John Green, Very Curious: God, I'm so old. Do you ever, like, see yourself in a zoom and just be absolutely shocked?

Dr. Katie Mack, Astrophysicist (chuckling before answering): Sometimes. But you can put on the - you can do the "touch up my appearance" thing on Zoom and then 

John: Can I do a baby filter that turns me into a nine-year-old? 

Dr. Mack: No, no, no, but it'll smooth you. So you'll be an inhumanly smooth person. That's always an option. 

John: It just goes in one direction... time. Well or maybe it doesn't actually, come to think of it. I don't know if I've learned anything in the last few weeks. It's that I have no idea if time goes in one direction. 

Dr. Mack: If you could see yourself from far enough away, you could see yourself as a baby, (chuckling)

John: Right. Yeah. 

Dr. Mack (chuckling before talking): You would have to have set up the mirror a long time ago, 

John: Yeah. 

Dr. Mack: but in principle. 

(theme music plays)

John: Welcome back to The Universe. So far, Katie and I have been talking about the astonishing amount of information that we have been able to learn about our universe, but in this episode, we're going to focus on a mystery of our cosmos: black holes. On one hand, black holes are a valuable tool. Their distinct properties, or lack thereof, help us to map the universe, but there are also things about them that by their nature, we are unable to learn about. So join me as I attempt to wrap my head around these spacetime objects that are infinitely compelling, literally, and yet also, ultimately, unknowable. Here's our conversation.

(theme music plays)

John: So today we are going to pause from our cosmic timeline where we've been going through the first seconds and then millions of years of the universe, and we're going to take a quick gander at something I have genuinely, 

John Green: deeply no understanding of. Which is black holes. I mean, you are painting onto a black canvas right now, Dr. Mack. 

(Dr. Katie Mack chuckles.)

Dr. Mack: Okay. Alright. So black holes are, they're one of these topics that (like) everybody is just constantly fascinated by black holes because they are genuinely one of the weirdest objects that we know exists in the universe. And they're everywhere!

John: So there are lots of them? That's the first thing I didn't know. I don't think I knew that there are lots of them. 

Dr. Mack: There are lots of them. We know there are a lot of black holes in our galaxy. There are somewhere around 50 that we can identify and point at and say, we know that there is a black hole right there. But the estimate is that there's probably closer to like a hundred million in our galaxy.

John: A hundred million?

Dr. Mack: Yeah.

John: Wow. Okay. 

Dr. Mack: The way it works. Some stars become black holes when they die. It's a process that happens often enough that black holes just kind of are everywhere in the universe. And I'll talk about how they happen, but we know that there are a lot in our galaxy. We know that there's a super massive one in the center of our galaxy, and we know that there are supermassive black holes in the center of the other galaxies, pretty much every large galaxy appears to have a supermassive black hole in the center. So they really are completely ubiquitous objects in the universe. They exist all over the place. And that's a fascinating thing because when you get into the details of how they work and what they are and what they are and what they represent, they're just an incredibly bizarre kind of thing to be in the universe. They really are one of the places where our intuitive understanding of physics and even our detailed understanding of physics gets very, very challenged. There are things about black holes we still don't understand especially in the interiors. There are aspects of black holes that we may never have information about because of

Dr. Katie Mack: the way that they sort of limit our ability to understand them. They are truly awesome and to an astrophysicist, they're incredibly useful, which sounds strange, but that's partially because in some ways they're some of the brightest things in the universe. And

John Green: Oh.

Dr. Mack: we'll talk more about that when we talk about the supermassive black holes, because supermassive black holes tend to be some of the brightest things that we can see in the cosmos, and there's some of the things that (supposed to) help us to map the cosmos.  

John: Now that is surprising

Dr. Mack: Yeah. 

John: from the perspective of somebody who's - who's interested in how these things get named. 

Dr. Mack: So. Okay. Let's just back up. So that's kind of the preview of why astronomers are excited about black holes, but let's go back up and (like) let's talk about what a black hole is. 

