that first explosion, it was no longer chemically pristine.

And that set the stage for everything else to happen, including us here talking today.

So there's a whole complex soup of elements now as opposed to just hydrogen, helium, and a little bit of lithium.

Yeah, so after the Big Bang, just hydrogen and helium.

We don't really need to talk too much about lithium because the amount was so small.

And after these very first stars formed and exploded, they, the heavier elements like carbon, oxygen, magnesium, iron, all of that stuff was suddenly present in the gas clouds.

But that actually helped, especially the carbon and the oxygen, to make the gas cool.

These atoms are more complicated than hydrogen, that's just a proton, and so it has cooling properties, can send out photons outside of the gas cloud so the gas can cool.

And when you have gas that gets colder and colder, you can make smaller and smaller stars, so you can fragment it and clump it and turn it into stars like the Sun.

And the cool thing about that is that when you have small stars like the Sun, they have a really long lifetime.

So those first low-mass stars that formed back then are still observable today.

I try to find these early survivors because they tell us what the gas looked like back then.

They have preserved that composition of these early gas clouds, the chemical compositions, until today.

So I don't need to look very far into the universe to study all the beginnings.

I can just chemically analyze the oldest stars and it's like unpacking everything that happened back then.

So to just reiterate, so in the very early days, in the first few million years, there was giant stars that's mostly hydrogen and helium, and then they exploded in these supernova explosions, and then they made these clumps.

So it took a few hundred million years for the first stars to emerge.

And then they exploded after a few million years.

And then it's like, I always consider the universe like a nice soup.

And then these first supernova explosions kind of provided the salt, just a little sprinkle of heavier elements, and that made it really tasty.

And that changed the physics of the gas, so that meant that these gas clouds that were surrounding the former first stars, they could now cool down and clump and form the next generation of stars that now included also little stars.

And as I just mentioned, the small stars have these really long lifetimes.

The Sun has a lifetime of 10 billion years.

Any star that is even less massive will have an even longer lifetime.

So that gives us a chance to still observe some of the stars that formed back then.

So we are testing the conditions, the chemical and physical conditions of the early universe even before the galaxy formed.

So what's the timeline that we're talking about?

What is the age of the universe and what is the earliest time we got those salty delicious soup, clump soups with heavy elements?

Well, the universe is 13.8 billion years old.

Well, you know, when I was in high school, the universe was 20 billion years old.

Do you think that estimate will evolve in interesting ways or no?

have mostly converged, yes, because the techniques are very different now, much more precise.

The whole business of precision cosmology by mapping out the cosmic microwave background, you know, that's a marvelous feat.

Maybe, you know, the digits will still move around a little bit, but that's all right.

Plus the gravitational waves and all that.

Kind of mapping out this detailed picture of the early universe.

And so we think the earliest little stars formed, I don't know, maybe half a billion years after the Big Bang, right?

Again, a few hundred million years for the first stars to emerge and then, you know, took some time.

And That was the time when sort of the very first proto-galaxies formed, early stellar structures, stellar systems from which the Mickey eventually formed, right?

So the Mickey was probably a bigger, slightly bigger 1.

And we know today that galaxies grow hierarchically, which means they eat their smaller neighbors.

So if you're the bigger 1 and have a few friends around, you're just gonna eat them, absorb them, and then you grow bigger.

And so all these little early stars kind of came into the Milky Way through that kind of process.

And that's why we find them in the outer parts of the galaxy today, because they're just kind of left there since.

So the old stuff is on the outskirts of the galaxy and the new stuff is closer to the middle?

So maybe just a step back, what is a galaxy, what is a proto-galaxy?

So the galaxy is a huge assembly of stars.

The Milky Way contains something like 200 to 400 billion stars, and most of the material and the stars are in the disk.

And when we look at the night sky, what we see as the Milky Way band on the sky, that is actually the next inner spiral arm Because we actually live in a spiral disk galaxy, so the Miku is a spiral disk galaxy.

And we're looking, actually it depends a little bit, in the northern hemisphere we're looking out of the galaxy.

So we're seeing the next outer spiral arm.

And as you can imagine, there's only dark space behind that, so we don't see it all that nice on the sky.

But if you travel to the Southern Hemisphere, let's say South America, you see the Milky Way and it looks so different on the sky because that's the next inner spiral arm and that's backlit by the galactic center.

The galactic center is a very big puffy region of gas, there's a lot of star formation, the galactic party is happening there, so it's very bright and it makes for this very beautiful Milky Way on the night sky that we see.

So actually, if you ever get the chance to experience that, I encourage you to almost like close your eyes while seeing this and imagining that you're sitting in this kind of disc in this pancake and you're just kind of looking right into it and you can really feel that we're in this 2D disc and then you can imagine that there's a top and a bottom and that we're really part of the galaxy, you can really experience that.

