It’s time for school! The Astro101 series will cover some of the most important questions in astronomy. In today’s lesson, we’ll have: What is a white dwarf? What is a neutron star? What is a black hole? I discuss these questions and more in today’s Ask a Spaceman!
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EPISODE TRANSCRIPTION (AUTO-GENERATED)
Class is in session. Now when you're faced with a new discovery, something brand new pops up in your experiment or your observations or your telescope or your laboratory or whatever, you have two choices. When something unexpected happens, you can either say, Eureka. That's exciting. Or you can say, shut up.
Observational science is built on finding patterns and structures. Like, this is what this is like a good chunk of what science is. It's just making observation after observation and looking for the patterns, looking for the correlations, looking for what connects one thing to another thing, to look for structures in everything from biological rhythms to celestial rhythms. So a lot of rhythms. But what happens when something shows up to to buck the trend, something that doesn't fit on the known patterns or structures?
This is it's so fundamental in so much of what I've talked about, not just in this series. And, yes, this is a part of a series called Astro one zero one where I'm talking about some of the basics in astronomy. So new listeners have somewhere to go to get up to speed, and then old listeners can just relive the glory days of their college freshman astronomy class. So much of what I talk about in this series and also this entire show, it is patterns and structures that we find in nature, and we learn a lot. It's through those patterns and structures that we are able to infer, that we are able to deduce, that we are able to ground our mathematical theories in a realistic basis, and then go out and make predictions and do all that fun science y stuff.
But what happens when something shows up that doesn't work? Well, no matter what, it means more work, which can be good because, you know, you get to discover new things and uncover new even deeper more fundamental patterns. And, you know, maybe you'll win a Nobel Prize or maybe a couple, or you'll just be famous and you're just you're just figure something new out. Or it can be bad because we really thought we understood what was going on, and now we don't. And now we have to go back to the drawing board.
We have to go back to the chalkboard. We have to revisit these old theories, and we thought we solved this and put it behind us, but now we have to talk about it again. In the early nineteen hundreds, we discovered white dwarf stars, which ruined a bunch of astronomers' dreams of how stars might work. I mean, keep in mind, in the early nineteen hundreds, we still had no idea how stars power themselves, but at least we're starting to have a classification scheme that made some sort of sense, if you remember last episode or a couple episodes ago ago about Annie Jump Cannon's amazing work in stellar classification and stellar means of the stars and can also mean, like, it was really fantastic classification, stellar classification scheme, where we were starting. And and it's amazing to think this was only a hundred years ago.
This is before airplane well, okay. About the same time as airplanes, but before nuclear power, like, before the world war like, just a hundred years ago, we had no idea how stars work. Eventually, we figured out the whole fusion nuclear fusion thing. But before that, we were starting to get some sort of handle on stellar classification, and we found some general patterns and structures that were starting to look pretty nice. You had small stars that were always red and dim.
Then you had medium stars that were always white and kinda sort of hot. And then you had big stars that could either be really, really, really hot, like blue giant stars, or you can have giant stars that were not so hot. They were red. And that was a separate problem. We had no idea what was going on there, but at least things made sense.
If you're a small star, you are dim and red. If you're a medium star, you are medium ish in temperature and you are white and medium ish in size. And then if you're a big star, you are either blue or red depending on the circumstances. At the time in the early nineteen hundreds, all small stars were what we classify as m type, which means they're dim and cool, and they are red dwarfs. So if you see, m type star, it's red.
It's it's a red dwarf star. It's small. Okay. And so in the early nineteen hundreds, they were doing surveys of this new class of star, this red dwarf star, and they're like, hey. Check out all these little stars over there.
Let's classify those. And then all these stars over there. Let's classify those. And then, oh, yeah. All these stars over there.
Let's classify those. And every time they're like, okay. That one is a m type red dwarf. That's a m type red dwarf. That one's a white dwarf.
That one's an m type red dwarf. Wait. Hold up. White dwarf stars were discovered completely on accident during a routine survey of red dwarf stars. And some people got a little bit upset because this was annoying.
Wait a minute. Wait a minute. You're telling me that there is a small star that is not dim and cool and red. Instead, it is bright and hot and white. What's going on?
