How Can We Tell The Distance To Stars?

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Speaker 1: You know, Daniel, sometimes I think how far is everything from us? Like how alone are we? We are nowhere in the universe exactly, or maybe right on the edge of the middle of nowhere. You mean, we're like in the suburbs. That's right. You have to drive pretty far to get somewhere excitings from where we are in the universe. You know, if you look at into the night skuy, it just looks black with little pinpoints, you know, Like, how do we know how far away these things are? Yeah, it's amazing. Some of these things we look at, the nice guy are pretty close by, you know, planets, other things are incredibly distant, you know, billions of light years away. We are sitting on this little ball of rock floating through space, and we are making these huge a statements about the structure of the rest of the entire universe, Like how could we possibly know all this? She's sitting on this little tiny rock. Hi am Jorge, and I'm Daniel, am my cartoonist. I'm a particle physicist. And this is our podcast called Daniel and Jorge Explain the Universe, in which we talk about all the things in the universe and how we understand them. Or if we understand them. Were actually most of the things that we don't understand to be on the podcast, we're going to answer a question from a listener. That's right. Listener Ryan Lynn wrote in with a really interesting question. He said, how do we know what we know? A lot of times in science you hear about an amazing discovery or something science has figured out. But this listener, Ryan always wondered, how do they know that? How they figure that out? How is it possible to know such crazy facts about the universe given that we're stuck on this tiny rock in one the will spot around the in the universe. Yeah, and just for the record, we may or may not have changed his name to protect his identity, and he may or may not live at one two three Question Drive, Atlanta, Georgia, And that's obviously made up. But yeah, if you have any questions at listeners, please send them to us. You can always write us at questions at Daniel and Jorge dot com. So this is a very broad question, how do we know what we know? But he had a very specific example, right, He asked, how do we know how far away the stars are? Yeah, which is a great question because as you look up with the night sky. A lot of the stars look similar, right, they're just pinpoints in the sky. Yeah, And so you might ask, like, how can you tell which ones are close by and which ones are are far away? In fact, your eyes sort of looks like they're all just painted on a ceiling right to your eye, to your brain, they just look like they're painted on a huge dome roof. Yeah, And I think for many thousand and some years that's what people thought. They thought they were looking up essentially the ceiling of their living room, right, and that the stars were painted on them. There's like a show and it's very uh, you know, anthrocentric that suggests that there's something created for us to experience, when in fact, of course, it's a mostly cold, empty universe that ignores and ignores us. I'd love to live in that house where your living room is the size of the cosmos. Right. But that's the thing. They had no idea how big it was, right. They thought the sky was, you know, a few miles or a a few hundred miles up there. They had no concept at the scale of the thing they we're looking at, you know. And that's the crux of the issue, is that when you look up in the night sky, you can't tell if something is really far away and huge or really close by and not actually that big, right, because like if you look out into a landscape, you can see a mountain, and you sort of know how big mountains are, so just kind of by the size of how it looks, you can sort of guess how far away it is, right, But a star is it's like you don't even see it as a circle or a ball or nothing. It's just like a pinpoint of light, that's right. And that applies even for closer up stuff. Like I was talking with my kids about this question yesterday, and I told my daughter, you know that the sun is much much bigger than the Earth, it just looks small because it's far away, And she was surprised. She didn't realize that the Sun was bigger than the Earth. And of course we know now it's much much bigger than the Earth. But in the sky it seems a lot smaller than the Earth, which is huge right in our perspective, But it only looks that way because it's far away. And if you didn't know, like, well, how big is it, then you would have no idea is it enormous and far away or kind of small and close up. Right. Yeah, so this is a good question, and it's not an easy question, right, and it's taken us a while to figure out how to tell how far away stars are. But before we talk about how scientists have done it, we thought we'd ask people on the street if they hadn't in any ideas. Do people know how the distance to far away stars is measured? Or do they just take scientists at their word. Think about it for a moment. You know, how would you tell how far away a star is? Well, here's what people around the U See Irvine campus had to say. I have no idea how to tell that. No, I don't know that at all. You use the telescope, I don't know. Stressed out but sorry, um by some scientific TV show something like that. Okay, yeah, I think so I don't know, like who count we metch with that? I don't know? Actually, okay, okay, So most people had no idea how this is done, which I love, right, And I could see in their faces when I asked them they're all of a sudden they thought, what, wait, that's a good