Daniel and Jorge Explain the UniverseDaniel talks about how astronomers can measure or deduce the mass of a star, and why it controls its fate.
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Did you ever eat astronaut ice cream as a kid? That stuff seemed so exciting, but when you put it in your mouth, it was always kind of gross and disappointing. I remember my brothers and I wondering if it was made four astronauts or maybe like made out of astronauts either. For hey, it makes me wonder about something. You know, how if you're on Earth and you eat too much ice cream, you gain weight? Right, Well, if you're in space, can you eat all the ice cream you want without gaining weight? I think I just invented the world's most expensive diet. For one hundred million dollars, or the price of a ticket to the space station, you can eat all the ice cream you want and still lose weight. I'm Daniel, I'm a particle physicist, and I'm not here to sell you some crazy weight loss scheme.
No.
Instead, Welcome to the podcast Daniel and Jorge Explain the Universe, a production of iHeartRadio. In our podcast, we try to explain to you everything we do know about the universe and everything we don't know about the universe. We bring you to the forefront of science and introduce you to the questions that scientists are asking today, the things that are puzzling physicists that are making them go what, how does that work? Because we think that everybody deserves to be out there on the forefront of knowledge, wondering with the rest of humanity, how there amazing and crazy and bonkers universe is put together. Unraveling that mystery, revealing the secrets of the universe to me, seems like the grandest journey of humanity, and it's amazing that it even works. That we can apply our minds and figure out the way the universe works, and as we look out into the cosmos, we see incredible stuff out there. And so our podcast tries to explain to you what's going on out there in the center of stars, around the edges of black holes, in all the tiny little particles, and we try to teach you also not just what science does know, but how it knows it, because you know, there is a pattern in the history of science where we think we know something, it's sort of established wisdom for a while, and then it gets overturned and we discover, oh, the universe is actually something different, it works differently from the way that we thought it did. Those are the best moments, if you ask me, in the history of science, when we're peeling back a layer of reality and discovering that the universe is pretty different from the way our intuitive experience led us to believe. And so when that happens, you have to wonder why did we think the old thing? Why did we get it wrong? So it's important sometimes to dig into how do we know what we know? And that's what we want to talk about on the podcast today. When we look out at the stars in the sky, we see some of them are bright and some of them are dim. But when scientists talk about stars, they're often not just talking about how bright they are. They're talking about how big they are, how massive they are. Stars with a lot of mass, stars with not so much mass, black holes with a huge amount of mass. But how is it possible to know that? So on today's program, we'll be asking the question, how do you measure the mass of a star? And it's not just me, and it's not just you who's wondering about this question. We actually got this question from a listener. Here's Dustin, Hi, Daniel and Jorge. I wonder how physicists come up with the weight of planets and stars. Thank you, Thanks Dustin very much for sending in your question. And this is a really interesting and really important question. It's not just an academic one because the mass of a star is really important. How much stuff you gather together to form that burning plasma in the sky determines its fate. Is it going to end up as a white dwarf? Is it going to turn into a neutron star? Is it going to collapse into a black hole? All of those outcomes are determined by how much mass it has, and so understanding how much mass there is in star ours is really important and turns out not that easy. But before we dig into how scientists actually do it, what they do know, and what they don't know about the mass of stars, I went out there into the wilds of the internet to ask people how they thought scientists did it or how they would do it themselves. So thank you very much to everybody who volunteered their brain to speculate how to measure the mass of something super duper far away that you could never hope to touch. And if you would like to participate in future baseless speculations for our podcast, please don't hesitate right into us to questions at Danielandjorge dot com. Here's what people had to say.
My guts, it would be to say that we know by how bright it is.
Perhaps it's done by measuring the gravitational lensing effect on light passing around the stars, or possibly with the help of mass spectrometer readings, So you know what elements you have in the star, and then you try to figure out how much of them you have.
I could guess that luminosity plays a part in it, and probably the orbit, but I'm not sure, so I'll say I don't know.
I imagine that if we can tell the distance to a star, we can probably figure out its diameter, and from its light signature we can tell its chemical composition, And from the brightness of the star itself, I would imagine we could tell from the combination of those three things what the masses somehow back it out from that information.
