Daniel and Jorge Explain the UniverseWhat did the night sky look like to the dinosaurs?
Daniel and Kelly talk about what the night sky would look like if you travelled back to the time of the dinosaurs.
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Daniel, how's it going.
I'm good.
Good.
Nice to talk to you again.
Yeah, nice to talk.
To you too.
You know, I was out the other day and one of the great things about living in the country is that when there's a cloudless sky, you really get to look up at the stars and see all the night sky. And I've been looking out there with my oldest kiddo lately, and I was wondering.
Do you and your kids like looking up at the stars too.
You know, I'm more into astronomy than they are. But back when they were younger, we did a lot of star watching.
Awesome.
So you asked a lot of questions, and so I wonder if your kids are like you, do they ask you some really cool questions?
They did, yes, sometimes some surprising questions and sometimes some kind of like embarrassing questions.
Embarrassing what could possibly be embarrassing about the night sky?
Well, I remember telling them that the stars are really old, like super duper old, like crazy old, Okay, And then their follow up question was, oh, really old? You mean like you and mommy?
Oh out out, kids are cruel, I know.
So I had to explain to them that we were incredibly old, but not like astronomically cosmically old, not quite yet.
It's a different scale entirely.
Really.
Hi, I'm Daniel. I'm a particle physicist and a professor at UC Irvine, and I like to think of myself as not astronomically old.
I'm Kelly Wiener Smith. I'm adjunct at Rice University. And my daughter asked the other day, when I couldn't hear what she was saying, if that was because I was getting old, and sometimes old people lose their hearing. I also prefer to think of myself as not astronomically old, but maybe old by people's standards.
Who knows.
And so, what did you say? Did you say?
Huh?
What? I didn't hear you.
Now I think I just gave her the stink eye.
That's a very powerful nonverbal communication.
That's right, that's right.
But one thing that's wonderful about the night sky and about the universe is that it does inspire the sort of kiddo's sense of wonder in all of us looking up at the stars and wondering what they are and how they work. It's sort of like a common thread through all of human existence, probably going back thousands of years.
It's true, probably some of the most profound and the most wish I was able to be profound statements have been said while looking at the night sky.
And Welcome to the podcast. Daniel and Jorge explain the universe in which we try to be profound about the nature of the universe. We stare out into the vast cosmos and wonder, how does it all work? What's going on out there? Can we make sense of any of it or all of it or none of it, and so on this podcast, we dig deep into the mysteries of the universe and try to explain all of them to you. My friend and regular co host Jorge can't be here today, but I'm very happy to be talking about the nature of the night sky with my friend Kelly. Kelly, thanks very much for joining us.
Thanks.
I'm happy to be here and I'm keeping my fingers crossed that we don't talk about anything that's going to scare me or my kids today, So I guess we'll see what happens.
We're going to talk about fun stuff today, but we're also going to talk about big cataclysmic events oh um uh oh.
Yeah, exactly, voting.
And we are going to try to touch back on that sense of awe and wonder that we have when we look up at the night sky, because something that I love when I see the night sky is thinking about how other humans saw it, you know, cave people or even proto humans looking up at the stars, wondering what they were not realizing how many secrets about the nature of the universe. They contain. The messages that are coming to us from outer space telling us about how the universe works.
Yeah, I wonder if it was mostly scary or mostly like relaxing to look up at the night sky back when we had no idea what it was.
Probably some combination of both. I had an amazing experience a few years ago because I got to be in the path of totality for the most recent eclipse that went through North America. And I was excited about it because you know, I'm a space nerd, but frankly, I was preparing to be a little bit underwhelmed. Like you know, fireworks and Fourth of July, for example, we've all seen them before. It's exciting, I guess, but it doesn't really blow your socks off. I was sort of expecting that kind of experience, but I was totally surprised and amazed at the depth of the experience, the like really shocking overwhelming nature of that experience. And part of it was thinking about what it must have been like for ancient peoples to have something like really central in their existence, the sun just like go away for a few minutes. What must have that been like if you didn't have any understanding about what was happening.
And I think it impacts more than just humans, Like I've heard that in some of the areas when the sun goes away in the middle of the day, insects will start chirping like it's nighttime. Like even they're kind of thrown like it's nighttime and the nighttime has gone away again, and what is the world doing. I don't know if they have a stress response, but it seems to throw all of us off.
It is very strange, and it makes me very glad that we have some understanding of why it happens, so we don't all have to be terrified that, like the basic deal in which we live is suddenly changed for reasons we don't understand. And I understand now, like why the Chinese and the Babylonians and the Mayans were so intent on understanding what was going on in the cosmos and keeping track of it and trying to predict the clipses and all this kind of stuff.
Yeah.
Absolutely, it's super fun actually to dig in the details of like how the Greeks did astronomy and how the Chinese did astronomy without understanding geometry and all this kind of stuff. Super fascinating things We can dig into another episode, but today we want to go even further back, not just wondering if ancient Chinese and Babylonian astronomers were looking at but the same night skies we were, and having the same feelings of on wonder we want to go deep, deep into the real history of the Earth and wonder like, what did the night sky look like a million years ago? Fifty million years ago? How much does it change? After all?