John: That would be super helpful for me because I don't even know whether to be anxious about all these black holes 

(Dr. Mack chuckles.)

John: until you tell me what they are. 

Dr. Mack: Okay. (chuckling while answering) It may not help when I tell you what they are. 

John: Sure. I don't expect it to help. I expect it to get worse, but I'm still excited. 

Dr. Mack: Okay. Okay, great. So a black hole is something that happens when a really massive star dies. So I'll tell you about how they form first, because that's kind of important. Stars go through a lifecycle and the lifecycle the star goes through depends on how much mass it starts with. The first thing that happens is a bunch of gas gets together in a proto star. It forms a star. A star is technically born when (you know) nuclear fusion starts happening in the center,  it lights up, that's the birth of the star, and stars can start out at a lot of different kinds of masses. The lowest mass stars start at about sort of 8% the mass of the sun,

John: Hmm.

Dr. Mack: something like that?

John: Okay. 

Dr. Mack: 0.08 times the... solar mass. That's the kind of lowest mass star that can still be burning hydrogen into helium in its core. And those really low mass stars, about 0.08 to about 0.4 solar masses. Those ones just (like) they burn hydrogen into helium in their core. They kind of burn slowly. 

Dr. Katie Mack: They don't have a whole lot of mass to (like) create a bunch of pressure in the core, so they kind of just glow for billions of years. So those red dwarf stars can just continue (to sort of) burning slowly for billions of years. Those are long lived stars that don't burn very hot. They don't do anything spectacular when they die. They just kind of burn out eventually, but over a very, very, very long time. 

John Green: Now, before you go any further, I just want to say that my great ambition as a person is to have a life that is as much like one of those small stars as possible,

Dr. Mack: Yeah, yeah. 

John: Long, 

Dr. Mack: Long, peaceful.

John: not too much drama.

Dr. Mack: Yeah.

John: Peaceful.

Dr. Mack: Yeah, exactly.

John: I don't want to have a ton of nuclear explosions.

Dr. Mack: Right.

John: I want to have just a steady, steady diet of them, 

Dr. Mack: Yes. Yeah, just a nice kind of (you know) constant burning, but not

John: Right.

Dr. Mack: sort of inflamed.

John. Yes!

Dr. Mack: Yeah. Yeah, that sounds great. When you start to get a little bit bigger, that's when things start to get more dramatic. 

John: Mhm.

Dr. Mack: So our star, the sun is kind of an intermediate mass star. So this is a range from about 0.4 solar masses to about eight. These are the stars like our sun, where it's burning hydrgen into helium in the core, and it's going to keep doing that for billions of years, but at some point it's going to run out of hydrogen in the core to burn, and there's going to be a few processes that go on. IT's going to burn a little helium for a bit as it's getting toward the end of its hydrogen burning life. It's going to be expanding and turning red. So eventually our sun will turn into a red giant star. So over the course of about a billion years, it'll get brighter and brighter and it'll become bright enough to burn off the oceans of the earth. So that's going to be it for us. 

John: Well, the earth part of us, 

Dr. Mack: The earth part of us. 

John: we'll be on so many planets by then, Dr. Mack?

Dr. Mack: Sure. I mean, we've got a billion yeas to figure it out, right? We'll sort something out. We'll find some other possibility. 

John Green: If we make it to the oceans boiling, I'm just going to say it. It'll be a dang miracle.    Dr. Katie

Mack: That's true. That's true. Yeah, that'll be a good run. 

John: I like our odds of making it through the next billion years at about zero. (Like) I would put it at about zero.  

Dr. Mack: (Yeah.) I don't blame you there. I think that would be impressive for sure. 

John: I think it'd be great if we're in the first quarter of human history. I don't think we're in the first 1%, but I've been wrong before. And one thing about me is I will not be around to find out.  Dr.

Mack: That's true. 

John: So these are the stars that are about the size of our sun that include our sun. 