All right. I love hearing people think on their feet trying to solve this really hard problem from nothing, figuring it out from scratch, And there's a lot of good ideas in there. You hear people talking about how you need to know the path of other stars nearby, or maybe you can and connected to the brightness, and hey, hey, are on the right track. So let's talk about how scientists can measure the mass of stars. Well, in general, if you're looking at an object in space, right, all you're getting is the light from that object, it's basically impossible to know how big or how massive that object is. Remember what mass is. Mass is like how much stuff there is in a star. It affects the star's inertia, like how it responds to forces, and it also affects the star's gravity. You know, how hard it pulls on stuff, but none of that can be directly observed from the star itself very easily. Mostly that's the effect of other stuff on the star, right the inertia, or the effect of the star on other stuff. And so the short answer is is that in general, it's impossible to directly measure the mass of an isolated star. We had the same problem when we were measure during the age of isolated stars. For example, right we talked about how to measure the age of stars, and it turns out that you need to do it in big groups because you're seeing how the stars which are probably formed together are probably dying together, and you can use that as a sort of clock for the population of stars. But individual isolated stars it's very hard to tell how old they are. It's a similar problem for measuring their mass. If a star's out there and there's nothing nearby it, nothing that can probe its gravity or be influenced by its gravity, or give it a push or a pull, it's almost impossible to measure its mass. But that doesn't mean the problem is impossible. What we do in this case is that we find a special category of stars where we can figure out how to measure their mass because they're near something. There's something nearby that's tugging on them, or that they are tugging on. We can measure the mass for that sort of special category of stars, and then we can try to fill in the gap and figure out a way to extrapolate to all the the other stars. All right, So first let's talk about how to measure it for a special category of stars where you actually can see them doing some tugging or some pulling, where their mass really is important for what we observe. And the most powerful way to do this is to see binary star systems. These are systems where you have two stars nearby, and it seems sort of exotic, like the kind of thing you might see in Star Wars, with multiple suns rising over the horizon. And you know, I love that they do that because it sort of gives you a sense that it's an alien world. And so we have the idea I think that binary star systems are unusual, that they are weird, and that's just because we're used to looking at one sun. It turns out binary star systems are not that rare. A lot of stars are born in pairs, and if you think about it, It makes a lot of sense, actually, because how is a star formed. You have a big collection of gas and dust and stuff that's swirling all around, and something happens to trigger its lapse and it rushes down and collapses. But stars are not formed by themselves. You have a huge cloud which usually forms many stars at roughly the same time, and so it makes some sense for those stars to be tugging on each other and even for those stars to end up in orbit around each other. So lucky for us, binary star systems are not that rare. So what you can do is look at the pair of stars, because if they are close to each other, even if they are really really far from us, then we can see the effect of their gravity on each other, which gives us a clue as to what their mass is. All right, so let's dig into it and figure out how that actually works. If you're looking at a binary star system, how do you use what you're looking at, what you see to actually figure out what the mass of those stars is. Well, the thing to remember about a binary star system, first of all, is that it's not one star orbiting the other. The two are orbiting each other. So there's some point in between them that the two of them are both moving around. That's the center of mass. So both stars are in motion. Right from the point of view one star, the other one's moving, and from the point of view of the second star, the first one is moving. And what that means is that we can measure their relative motion. As the star moves further away from us, its light gets stretched out, the wavelengths get a little bit longer, and when a star is moving towards us, its light gets compressed a little bit, the wavelengths get a little bit shifted. This is called red shift when it gets longer wavelengths as it moves away from us, and blue shift as it gets shorter wavelengths when it's moving towards us. This is very similar to how you discover a planet is orbiting around a star, because you see the gravitational effect of the planet makes the star wiggle a little bit and that changes the light that we see. Now, in that case, you only have a single star, and you're deducing the presence of the otherwise invisible planet by looking at these Doppler shifts in the light from the star. In this case, we have both stars, So what you can do is look at the Doppler shift from both stars independently, So you get both of these curves right, and you can see how these curves change. You can see, for example, oh, the star is moving away from us. Oh, now it's moving towards us. Oh it's moving away from us. And now it's moving towards us. And so from those curves you can get some really interesting information. First of all, you can figure out the period of each star. How long does it take to move around the other one. This is because you can see when the velocity turns around, right, it gets red shifted, then it gets blue shifted. What it flips over is that point when it's turning around, And so from those flip over points you can figure out what is the period of this star moving around the other one. And you can also actually measure the velocity of each star around the other one. This is determined by the amount of red shift and blue shift.
Right.