That's so?
How far back are we going?
So?
On today's episode, we're gonna ask the question what did the night sky look like to the dinosaurs? Whoa exactly if you were a dinosaur astronomer, I guess that would be like a dino astronomer or an astronosaurus. I'm not sure what you would call that. What would you see if you looked up at the night sky? Are we looking up at basically the same night sky as a dinosaurs? Or did the dinosaurs live long enough ago that enough cosmic time has elapsed that things could actually look different?
Well, you asked a bunch of smart people this question. Should we see what they had to say?
I did exactly that. Thank you very much to our listeners who participate in this segment of the podcast, Give Me MAKEE does a sense for what people out there know and do not know about the question of the day. So thanks very much to everybody who participates. If you would like to hear your voice speculating basisly on the podcast, please write to me two questions at Danielandjorge dot com. Everybody is welcome. So before you hear these listeners' answers, think to yourself, do you think astronosauruses were studying the same night sky as we were.
Well, that's an interesting question. I know that the constellations that we see in the sky are not static. The universe is moving, the galaxy is moving, our Solar system is moving with respect to the other stars. It's just it's happening on a very slow scale. So on the scale of dinosaurs sixty five million years ago, they probably didn't see the same constellations that we see. Some of the stars that we see today probably they saw as well, but they probably saw other stars that are no longer there.
I'm guessing that the night sky looked quite similar. However, some stars were likely brighter on the account that the universe hadn't expanded as much.
I don't think the sky looked really different for dinosaurs. I mean sixty five million years, not a lot for the universe, So I think the skyes looked pretty much the same, maybe a little more stars here and there, not so different.
I think in the time of the Dinosaurs, this night sky probably looked a bit more condensed with stars, like the constellations were maybe more close together, although there were other stars that were visible that on.
Anymore, I can imagine there'd be a lot of light pollution, so you'd get to see a lot more stars, probably different nebulas than we have now, different constellations, and I can imagine there was an asteroid approaching at one point.
I think that there's ki for Dino's. The night sky for Dino's would look like filled with more stars because then there was no light pollution, and distant galaxies were slightly closer to worth some billion years ago.
So I was really surprised by how diverse the answers were here. There's a lot of different you know, the answers ranged from pretty much nothing has probably changed to different stars. And I hadn't actually even considered that maybe our conversation would include light pollution. So I thought that was an interesting thing to include, and a very good point. But yeah, so they you know, they were thinking about what are Cosmolalosaurus. There's got to be a way to shorten it, but what they were looking at, So, can can you give me a sense of like how long ago we were talking here with the dinosaurs?
Yeah, so the dinosaurs are really deep in the past, right. We're used to thinking about humans arising fifty one hundred thousand years ago and the evolution of our ancestors over the last few million years. But dinosaurs, we think were wiped out about sixty five million years ago, and they lasted for a long long time before that. We're talking about like the Triassic Period started about two hundred and fifty million years ago Jurassic, which is when we had like stetosauruses like two hundred million years ago, and then the Cretaceous period, when we had like the t rex and the triceratops that's like one hundred and fifty million years ago till about when that asteroid hit like sixty five million years ago. So that's like really a long time compared to human existence, though of course it's still pretty short compared to like the Age of the Universe, which is fourteen billion years and even the age of the Earth, which is four and a half billion years. One of my favorite things about astronomy is just like the length of these crazy timelines. You know, so much happens on Earth in basically the blink of an eye from the point of view.
Of the cosmos.
I am so bad thinking about like units and how to compare them, Like, you know, my husband laughs, if we have to say, you know, oh, how much is two.
Inches, We'll be like, no, no, don't.
Even ask Kelly, because I'll be like us anywhere from you know, like I don't know, my fingers are showing like a foot to a centimeter, and I'm just.
Like, I'm pretty bad at this stuff.
So my brain has a little trouble wrapping itself around questions like this. So, so how do we start thinking about this question?
Yeah?
I think maybe we should start by thinking about what we are seeing in the night sky, like what is up there that we can see, and then we can dig back into the past and think about how things are changing. Because you're right, the listeners give us a lot of really cool clues, like thinking about light pollutions and the expansion of the universe and the constellations, and I think there's a lot of great stuff to dig into, but maybe we should start with what we're seeing in the night sky, Like what is up there in the night sky for us to see? When you stand out there with your kids, staring up the night sky, sipping your hot coke, what is it exactly that you're seeing? And I think people will be surprised to learn what we're seeing and what we're not seeing. Like if you ask people how many stars are visible in the night sky, they typically come up with like really big numbers, thousands and thousands, millions and millions of stars, because they know that there are lots of stars out there in the universe. But when you look up at the night sky, they're only about a few thousand that you can see with the naked eye.