question. I have no idea, not only how you would how scientists do it, but how you would even do it right? Most of the people reacted that way. It's all familiar words. You know, how far away something? Stars? Who know stars? But when you think about it, like, it's really not that intuitive to know how far away stars? Yeah, it's not that easy, though. I love that some people had ideas, like one guy's like, well, just watch scientific television shows and we'll tell you. Just listen to a podcast. I mean, that's what scientists do, right, you wanted the answer to question, just just turn on science TV and listen for the answer. Then you write it up in the paper. It's like a snake eating its own tail. That's how all science has done. But the point is that it's not an easy problem and that most people don't know the answer. Yeah, maybe we should start by talking about things that are close up, Like how do we tell how far away things are? Well, we have two eyes, right, So say, for example, Uh, somebody's throwing a basketball at you. How do you know how far away the basketball is? Well, one thing is you know how big a basketball should be, So as it gets bigger, you're imagining it's getting closer. But say somebody throws something at you you've never seen before, you're not familiar with. Right, has your brain know how far away it is? The key is that you have two eyeballs and not just one. Yeah, so you you your brain looks at the difference between what your left eye and your right eye are seeing. Right. Yeah, you have to do this experiment. Hold out one finger in front of your face and then look at it with your left eye only and then your right eye only, and you'll see it move right. You get two different images, and you see a little bit different. You see a little bit more of one side of the finger with one eye and a little bit more of the other side of the finger with the other eye. Right, so you get its binocular vision right binocular meaning two eyes. You get binocular vision, and your brain compares these two pictures. And if these two pictures are really different, that means the thing is pretty close, right, because you're looking at the thing from very two very different angles. Um, only if it's really close. But now move your finger as far away as your arm will allow. Right. Please don't chop off your finger and throw a bast room or over driving. Then you do this as a mental exercise. Please or pull over. Yeah, So now with your finger further away, do the same thing where you look at it only with one eye or the other, and you'll you'll notice that the two images look more similar. And as your finger gets further and further away, the two images look more and more similar. Something that's really really far away looks the same to both eyes, right, because the distance between your eyes gets really small compared to the distance to the object. Yeah, and it's more noticeable if you switch eyes very quickly, right, Like, if you go blink blink, blink, blink, blink, switch between eyes, you can really see how things change the closer they are too, that's right. It also kind of makes you look like a crazy person. So if you're listening to this podcast out in public, you know, maybe get a little privacy. I'd love to imagine there's some some car pulled over by this side of the road with a person blinking back and forth, and somebody is now calling Homeland Security saying because of somebody's suspicious behavior. Right, let's take a quick break. So yeah, that's called parallel right, which is is not a comic book villain. It's like an actually, oh man, this should totally be a comic book villain with like lots of sets of eyeballs or something. Parallax. Yeah, that's um, that's called binocular vision, right end, or parallax. And the idea there is that you see from different angles if you have two views of it. So that works for your eyeballs because they're spaced, um fairly, there is space fairly wide. Right, It's kind of like a triangle, right, Like if you draw a line between your eyes and then another line from each of your eyes to the object, you form a triangle, right, And that's how you it's called triangulation for because then with the triangle, you can tell how far away it is, that's right. And that's why if you lose an eye or close an eye, you don't have very good depth perception, right, because you need both of those views to see how far away things are. So people with one eye or people with an eye Patriot ever, you know, they stumble more often for this reason, and they have developed other techniques for knowing how far away things are. So that also works for the stars, right, and maybe you're thinking a whole lot of second, the stars are super duper far away, right, My eyes can't measure the distance to a mountain. How can my eyes measure the distance to the stars? It seems almost impossible. Well, what's happening when I look at the stars with my naked eye? Like, why can't I just resolve the use the same technique? Right? And the reason is that the distance to the stars compared to the distance between your eyes is almost infinite. Right, that triangle you talked about a tiny little side which is the distance between your eyes, and then you know the other two sides that extend all the way to the stars. It's like light years and light years and light years. So basically those photons are parallel to each other, right, And do you see the same image? Technically your eyes see different images. It's just that maybe the difference is so small your brain and your eyeball can't tell the difference exactly. So in theory, if you had super duper vision, then maybe you could use that