How do we measure the sizes of stars? What are the biggest ones today, and how big could stars have gotten in the past? Is there any way for a star to cheat and get even bigger? I discuss these questions and more in today’s Ask a Spaceman!

Support the show: http://www.patreon.com/pmsutter

All episodes: http://www.AskASpaceman.com

Watch on YouTube: http://www.youtube.com/PaulMSutter

Read a book: https://www.pmsutter.com/books

Keep those questions about space, science, astronomy, astrophysics, physics, and cosmology coming to #AskASpaceman for COMPLETE KNOWLEDGE OF TIME AND SPACE!

Big thanks to my top Patreon supporters this month: Justin G, Chris L, Alberto M, Duncan M, Corey D, Michael P, Naila, Sam R, Joshua, Scott M, Rob H, Scott M, Louis M, John W, Alexis, Gilbert M, Rob W, Jessica M, Jules R, Jim L, David S, Scott R, Heather, Mike S, Pete H, Steve S, Lisa R, Kevin B, Michael B, Aileen G, Steven W, Deb A, Michael J, Phillip L, Mark R, Alan B, Craig B, Mark F, Richard K, Stace J, Stephen J, Joe R, David P, Justin, Robert B, Sean M, Tracy F, Ella F, Thomas K, James C, Syamkumar M, Homer V, Mark D, Bruce A, Tim Z, Linda C, The Tired Jedi, Gary K, dhr18, Lode D, Bob C, Red B, Stephen A, James R, Robert O, Lynn D, Allen E, Michael S, Reinaldo A, Sheryl, David W, Chris, Michael S, Erlend A, James D, Larry D, Karl W, Den K, Tom B, Edward K, Catherine B, John M, Craig M, Scott K, Vivek D, Barbara C, Brad, and Azra K!

Hosted by Paul M. Sutter.

 

EPISODE TRANSCRIPT (AUTO-GENERATED)

The largest star in the known universe, bigger than any other star by a factor of 20,000, it's a true beast, is the Sun. Thank you for listening, and I'll see you next time for more Complete Knowledge of—wait, wait, wait, wait, wait, hold up. That really can't be the end of the episode, can it? No, it's not. But just like that professor in college who always asked rhetorical questions seemingly just for the sake of annoying you, I'm about to ask a rhetorical question seemingly just for the sake of annoying you. And that question is, what does it mean for a star to be big? The Sun is by far the biggest star on our sky, and I know you're thinking, Paul, come on, it's big because it's close, not big because it's, you know, big. It looks big on our sky, the same way an ant can look huge if you hold it up to your eye, but when you put it back on the ground, it looks small again. But let me ask you this, so what? So what if it's only big because it's close?

Sure, the sun may not have the largest physical diameter of any star in the universe, but to us, it's big. And that means something. The sun gives us heat and light and gravity in a way that no other star does. The sun is the biggest star on our sky, and it's the biggest, most important star to us. If all the other stars disappear, blinked out of existence, It'd be annoying, and nights wouldn't be as pretty, and maybe ancient navigation would be a little more challenging, but life would go on. If the sun were to blink out of existence, well, life would not go on. What does it mean for something to be big? The sun is big in almost every respect except for physical diameter. But it's still big. And we know that the sun is big on our sky because we can measure it. We use a measure, and this is jargon alert here, we use something called angular diameter. It's the width that something takes up on the sky. And we can measure that with rulers. It's pretty straightforward.

You just hold up a ruler and you can say, well, the sun is yay far, yay wide on the sky. That's it. And when we want to compare the Sun to other stars, what we'd like to do is the exact same technique. We would like to just hold up tiny little rulers to all those stars on the sky and measure their width. Now, their width can change depending on how far away they are, just like the Sun looks ginormous on our sky, even though it doesn't have the greatest physical diameter, or radius, or circumference, or any way you want to measure this on the sky. So, if you go up to some random star, and you hold up a tiny little ruler, you can measure its angular diameter, but in order to get its actual size, you have to get its distance, and I just want you to bookmark that thought, because I'm going to return to it in a second. But measuring the angular diameter of stars is actually really, really challenging because stars are stupid far away.