WHOA, I would have guessed that number was much bigger, And so is we talking like two thousand?
So there are nine thousand stars in the Milky Way that are bright enough to see with the naked eye. The brightest one is called Vega, but it's still pretty dim. It's like one hundred and twenty five thousand times dimmer than the full moon. And most of the stars that are out there are like less than a percent as bright as Vega. Now, I don't want to be dissing these stars, right, Remember, these guys are really far away. We're talking about light years or tens of light years or thousands of light years away. So it's amazing that you can see this ball of gas burning from light years and light years away from billions and billions of miles, right, Like, that's just another sense of awe. I remember teaching my kids that, like, the Sun is crazy far away, like ridiculously far away, but it's so big that you can still feel it through ninety million miles of space. Right. Even just that fact still amazes me when I think about it. So, these stars are out there, they're burning bright. It's incredible that we can see them over such great distances. But there really are only nine thousand that are capable of being spotted on a given night, and so if you're standing on the Earth looking up, you're seeing some fraction of those. So it's really just a few thousand.
Stars, to be honest.
This is making me wish that we had another kind of biologist expert on the show right now, like someone who could tell us about eye vision. You know how many of those nine thousand stars that we can see with our naked eyes. Do we think that maybe the dinosaurs would have been able to see based on like the size of their eye sockets and what we know about vision and birds and reptiles or something now, but we're gonna have to let that go for now.
Yes, we'll ask Katie that question next time she's on the podcast.
There you go.
Yes, dinosaurs with glasses, dinosaurs with like binoculars. You know, there's a whole great comic vein to mine there, you know, with t Rex even be able to put glasses on because his arms are so short.
Maybe maybe that would favor the evolution of social behavior, you know, so that you could have someone help you put your glass on.
Or maybe the reason t Rex has never developed astronomy was just because they couldn't hold the telescope up to their eyes. Otherwise it might have saved them, right, if they had developed astronomy and seen the asteroid coming, boy, life could have been.
Different, absolutely so. But we're lucky that they didn't.
We are we are all lucky. We have no complaints about the history as it happened. But you know, we know that there are other things up there in the night sky as well, right, Like, we know that the galaxy is just one of many galaxies in the universe, and we're used to talking about the other galaxies here on podcast and how the universe is expanding. We're part of a cluster and a supercluster and et cetera. But the interesting thing is that when you look up at the night sky, you mostly can't see those other galaxies. I mean, this is an incredible number of them, but they're really, really, really far away. It takes special equipment to see them. You might imagine, for example, the Hubble Deep Field. That's this famous picture where the Hubble focused on a single spot in the sky for a long time, gathered all those faint photons, and it came away with this shocking image. Shocking because it's so chock full of galaxies. It's just like grains of sand on the beach. You know, there's just galaxies everywhere, but you can't see them mostly, And you definitely can't see them with the naked eye because so few photons arrive from them that you need like a really big aperture and a really long time to even see a single photon from those distant galaxies.
And the dinosaurs definitely didn't have the web telescope.
So, and there's something really interesting and sort of quantum mechanical there that some listeners write in and ask me about. They like to think about the photons flying away from that galaxy. And you know, in principle they're flying in every direction, but photons are discrete, right, They're little bundles, and so they fly like in one direction and in not another, which means if you're really close up to the galaxy, you're getting lots of photons from it, But as you get further and further away, there can start to be gaps places where like no photon from that galaxy is now arriving. You wait long enough, eventually you'll get lucky and a photon will spray out in just your direction to fly across the universe and hate your eyeball, But you're not guaranteed, right, there are moments where there are zero photons arriving from a distant galaxy, and that's why you just can't see them. You need to wait long enough or like point hubble at it long enough to sort of integrate enough photons. These things emerge from the blackness.
Like a cosmic invisibility sort.
Of, and even the nearby galaxies are not easy to spot, like our neighbor and Dramata. It's a few million light years away, but it's a huge, huge, monster galaxy. It's much bigger than our galaxy. In fact, it's so big that if you could see it, it would be larger in the sky than the full moon. WHOA, Yeah, but it's just not that bright because it's so far away, so it's basically invisible to the naked eye. But you know, you set up a telescope and point it in the right spot for long enough, you can get really amazing pictures of Andromeda. And I feel like that would change our whole concept of galaxies and what they are if we could have seen Andromeda. Imagine just like having another galaxy out there for us to look at from the outside would have given us such a huge clue about like what was going on out there in the universe.
I can also imagine that, yeah, being very scary if you didn't know what it was.
That's true.
And you know, it was only like one hundred years ago that Hubble realized that there are other galaxies out there. Until then, we thought our galaxy was the only one just floating out there in space. And it's when Hubble realized that those little smudg that were just barely visible in his telescopes, who were actually super duper far away. He and some other astronomers developed a way to measure the distance to them. That's when he realized, Oh, those little smudges are actually entire other galaxies, super far away. That's how hard they are to see and to study, and how recently we've understood that they're even there. So our ancestors, Chinese astronomers, Babylonian astronomers, had no idea that there were other galaxies up there in the night sky.