information to tell the distance to the stars. Or if your eyes were really really far apart exactly, if your eyes are really far apart, or you have really really good vision, those are two ways to make this distance measurement measurement more possible. So that's exactly what we do to make our eyes further apart. We don't just look at the star up at the night sky. We wait for the Earth to go around the Sun, and we look at the star from both sides of the Sun. So, you know, you look at the star in in June and you're one side of the Sun, and then the Earth goes around the Sun. You look at the same star in December. Now you're looking at the star from two astronomical units apart, right, as if your eyeballs were two astronomical units apart, right opposite sides of the Sun. So that's pretty good distance. Yeah, it's like in December you open your right eye and you look at the star, and in June you open your left eye and you look at the star, and you compare how those two images are different. That's right. And I hope that you have things to do between December and June other than just standing outside waiting for six months to open and the other isn't that? I thought? It depends on how devoted you are to science, you know, like people, if you really care about this stuff. No, that's exactly what it's like. And um, so you take one piece of data in one part of the year, and the other piece of dating the other part of the year, and that's effectively like making your head you know the size of the solar system, and so that's a huge additional leverage Toto seeing things that are really far away. I wonder if that's how they measured how far the moon was, do you know what I mean? Like maybe not waiting, waited until a whole half year, but just kind of like looked at the moon, talk to somebody who was a couple of miles away and see what the what the difference between what he saw and you saw That would tell you how far away the moon is. Right, Well, there's a couple of things there. One is them waiting part of the year won't help you with the moon because the moon moves with the Earth, right, we don't leave the moon behind. And the Moon is actually so close up that you can do cool stuff like bounce a laser off the moon or bounce radio waves off the moon, and to measure the distance, that's actually the best way to measure the distance to the moon. And what they've discovered actually is that the Moon is getting further and further every every year. We're losing the moon, like the Moon is orbiting the Earth. But what orbit grows very gradually? Yeah, when when are we going to lose the moon? Let's see what time is it now? You know, um, it's gonna be a long time. We're gonna have the Moon around for a while. You don't have to worry about it. And if you bought real estate on the Moon, you're fine. But I think by a centimeter of years is the number. I remember. The distance from the Earth to the Moon is growing. That's not nothing. That's not nothing. But also the astronauts put mirrors on the Moon when they landed there so that we can shine lasers at those mirrors and do cool tests stuff like a global selfie. That's exactly right. Yeah, And so so we're saying, if you want to get better measurements of UM using this parallax system, you either need to have your eyes further apart and when you do that is so the June and December, or you need better eyes. So, of course we don't just use my eyeballs or Jorges eyeballs or my students eyeballs. We use telescopes and telescopes out in space that can tell the difference between really really small images, right, that can look really really far away and measure very precisely where these stars are at different times of the year. Yeah, I just think it's amazing that you can see something so far away. You know, like here on Earth, you're used to far away things looking blurry or faded or faint. But just the idea that you know, millions trillions of light years away, you know, a photon left the star, traveled throughout the entire cosmos and then arrived into your eyeball, right when you're looking at the night sky. That's one of my favorite things of the night sky is that it's the world's greatest view. It's the universe is greatest view. You know, you are seeing across billions of light years of space. It's it's amazing to me. I totally agree that those photons traveled unimpeded for so long and then finally just get absorbed by your eyeball and then you just a little a glance away, you know. Okay, So then what are other ways? How do we tell how are weight things are beyond a few thousand light years? Well, that's really the best method we have, is this parallax method. And so science has been working on that really hard, and it's actually cool because our ability to do this is improving pretty rapidly. It used to be we could only see things to like maybe a thousand of thousand light years away, and then we've got better telescopes and navigate they see things pretty reliably up to several thousand light years, And now that we have even better telescopes, we're seeing some things up to like ten fifteen thousand light years away. So as we get better and better telescopes, we're gonna get better and better measurements of this than Parallax is really the crux. That's the way that we really believe that it gives us the most reliable estimates of distance, so everything is built on that. Beyond that, when things are further away, then there's you know, fuzziness, there's questions and people out there might get a little skeptical and how do we know some of these things? But essentially what you need to do is what we talked about earlier, is find something that's a reference