Some people are just annoyed, and other people's were exciting. Some people said, well, exceptions are where we learn new things. And other people said, well, I don't wanna learn new things right now. I'm tired. So people started studying these.
They very quickly got the name white white dwarf because what else do you call? It's a dwarf and it's white. People started studying it more. And things got especially funky with these white dwarf stars once we started to calculate their size. And you can calculate the size of a star by one if you know how far away it is.
If you know how far away a star is, then you can compute its absolute brightness so you know exactly how bright it actually is as if you were standing right next to it. Then you can also measure its temperature. You get this from the color of the star. In the case of a white dwarf, that means it's hot. And then come based on its brightness and its temperature, you can do a quick little bit of math, and you can get a surface area of the star.
And once you get the surface area, then you know its diameter and its radius. And if it happens to be orbiting anything else or anything else is orbiting it, you can use that orbital mechanical motion to get its mass, and you put together its size and its mass and you get the density. And these things, the white dwarf stars, were the strangest things ever discovered in astronomy at the time. They were just weird. There is a very, very famous quote by sir Arthur Eddington.
Sir Arthur Eddington was an absolutely famous super genius astronomer extraordinaire. He didn't win a Nobel Prize because, I don't know, because he didn't really do a lot, like, Nobel Prize worthy. I guess, I don't wanna say that too much or too loudly. Anyway, I could do another episode on sir Arthur Harrington, but he did lots of stuff. He's the one that that did the expedition to show that Einstein's theory of general relativity was correct.
Like, he's just a really good astronomer. He has this wonderful quote. He said, and I quote, please go to patreon.com/pmsutter to keep supporting the Ask a Spaceman podcast and YouTube series. Paul sincerely appreciates it. Signed, sir Arthur Eddington, astronomer who lived a century ago.
I'll tell you the real quote. Quote, we learn about the stars by receiving and interpreting the messages which their light brings to us. The message of the companion of Sirius, which is a white dwarf star, when it was decoded ran, I am composed of material 3,000 times denser than anything you have ever come across. A ton of my material would be a little nugget that you could put in a matchbox. What reply can one make to such a message?
The reply which most of us made in 1914 was, shut up. Don't talk nonsense. That was the reaction of the astro astronomical community to the discovery of white dwarf stars because they made no sense. They bucked the trend. This is not only did your fancy classification scheme fail to account for this, so now you have to figure out what the heck is going on.
But then when you actually start to study their properties, they're weirder than anything you have ever encountered in your entire life. So some people were understandably a little bit put off, but others were excited. People like Subramanian Chandrasekhar. He was an Indian physicist. His uncle got a Nobel Prize.
His uncle was Raman, which we have Raman scattering named after, but that's another show. And he get a scholarship. He was so accomplished in his undergraduate studies. He got a scholarship to go work on a PhD from Cambridge. And at the time in, like, the nineteen tens, this was hot stuff.
This question of how white dwarf stars were, like, what or just what is a white dwarf star? Who who who asked for this? How do we explain it? What kind of physics do we invoke? This was a big topic of debate in the astronomical community.
So Subramanian Chandrasekhar got wind of this and was kind of interested, And he was taking a boat ride, which I think was, like, three weeks long to get from India over to England so he could start his PhD work. And on the boat ride, he solved the whole thing. He ended up winning a Nobel Prize for his work. This is work that he got before he even started his PhD while he was killing time on a cruise ship. He's kind of smart.
In order to solve what the heck is going on with the White Dwarf star, Supermanian Chandrasekhar had to combine the hottest of the hottest math at the time, the time being, by now, like, nineteen tens into the twenties and into the thirties, he had to combine relativity and quantum mechanics. These two fields of physics were just getting started. I mean, I I know Einstein released special relativity or started special relativity in nineteen o five, and then a decade later ended up with general relativity. Quantum mechanics was going through several revolutions, and I would love to to do a podcast series on quantum mechanics. I know I'll get to it one of these years.
But at the time, quantum mechanics was going through a few revolutions, and we're gaining insights into the subatomic world. People were just starting to put the pieces together to combine relativity and quantum mechanics, not general relativity that we haven't cracked that, but special relativity and quantum mechanics to get a complete more complete picture of nature. And Subramanian Chandra Sekhar was at that bleeding edge of the combination of mathematics and was using it for a real life application. So this wasn't in a laboratory. We're not, like, shooting beams of electrons and trying to understand it.