And this is why it's great that we fund science more now. So that's what things look like. Now, let's take a break, and when we get back, let's talk about how that view changes over time.
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Okay, so we talked about how we can see about nine thousand stars in the sky with our naked eye, and some of them are pretty dim and stuff that's far away is too faint to see, including galaxies.
How did that view change over time?
So there's a lot of really interesting aspects here. You know, there is a cousdent of light pollution. There's the question of the moon. There's how the constellations might change. There's how the galaxy rotates. There's the expansion of the universe. There's asteroids, you know, coming to hit the Earth that we or dinosaurs might have spotted. So let's take them one at a time and dig into each one and think about how it impacts our view of the night sky and the dinosaur's view of the night sky. The first one, because I love of this response from the listeners, is, of course, light pollution right now. Clearly, if you're living in New York City or a big city and you look up by the night sky, you're not seeing a whole lot of stars. Maybe you saw the recent conjunction of Jupiter and Venus, maybe you see the North Star. But you know, just most of the stars out there are sort of overshadowed by the light from your city. And this happens during the day too, Right, you look up at the sky during the day. The stars are there, they're sending photons at you. It's just that you can't see them because they're not overshadowed. With the word over photon or out shown there you go, they're out shown by the sun and go you're blinded by the light of the sun, and so you just can't see them. And you know, we imagine dinosaurs of course did not build large metropolises, you know, or metropolisauruses, and so they didn't have light pollution. Unless you're a believer in like you know, atlantis or like ancient aliens or some other kind of nonsense.
That's for a different show, a different show.
That's for a different show where it's sticking to the actual science here. Then probably the dinosaurs didn't have to contend with light pollution, and so they could see the night sky as well as we can under the best circumstances. And while if you live in a city you're not seeing it under the best circumstances, there's still a lot of places in the country and in the world you can go to get a pretty good dark sky. It's unfortunately getting smaller and smaller as humans encroach on every last piece of the planet, but it is still possible, and if you go camping you can get a pretty good dark sky experience.
I consider this one of the biggest perks of living out in the middle of nowhere is the lower light pollution.
Yeah, and if you're not somebody who regularly sees that kind of sky, it can be kind of alarming or shocking or just you know, awe inspiring. If you do go out camping and you look up, you're like, wow, all these stars are here, like all the time. It's an incredible view, and I think most people underappreciate it. You know, you stand on like the top floor the Empire State Building, You're like, Wow, what a view I can see for miles? Right If you look up in the night sky, you're seeing for billions of miles, Like, what of you that is? You know, It's incredible, and we're so lucky that it even exists, that everything is set up in the universe for there to be stars that shine photons, and for it to be mostly transparent so that light can get to us. I'm like totally proselytizing for astromy here, but I just feel like people don't experience and appreciate that enough.
I think it's one of the greatest experiences in life when you get a really good clear night sky. I was in Costa Rica once and I could see it was the clearest I've ever seen the night sky. And it was my job to tend the turtles that were hatching, and so I would like see shooting stars and then help some turtles get to the ocean. And it was like the greatest mix of space and biology, like like my two loves coming together at once, and it was the happiest I think I've ever been anyway.
Well, something that could really impact your ability to see the stars in the sky is the moon. You want to have like a really good dark night experience, you need new Moon because the moon is crazy bright. As we said, it's like thousands and thousands of times brighter than even the brightest star in the sky, so it can really spoil in her dark night. But it's also interesting to think about, like what the Moon looked like to dinosaurs, because while we think about maybe things out in deep space, other galaxies, whatever, changing over time, the Moon is also not static, like it's been orbiting the Earth for lots and lots of years, but its orbit is actually.
Changing every year by how much?
So the Moon is orbiting the Earth and its orbit gets larger by a couple of centimeters every year. Now, I know, Kelly, you're not good with units, but like if you open your fingers.
You know, uh huh, all right, I'm doing it.
It's like the height of a really thick pastromi sandwich or something, you know, so like a foot Wow, I want to eat a stromy sandwich at your house sometimes, yeah.
Yith house, I believe it, and some of Zach's homemade rye bread of it.
So you might imagine like two and a half centimeters a year doesn't sound like a lot. Bub we got a lot of years here, right, Like the dinosaurs lived a long time ago, and one hundred million years adds up to like two thousand, two hundred kilometers over all that time. So the moon is further away from the Earth by twenty two hundred kilometers than it was when the dinosaurs were looking up at it.
Oh my gosh, So what is the total distance? Like, what is the percent difference?
Yeah, that's exactly the right question, right, It doesn't make any difference at all. Because if your friend was right next to you and then a moment later they're twenty two hundred kilometers away, it's going to make a big difference. But the moon is not right next to us, and was never right next to us on these time scales. Its current distance is like three hundred and eighty three thousand kilometers, so we're talking about like two thousand kilometers more than the dinosaurs used to see it. That's only like a half a percent difference.