We're not just lost in space somewhere, but we're really a part of it.

And knowing a little bit about the structure of the Miku'e really helps.

Do you feel small when you think about that?

When you look on that spiral on the inside of the Milky Way and then you look out to the outside.

I feel in awe and I feel I'm a part of it because I can really feel that I'm a part of it.

I think for many people they think like, oh, there's just the planet and then there's nothing.

And that's almost a little bit sad, but that's really not the case, right?

Because there's so much more, and I really like to imagine, wow, I'm sitting in this big galactic merry-go-round, and we're going around the center, and I can see the center above me, right?

And I can almost feel like we're going there.

Of course, we can't really feel that, but the sun does circle the galactic center.

But there's a kind of sadness to like looking pictures of a nice vacation place.

Do you feel like sad that we don't get to travel or you and I will not get to travel there and maybe humans will never get to travel there?

Yeah, I always wanted to travel into space and see the Earth and other things from up there.

They're certainly that, but I don't know.

It's also okay to just be at our vantage point and see it from here.

With the sensors, with the telescopes that we have and explore the possibility.

I mean, there is a kind of wonder to the mystery of it all, what's out there, what interesting things that we can't possibly imagine.

You know, there could be all kinds of life forms, bacteria, all this kind of stuff.

I tend to believe that, You know, it depends on the day.

I tend to believe there's just a lot of very primitive organisms just spread out throughout and they built their little things like bacteria type organisms.

And just to think what kind of worlds there are, because they're probably really creative living organisms.

Because the conditions, I guess the question I'm wondering to myself when I look out there to the stars, how different are the conditions on the different planets that orbit those stars?

We know now that I think it's about every other star has at least 1 planet.

I already mentioned the number of stars in the galaxy.

I mean, you know, It's a huge number of planets out there.

All we know is that there is a lot of variety.

We don't quite yet understand what drives that, what governs that, why that is the case, why is it not all 1 size fits all?

Or you mean the dynamics of planet formation, like exoplanet formation, or star formation, the whole shebang?

Star formation remains a much researched topic.

But the details are so varied per gas cloud, right?

It's very hard to come up with very detailed prescriptions.

You need a gas cloud, you need to cool it, something clumps and fragments, and somehow it makes a star with planets or without.

But the dynamics of the clumping process is not fully understood.

No, no, and the local conditions are so varied, right?

I mean, it's the same with, you know, all people look like people, but individually we look very different.

So even the subtle diversity of the formation process creates all kinds of fun differences.

Yes, so we just don't know how this turned out in an individual case and it's kind of hard to figure it all out and to take a look, certainly with planets, right?

The chance to ever actually take a picture of a planet.

So I'd say there's a lot of possibility out there, but we have to be a little bit more patient.

Come up with technologies where patience becomes less necessary by extending our lifetimes or increasing the speed of space travel, all that kind of stuff.

Well maybe just to linger on what a galaxy is.

What should we know about our understanding of black holes in the formation.

Is that an important thing to understand in the formation of a galaxy?

So all the orbiting, all the spiraling that's going on, how important is that to understand?

That's what makes astronomy really hard, but also really interesting, right?

No day is like another because we always find something new.

I want to come back to the idea of the proto-galaxy because it actually matches or relates to the black hole formation.

So most large, well, pretty much all large galaxies have a supermassive black hole in the center.

And we don't actually know, we don't really know where they come from.

Again, we know that they are there, but how do we get there?

So we go back to the early universe, right?

We had a little galaxy that just sort of had some small number of stars.

It was the first gravitationally-bound structure that was held together by dark matter because dark matter actually kind of structured up first before the luminous matter could, because that's what dark matter kind of does.

And it started to hold gas and then stars sort of together in these first very shallow what we call potential wells, so these gravitationally-bound systems.

And then the Mikue grew from absorbing neighboring smaller, even smaller systems.

And somewhere in that process, There must have been a seed for 1 of these supermassive black holes, and I'm not actually sure that it's clear right now what was their first, the supermassive black hole or the galaxy.

So lots of people are trying to study that.

And of course, the black hole wasn't as massive back then as it is these days.

But it's a big area of research, and the new James Webb JWT, the telescope, the infrared telescope in space, many people are working on that to figure out exactly what happened.

And there are some surprising results that we really don't understand right now.

So to solve the chicken or the egg problem of, do you need a supermassive black hole to form a galaxy or does the galaxy naturally create the supermassive black

Yeah, I mean, I think to some degree we can answer that because there are lots of little dwarf galaxies out there.

The Milky Way remains surrounded by many dozens of small dwarf galaxies.

I have studied a bunch of them and to the extent that we can tell, they do not contain black holes.

So There certainly were gravitationally bound structures, so either you can call them proto-galaxies or dwarf galaxies or first galaxies.