This is, as far as I can tell, the first application of both relativity and quantum mechanics to explain something that we just observe to already exist, not something that we're creating in a laboratory. I could be wrong on that statement, but I feel good saying it. He used quantum mechanics and relativity to explain what is happening inside of a white dwarf star. And the answer is white dwarf stars are not stars. Stars are places where nuclear fusion is releasing energy, and that energy is able to support the star against the inward gravitational pull of its own weight.
That is not what is happening with a white dwarf. Instead, white dwarfs are not supported by the release of energy. They are supported by electron degeneracy pressure. I've done episodes on it before, but I'll do a quick review. Basically, electrons hate each other.
These little subatomic particles hate each other on a very fundamental quantum level where, yes, they have similar charges and so they don't like to be close to each other. You can if you work hard enough, you can take two electrons and push them as close together as you want just based on their electric charge. They'll they'll repel each other, but that's easy to overwhelm, which is, you know, just add some mass, add some gravity, and they'll squeeze together. But then you run into another limit, which is based on quantum mechanics and actually a relativistic application of quantum mechanics, which says that electrons can't share the same quantum state. They can't occupy the same energy level as another electron.
They just can't. They just won't. And if you try to squeeze electrons down closer together to try to make them share the same energy level or same quantum mechanical state, you will fail, and they will resist you. This resistance has a pressure, and this pressure can support an object against gravitational collapse. This problem, this solution that's Chandrasekhar found is kind of difficult, but Chandrasekhar was also kind of brilliant, so he's able to do it on a boat trip before he even had a PhD.
I'm not jealous. Sometimes I look at these historical intellectual giants, and I just feel, well, I don't know. That's another show. So, anyway, Chandrasekhar was able to explain the relationship between a white dwarf size, its brightness, and its sense. Using all this fancy mathematics, he's like, oh, yeah.
Yeah. Yeah. So we have all these white dwarfs. They have different sizes, different brightnesses, and different densities. Look.
They're all connected by this fancy mathematics. His idea, Chandrasekhar's idea, met a lot of resistance in the astronomical community, which Chandrasekhar himself suspected was partly at least, racially motivated, but it ended up sticking. And like I said, Chandrasekhar ended up winning the Nobel Prize in physics, so he did pretty well for himself. But, also, a big chunk of the controversy came in a surprising conclusion of Chandrasekhar's math. There was a limit to how big black there was a limit to how big white dwarfs could get.
This this electron fancy newfangled degeneracy pressure thing was really, really good at holding up objects, but only so good. There was a limit. If you did pile on too much mass, the limit today is, as we understand, is around 1.44 solar masses, so, like, 40% heavier than the sun. You can overwhelm this degeneracy pressure. The electrons will say, okay.
Fine. And then it just goes haywire, and the star collapses. It doesn't form a new object. It just collapses. And what is the ultimate endpoint of an object that keeps collapsing and collapsing and collapsing?
That ultimate endpoint is a black hole, and nobody at the time wanted black holes to exist. Why? Because they're points of infinite density. Like, they black holes appeared in the mathematics of Einstein's general relativity, and nobody liked them. They're like, oh, Einstein.
Yes. Your math is great. It's doing a great job at explaining the solar system and the expansion of the universe and all this other stuff. And, yes, high you know, hypothetically, in general relativity, you can make a black hole form, but, of course, nature isn't gonna really make black holes. But then when we look at white dwarfs, we see, Okay.
Yeah. So if if you have a bunch of material compressed together, then we have this degeneracy pressure, it can hold itself up. But then you can overwhelm that, like, all you need to do is, like, sprinkle a little bit of salt on a white dwarf, and then all of a sudden the electron degeneracy pressure gets overwhelmed collapses as far as we knew at the time to a black hole. And so astronomers were torn because they desperately wanted to explain how white dwarfs work, but they didn't want black holes to exist. And the best explanation for how white dwarfs work opened the door to the existence of black holes, so they were kind of stuck.
You couldn't have one without the other. If you want to accept Chandrasekhar's models of how white dwarfs work, you have to accept that there's a limit, and then you have to accept that stars might be able to collapse to infinity and form a black hole. Unless unless there was something else that could stop ultimate gravitational collapse, like, maybe maybe if you sprinkle that little bit of salt on a white dwarf and it starts to collapse, maybe it turns into something else. Maybe maybe nature does really, really, really not want black holes to form, and so something else happens. But what could it be?