The larger the velocity, the more red shift and blue shift you get as it swings around the other star. So we can just by watching the color of the light from these two stars change, figure out what the period is, how long it takes for them to orbit each other and their velocities. And that's really awesome because then we could just plug it into an ancient equation. Kepler's third law tells you exactly what the combined mass of the system is if you know the period and you know these velocities. So that's pretty cool. Now we know just from the period and those velocities, we know the total mass of this binary star system, but we also know their relative velocities. We know which one is moving faster than the other one. Say, for example, they don't have an equal amount of mass. It's not like two equal binary stars, but instead maybe one of them is much bigger than the other one. The bigger one is going to be moving slower, and the smaller one is going to be doing more of the moving around the bigger one. So in the case when the two stars have the same mass, they'll have the same relative velocity. In the case when the two stars have very unequal masses, when it's very asymmetric, then one of them will be moving faster than the other one. And so from this relative velocity, which again we know, we can figure out how to split up the total mass of the systems into the two masses, and so boom that means from a binary star system. Just from watching their light wiggle, we can figure out what is the mass of each star in the system. And one of my favorite things about this is that it doesn't just work for stars that are close by. It also works for stars that are super duper far away, because you just have to look at the light that's coming from the star. It's not like you need to see the gap between the stars or anything. You just need to look at the light pattern. And the cool thing about the Doppler shift is that it doesn't like disappear. If there's a Doppler shift pattern in light that comes to us from something really really far away, there's still that same shit when it gets here. It will persist over billions and billions of light years of space. And so this is powerful because it lets us look at even further away stars, so we get like a larger sample, so we can learn like a more general trend rather than just understanding something that's happening in our neighborhood. And also we can tell whether it's something that's true here and something that's true far away. We always want to be open to surprises when we look out into the universe. We don't want to draw too many conclusions just by looking at our cosmic neighborhood. So it's important to have a technique that works for nearby and also works for really far away stuff, all right. So that's how we measure the mass of binary stars of special star systems, where we have two stars that we can see and we can measure their velocities from the wiggles in their light. But we're interested in all the stars. We want to know what is the mass of any given star we see out there in the universe, even the ones that are not in binary star systems. So how do we do that. We'll talk about that in a moment, but first I want to take a quick break. With big wireless providers, what you see is never what you get. Somewhere between the store and your first month's bill. The price you thoughts you were paying magically skyrockets. With mint Mobile, you'll never have to worry about gotcha's ever again. When mint Mobile says fifteen dollars a month for a three month plan, they really mean it. I've used mint Mobile and the call quality is always so crisp and so clear. I can recommend it to you so say bye bye to your overpriced wireless plans, jaw dropping monthly bills and unexpected overages. You can use your own phone with any mint Mobile plan and bring your phone number along with your existing contacts. So dit your overpriced wireless with Mint Mobiles deal and get three months a premium wireless service for fifteen bucks a month. To get this new customer offer and your new three month premium wireless plan for just fifteen bucks a month, go to mintmobile dot com slash universe. That's mintmobile dot com slash universe. Cut your wireless bill to fifteen bucks a month. At mintmobile dot com slash Universe, forty five dollars upfront payment required equivalent to fifteen dollars per month new customers on first three month plan only speeds slower about forty gigabytes on unlimited plan. Additional taxi spees and restrictions apply. Seementt Mobile for details.
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And we're going to use that light to figure out the answer to a really interesting question, which is how big is that star? How massive is it? What is its future? Is it a huge blob of hydrogen which will eventually collapse into a black hole or is it going to end up a glowing white dwarf for trillions of years? And that is entirely determined by how much mass it has. So we talked about how to measure the mass of a special category of star stars where we identify two of them near each other that are orbiting each other, so we can use their gravitational effect on each other, which determines their relative periods and velocities of their orbits to figure out how much mass there is exactly. But now we want to move beyond that. We want to understand can we talk about any our arbitrary star. We see a star in the sky and we want to know how much mass does it have? How can we figure that out if it doesn't happen to have a big massive object near it that lets us directly measure its mass. And so here we have to be very careful. We have no direct way to measure the mass of those stars. But what we do is we look for a connection between what we can measure, like the brightness of a star, and what we want to know, like the mass of the star. And we look for that connection not in the actual stars out there in the universe, but in the stars here in our computers at home. Because we have an idea for how stars work, we have some theory about it. We know the nuclear fusion that happens inside stars. We think we understand something of the gravitational pressure that's pulling these things down and making these things happen. We compare a lot of these models to what we observe about nearby stars. So we've been spending decades developing these sort of like theory of how all stars work. And in that theory. At least in our computational models of stars, we do see a relationship. We see a connection between the mass of the star, which is the thing we want to know but can't directly observe, and the luminosity of the star, the brightness of the star. In fact, we see a pretty direct relationship. We'll talk in a minute about what that relationship is and why we think it makes sense the sort of physics that underlies the connection between the mass of a star and its brightness. But first let's make sure we're understanding the larger strategy. What we're going to do here is get a connection sort of in our theory