Dr. Mack: Yes. Yeah. So in the future, (you know) in a few billion years, the sun will turn into a red giant star, and at that point it'll go through changes in the core. It'll sort of blow off the outer layers of the star, and that'll be really cool because that'll create this (like) big nebula.

We see a lot fo these in the sky. They're called planetary nebula. They don't really have anything to do with planets, but it's in historical term anyway.

So there'll be this big colorful nebula created from the slough off outer layers of the star, and the core of star will collapse into an object we call a white dwarf. So a white dwarf is the super dense core of a star, and what happens is that when you stop having nuclear reactions to kind of puff out the gas, then that gas can collapse on itself. Right now, the sun is kind of help up against its own gravity by the pressure from these nuclear reactions in the center.

So there's this balance between the outward push of those nuclear reactions and the inward push of the gravity of just all of the stuff trying to fall together and collapse together under its own gravity.  And so when you get to the point where you stop being able to have those nuclear reactions holding everything up, it can collapse and it can get really, really dense. So there's a kind of maximum density of

Dr. Katie Mack: regular matter where you can put a whole lot fo regular matter together and make it really, really dense. And we have materials that are super dense, really heavy metals can be very, very dense. But there's a point where even that can't hold up the matter anymore, where the electromagnetic forces that hold up atoms and molecules can't hold up that matter anymore, and it gets condensed even more. That creates what's called a white dwarf, where it's called degenerate matter, electron degenerate matter, where you kind of push everything together in this way that's sort of denser than ordinary matter. It's called degenerate because it has to do with how the electrons are kind of in each other's energy levels in this weird way. It's a different kind of matter that's just super, super dense. So a white dwarf can be about the mass of the sun in about the volume of the volume of the earth. 

John Green: Oh. 

Dr. Mack: So it's condensing a whole lot of matter into a very small space 

John: Yeah. 

Dr. Mack: and creating this weird form of matter, this electron degenerate matter where you've got kind of the protons and neytrons and electrons kind of in this sort of strange space.

John: And just to do a quick though experiment. If I were to visit a white dwarf, I assume it would be a big problem for me because the gravity would be really intense, even though it would be an earth-sized object, potentially, it would not be an earth-like experience for me as a visitor? 

Dr. Mack: You would definitely be crushed. You would be sort of squished onto the surface of a white dwarf. It would not be a pleasant thing to be because it would just be this super, super dense object that you're very, very close to.

John: Okay.

Dr. Mack: In general, (you know) the more mass in the smaller amount of space, the worse the gravity is. Those words I did not put together in the right way, but 

John: But I know what you mean. You mean the more extreme the gravity is? Because there's still all the same mass in a 

Dr. Mack: in a small space. 

John: small space. Okay, I got it.

Dr. Katie Mack: Yeah so it's this compactness that's the issue. So a white dwarf is much more compact than a star. 

John Green: Okay. 

Dr. Mack: It's a way that you can pack in a whole lot of matter into a small space by kind of messing with how the electrons interact with each otehr so that they're not kind (of) held up in these levels the way that regular matter is. Now, there are some stars that are more massive than that, 

John: Hmm.

Dr. Mack: that have a different cycle that they go through. So a higher mass star, more than about eight times the mass of the sun. So when it goes through its life cycle, it starts out doing the same thing. It (sort of) burns hydrogen in the core, but when it finishes up all the hydrogen, it can start to burn higher elements, heavier elements too.

So instead of just giving up and collapsing when it finishes its hydrogen, (it has-) there's enough matter pushing everything in. There's enough pressure and temperature in the core that it can burn heavier elements, so it can go through carbon and nitrogen and oxygen, and it can create these kind of shells of burning of different elements as it goes through its lifecycle. So these really massive stars, they can burn heavier elements, and this is part of how we get a lot of these heavier elements in the universe is through burning inside really massive stars also through the death of stars like our sun.

When our sun (you know) sort of blows off its outer layers, it creates some heavier elements in that whole process too. But higher mass stars are doing this kind of interior to themselves, but when a higher mass star gets to a certain point, it's burning through heavier -elements. When it gets to iron, it can't burn iron into a heavier element.