Well, this question was largely put on pause for a couple decades because of a little dust up known as the second World War, but there was some work in this direction. I I mean, there's more to the subatomic world than electrons after all. At about the same time we were figuring out all this white dwarf business, we discovered the neutron. I mean, that's how new the neutron is. Like, we knew about white dwarfs at about the same time we discovered the neutron.
Just just let that sink in for a bit. The neutron is the eight neutral particle, you know, whatevs. And two astronomers who were paying attention to the physics journals at the time, Walter Bade and Fritz Zwicky, proposed what they called a neutron star, which is basically a giant ball of neutrons. And they were using the same kinds of arguments that Chandrasekhar made to explain white dwarfs just like more neutron y. And because neutrons are heavier than a an electron, you can cram them down more tightly than you can electrons.
And so this hypothetical neutron star would be smaller and potentially dimmer, in fact, probably a lot dimmer than a white dwarf. Okay. People wondered aloud if these things could exist. Like, I wonder if neutron stars exist, But they didn't really dwell on it because they thought it would be too difficult to detect. Like, you can't just go out and see a neutron star because it'd be too small and too dim.
Like and we're also busy with this whole white dwarf thing, and they were largely correct. Like, you really can't just go out and snap a picture of a neutron star. That's not really a thing. So we have to fast forward thirty years. Thirty years Two astronomers, Anthony Hewish and Jocelyn Balburne, found they were radio astronomers.
They were radio astronomers. They were doing just normal radio astronomy survey things. I know this. I've told this story before, but it's a fun story. And they saw a source in the sky, a signal in the radio that was repeating on a very regular basis.
I think it was, like, a little over a second, just like beep beep beep. It wasn't literally going beep. It was flashes of radio emission, but because this is an audio podcast, I'm gonna say beep a lot. Beep beep. It was pulsing.
It was pulsing, and it was so regular and so perfect that they thought maybe this is, like, the signal from an alien civilization saying, here we are. Beep beep beep. That's us saying hello. Beep beep beep. And so they nicknamed the source LGM one for little green men one.
They ended up finding LGM two and three and four dozen, and they realized, okay. So we're probably not surrounded by alien civilization. This is probably some astrophysical object that we've never encountered before. But they called the thing pulsars because they pulse in the radio. The astrophysicist, the theorists, got to work.
By the way, Anthony Hewish ended up winning the Nobel Prize. Jocelyn Bell Burnell did not because basically sexism, but that's another story. Astrophysicist got right to work, and they realized that the only way to explain pulsars was to make them out of neutron stars. Nothing else could explain the insane physics. And and what they realized is that if you have a ball of neutrons and it's large and it's rotating rapidly, it can generate strong electric and magnetic fields.
It can generate these beams of radiation that can then spin around like a lighthouse. And then when that light washes over the Earth, we get a boop, or a beep, take your pick, of radiation in the radio. And so every time, it's just rotating and rotating and rotating and rotating. And then every time it washes over the Earth, we get the beep beep beep. Again, it's not a literal auto audible beep.
Yeah. Anyway. So neutron stars became a thing in the nineteen sixties, and neutron stars are just wacky. They're just wacky. Like, we we talked about how sir Arthur Eddington wanted you to go to Patreon and how he he was just absolutely flabbergasted by white dwarfs.
Well, neutron stars take that to eleven. Like, a truckload of neutron star material can outweigh the entire Earth. The gravitational pull around neutron stars is so strong that it can bend light in a circle. Like, light can orbit a neutron star. The gravitational pull is so strong that mountains on neutron stars are, like, a centimeter high.
And then if you were to fall off the mountain, by the time you fell that one centimeter, you would traveling be traveling a healthy fraction of the speed of light. And neutron stars, what are they? They are the largest atomic nuclei in the universe. Like, we think of nuclei like, oh, look at that little oxygen nucleus with its protons and its neutrons. Well, look at this neutron star with its, like, 10 to the 57 neutrons.