or in our simulations, between the mass and the luminosity, and then we're going to calibrate that. We're going to make sure it's right by looking at binary star systems. So binary star systems give us a way to actually measure both the mass and the luminosity. Then we have these calculations we can do that can connect mass to luminosity, and we'll use the app actual measurements to make sure that calculation is correct at various points. So we have like sort of a string that connects mass and luminosity, and then we have various pushpins we're going to put into it to make sure that it's sort of nailed down by actual measurements in reality. And between those pushpins, between the actual measurements we make for the binary stars, we're basically extrapolating. It's a little bit of guesswork and a lot of complicated nuclear theory, but it's not something we actually know, and so there's, for example, a gap where somebody could come in later and decide, oh, it turns out our model for stars was actually wrong and these extrapolations didn't quite work. So it's important for you guys to understand, well, we actually know what we actually can measure, which is the mass of a few binary star systems, and where we get the rest of the information which comes from this nuclear theory, which helps us sort of interpolate between the examples that we can't measure directly. All right, So we have a connection in our models between the mass and the luminosity, and it's really kind of fascinating. It tells us that the larger the mass, the brighter the star. Right, the bigger your original scoop size of hydrogen, the brighter the star is the faster it's gonna burn, And that means something else really cool, which means that big stars burn bright, they burn hot, but they don't burn for very long. So the big ones are like flashy and exciting and very very bright and shine their love into the universe, but not for that long, whereas the little star that could it's sort of out there pumping not nearly as much light, but it can do it for a much much longer time. These little stars can last for billions or maybe even trillions of years, whereas the really really big stars only last for like a few one hundred million years before the party is over. So what does that actually mean? Well, the relationship for stars, like around the mass of our sun are a little bit less and then up to about fifty times the mass of our sun, the luminosity of a star goes like the mass to the fourth power. That means that a star twice as massive as our sun will be like almost sixteen times as bright as our sun. A star twenty times the mass of our sun will be one hundred and twenty thousand times as bright. That's right, you double the mass of the star. You don't just double the brightness. Right, it goes up by the power of four, and so it increases very very quickly. And that gives you a sense for why these really big stars burn out so quickly because they are incredibly bright. The bigger the star, the brighter the star, and the faster it dies. It also means, if you turn your attention in the other direction, that stars that are less bright than the Sun are much much dimmer and burn much much longer. For example, there are stars out there that have a fraction of the mass of our sun and they burn like one ten thousandths as brightly as our sun. Imagine living on a planet around the star that was one ten thousands the brightness of our star. Right, you could be much much closer to the star without it being brighter than the Sun, which means it would fill up a much bigger area in your sky. Right, So you could have, for example, a nice, warm, toasty day in front of a huge sun in your sky without actually getting burned. So the opportunities there for like crazy science fiction ideas for what it would be like to have a huge star in your sky seem pretty wide open to me. And so this relationship, this force power relationship, is true for stars that are like half the mass of the Sun up to about fifty times the mass of the Sun, and then there's some kinks in it. Like above fifty times the mass of the Sun, there's a different relationship. It actually becomes linear, it doesn't grow as quickly, and below half the mass of the Sun, the relationship changes again. It goes more like mass squared. And so there's some details there, but let's understand sort of the general idea. Why is it that a more massive star would burn brightly and so much more brightly. Why isn't the relationship just linear. Why isn't it that if you have twice as much mass in the star, it burns twice as bright and twice as hot. Well, the answer comes from the details of the nuclear theory. So let's dig into what's going on on the inside of this star, right, Why is the star made at all? Right, a star comes from the gravitational pressure. You have a big collection of gas out there in space, and it's gravity that's tugging it together. But gravity is not the only thing active in a star, right. If it were, then every star in every object would just collapse into a black hole. You a star is a pretty steady state object. It can last for millions or billions or maybe trillions of years because there are two forces is at play. There are two things they're pushing against each other, which is what keeps the star stable. So there has to be some sort of outward pressure to keep a star from collapsing. In the case of most stars, that pressure comes from the fusion, from the nuclear reaction that's powering the star itself. And it's fascinating to me because it's such a give and take, Like gravity pushes on the star, squeezes on the star. That's what actually causes fusion, and that fusion releases a lot of radiation. That radiation, that brightness, that energy pushes the star back out right. So gravity causes fusion, which creates a sort of like back reaction, which pushes out on the star to keep it from collapsing. And there's a whole fascinating rabbit hole you could go down there, right because then that fusion creates heavier elements, which increases the gravity dot dot dot. But there's a whole other podcast episode about the life cycle of stars. Let's focus right now on just what's happening inside a star. And why if a star gets more massive, it also gets brighter. So the reason simply is that a more massive star has more gravity. Right, there's a bigger helping of mass, and each of those little particles has a little gravitational force on it. Each of them is then squeezing the center of the star. So if you take a star the mass of our sun and you wrap it in another helping of hydrogen, another solar mass of hydrogen, there's going to be additional gravitational forces. Right, It's going to increase the pressure on the center of the star. And so in order for the star to be stable, for it to avoid collapsing into a black hole, it needs more pressure outwards. Right, It needs to be sending out more radiation. It needs to be brighter. And you might think, well, just because it needs to be brighter doesn't mean that it does.