There's this thing that happens in nuclear physics, (that's -) I guess it's a little complicated to explain. If you put together light elements into heavier ones, that creates energy up to a point. So all the elements lower than iron.

If you put the lighter elements to make heavier elements, that creates energy. But on the heavier side, on the higher side, If you try to put together things heavier than iron,

Dr. Katie Mack: it would take energy to do that. And so on the higher side, if you split the nucleus apart, that's what creates energy. This is why you can create a bomb out of either splitting uranium or plutonium or whatever, splitting the big heavy elements or by using hydrogen and fusing it into helium. So those are two different kinds of big bombs that you can do because of the low end fusion, putting elements together creates energy on the high end, fission pulling elements apart creates energy.

So what that means is that when you get to iron, when you're burning up all these heavy elements and you get to iron, you can't fuse beyond iron. You can't just kind of push things together and create energy beyond iron. So when you get to iron, you're not creating any more energy.

When you're trying to push those elements together anymore. You're not creating new pressure to hold up the star. And so at that point, the star can't hold itself up anymore, and that's when it starts to collapse.

So for these really massive stars, they start to collapse once they get to the iron burning stage and the way that collapse happens, it's a really massive thing. The collapse is more violent, it implodes, and then it explodes spectacularly. There's like a bounce off the core, and this creates a supernova.

So it's only these high mass stars that can do that supernova as the end of their life. There's another kind of supernova that can happen that involves white dwarfs sort of gathering more mass from neighbor stars, and that can create a supernova too. But that's a different process.

That's not an end of life supernova. That's something else that can happen, that you can destroy white dwarf stars. But in terms of the end stage of a star, it's only the high mass ones that die by blowing up as a supernova.

And a supernova is just, it's an explosion of the star that creates a really bright explosion. It outshines its own galaxy for a short time. It can be seen billions of light years away.

It's a spectacular thing. Now, our star is not going to do that. 

John Green: Our star is not going to do that.

John Green: When you say for a short period of time, sometimes that means (like) a picosecond, and sometimes that means (like) 2 million years.  

Dr. Katie Mack: It's days. It's like several days.

John: (Okay.) So you've to catch a supernova in a relatively short frame of time to be able to enjoy that beautiful explosion. 

Dr. Mack: (Yeah.) Yeah. That's right. 

John: (Okay.) And so just to make sure I've got it, stars that are larger than eight times the size of our sun, which is a fairly large number of stars.  

Dr. Mack: Yeah. I mean, most stars are low mass. Most stars are lower mass than our sun, but some stars are heavier, so there's a range, (you know) and they can be much more massive than our sun. So there's a range. It's kind of weighted toward the low end, but there's quite a lot of stars that are more massive than our sun. Yeah.

John: (So) And when those stars die, they kind of run out of the elements that they can burn. They get to iron, they can't burn it, and then there's this massive implosion followed by a massive explosion that becomes the brightest thing in that galaxy for a few days. 

Dr. Mack: Yeah, yeah, exactly.

John: Okay. Alright. I mean, so far it seems fine because this seems like other galaxies' problems.  

Dr. Mack: Yeah. Although, I mean, we're a little overdue for one in our galaxy. 

John (grunts): Mmmm.

Dr. Mack: We haven't had one in a while, and 

John: Great. 

Dr. Mack: We're - we're kind of crossing our fingers because it would be, 

John: Whoa, whoa, whoa, whoa, whoa. Wait. Are we crossing our fingers in hopes that it does happen

(Dr. Mack: Yeah!)

John: or in hopes that it doesn't happen?

Dr. Mack: Yeah, because we would learn a lot. 

John: Oh, okay. 

(Dr. Mack chuckles.)

John: And we wouldn't die in the process. 

Dr. Mack: We wouldn't, no.

John: Great.

Dr. Mack: So to give you some comfort, there are no stars within the (like) lethal range of us that we think could go supernova anytime in the next, (like) I dunno, some ridiculously long number of years. 