They were definitely there, but there must have been bigger things like the proto-Milky Way where something was different, right?

What made them more massive so that, you know, they would gravitationally attract these smaller systems to integrate them.

How do we look into that, into the dynamics of the formation, the evolution of the portal galaxies?

I mean, what are the set of data that we can possibly look at?

So we got gravitational waves, which is really insane that we can even detect those.

So that would fall into the category of observational cosmology.

And the JWST is the prime telescope right now, and it promises big, big steps forward.

This is in its early days because it's only been online like a year or so.

But that collects the infrared light from the farthest, like literally, proto-galaxies, earliest galaxies.

That light has traveled some 13 billion years to us, and they're observing these faint little blobs.

And folks are trying to, again, study the onset of these early supermassive black holes, how they shape galaxies.

So they're seeing that they were there, surrounded by already bigger galaxies.

Ideally, I'd like for my colleagues to push a little bit further.

In terms of looking towards older and older

Yeah, yeah, more and more sort of primitive in terms of the structure.

But of course, as you can imagine, if you make your system smaller and smaller, it becomes dimmer and dimmer, and it's further and further away.

So we're reaching the end of the line from a technical perspective pretty quickly.

But it's dimmer and dimmer means older and older.

Yes, in a sense, because it all started really small.

it's smaller and smaller, which correlates to older and older.

In that phase of the universe it would, otherwise it doesn't,

Just to take a small attention about black holes, and you know, because you do quite a bit of observational cosmology and maybe experimental astrophysics, what's the difference to you between theoretical physics and experimental?

So there's a lot of really interesting explorations about paradoxes around black holes and all this kind of stuff, about black holes destroying information.

Especially when you step away from your work and kind of think about the mystery of it all.

Well, at first glance, there isn't actually much cross talk.

Personally, I mostly observe stars, so I don't usually actually think too much about black holes.

And stars is a fundamentally kind of chemical, physical phenomena that doesn't.

That's right, the physics is kind of different.

I mean, you could consider nuclear fusion sort of be perhaps extreme, you need to tunnel.

That has some interesting physics there, but it's just a different flavor and I don't do these kinds of calculations myself either.

I very much like to talk with my theory colleagues about these things though, because I find there's always an interesting intersection.

And often it's just, I've written a number of papers with colleagues who do simulations about galaxies, and so they're not quite as far removed as, let's say, the black hole pen and paper folks.

But even in those cases, we had the same interest in the same topics, but it was almost like we're speaking 2 different languages.

And we weren't even that far removed, you know, both astronomers and all.

And it was really interesting just to take the time and really try to talk to each other.

You know, even amongst scientists, we already have trouble talking to each other.

Imagine how hard it is to talk to non-scientists and other people to try, you know, to...

We're all interested in the same things as humans at the end of the day, right?

But everyone has sort of a different angle about it and different questions and way of formulating things.

And sometimes it really takes a while to converge and to get to the common ground.

But if you take the time, it's so interesting to participate in that process.

And it feels so good in the end to say like, yes, we tackled this together, right?

We overcame our differences, not so much in opinion, but just in expressing ourselves about this and how we go about solving a problem.

And these were some of my most successful papers and I certainly enjoyed them the most.

I mean, there's a, I think you put it really well in saying that we're all kind of studying the same kind of mysteries and problems.

I mean, I see this in the space of artificial intelligence.

You have a community, maybe it seems very far away, artificial intelligence and neuroscience.

You know, you would think that they're studying very different things, But 1 is trying to engineer intelligence and in so doing try to understand intelligence.

And the other is trying to understand intelligence and cognition in the human mind.

And they're just doing it from a different set of data, a different set of backgrounds, and the researchers that do that kind of work.

And probably the same is true in observational cosmology and simulation.

So it's like a fundamentally different approach to understanding the universe.

Let me use for simulation, let me use the things I know to create a bunch of parameters and create some, just play with it, play with the universe, play God, create a bunch of universes and see in a way that matches experimental data.

It's a fun, it's like playing Sims, but at the cosmic level.

So, but, and then probably the set of terminology used there is very different, and maybe you're allowed to break the rules a little bit more.

Let's have, you know, yeah, take the Drake equation.

You kind of come up with a bunch of values here and there and just see how it evolves and from that, kind of intuit the different possibilities, the dynamics of the evolution of a galaxy, for example.

Yeah, but it's cool to play between those 2.

Because it seems like we understand so little about our cosmos.

And everyone kind of has their little corner and they do things, but we're all in the same sandbox together at the end of the day.

But in that sandbox does have super powerful and super expensive telescopes.

That everybody's also, all the children are fighting for the resources to make sure they get to ask the right questions using that big cool tool.

Well, can we actually step back on the big field of stellar archeology?

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Anna Frebel: Origin and Evolution of the Universe, Galaxies, and Stars | Lex Fridman Podcast #378