They're so far away that even with our most advanced telescopes, our widest lenses, you know, we throw a James Webb at this, we do an extremely large telescope over here, we do our most powerful telescopes in the world, and they still only appear as literal points of light. And if you've ever tried to measure the width of a point, you will find then you know that it is somewhat challenging. There are some stars that are close enough slash big enough on their own for us to get an actual angular diameter measurement of it. We can hold up a little ruler and we can measure how wide it is on the sky. It's a select few stars. We can do this. I like Betelgeuse. We actually have an image of Betelgeuse. It doesn't look like a point to us with our most advanced telescopes. It looks like, like a blob that's slightly larger than point. Look, it's not the most impressive. pictures in the world like stars are really far away, but we can still measure their angular diameters.

But that doesn't always give us a reliable measure of the size of a star. And that's because, you know, stars are dynamic living things. They don't just they're not like static. spheres of metal that just sit there. And once you measure it, you know, it's the measurement they especially large stars, which is the topic of today's episode. Large stars are like pulsating and vibrating. They're breathing in and out. Some days or weeks, they'll be bigger. Some days or weeks, they'll be on a slimmer side. They're like on a yo-yo diet here and no judgment there. It's just it's just the way it is. So it's hard to pin down a reliable measure.

of their width and also also when you look at stars at different wavelengths they might have different sizes like just look at the sun like how big is the sun you're like okay we'll look at the photosphere the part we can see and it's yay big but if you switch to like x-rays then you see more of the corona the atmosphere of the sun And the corona extends for twice the radius of the Sun. So which is it? How big is the Sun? Is it the photosphere, the part we can see, or is it the corona, the part we can't? That's still a part of the Sun. So how big is it? Other stars, especially big ones, if you look at them in different wavelengths, if you look at them in visible light and then infrared and then maybe some high energy stuff over here, You get different size estimates even with the straightforward case of just holding up a ruler. It turns out that measuring the sizes of stars is really, really hard. We have another technique at our disposal, which is ironically enough to use the moon.

If there's a chance alignment, if we're looking at a particular star and the moon occults that star, so as we're watching the moon is doing its or it has no idea our plans or intentions, but it just happens to cross in front of our field of view and then block the star. we can take how long it takes for that star to be blocked by the moon like we see a little bit of light being carved out and then a little bit more the star looks dimmer and dimmer and dimmer and dimmer as this edge of the moon crosses in front of that star. Now this all happens in like a blink in less than a second but if you're paying careful enough attention and astronomers are pretty good paying careful enough attention, you can measure how long it takes for that star to get blocked out and then translate that into a size. But again, that's challenging because you need very special alignments.

Not every star you want to target, especially ones that you think might be really, really large, are just not going to be in the path of the orbit of the Moon and then you're stuck. So for the vast majority of stars, We have to turn to other, even less reliable methods. Less reliable than holding up tiny little rulers. Less reliable than using lunar occultations to block their light. Again, with all these caveats that stars' sizes can change with time, that if you look at different wavelengths, you might get a different measure. Sometimes stars are just really lumpy, so they might look big in one direction than another. So to try to get around all that, astronomers use a technique where they combine their temperature and brightness. So if you observe a star, you can measure its temperature, just how hot it is. Okay, once you have your temperature, you're good. And then you can look at how bright it is. Now, if you know how far away a star is, then you know how bright it actually is.

I mean, to us on our sky, the star looks really, really dim because it's really far away. But if you know how far away it is, you do a little bit of math and you can figure out how bright it actually is as if you were standing right in front of it. And then you can combine this brightness measurement with the temperature measurement to get an estimate of the size. And we can do that because the brightness of a star is determined by its surface area, by the amount of surface on the outer layer of a star that is emitting light. That sets how much light can be emitted by a star. A bigger star with a bigger surface area has more room to give off light and so it can look brighter. But you need to calibrate that with the temperature because a hotter star is naturally going to put out a lot of light while a cooler star is not going to put out a lot of light.