point. Find something where you know how bright it is, so you can tell like a mountain, like, you know how big amountain typically is. Yeah exactly. Um, yeah, So if you if somebody was, for example, standing on a mountain and shining a really bright light, and you knew how bright that light was at the source, then you could measure how dim it is where you are, and you could tell the distance, right, because the brightness falls like one over the distance squared, because the photons go out in every direction, and the surface area of a sphere around the star goes like radius squared, and so the same amount of light is spread over more and more areas. You just get fewer of those photons the further away you are, even if with no atmosphere, no air between you, that's right, things just spread out. And so, as you said, we get just a little stream of those photons. Most of the photons from stars are going somewhere else, right, something somewhere else in the universe, and eyeball, we hope is picking up one of those photons. So we're only seeing a tiny little slice of the photons that come from that star. So, as we were saying, if you want to know how far aways something is, you have to know how bright it was originally, how bright it is at the source, and then compare that to the brightness you're measuring on Earth. That's pretty tricky because the universe is filled with weird stuff that we don't understand, right, and so it's sort of chicken and egg, Right, you want to know how far away is that stuff? When what is it? Well, you don't know either one. You're you're sort of in a in a pinch. It just looks like little dots. It just looks like little dots. But we found a few things that we can use for reference points. But maybe let's take a break and we can dig into that in a moment. Okay, So what are the other ways we can tell how far away stars are? So one of the ways is with these really weird kind of stars that are variable stars that don't shine the same amount of brightness all the time. And the reason is that, well, there's some really complicated astrophysics that's beyond me, frankly. But the thing that's important to know these stars, which are called sapid's um, they pulsate, and the rate at which they pulsate is very closely connected to their brightness. So if you can measure how fast they are pulsating, you can know their brightness. It's got this internal layer that stores and releases energy so that the whole star expands and contracts, and that's what makes it pulsate, kind of like a lighthouse. Yeah, just like a lighthouse exactly. Um, it's just like a lighthouse. So these things are like lighthouses out there in in our galaxy and in other galaxies. And people figured out by using parallax that the ones that are pretty close up that there's a relationship between how bright they are and how fast they pulsate. And that's really cool because then there are ones that are really far away where we can't use parallax to tell how far away they are, but we can tell how fast they are pulsating. Right, that's not hard to measure. You just watch it and you see blink on and off. You can then say, you know how bright it is at the source. You know how bright it is at the source, and you know how bright it is here on Earth. Then you can do some simple math to figure out how far away it must be. Like it's telling you using Morse code how bright it is, right, Like it's like I'm really right, I'm really dim yeah, and so and that's the key. These things are called standard candles, and they're just ways to know how bright something is at the source without knowing how far away it is, right, that's the key. You have to have some other way of knowing how bright they are. And so these were discovered, you know, almost a hundred years ago, and it was Hubble himself, Edwin Hubble, who used these and found them, a bunch of them that were surprisingly far away. He looked up in the night sky, and back then people thought the whole universe was just the Milky Way Galaxy. That was it. You know, there was just a bunch of stars and we are galaxy and nothing else right like there was it was all concentrated around us. Yeah, they thought that was the whole universe. And you know, even that was mind blowing to people. Right If if all you thought was, oh, it's just just the Earth and a few other planets and everything else is sort of painted on the living room of the sky, then it's mind blowing to think, what, there's a whole galaxy of zillions of stars, right. So people were just are slowly accepting that. And then Hubble he looked to try to measure these these syfids and see how far away they were, and he got some really weird results. He got results that suggested that these things were crazy far away, far away than any star anybody has seen before, and so we thought, well, maybe these things are not just like weird nebula or weird other stars. Maybe there are other galaxies. And that must have been a mind blowing moment for him, Right, Wow, It's like, it's not just us in our living room, there's other houses around us, exactly, exactly. And that's what I love about this question is figuring out how far away the stars is, gives us a three D map of the universe, right, tells us where everything is, what is the structure where we living, Like, are we in the suburbs? Are we in the exciting downtown hip area of the universe? Right? So it's so important, and it's exactly what's led to these moments of realization where you discover or that the universe is totally different from the way you thought it was. I hope to I hope to have one of those moments myself in my science career. It's it's let us map the universe