John: Great. 

(theme music)

John: So you know what that means. As a planet anyway, we're going to be here for a while. Long enough, 

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Policygenius, because there are no nearby stars about to go supernova. 

(theme music)

Dr. Katie Mack: So when that supernova happens, the remnant, the core of that star that did the implosion, it can go a couple of different ways. (So) It can either become a neutron star or a black hole. (And) I'm going to say a little bit about neutron stars first, because neutron stars are these really amazing objects. So if the star is less than something like 20 times the mass of the sun, but more than eight, then when that supernova happens and the core collapses, it's too massive to be held up by even electron degeneracy pressure. Even this white dwarf thing, it's too massive to be a white dwarf. It compresses even further.

And the thing that holds it up when it compresses even furtehr is called neutron degeneracy pressure. And basically what happens there is the whole star core part of the star becomes like a giant nucleus.

(John: Hmm.)

Dr. Katie Mack: So it smashes all of the protons and neutrons together. It turns most of the protons into neutrons. You have this giant kind of nucleus of matter, so it's as dense as an atomic nucleus, but it's about, it's (like) the mass of the sun or a little bit more. So it's this supermassive nucleus basically. So it's held up basically by the fact that you can't have two particles in the same place at the same time, just sort of quantum mechanically. There's this weird thing that it's just about trying to (like) keep the particles existing that (it) holds it up. So it is this very, very extreme form of matter, and we don't understand all of the details of what that matter is. There's all sort of theories about what's going on inside. It might be a super fluid inside, (it) could be these weird vortices, could have a kind of strange structure that's like a sort of lasanga structure. There are all these models with (like), they call it nuclear pasta.

(Both Dr. Mack and John chuckle before Dr. Mack continues talking.)

Dr. Mack: What the configuration of stuff inside the neutron star is. It's a really weird kind of stuff, but it's so dense that you can take the mass of the sun, and if you turn that into a neutron star, it's now the size of a city. 

John: Wow.

Dr. Mack: It's like a couple of kilometers or 10 kilometers or something 

John: Wow,

Dr. Mack: that is extraordinarily dense matter, extraordinarily compact.   

John: wow,

Dr. Mack: And one cool thing about neutron stars is a lot of times when they're formed, when they're compressed that way, the magnetic fields of the star are kind of compressed and twisted around, and the star is born (like) spinning really rapidly. So you can end up with this (like) strong magnetic field in the neutron star, and it's spinning like a little magnet, and it can create these jets of radiation from its poles, from its magnetic poles that (like) throw out radiation (like) positrons and gamma rays. And that jet of radiation can (like) 

Dr. Katie Mack: spin around because the magnetic pole and the rotation pole might not be quite lined up. So it's a little bit off kilter. And that means that, that jet of radiation can (like) sweep through the universe like a lighthouse, like a lighthouse beam. And if we are lined up with one correctly, then we see this pulsing beam of radiation coming to us.

Every time the neutron star spins, they can spin at period of milliseconds. So it depends on the stage of the neutron star that it's at, but it can be these millisecond pulses. These are called pulsars, these kinds of stars.

And we can use them kind of like clocks. They have a very regular pattern. (Because) They're just rotating. And every time they go through a rotation, they flash because of this beam of light.

And so you can use them kind of like lighthouses, like clocks, and you can use that to map out a lot of things about the galaxy by seeing these across the galaxy, these (you know) dozens of neutrons stars, these pulsars, and I'll talk later about how we can use pulsars to learn about supermassive black holes. (chuckles before continuing) There's a kind of cool connection by, (you know) we use these really super extreme objects as sort of markers throughout the universe because we can see them really well and they have these very distinct properties. 

John Green: Wow. 

Dr. Mack: So that's a neutron star. So that's what happens when the initial star is somewhere around 20 or fewer solar masses.

John: Okay.