But once you combine those two bits of information, once you know how much light a star is capable of producing, And then you know its overall brightness, then you link that to its surface area, you put those two numbers together, you get that surface area, you get the size of a star. That's pretty fine. It's a great method if you know exactly how far away the star is. Measuring the temperature is pretty straightforward. Measuring its true brightness, getting its true distance is actually really challenging for individual stars. It's very hard to pinpoint a distance. You know, this comes up if you've ever heard of the Methuselah star, this star that is, you know, estimates say it's older than the universe itself, which is impossible and hence this like big mystery. No, the big mystery is we don't know how far away that star is. And if you change its distance just a little bit, you get radically different pictures of its total brightness.

And from there you get, you put that into stellar evolution codes and you get different estimates for how old it is, but it all depends on how far away the star is. Yes, we have amazing surveys of stars like the Gaia satellite, which deserves its own episode. Please feel free to ask that has measured the precise distance to something like two billion stars and counting. That's an impressive number. It's also less than one percent of all the stars in the Milky Way. So the vast majority of stars, especially ones that might be candidates for largest stars, you know, they just don't we don't have reliable distance measures to And so, you know, it's the frustration of astronomers for hundreds of years. It's really, really hard to get a pinpoint distance to any old random object on the sky. You need a lot of tricks. You need to play a lot of games to get a reliable distance measure.

And so that's why I'm telling you all this, because anytime you see or hear anything about the largest star in the universe. You can google it right now if you feel like it. What is the largest star in the universe? You'll get an object. You need to know that there are a lot of caveats to that statement. There is a lot of uncertainty, a lot of unknowns, and any one reigning champ is unlikely to hold that title for long. Not because we found a larger star. It's not like a new heavyweight champion comes on the scene and knocks them out. No, it's more like we reviewed the tapes and we found that a star was cheating because we didn't have an accurate distance measurement to it. Or it was pulsating and that was at a maximum phase and now it's at a minimum phase. It's really hard to pin down the sizes of stars. Like, really hard.

And there are a lot of uncertainties, and as time goes on, we find mistakes, we find ways to clamp down on the uncertainties, we find a new trick, and so that's why this list shuffles around a lot. Now, if you ask Google, or you go on Wikipedia, or you start digging what are the largest stars, you'll get a list. And I have a favorite for what I think is the actual reigning champ. That star is VY Canis Majoris. It's about 6,000 light years away from the sun. It's big. Now, I pick this star. Sometimes it's placed second, sometimes all the way down in fifth place depending on the particular list, depending on when the list was made and how it's quoting certain values. It's never in the number one spot, but I think this star has the highest chance of being the genuine largest star that we know of, and I pick it because it has the tightest uncertainty, has the smallest uncertainty, so it's up there in size, and compared to its size, it has a very, very small uncertainty.

We have a somewhat reliable distance measure to VY Canis Majoris. somewhat reliable. I mean, we're still talking like 10% uncertainty here on its size, but for these kinds of measurements, that's actually doing pretty good. A lot of astronomers are satisfied if you get within like a factor of 10, you know, and to only have 10% uncertainty on something is pretty stellar. I crack me up sometimes. There are other stars that have larger quoted sizes, but their uncertainties are larger and so I'm a little worried that they won't be able to hold on to that title. So I like VY Canis Majoris, that's my bet if I had to put money down for current stars, currently mapped, currently known, VY Canis Majoris is the largest. It's 1500 times wider than the sun. It is 1,500 times wider than the sun. That puts its volume at over a million times greater than the sun.

If you dropped VY Canis Majoris, and I really enjoy saying that name, If you dropped this star in our own solar system, it would stretch to the orbit of Saturn, which is just insane. Imagine a single astrophysical object, a single compact body like a star placed in our solar system stretching to the orbit of Saturn. That's like the sizes of supermassive black hole event horizons. This is insane. Now here's the thing though about VY Canis Majoris. and why I annoyed you earlier with that rhetorical question about what does it mean for a star to be big. Yes, it's probably the largest star in diameter that we know of, but it's not the largest star in mass. In fact, it's only about 17 solar masses, which is indeed large. Trust me, 17 times more massive than the Sun is on the beefy side when it comes to stars, but it's not even in the heavyweight class. It's a middleweight champ when it comes to mass.