Okay, And so like if I went back in a time machine, I probably wouldn't be able to tell the difference.
That's a great way to think about it. Like if you get a time machine and somebody spins the dial and sends you back to an arbitrary time, could you use the night sky to figure out like when it was, would you notice any difference? So if you went back to like the Cretaceous period sometime in the middle of that. Then you look up for the night sky, you would notice the moon would be like one percent bigger and like one person brighter than it is today. And so unless you're like extraordinarily sensitive to moonlight, you're like a werewolf or something, then you probably wouldn't notice. But be careful about sending your werewolves back in time.
Yeah, no, no, you shouldn't know. I'll be very careful.
Any and fiction writers out there, if you have time traveling werewolves, I want you to include this detail. I mean we're talking hard science fiction here.
Hard to read.
Kelly, somebody out there's pouring their heart into a time traveling werewolf novel and you just dissed it without even reading it.
I'm so sorry. I promise I'll have an open mind. It might end up being great.
Please send us your fiction about time traveling werewolves. I promise to read it with an open mind under a very bright full moon.
That's right, that's right.
Okay, I think we've said everything that needs to be said about the moon, and then maybe a lot more. How about constellations, Are they gonna be the same? Is Orion's belt still going to be there for our Cosmololosaurus to find.
So this is where the big change really happens. And the short version is that the night sky could look dramatically different to the dinosaurs than to us for a few reasons. Number one is that stars are not infinitely old, and they don't last for an infinite number of years. Remember, stars form when you have like a big cloud of gas and dust that's cold enough that gravity can like pull on it. For gas that's too hot, the particles are just moving too fast for gravity to like gather it together into a star, and this can take a long time. Remember, our star was only born four and a half billion years ago, which means there were ten billion years in the life of the universe in like nine billion years in the life of our galaxy before our star even existed. Right, So stars are being born all the time, and so some stars that we see in the night sky didn't even exist when Tyrannosaurus Rex was trying to invent a telescope that he could lift to his eyeballs.
Wow, and we can age stars, so we can get an estimate for how many didn't exist, is that right?
Yeah, we can have an idea for how old stars are because we have a model of star formation and how stars burn, and as stars burn, they tend to get like bigger and brighter, or there's some variation depending on the initial clump of stuff they had. But there's a whole deep knowledge now of like the life cycle of stars, and so you can look at stars, and you can look at stars nearby and understand how they form together. We have a whole episode about how to tell the age of stars. It's really a fascinating science. Mostly you have to look at stars nearby and understand how they all form together. But sometimes you can get clues from an individual star. But you know, to take a particular example, the Pliades, which are a famous constellation. They're only like thirteen million years old, and so we're looking up at the night sky, we're seeing that constellation. It appears in like you know, ancient astronomical texts. People have seen these. You can even find like neolithic records of this constellation from like Bronze Age shields where they have like hammered the night sky in a pattern onto the shield and you can see the Pleiades on it. It's pretty awesome. So humans have always seen this constellation, but dinosaurs never did. It just hadn't come out yet.
You know.
It's sort of like asking if dinosaurs have seen the Star Wars movies.
You know, yet one more reason why it's good to not be a dinosaur.
Although Star Wars happened a long long time ago, so it might have been like actually occurring while the dinosaurs were here. It just took a long time for the movies to get here from that galaxy far far away.
An excellent point.
And you know, stars also die. We see supernova which are the endpoint of stars, stars that have burned for so long that they've basically turned all of their fuel into ash. You know, Like our star can burn hydrogen and it creates helium, but it can't burn that helium, so the helium falls to the center of the star and basically tries to put the star out. So that's why our star then starts to burn on the outer layers where there's still hydrogen, and it puffs up and gets big, and all stars will do this, and eventually they'll burn through all of the gas that they can burn, and there be so much gravity that they collapse in a huge gravitational implosion, which then bounces back into an explosion, and you get these really dramatic events in the sky. We call them supernova and what's left is not a star. We can actually look back in history and see records of these things dating back to like a thousand years ago. There was a huge supernova in ten fifty four that the Chinese recorded, and like, this must have been a very dramatic event. It lasted for a few months in their sky, this sudden appearance of something very very bright. They noted it as a guest star. And when we look up at the night sky in the same spot, we see a nebula, which is a huge spray of material that erupted from that supernova. So that's pretty awesome. Actually, because we have like a thousand years of observations of this thing, we can see it exploding sort of over time. But you know, the star that was there is now gone. And so if you looked back million years ago, you would have seen that star, and today that star is no longer there. So the night sky is definitely changing on the thousand or million year timeline.
Okay, so the dinosaurs both saw stars that we never got to see and don't even know existed. Because supernovas don't leave signatures for quite for millions of years, and we see some stars that they don't see. So do we have a general like, is it like about ten percent of the visible stars different now or is it just no impossible to get a number like that.