They're the largest atomic nuclei in the universe, and they're supported against gravitational collapse the same way that white dwarfs are, but with neutrons instead of electrons. Oh, and but did I mention these things are like the size of a city, weigh more than the sun, several times the mass of the sun? They can rotate at thousands of revolutions per minute. These things are crazy. And, of course, astronomers being astronomers, I didn't get into the fact that there are different subclassifications of white dwarfs because why bother?
But there's all sorts of different kinds of neutron stars. There are the rotation powered pulsars. There are the magnetars. There are the radio quiet neutron stars. There are the X-ray pulsars.
There are the millisecond pulsars. There are the X-ray burst stars, which I'm sure were named because astronomers stay up late watching too many sci fi movies. And we won't get into that because all that because who has time for that besides the professional astronomers? So, like, this road to black hole was stopped because there is a stopping point. There is something preventing stars from collapsing entirely into a black hole, except for the fact that just like white dwarfs, there's a limit to how big a neutron star can get before it collapses.
That, that limit is a little bit less well known than the white dwarf limit because the physics gets even nastier and less well understood, but it's around five solar masses. If you if you have a neutron star at five solar masses and you add a little bit of salt, something nasty is gonna happen. And so slowly over the course of half a century, we realized that you just can't prevent black holes from forming. And black holes are the ultimate end state, the thing that nobody really wants. The good news is that if you make a white dwarf or a neutron star, it will generally just hang out being a white dwarf or a neutron star for trillions of years.
I mean, the bad news is that you have to be a white dwarf or a neutron star for the rest of your life, but, hey, at least you're not a black hole. They are very, very stable objects, and unless they accrete a lot of matter, they're just not going to transform themselves. They all simply exist. But could the universe actually make black holes? Like, what if it did somehow, in some pocket of space, assemble enough material to overwhelm electron degeneracy pressure, neutron degeneracy all the degeneracy pressures and just collapse.
Could the universe make black holes? This nightmare found in Einstein's equations. It was debated for decades. Yes. On one side, they were theoretically possible right there in the math plain as day, but nobody knew if nature could actually produce them.
The big clue about how nature might form black holes actually came once we realized how white dwarfs and neutron stars were made. White dwarfs and neutron stars are made from dead stars. When stars die, like we talked about in the last episode, Some stars die by forming a core of carbon and oxygen and then ejecting their material in the form of a planetary nebula. That's how you do it if you're a medium sized star. Some giant if you're a giant star, you build up a core of iron, then the iron gets compressed, then that neutron degeneracy pressure kicks in and triggers the supernova explosion.
So medium stars are able to make white dwarfs. Larger stars are able to make neutron stars. They're the leftovers. But once we started to get this general picture, we realized, like, oh, okay. If there's a big enough star, when it goes boom, it informs that iron core.
I mean, maybe it can be big enough to overwhelm that neutron degeneracy pressure, and you still get the boom, but then what happens to the core? We realize that there's a certain size limit. Like, neutron stars can't form if if the or can only form if the star is smaller than like, around eight times the mass of the sun. And if they're bigger but there are obviously stars bigger than eight solar masses. So what happens when they die?
The answer is they make black holes. For decades, this was a scary thought. They we we were convinced that nature would come up with some way, some other thing, some physics we hadn't thought of yet to prevent the formation of black holes. But then we discovered Cygnus x one, which is an incredibly bright X-ray source. And we are able to observe this X-ray source and realize that it's a binary system.
There's a big star, and then there's a small companion. And material, atmosphere, gas from that big star is flowing down onto that small companion and compressing and heating up, and we see it in the X rays. And from the brightness of the central object, of whatever is sucking down that material and then its mass, we realize it's it's too massive to be a neutron star. It's too massive to be a neutron star. That everything we know about neutron star said, once you get above a certain mass, you can't be a neutron star anymore.
You are overwhelmed neutron degeneracy pressure, and Cygnus x one was exactly that kind of object. And then we discovered quasars or started to understand quasars and realized that the only way to power them was through the gravitational attraction of a giant black hole. Then we start doing observations of Sagittarius a star. This is that bright radio source in the center of the Milky Way galaxy, and we start watching stars orbit that source. And we can't see the source itself because it's dark, but it's really, really massive.