Instead, VY Canis Majoris got big because stars just tend to get big when they're about to die. Our sun is going to do it, it's not going to stretch to the Ikanis Majoris scale, but it's going to expand to something like the size of the Earth, the orbit of the Earth, that's pretty big. And this happens because near the end of a star's life, you know, there's fusion in the core of every star turning hydrogen into helium, this helium builds up in the core, it's like pollution, it's like a byproduct of the fusion reaction, it builds up, it gets in the way of the fusion reactions. And so as the helium builds up, the fusion reactions have a harder time doing their thing because the hydrogen keeps running into helium and messing things up. And so it runs at a faster pace to keep the star stable, and it runs at a faster pace and it runs in a larger volume. core of the sun expands and gets hotter because the fusion rates have to work around all of that helium pollution.

And once the core gets hotter and bigger, it pushes the rest of the star outwards and stars get big. And VY Canis Majoris, for whatever reason, got really big. But one of the reasons is it's already massive, so it's already big. Even in normal times, it would be pretty large for a star. And it's very near the end of its life. We usually don't catch these kinds of stars because this phase doesn't last very long, so they're very rare. If you just have one snapshot of the sky, which we essentially do, you're going to catch stars in all different phases of their lives. We happen to catch VY Canis Majoris right before it's about to go supernova, and so it's at its maximum extent, and its maximum extent is pretty dang big. And in fact, sometimes stars can get even bigger than VYK and S Majoris, but that's totally through cheating.

This is something we call a failed supernova, where we're not exactly sure of the physics behind a failed supernova, except that when they happen, a star puffs out like it's about to blow up, it's about to detonate and then it just doesn't and it stays intact. So we think what happens is like all the mechanisms are in place for a star to go supernova and it destabilizes but then it just doesn't reach that critical threshold. And so when this happens, the star like really blows up like an overblown balloon, but then gently comes back down to normal sizes. Sometimes they later do go supernova. Sometimes they just fade away and then that's it. They're just like total duds. like the cheap firework that you got and you're all anticipating and you light the fuse and then nothing happens.

I consider that cheating because that's a very temporary phase, but when these happen, you know, a star can stretch to like two or three thousand times the radius of the sun, which is gigantic, but it just doesn't last for long. But what about Mass? VY Canis Majoris, big star, but not the biggest star when it comes to Mass. Mass is a totally valid way to rank how big a star can get. And the reigning champ in this category is called R136A1. It's a catalog designation, don't worry about it. These stars are so far away that they don't have cool names like Archeron or Beetlejuice, they just have catalog designations. And don't worry, just like with size measurements, radius measurements, angular diameter measurements, there's plenty of uncertainty and unknowns in any kind of mass measurement for a distant star. But in this case, just like VY Canis Majoris, it's my favorite one.

It's the one I would place bets on to be the largest known star in the universe, and that's around 1,500 times wider than the Sun. R136a1 isn't all that big in size. It's only about 40 times wider than the Sun, which is gigantic and mind-bending to even think about, but that's, you know, on the scales we're talking about, that's not all that impressive. It's puny compared to VY Canis Majoris, but in terms of mass, it's roughly 300 times more massive than the Sun. 300 times more massive than the Sun. It would take 300 Suns to equal the mass of a single R136a1. More massive stars burn hotter because their fusion rates can go much faster because there's so much more gravity pulling in. This makes them brighter. R136a1, which is not nearly as much fun to say as VY Canis Majoris, is 4.5 million times brighter than the Sun. 4.5 million times brighter. It is something like 7 times hotter, making it the most massive, the most luminous, and the hottest star ever known.