Right.
Well, stars that don't get brighter, that don't provide those pressures, they do collapse into black holes. Right, So if there's a mechanism to provide that pressure, that's what keeps the stars stable, and in fact there is because increasing the pressure on the core of the star increases the temperature and when you increase the temperature at the core. The nuclear theory tells us something happens the fusion that's happening at the core of the star, the merging of hydrogen into helium and then helium into heavier elements to release more energy. The rate of that fusion depends very sensitively on the temperature, and so here's where that nonlinearity effect comes in. Here's why doubling the mass of a star doesn't just make it twice as bright, because the nuclear fusion at its core is very very sensitive to the temperatures. That's the nonlinear response. If you crank up the temperature of the core of the star by just a little bit, you can lead to a very large increase in the reaction rate. And this is something that's very complicated and difficult to wrap your mind around, just because there are so many particles involved. I mean, you have to sort of like take your mind and go deep into the center of the star and imagine all of those hydrogen atoms pushing against each other. Because remember, hydrogen doesn't like to fuse. Its nuclei are positively charged. You've got a bunch of protons in there, and protons push away from each other. In order to get the hydrogen to fuze and helium, you got to really squeeze it down. You got to have a lot of pressure. At the same time, you have to have a lot of energy. You need a high temperature because you need those hygen atoms to be banging into each other, and so you can have conditions at a very high pressure and temperature but without fusion, and then all of a sudden fusion happens. You've sort of overcome a threshold. And then once that threshold happens, more fusion leads to more fusion because more of it gets to that high pressure and temperature that you need. So there's a very nonlinear effect there, because hydrogen doesn't want to fuse at all, and once you've created the conditions for fusion, it can lead to sort of a runaway process. So that's the basic idea. The reason that brighter stars are more massive is because because there's a higher temperature at their core created by this incredible gravitational pressure from the additional mass, and that higher temperature leads to a faster nuclear reaction. And of course that faster nuclear reaction is not just going to make the star brighter which provides the pressure you need to keep the star alive. It's also going to eventually kill the star because that faster nuclear reaction uses up the fuel. These stars are made out of a lot of hydrogen, but there's not an infinite supply of it, right, Eventually the star is going to fuse so much hydrogen into helium that it's going to get a helium core, and that helium is going to fuse and you're going to get something denser and denser and denser, And so that's just a ticking clock, right, You have a fixed amount of fuel. Once you've formed a star, it's difficult for it to like accrete more matter unless it has some like very close binary it can steal matter from. So then it's just a question of like how many fusion cycles can it have. If it's a really big star, it'll burns through those really really quickly and eventually get to the stage where it collapses. Or if it's a little star and it's burning in a much lower temperature and brightness, then it's sort of like biding its time and it can last a lot lot longer. You might expect it to be the opposite, right, you might expect that big stars have more fuel and so they can last longer into the deep future of the universe, but it's sort of surprisingly the opposite. Those big stars do have more fuel, but they also burn more fuel, and they burn more fuel per second because they're burning hotter. So that's an idea for why there's this connection between the mass of the star and the brightness of the star, and that helps us sort of connect the dots between the things that we actually can observe. Those binary star systems, the ones where we see the masses have an effect on each other, and that tells us whether we get in these calculations correct, and it gives us confidence that we can interpolate between the things that we can see. But people are always working to improve these measurements. People are always looking for other ways to double check it, to figure out if this is wrong, to find another way to measure these things. The best thing you can do in sciences have two totally independent measurements, ones that make maybe different mistakes or different assumptions that cross check each other. And so there are a couple of other ways we can get a handle on the mass of stars, but they're even more specialized than the binary star system method. One of them is called gravitational micro lensing. This is a super fun one. It's when a star acts like a lens. Because remember that the gravity of a star isn't just a force pulling on things. It actually is bending the shape of the space around the star. This is Einstein's idea of how gravity works. Rather than being a force, it's a change in the way space is curved. In Einstein's idea of gravity, space is not just like a backdrop. It's a thing that bends as mass gets around it. And there's a complicated dance there, right, because mass tells space how to bend and then bend a space tells mass and other things how to move, including light. And so a star out there in the universe bends the space around it, and so a photon coming to us from for example, a background