Then we get gravitational waves. Then we get the event horizon telescope, taking pictures of things. Black holes were first proposed about a century ago, and it's only now that their existence is, like, truly unquestionably confirmed as evidenced by the 2020 Nobel Prize in Physics, which went to work on black holes. Nobody wanted black holes to exist, but then there they were. And we kept trying to concoct reasons for them to not exist.
But the story started with the discovery of the white dwarf. Because once you explain how white dwarf works, you realize that black holes can exist. What is a black hole? Well, it's defined by the singularity. It's that point of infinite density where all matter has compressed into an infinitely tiny point.
That singularity is wrapped in something we call the event horizon, which is not a real thing. It's not like an object you can reach out and touch. It's just a mathematical boundary. The gravity the gravitational pull of a black hole is so intense that it exceeds the speed of light. And the distance from the singularity where it exceeds the speed of light is called the event horizon.
Once you cross the event horizon and you try to turn around and leave, you can't. Sorry. You're stuck because in order to leave, you have to go you would have to go faster than the speed of light, and you can't. Black holes are weird. Black holes are scary.
We do not understand black holes. It is a one way trip inside of a black hole. Once you cross the event horizon, you do not get out. In fact, once you cross the event horizon, the singularity lies in all of your futures. So not only do you are you unable to leave a black hole, but once you enter a black hole, you will go to the singularity, the point at the center, regardless of what you do or how hard you fight because it is simply in all of your features.
No matter which direction you look, the singularity is always ahead of you, and you can't help but move forward. You can't stay still inside of black holes. Like, they're just messed up. No wonder nobody wanted black holes to exist, because they don't make any sense, and we still don't understand. Even though we know that they exist, we have all this evidence.
We have a plethora of evidence for the existence of black holes, but we don't know exactly what they are. We don't know what's going on at the event horizon. We don't know what's going on at the singularity, but they do exist. Collectively, white dwarfs, neutron stars, black holes, they're the leftovers. They're the what happens to stars when they die things.
When stars die. Stars are the things that are burning nuclear fuel in their course. They are achieving nuclear fusion. This is how they live. This is how they generate energy.
This is how they prevent themselves from collapse. Eventually, they run out of fuel and they die, and then they leave behind something else. White dwarfs are by far the most common because they are come from stars about the mass of the sun, you know, plus or minus a little bit, and stars like our sun are pretty dang common. So white dwarfs are gonna be very common. The nearest one is called Sirius b.
That's the one that sir Arthur Eddington was talking about, which is just, like, eight and a half light years away. It's, like, right next door. Like, the nearest white dwarf is right over there. It's no further away than the nearest stars. Neutron stars are far more rare, but ironically, when they light up as a pulsar, they're easy to see because they generate so much energy.
Black holes, we actually don't know how common or rare they are. There could be millions of them, potentially billions of them in the Milky Way galaxy. There's the big ones, the giant ones, things that are hundreds of millions of times more, millions to hundreds of billions of times more massive than the sun, those lie in the centers of all almost all galaxies or all galaxies as far as we can tell. And then there's all sorts of other black holes a few times more massive than the sun scattered around everywhere. And then there's a bunch of neutron stars and then a whole bunch of white dwarfs.
And there are all sorts of interesting binary systems, like white dwarfs orbiting each other or objects orbiting a black hole. We get all sorts of cool mergers and kilonova and nova and supernova resulting from all those systems. And someday, these white dwarfs, the neutron stars, and the black holes, they'll inherit the universe. But that's another lesson. Class is dismissed.
I'd like to thank my top Patreon contributors this month. That's patreon.com/pm. Sorry to keep this show going. It's Matthew k, Justin z, Justin g, Kevin o, Duncan m, Corey d, Barbara k, Nuder Dude, Robert m, Nate h, Andrew f, Chris l, Cameron now, Nalia, Aaron s, Tom b, Scott m, and Rob h. It is their contributions and everyone else's that is keeping this show going.
Also, I do have copies autograph copies of my books available, both your place in the universe and how to die in space. Go to pmslitter.com/book. Or if you don't need the autograph copy, just go to Amazon or Barnes and Noble, Books A Million, all the other places. Also, audiobook available on Audible for all your book consuming needs. And keep sending me questions.
Hashtag ask a space man on all social media channels. You can also email askaspaceman@gmail.com. Check out the website, ask a space man Com, and I will see you next time for more complete knowledge of time and space.