It's also the greatest contributor to Patreon. That's patreon.com slash PM Sutter. What makes a star big? Maybe it's in terms of its monthly contributions, of which I am forever grateful. That's patreon.com slash PM Sutter. Thank you. But this star is so hot. It's so hot. How hot is it? It's so hot that most of its light is in the ultraviolet portion of the spectrum, which makes most of its light invisible to our eyes. And so visually it's only, and only it's not really capturing what's going on, it's only 164,000 times brighter. to the human eye than the sun, that's because most of its brightness, most of its luminosity goes to ultraviolet radiation. So if you were near this star, yes it would seem brighter, it would seem 164,000 times brighter, but it would be 4 million times more luminous, so even though maybe you put on your super sunglasses to block the light, like your skin is gonna notice, you're gonna tan up in an instant, you need SPF like 5 billion to block this thing.

If we were to put this star, R136a1, 40 light years away, it would still outshine Venus. If it were placed at Proxima Centauri, it would be brighter than the full moon. We can't even see Proxima Centauri with the naked eye, we need telescopes to see it. You put this star there, it's brighter than the full moon. It lights up like a supernova every single day. In fact, R136a1 is so big that it's pretty much the most massive star that the universe can build. Remember that the universe wants to build big stuff. Once a process gets going, unless there is something in the way to stop it, the process is just going to keep going. So if the universe is going to start, say, collapsing a cloud of gas to form a star, it just wants to keep collapsing it until something gets in the way. And what gets in the way is a balance between radiation and gravity. Gravity pulls a star inwards, which keeps a star, you know, being a star.

But a star is also a star because it's glowing brightly, so light, heat, radiation is pushing outwards. And there's a balance here that keeps stars stable for millions or billions of years. But the more mass you add to the star, the more gravity there is, the more pressure there is at the core, the more intense the fusion reactions are, the more heat that's released, the more radiation that's pushing outwards. If you tip this balance too far, then light simply disintegrates a star from the inside out. This balance breaks down right around 200-300 solar masses. If you try to make a star bigger than that, say you're saying, I'm going to make a star that's 400 solar masses, I don't care what the universe says, I'm going to do it. You can do it, you can build it, and then once the fusion reactions get going, it will be so hot and produce so much light that the star will just disintegrate. So yes, R136a1 is right around the limit of what a largest star could be in the universe.

But that's the biggest that a star can be today. The early universe was a different story. If we rewind the clock, Before multiple generations of stars, things are different. And if the universe is different, maybe stars are different too. Billions of years ago. Here's the thing, all this talk about the competition between radiation and gravity depends on how easily the light generated in the core of a star can escape. And that depends on what the star is made of. Light can escape glass easier than light can escape, I don't know, bricks. And the limits that we calculate to figure out what is the most massive star you could possibly build assume that stars have a certain fraction of heavier elements in them. You know, everything that the astronomers call the metals. Anything that's not hydrogen and helium. In the early universe. Before there are multiple generations of stars around to make these heavy elements, the first generation of stars were made from pure hydrogen and helium.

You need a lot of generations to create the pollution that mixes into every galaxy so that every star born today has a certain minimum proportion of heavy elements in this. And this changes things. because heavy elements are a little more opaque. They block light more easily than hydrogen and helium. Light can filter through hydrogen and helium easier when it's pure hydrogen and helium. Once you sprinkle in some oxygen and some carbon and some iron, light has a harder time escaping. So a star with heavier elements, a modern day star, a polluted star, blocks light more effectively and that sets the limit of how big a star can get before light just disintegrates it. But if light can escape a star more easily, you can have more light produced in the core before the star starts to fall apart because it just slips through. It doesn't affect it, it doesn't push it apart, it doesn't disintegrate it, it just slips through.

So the very first stars in the universe, the ones that didn't have any pollution, didn't have any elements, were just pure hydrogen and helium, this calculation changes. And the biggest star that you can produce in the first generation is big. Like way big. Like a few hundred solar masses at minimum. So stars can be much bigger in the early universe. They can also form much more easily. And that's because in order to build a star, you need to take a giant cloud of gas and you need to collapse it. You need to squeeze it down. But when you squeeze a cloud of gas, you know, go take a cloud of gas and start squeezing it, what you're going to find is that gas heats up because of all the friction as you squeeze it together. And it's going to say, no, I don't want to be squeezed anymore because now I'm hot. And that heat, that pressure is going to resist you trying to squeeze it further. So if you want to squeeze it down to make a star, ironically, you have to cool it off.