That's a great question. I don't have a number, but we do have a sort of a sense of it by thinking about the kind of stars that are out there in the universe. Like in our galaxy, most of the stars are red dwarfs. These are smaller, cooler stars than ours, and they also last a lot longer. Our star will burn for ten billion years. These red dwarfs will burn for much much longer, sometimes one hundred billion years for example. So we think the red dwarfs that are out there that we're seeing will last a long time, and any red dwarfs that exist at the time of the dinosaurs probably also exist. But the flip side of that is when you look for the night sky, you don't see any red dwarfs because even though they're the most common star in the galaxy, they're so dim that you really can't see any of them with the naked eye. So that means that the stars we're seeing are the brightest stars, which are the ones that are shortest lived, and so like, the typical life cycle of these stars is much much shorter, though we're still talking typically hundreds of millions of years or billions of years. And so you know, of the nine thousand stars that we can see, I'd be surprised to it was more than ten percent that had burned out or been born in the time since the dinosaurs.
Okay, so now we have a good sense for how many of the stars both the dinosaurs and us got to see, and when we get back from a break, we'll talk about whether or not they're in the same spots.
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All right, so you told us that the dinosaurs saw some stars that we didn't get to see, and that we see some stars that the dinosaurs didn't get to see.
Are the stars that sort.
Of overlap in that ven diagram, are they in the same spots.
So the short answer is that they're not right. Stars in the galaxy are not static. It's not like they're all in the same place. Remember that the galaxy is rotating and that stars are moving through the but not all together. It's not like we're all in a merry go round that everybody's picked their horse and they're going up and down and they're just in the same place. It's more like a crazy traffic circle in Europe where everybody's driving a different speed, and some stars are like weaving through other stars, and some stars are going in the wrong direction, and so it's a lot more chaotic than you might imagine. And over the timescale of like hundreds of millions of years, a lot of these stars move relative to Earth, and so we will not see the same picture of the sky all right.
So, first of all, that sounds awful. I do not like those traffic circles one bit. It sounds very chaotic, and they're probably running into each other. And it's amazing that we can plot the paths of any of these. So how much have.
Things moved since the dinosaurs were around.
Yeah, maybe instead of traffic circle, we should say like mumper cars. It's really much more dramatic than a typical traffic circle where we hope nobody's colliding with each other. Well, there's a big variation in the speeds of these stars. The typical speed of stars relative to the Earth, it can be like ten or twenty kilometers per second, but some of them are super duper fast. There's some stars they call like hypervelocity stars that can move up to like one hundred kilometers per second faster than like the typical speed in the Milky Way. These are stars that have like had some gravitational event happened to them, maybe they whizzed through the center of the galaxy and got accelerated by the black hole or had some near approach to something else which really changed their direction. And so like a hyper velocity star would like move across the sky very quickly on the timescale of us in the dinosaurs. But in general, things tend to move sort of in blobs. Like stars are born not individually. Stars tend to be born together. You have like a star forming region we call these stellar nurseries where a bunch of stars were all made at once. We painted this picture of stars forming from gas clouds collapsing, But it's not like one gas cloud equals one star or one solar system. You tend to have like a huge gas cloud and it collapses into multiple stars all at once, a bunch of stars with like a common origin, and then they tend to have like common kinematics. Right, They move sort of as a group for a little while until they dissipate and get absorbed into the general flow of the Milky Way. So instead of thinking of the Milky Way as like a merry go round, think of it like a bunch of different gangs on the bumper car floor, like all moving together communicate with each other.
So this might be a silly question.
Our constellations usually from the same nursery, or are constellations made up of stars that are much farther apart than that, But our brain just makes patterns with them.
Anyway, Oh, it's a great question. The short answer is that you're not guaranteed at all that constellations in the sky have stars that are near each other, right, They just happen to line up from our perspective, and so they might not be near each other at all. There could be one that's fairly dim but close to us, and we can see it in another one that's super bright but far away, and so it lines up in the sky near us. And I always felt like constellations, I don't understand why they're typically taught in the beginning of like astronomy courses, because there's nothing to them, right. It's like casting a bunch of dye on the floor and saying, like, let's learn about the universe from these random numbers. That really tells you nothing about what's happening in the universe. I mean, I guess it's like accessible, and that's how people like latch onto astronomy by looking at the stars. But I feel like there's so much actual science you can teach people instead of like basically astrology. But some of the constellations are moving together, like the Big Dipper. The Big Dipper, most of those stars were formed about the same time, and they're in a group that are all moving together. And it's actually a very big, very fast moving group of stars that are all sort of like running together across the universe.
Are they all moving at hypervelocity?
Not quite at hypervelocity? But definitely tens of kilometers and it's one of the largest and closest of these kinematic groups. They call it the URSA major moving groups, so it includes the Big Dipper, and it's moving so fast that if you rewound the clock, like even just one hundred in fifty thousand years, you would not see the Big Dipper in the sky.
No.
Yeah, it's a good thing to check for when you get out of your time machine with your werewolves. Yeah, or fine, if you don't like traveling with a werewolfs, leave it at home, but still look up at the night sky and check for the Big Dipper when you get out of your time machine.