and where we are in exactly. That's a big exactly, that's a huge deal. And that really was the birth of modern cosmology. You know, knowing there were other galaxies and wondering how many are there and how did this all come together? And you know, and obviously if there are other galaxies, then maybe we're not the most important one or at the center of anything or all those questions were created just at that moment when we discovered that there were other galaxies. Okay, so that's a really cool trick is find an astronomical object that somehow tells you how bright it is, not by how bright it is, but through some other information. Yeah, exactly, and so you have to really know the astrophysics of it. And the cepheids were calibrated by comparing to the parallax scale, So parallax works up to you know, maybe ten tho light years, and in that sphere there are some of these stars, these variable stars. Once you know that, then you can look at the ones even further out using this technique exactly exactly. An astronomers call this the cosmic distance ladder. Because there's a bunch of different techniques that use for things at different distances, and then you try to overlap them and stitch them together. There's no one technique that will work for everything really close up stuff. You've got parallax for stuff that's a little further away. You got these these variable stars, the cfids, and then after that you have to use GPS. After that you just watch a science TV show that you have to do everything. Um, the problem is that cefids are just stars, and so they're bright, but they're not that bright and you want to see something like in another galaxy or really really far away galaxies. You can't resolve individual stars in super duper far away galaxies, so that has a limit to Yeah, so then they needed something super crazy bright to serve as a standard candle for the rest of the universe because these stars, even though they blink, they get lost in the light from the rest of the galaxy they're in. Yeah, exactly, they're not particularly bright kind of stars, and so that's why the end of the cosmic distance Ladder is dominated by supernova. Supernova is when a star goes boom, right when it's time to check out. It's had all of its fun and it's decided we're done with all this fusion stuff. Let's just blow it on one last big party. It collapses basically right like it runs out of fuel and then it just yeah, and we did a whole fun podcast episode on house stars and their lives um which you should go out and listen to you're interested in that kind of detail. But the critical thing is that one kind of star ends in a very particular way, and it's called a type one a supernova, and the supernova are extraordinarily bright. The thing you have to understand is that the stars like using up all of its fuel in a very short amount of time, and so it's extremely bright, and a single supernova can be brighter than the entire galaxy that it's in. So that's how we can see it. I mean, if you look at the night sky, you can see these these little fuzzy things that are the other galaxies, but you can't really resolve any particular star in it right. Well, for the close up ones you can, like for Andromeda, which is whe of the nearest galaxies, you can see the shape of it, you can see individual stars, but you're right. For the furthest ones, they're just little smudges and you can't resolve individual stars except when one of them goes boom, and then you can see it and it's brighter than the hundred billion stars combined. Right. That's it's incredible to me how bright these supernov are. And you don't want to be close to any of these things, right, any Anybody near these things gets instantly sterilized. So we would see it as a like a little dot in the inside of a galaxy which is flash like it will just yeah, And in the night sky you see it as maybe a new star. Right. There could be like a little smudge there you never noticed from a really far away galaxy, and all of a sudden, there's a bright star there. And you can look back in history and see the record where ancient people's saw these things in the night sky. Right, we have a history of supernov explosions that goes back more than a thou and years because like Chinese astronomers are noticed, hey, on this date, a new star appeared in this guy and the only lasted for three weeks and then it was gone. Boy, that was weird. And the trick is that these supernova are always the same, right, It's not like do you have small supernovas and big supernovas. It's like, if you're gonna have a supernova, it's always this bright. Well, there are a lot of different kinds of supernova. But this one kind, called Type one A always has the same sort of curve, this light curve when we talk about how bright it gets and then it hits a peak brightness and then dims away, and the whole process last days. But it always has the same shape and always has the same peak brightness. Now, for those of you who know a lot about this, there are some technical details and in the variation of the peak brightness, but that can get calibrated away, and we can talk about that another time. But the basic version of the story is that they always have the same peak brightness because it's always the same kind of process. It's always the same size star with the same amount of fuel in it. It's not like every star those supernova. It's like only once with a particular size and stuff, and it will collapse at some point right in a very particular way. That's exactly right. There's lots of different kinds of supernova, but one kind which comes from binary star systems, in which one of them is a white dwarf