Dr. Mack: If it's bigger than that, if the star starts out more massive than that, and I don't know exactly where all the boundaries are on these things, but if it's a little bit more massive than that, then when that collapse happens at the supernova stage, you can get a situation where there's so much matter that extreme neutron degeneracy pressure cannot hold it up. So even by turning the whole thing into a nucleus, it's still, there's too much gravitational force. It compresses it more than that.

Dr. Katie Mack: And then there's, there's no force known to nature that can hold it up. There's nothing at all that we know of that could provide an outward push that can counter that much gravity. So you get this runaway, just runaway collapse. 

John Green: You get this sort of cycle of collapse.

Dr. Mack: (Yeah.) So there's just nothing that can stop it. And so all that matter, that would've formed the core of that star just keeps going inward, keeps compressing and compressing and compressing. 

John: Like infinitely? 

Dr. Mack: Well, yeah, because there's nothing to stop it. (Right?) So this is the theory,

John: Okay. 

Dr. Mack: and this where things get fuzzy, because at some point we cannot learn more about that kind of matter, about that process because based on the theory, if there's nothing to stop that collapse, it'll (just) come to a point in the center, the center of that gravity, and it'll just become an infinitely dense point. We call that a singularity. 

John: (Mhm.) Sorry. I got really anxious there. 

Dr. Mack: (chuckling before answering) It's okay. There's a lot of weird stuff about black holes. 

John: So how come they don't suck in everything?

Dr. Mack: Well, because not everything is already falling toward the black hole. 

John: (Oh! All right.) So it's only things that are already falling toward the black hole. It doesn't (like) expand infinitely. 

Dr. Mack: (Yeah.) It can't reach out to distant things.

John: But if I got near it -

Dr. Mack: If you get near it, it's a problem. 

John: If I got in a spaceship and got close to it, that would be a big problem. 

Dr. Mack: Yeah. The way that gravity works, (like) let's say that you're in, (like) I dunno, a giant cloud of gas or something, and there's a whole lot density at the center, but you're kind of toward the outer edges. You're going to feel the gravity of everything within (like) closer to the center than you. (Like) You can draw a sphere and you're at the edge of that sphere and everything close to the center, 

Dr. Katie Mack: you're going to feel the gravity of that. If you get closer and closer to the middle, you're just going to feel the gravity of the stuff interior to you. And so if you can get really far away from that gas cloud, you're going to feel as much gravity as if all of that whole glass cloud was compressed to a single point in the center. The amount of gravity that you'll feel is the same because you're still just feeling all of the gravity of this stuff interior to you. 

John Green: (Mhm.) Okay. Okay. 

Dr. Mack: So what matters is how close you are to the center of (like) where the matter is concentrated. So (like) on the earth, we're not that close to the center of the earth. (Right?) We're feeling the gravity in a sense of all of the matter interior to us. So (like) the whole planet, we're feeling it the same as though all of that mass were concentrated at the center. The amount of gravity we feel from it is the same. So if we could compress all that matter to a smaller space, we could get closer to the center. But as it is, if we got closer to the center of the earth, there would be less matter between us and the center of the earth. So we wouldn't feel all of the matter of the earth (you know) interior to us. 

John: (Okay.) So if you made the earth half the size that it is now with the same density, 

Dr. Mack: Well, the same amount of stuff -

John: same overall amount of stuff, but in half the size, I would still walk around the earth feeling the way I feel now. 

Dr. Mack: Let me put it this way. If you could keep the outer layer of the earth, but compress all of the rest of it into a smaller space, you would feel the same. 

John: Right, right. (Okay.)

Dr. Mack: But if you actually compressed it all and then you were down there, right? If you compressed everything and they were down there, then you would feel more gravity because you're closer to the center of where all that gravity is.

John: Okay. Okay.

Dr. Mack: However you redistribute the matter, as long as it's till kind of interior to you in that sort of sphere, it's how far you are from, I mean, to first approximation, there are things that can change, but (like, you know,) how far away you are from the center.  So a neutron star, a white dwarf, (you know)