galaxy might get bent as it passes around that star, causing a distortion. So if you're looking at a galaxy really really far away, for example, and a star passes right in front of that galaxy, it'll distort that galaxy in a very particular way, and in a particular way that depends on its mass. So this is one way we can measure the mass of an object without anything else necessarily being near it, by seeing how it distorts light from background galaxies, because the distortion of that light depends on the mass. The higher the mass of the star, the more it will distort that light. And then of course we can look at the brightness of that star, we can look it up on our chart and we can say, oh, does this follow the pattern that we expect. Does this help us understand that the connections between the luminosity and the mass of the star. And so this is more difficult to do because it's not something that happens very often. It's not something you can necessarily predict. It's a sort of a chance encounter a star wandering in front of another system that we were already looking at. But when it does happen, it's a very nice calibration, and it lets us look at a different population of stars and get a direct measurement of their mass as well. And then there's one extra cool, sort of crazy idea for how to measure the mass of stars, and that's using astro seismology. These days, we can study in real detail the light coming from stars, and some astronomers think that variations in the light that comes from a star might be due to seismic activity on the surface of the star. That's right. These stars have sort of like crusts, and you know how our sun, for example, sometimes blows out big coronal injections or flares up and flares down. If you're study the light from that star from really far away, you can tell when one of these events happens because it affects the light that you see. Because the star is spinning, there's a variable effect on the light. So there's like a hot spot on one side of a star. You'll see it as the star spins, and then you won't see it as it spins away from you. So you can see this sort of pattern in the light from the star because of crazy effects on the surface of the star. And there's some models that tell us that there's a connection between the sort of rate of these effects, how often this happens seismic events, and the crusts of stars, and the mass of those stars. And so this astro seismology looking at the pulsation on the surface of stars might also be able to give us clues as to the mass of those stars. All right, so we've been talking about how we measure the mass of stars. I want to dig into what we've learned, what is the range of mass of stars and why that's important and what it tells us about the hit and the future of our universe. But first, let's take another quick break. When you pop a piece of cheese into your mouth, or enjoy a rich spoonful of Greek yogurt, you're probably not thinking about the environmental impact of each and every bite. But the people in the dairy industry are. US Dairy has set themselves some ambitious sustainability goals, including being greenhouse gas neutral by twenty to fifty. That's why they're working hard every day to find new ways to reduce waste, conserve natural resources, and drive down greenhouse gas emissions. Take water, for example, most dairy farms reuse water up to four times the same water cools the milk, cleans equipment, washes the barn, and irrigates the crops. How is US dairy tackling greenhouse gases? 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Okay, we're back, and we're talking about how we know what we do know about the universe and how well we know it, And specifically today we're digging into the question of how do we know the mass of a star? How do we measure that, how do we figure it out when we can't measure it, and how well do we actually know that? And so we've been talking about how we measure it using binary star systems and gravitational microlensing, and how we interpolate between those measurements using nuclear theory models about how a star works, and now let's talk about what that means. It turns out that the mass of a star is basically the most important thing about it. The whole future of the star, what's going to happen to it, it's whole life cycle, it's life span in fact, and then the future of any civilization living around that star depends entirely on how much stuff went into the star, what the mass serving was for the material that ended up in that star. And there's a few outcomes that are possible for a star. But there's this awesome stellar life cycle chart which I encourage everybody to google and take a look at. But the end points of this are super fascinating. Basically, once you become a star, you have a few possible outcomes. You can even become a brown dwarf, which is like a failed star that never really takes off and has the same kind of bright fusion that's for very low mass stuff. Or you can become like a normal star, which ends up as a white dwarf after it blows out its outer layers and then eventually, after trillions of years, cools to a black dwarf. So you've got brown dwarfs, white dwarfs, black dwarfs. Or if you're even bigger, you can end up as a neutron star, which might eventually turn into a pulsar. Or you can end up a black hole, and in some scenarios you end up as just like this big super nova remnant. The whole thing just sort of blows up and spreads out into the universe. But the one that you end up. Your eventual faith if you're a star, depends just on how much stuff you have, because it's the amount of stuff that drives the whole process, right, that makes you burn hot and