I feel like this is almost like rattling to me.
You know.
One of the first things I learned about the night sky was the Big Dipper, Just like the idea that it wasn't there for the dinosaurs is yeah, it makes sense, but I guess I'd never thought of that.
That's okay, yeah, and.
It means that they definitely saw other constellations, like you would not recognize the night sky from the dinosaurs times. Because it's dynamic, we tend to think of it as static. And I look up at the night sky, and I wonder if you know ancient astronomers or chromagnet man looked up the night sky and saw the same thing, and most likely they did. But you go far enough back in time, in deep cosmic time, then you'll notice that these things are swimming. I would love to see like a time lapse of the sky dating back, like in fifty million years, because you would see things move, right, and they are moving. It's just moving so slowly that from our perspective it looks like nothing's happening. It's like checking in on the Grand Canyon every couple of years, being like, oh, this thing's not changing, whereas we know right that over time, of course, it's changing and growing, and a time lapse would reveal it to be a dynamical system in progress. Same thing with the stars. They're up there swirling around and changing and swinging around each other, and the night sky is definitely in motion.
It would be so cool to see that time rest.
H that's never going to happen unless you get both the werewolf and a time machine, or.
And there are other effects going on. It's not just the night sky is changing around us, that these stars are moving relative to us we're also moving relative to the galaxy.
We're not in the center.
We're not in the center of the galaxy for sure, right We're like halfway out from the center. And you should be glad we're not out in the center because it's crazy in there. You know, it's like very dense with star and radiation, and our kind of life could not survive. We're safe out in the suburbs of the galaxy, not so far out in like the excurbs with the star density is very very low. We're like, you know, halfway out from the center of the galaxy. But we go around the galaxy and it takes like two hundred and fifty million years for the galaxy to rotate. But while that's happening, the Sun does this really interesting and weird thing is that it goes up and down. So imagine like the plane of the galaxy, the Milky Way is like flat disk. The Sun is going through that disk and then back out the bottom and then coming back up again and through the top of it. It's going like up and down in a sort of a sane wave through the disk of the galaxy.
The solar like the galaxies are dancing. Why does it do that.
It's sort of like on the Merry Go Round where your horse goes up and down, and it's just oscillating. Like most of the gravity is at the center of the galaxy, and from this perspective we mean like the plane of the galaxy, and so it's attracted to the plane of the galaxy, which slurps it in it applies basically a force to it. But then when it gets to the planet of the galaxy, it's moving fast because it's been tugged on by that gravity, and it passes through it, and then the galaxy slows it down as it goes through the bottom and turns it around and pulls it back. So it's basically like a pendulum. The Sun is oscillating around the center of the galaxy over like a thirty million year cycle.
So does that mean that if you look out that like for the dinosaurs, some of the stars were higher in the sky and then later they're lower in the sky or no, because the Milky Way moves all as one and it's the rest of the stuff that's moving relative to us.
Yeah, it's a great question. It's not that the Milky Way moves all at one, right, there's all these blobs of kinematic groups that are moving, and the Sun is basically moving through it. And so as we go down, all the stars that are near us change, and then we come back up. Some of those are oscillating on different cycles than hours, So like every thirty million years, you like go into water and come back up, things can really have changed. In the meantime. We're not the only star that does this kind of oscillation, so there's a sort of a lot of change in our galactic neighborhood on a million year timescale.
I feel like I've never wanted a time machine more than during this conversation.
It would be so cool to see this stuff.
There's another really interesting effect just from the rotation of the galaxy. Like you look at the galaxy like Andromeda or most galaxies, you see that they have these arms, right, there's the central bar and then these arms where the stars are denser than other things. And we know that galaxies are rotating. I think that most people imagine those arms to be like physical structures. They have a bunch of stars in them, and as it rotates, those stars all move together the way like if you spin your arm around your body. All the cells in your arm move together, right, your arm stays together. But the galaxies arms are not like that at all. Those arms are not physical structures. They're density waves. They're more like if you go to a football game and people do the wave, right, the wave moves through the crowd, but the crowd doesn't move right. It's those people's arms going up and down that creates this impression of a wave, and.
There's all those people that don't keep up.
Exactly, And that's what's happening in the galaxy. It's not that the stars in those arms are all moving together. Instead, there's a density wave with things pulled together, and then that dense blob moves through the galaxy. So it's sort of like a traffic pattern, right, you know, like when you're driving on the freeway and somebody slams on their brakes and then the cars bunch up and then they speed up, but the cars that were behind them end up bunching up at the same place. Right. So stars are moving through this density wave and then out of the density wave on the other side, which means something really interesting for star watching. It means that sometimes you're in the arm, which means there are more stars near you, and sometimes you're out of the arm, which means there are fewer stars visible to you.
WHOA, Okay, so what kind of time skills are we talking about?
How long does it take to get in or out of the arm.