You have to cool off a cloud of gas to squeeze it down so it can become a star. And what's really good at cooling off gas? Heavy elements. They can emit lots of different kinds of light. If you sprinkle in heavier elements, carbon, oxygen, iron, potassium, you know, whatever you want in a cloud of hydrogen helium, you can more efficiently squeeze it down because it has an easier time releasing its heat. It has an easier time glowing, getting rid of its heat, which lets you squeeze it down more. In the modern universe, this process of star formation is super duper efficient. It's so efficient that if you take a single gas cloud and squeeze it down, it cools off so rapidly like in a million years, but like rapidly that the cloud just fragments into bunches of little tiny pockets with each tiny little pocket becoming its own star. So if you take a gas cloud that has a thousand, 10,000 solar masses worth of stuff inside of it, you don't get a single star out of it.

You get a whole bunch of red dwarfs, you get a whole bunch of solar mass stars, and then you get a few giants. Because this giant cloud is so good at cooling that the cloud can fragment, it can break up into tiny little pockets. But in the early universe, this process was not efficient. It was not smooth. You take a giant gas cloud weighing thousands of solar masses and it can collapse in one go. Now how it cools off, how it actually proceeds in star formation is a big open question in astronomy right now, but you can imagine the possibility where a giant gas cloud weighing thousands of solar masses collapses as a single unit into a single star. How big? Maybe the first stars were thousands of solar masses. Maybe the first stars were so big that they were essentially like galactic cores in their own right. None of these stars survive to the present day. The first generation of stars died a long time ago.

We see no evidence, we see no star in the modern day universe from that first generation. One of the reasons we think that the first stars were so big is because they are gone. Giant stars live short lives. And if the first generation of stars happen to be small, small stars can live for very long times. They can live longer than the present age of the universe at 13.8 billion years. So if the first generation of stars were small, we should be seeing them today. We've mapped enough. Once you map one or two billion stars, you ought to pick up a few of the first generation. We ought to see some stars that are essentially free of any heavy elements whatsoever. We don't see any star like that in any survey. So we strongly suspect that the first stars were very, very large, very massive, led very brief lives and died. How big were they? We don't know.

This is the job of the James Webb Space Telescope, future surveys pushing back into the cosmic dawn, the first stars to appear on the cosmic scene. We know that they were likely very big, probably hundreds of times the mass of the sun, possibly up to tens of thousands of times the mass of the sun. But for now, these giants are only dinosaurs, they'll have to live in our imaginations. And observations, you know, because we might still see them in a deep survey of the early universe. But that's another story. Until then, when someone asks me what's the biggest star in the universe? Is it R136A1? Is it UV Canis Majoris? No, to me. The biggest star in the universe is always going to be the Sun. You can also support this show. You can do it by a bunch of ways. You can tell your friends or your enemies, it doesn't matter, about this show. You can also drop a review on your favorite podcasting platform, and you can also contribute to Patreon. That's patreon.com slash P-M-S-U-T-T-E-R.

That is my name. Paul Sutter. P.M. Sutter. And thank you. I'd like to thank my top Patreon contributors this month. They are Justin G, Chris L, Alberto M, Duncan M, Corey D, Michael P, Nyla, Sam R, John S, Joshua, Scott M, Rob H, Scott M, Louis M, John W, Alexis, Gilbert M, Rob W, Jessica M, Jules R, Jim L, David S, Scott R, Heather, Mike S, Pete H, Steve S, Lisa R, Kevin B, Michael B, Eileen G, Don T, Steven W, Deborah A, Michael J. and Philip L. It's all of you that make this show possible. I can't thank you enough. And I'll see you next time for more Complete Knowledge of Time and Space.

Comment