fast, or makes you burn slow and cool. So the mass of the star is actually really, really important, and it's not just important for understanding the fate of an individual star. It's also really important for understanding the whole collection of stars. Right. We want to know the mass of stars because we want to understand, for example, how the galaxy is rotating. Think about one of the deepest and most amazing discoveries and astronomy in the last one hundred years, which is that galaxies are mostly not made out of stars and gas and dust, that they're made out of something else. This is an observation that came out of just looking at the rotation of those galaxies and asking how fast are those galaxies rotating and is there enough mass in the galaxy to explain how fast it's rotating? Right? If you, for example, put a bunch of pingpong balls on a Merry Go round and spun it, they would fly off into space. The reason that doesn't happen for stars in the galaxy, which is basically a huge cosmic merry go round, is because there's something holding them in. But you can measure the speed of our galactic merry go round and ask is there enough mass in the galaxy to provide the gravity you need to hold the stars in place? Well, how do you do that calculation? All you can do is look at the stuff. You see. You look at all those bright points of light in the sky, and you add up their individual masses, And that's exactly the point. You need to know the mass of all those stars in order to do that calculation. If we had no idea what the masses of stars were, we never would have discovered dark matter because we would have looked at all the stars in the sky and said, well, we have no idea how much mass there is, so maybe there's enough to provide the gravity we need to hold the galaxy together despite how fast it's spinning. But no, we do know the mass of those stars, and so we were able to calculate how much mass there is in stars. And we looked at that and we said, well, is there enough mass to provide the gravity needed to hold the galaxy together, and of course you probably know by now the answer is not even close. The galaxy is spinning at a crazy fast rate, and the mass of the stars we can see does not provide enough gravity to hold it together. That was the essential clue, the first crack that told us that there was something else out there in the universe. So doing these kinds of studies, things that might seem a little boring, like hmm, our stars as bright as we expect them to be, how much mass do they have? Can we explain? It doesn't make sense, seems sometimes like sort of scientific busy work, But sometimes the answers don't add up, and they reveal a huge cosmic mystery. So that's why it's important to know the mass of stars. That's why the mass of the stars determined not just the fate of the individual objects, but potentially how everything works. And now we know that it's not actually the mass of stars that's determining like the whole shape and future of our universe. It's actually all of that dark matter. Because there is five times as much dark matter in the universe as there is hydrogen and helium and the kind of stuff that makes stars, and it's that dark matter that controls like why there's a galaxy here and not over there. The reason is that there was a big blob of dark matter and it attracted all the hydrogen and helium, and it helped compress all that stuff, and it started the fire in those stars. So we wouldn't even have stars and galaxies the way we do today if it weren't for that dark matter, And we wouldn't know about the dark matter if we didn't have a careful measurement, a way to figure out the mass of individual stars. All right, so then let's talk about what the mass of the stars actually are. Right, we look at our Sun. That's a pretty standard candle. We think, let's use the Sun as an example star and measure everything in terms of like one solar mass. Well, right off the bat, we discover the Sun is kind of unusual. It's sort of a large star. The most common star out there in our galaxy is one called a red dwarf. It's significantly smaller and colder than the Sun, and it's hard to see them. Right. These stars are much much dimmer than the Sun. Something that's half as bright as the Sun is going to be one sixteenth as dim, and there are a lot of stars out there that are smaller than half the mass of the Sun, and so they're much much dimmer, which makes them hard to see. So we don't even really have a complete catalog of all of those stars. But none of the stars that are very nearby are much larger. In fact, none of the stars that are within thirty light years of the Sun is very much bigger, Like the biggest star in our neighborhood is about four times the mass of the Sun. And as you explore out there into the universe, you discover something really interesting, which is that there seems to be sort of a limit to how big stars can be. And we discover this just because there aren't stars that are much much bigger. The limit seems to be somewhere around one hundred two hundred, maybe up to two hundred and fifty times the mass of the Sun. The biggest stars out there are just about that big. And the reason is that stars that have huge amounts of mass burn really really hot and bright, and they push so hard from the inside that they're basically unstable and blow themselves apart. Anything above that it basically blows the excess off, so you just can't have a stable blob of matter that's more than like two hundred times the mass of the Sun. But we're going to dig into that in a whole other podcast episode about the biggest stars in the universe. It's also actually fun to think about the history of the