that leads to Type one A supernova, and which just happens to be very regular, and it happens to be very little variation between the brightness of different type one A supernova. And this is something people realized, you know, like twenty thirty years ago, but it took some work. You know, people are constantly out there looking for new ways to find distance metrics, new ways to figure out how far away things are. And people we're working on Type one A supernova and other people working on this kind of thing. So people work on the other kind of thing. People today right now are working on new ways to measure distances because we always want to know more precise information. So behind the scenes, grad students were slogging away, can we figure out how far away type and supernova are? Can we calibrate they're they're like her, so that they all look the same. And then about in the late nineties, people figured out how to do it, and the technology became possible and they started collecting this information, right, and so all of a sudden, a whole new window into the universe. We could tell how far away things that are super far away we're because the supernova are so bright. Right, But it's sort of interesting because it came from kind of a random occurrence in the universe, right, Like people just cataloging and observing supernova just for the sake of science. Suddenly they realized that this gives us a tool for mapping the universe exactly. And that's what astronomy is all about. It is like, let's figure out how to use the idiosyncrasies of the universe to give ourselves clues, right, And so yeah, it's just luck, right, I mean, I mean it's luck. What is it revealing that we underneed the surface exactly? That is that is pure science right there. It's like, let's nail down facts about the universe by things that accidentally reveals to us. I mean, everything the universe tells us is an accident, right, nobody's purposely sending us information. We're just a sit unless we're living in a simulation, in which case everything is on purpose. Um. Yeah, So Type one A supernova stand out really really far. The best distance measurements we have come from type one A supernova. So this sort of three step louder there. There's the parallax for close up stuff, the cepids for medium range stuff, and then the type one A supernova for super far away stuff. And you know they overlap, which allows us to calibrate. And it's built one on top of the other, right, like what we know from supernovas is built on what we know from these blinking stars, which is built on what we know about from parallax, which is built on our eyeballs. That's right, exactly, it's built on our listeners eyeballs. And now we actually have a new way to measure distance, which is really cool, which has only been possible recently, and that's um from gravitational waves. Gravitational waves are these ripples in space time that we can measure by seeing how these observatories shrink and expand by my new distances as the gravitational wave passes. And just like with type one A supernova or with cepheids, we know something about the strength of the gravitational wave based on what it looks like here, based on not on its brightness, but like on how fast it's wiggling, because because gravitational waves wiggle right there waves of space, so something about how they're wiggling gives you a clue as to how bright, how intense they were at the source. And then we can by measuring how the intense they are here, we can tell how far away things were. So this is a whole new handle. We only recently added to our cosmic distance ladder. You sound pretty excited about gravitational waves. Um, I am, I am. It's a it's a heavy topic. Um, but it's sort of cool. I guess just taking a step back just to think that we went from like a two D v of the universe just looking at these things that we thought we're painting on the ceiling, to this now really sort of incredibly rich and deep three D conception of the entire cosmics. Right, It's absolutely incredible, and it's more than three D. Actually, it's actually kind of like four D because when we look out into space, we don't just look at it where things are. We look at where things used to be, right, so we see things in sort of these spheres, like the things that are nearby are recent, things that are far away are old. And so we're not just looking out at where we are in the universe like a three D map. We're looking at these shells that get further and further back in time, and so this allows us to see what the universe used to be like, so absolutely gives us a sense for the structure the universe, but also gives us a sense for how that structure is changing, and why you cannot get more rich information about the formation of the universe in our context and why we're here and all that crazy important stuff. Then understanding how the universe came to be the way it is, right, all that just from looking up. That's right. So next time you're looking at the night sky, wonder is there more information there that I'm seeing? If they're more information than even scientists know. Bobile, future scientists laugh at us for missing what's going to be in those futures science shows what's going to be on a future science podcast. All right, thank you very much for listening, and thank you to Ryan for sending this question. Thanks for listening, See you next time. If you still have a question after listening to all these explanations, please drop us a line. We'd love to hear from you. You can find us at Facebook, Twitter, and Instagram at Daniel and Jorge that's one word, or email us at Feedback at Daniel and Jorge dot com.