So the Sun is actually moving through the galaxy like twice as fast as the arm, so we like pass through the arm and out the other side. So the Sun is moving like twenty nine kilometers per second through the galaxy, and these density wave arms are moving like seventeen kilometers per second, but they're pretty big, So we're talking like tens of millions of years to go through the density wave. It also means something really cool about beyond the galaxy. Right, the galaxy rotates every two hundred and fifty million years on average, and the dinosaurs lived, you know, like one hundred and fifty to two hundred million years ago. That means that the dinosaurs basically lived on the other side of the galaxy or like when the galaxy was spun around halfway around.
Right, I have nothing to say.
And so even though we can't see beyond the galaxy with the naked eye, our view of other galaxies has changed completely in one hundred million years. We're like looking at a completely different part of the universe. When we look out from our galaxy is as big a difference as like winter and summer galaxies. Right, You're just looking at a completely different part of the sky. So dinosaurs basically evolved on the other side of the galaxy. They were here, right, Those dinosaurs never existed in this part of the galaxy.
I'll be honest.
When we said we were going to do this topic, I thought like, maybe there'll be some more stars or fewer stars, and maybe things have shifted around. I did not expect the answer to be.
The sixth stree.
There's some other fun effects too that I want to get into that the listeners talked about. Somebody mentioned the expansion of the universe, right, because the universe is getting bigger, and that expansion is happening faster and faster every year. And so that's true that galaxies are getting further away. And again, we can't see galaxies with the naked eye, and the dinosaurs definitely didn't have the technology. But it's interesting to imagine, like, could those galaxies have been closer during the dinosaurs times? Could maybe they have seen them with the naked eye. Well, the answer is, unfortunately probably not. The structure of the universe on a really, really big scale is dominated by dark energy, this thing tearing the universe apart, making it expand. But the organization of our galaxy and the neighboring galaxies altogether is mostly just dominated by gravity. Like, dark energy is not powerful enough to really affect the structure of our galaxy or of our relationship with nearby galaxies like Andromeda is headed towards us, despite dark energy expanding the space between us, because gravity on that scale is more powerful than dark energy. Shorter distances like between galaxies, gravity dominates, and longer distances like between clusters of galaxies, that's when dark energy really takes over. The other issue is that like two hundred and fifty million years sounds like a long time, but it's only like two percent of the age of the universe, so there hasn't really been time for dark energy to dramatically change the arrangements of galaxies in that time.
Well, there's one difference in the night sky that I'm sure the dinosaurs. I don't know how good or bad their vision was, but I'm sure they didn't miss this. How about that asteroid.
That asteroid probably was totally visible to the t rexes and whoever else was around, and they're proto astronomers. We imagine that asteroid was probably moving around thirty kilometers per second, which means that if it's approaching the Earth, it gets closer by like two point six million kilometers every single day. And they've actually done a reconstruction to think like how far away was it when it was visible, and they think that probably it was visible to the naked eye for like three days before it actually entered the atmosphere because it probably reflected enough sunlight to be visible in the night sky. It's like, huh, what's that? And so you know, if they'd been paying attention, they had at least three days to build their bunkers.
Oh boy, I don't think that's enough time.
It's definitely not enough time, which is why dinosaurs needed to really fund their astronomy programs and think about this stuff so it had like more warning before something hit them. Absolutely, But you know that's a moral question, like if you go back in time in your time machine to the dinosaur time, should you lobby dinosaur Congress to fund astronomy which might prevent the evolution of mammals and you and your kids.
I think they might just eat that's my biologists perspective here.
Even if you come with valuable information, they're just going to gobble you up.
Yeah, you know, I think probably that's how it would go.
I guess there's a good lesson there. You know, if we get visitors from the future, you know, future reptiles or whatever is going to replace humans when we kill ourselves, we shouldn't just eat them. We should listen to them first. That's what you're saying.
Yeah, don't eat the visitors.
That's right, and that's the lesson for your kids. You know that we should fund astronomy today so that we can be better prepared than the dinosaurs were when they looked up at their night sky. And also, don't eat the future time traveling were wolves.
Oh my gosh.
We covered a lot of ground today, both fictional and otherwise.
So I think it's really fun to think about the depth of the night sky and how it changes over time. And it's sort of cool this coincidence that, like astronomical time scales are very very different from the time scales of a human lifetime or even human history. But they do kind of line up with archaeological time scales, right when we think about the dinosaurs one hundred two hundred million years ago, that's long enough to really see a change in our astronomical neighborhood.
Quite a big change.
Quite a big change indeed. All right, Thanks very much everybody for coming along on this tour of deep time and this trip into our imagination for what dyn astronomers might have seen. And thanks very much to Kelly for going along and laughing at our time traveling werewolf jokes.
Thanks for having me. It was a blast as always.
All right, everyone, thanks for listening. Tune in next time. Thanks for listening, and remember that Daniel and Jorge Explain the Universe is a production of iHeart Radio. For more podcasts 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. House 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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