universe. It turns out that the massive stars changes as the universe gets older. In the very very early universe, we think that the stars that were first born, the first population of stars that came together out of that hydrogen, were actually typically much much bigger than the stars we see today. Those are these so called population three stars. They're called population three stars because our stars today are called population one, and then the stars that they came from are called population two. So we're sort of counting back to the beginning of the universe. It might make more sense to you to think the first generation of stars are population one and we should be population three, but for whatever reason, astronomers are counting backwards. So population three stars are the first ones to form in the universe, and those were much much bigger because there wasn't very much metal around in the universe right after the Big Bang. The universe was mostly hydrogen with a little bit of helium and a tiny bit of lithium, but overwhelmingly hydrogen, and hydrogen doesn't clump as well as heavier metals, right, it makes it harder to collapse into a small sample. Because molecular hydrogen doesn't collapse as well, it's harder for it to cool. Right, you get this big blob of gas in which you need to form a star is for it to collapse gravitationally, And it turns out that that's easier to do in smaller clumps when you have little blobs of metal that sort of like seed smaller pieces. If you just have a huge mass of hydrogen, that it's harder for it to collapse into smaller clumps. It tends to collapse into these much bigger objects because it's harder for it to cool off. And so in the very early universe we had really really huge stars, much more massive stars than we typically see today. And then the second generation of stars had more metal in them, right, because the first generation stars burned and fueled helium production of heavier stuff, and then that seeded the next generation of stars. And because there was more helium and heavier metals around when the second generation formed you tended to get smaller clumps, so that second generation of stars, some of them are still around. They are still burning in the universe. We can see them. Check out our podcast episode about the oldest stars in the Universe, and you'll see that we've found some that we think are second generation stars that are burning for billions and billions of years. Those first generation we think only lasted a few hundred million years, if at all. And the crazy thing is that we've never even really seen one of them. Nobody's ever seen a population three star, and that's, of course because they're all burned out, so there are none of them in our neighborhood. In order to see when you need to look really really far away, so that light from that population three star would just be arriving to our eyeballs now, billions of years after it already died out. Problem is that those galaxies are super duper far away. We're talking about things that are thirteen billion years ago, so they're basically at the edge of the observable universe. Those galaxies are hard to spot. We can see the galaxy, we know it's there. We're getting light from that galaxy. We suspect it's filled with population three stars, but we haven't ever identified an individual population three star from the early universe, and it would be super fascinating if we could. We had an understanding of how big were these things, how much mass did they have? All right, So today we've dug into the details of how scientists know the mass of these stars. It turns out to be really important to the life of a star. It completely controls what's going to happen to it, whether it's going to be a black hole or end up as a white dwarf. And eventually a black dwarf is just turned by the original helping of hydrogen and other stuff that it got when it was formed. And we can figure that out by looking at binary star systems, by looking at gravitational microlensing, and then sort of extrava in between what we do know to guess about what we don't quite know. But there's still a lot of uncertainty because these calculations we do they're hard. There's a lot of approximations we have to make. We think they're probably not completely accurate. We have some confidence that they're not way off because we can calibrate them using the stars that we do see where we can measure their mass. But there's always room for improvements, and so it's important to think about the knowledge and also the uncertainty, because hey, most of the universe is uncertain. Most of what we will learn about nature and stars and the universe is ahead of us. So thanks for coming on this ride to explore what we do know and what we don't know, and our speculation about what we will eventually know. Thanks everyone, tune in next time. Thanks for listening, and remember that Daniel and Jorge Explain the Universe is a production of iHeartRadio. For more from iHeartRadio, visit the iHeartRadio app, Apple Podcasts, or wherever you listen to your favorite shows. When you pop a piece of cheese into your mouth, you're probably not thinking about the environmental impact. But the people in the dairy industry are. That's why they're working hard every day to find new ways to reduce waste, conserve natural resources, and drive down greenhouse gas emissions. How is us dairy tackling greenhouse gases? Many farms use anaerobic digestors to turn the methane from manure into renewable energy that can power farms, towns, and electric cars. Visit you as dairy dot COM's last sustainability to learn more.
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