It is good practice as anyone who's taught, knows to start with the thing that you want the audience to take away. So I'm gonna give away the ending of my three years of Gresham lectures. The universe, we're pretty sure is really big. And if you remember nothing else from these lectures, I want you to know that it's really, really, really big. And that's the topic of this evening. And it's much bigger than anyone would've thought. When this series of lectures started in the late 16th century, we've come to realize that far from being in an important part of the universe, we live on an ordinary star in an ordinary galaxy. And though it's beautiful to look at in the night sky, when you look up, you should realize that you're looking at some, a tiny selection of a few hundred billion stars that make up the Milky Way galaxy. And that that galaxy, which we once thought was all there was, is one of hundreds of billions in the entire observable universe. And in fact, there's good reason to believe that the observable universe is a tiny fraction of the whole. That wasn't enough. You can sort of add this up. Um, it's often said that there are more stars just in the observable universe than there are grains of sand on earth. And as I know, aggression, audience likes accuracy should say, we're not sure whether that's true or not. Um, but not 'cause we dunno the number of stars. It's 'cause we dunno the number of grains of sand on earth. Um, so if there are geologists in the room, we can talk about that later. I, I feel there's some slacking going on, but this is a, a really strange position to find ourselves in. We stand, we marvel at the cosmos. We enjoy looking at the night sky and the wonderful pictures that we get from telescopes. But it's hard not to feel lost in this vastness of the universe. I'm a great fan of the, the 1930s philosopher and slightly strange sci-fi writer, Olaf Stapleton, uh, who wrote about feeling something awesome but terrible in looking at the night sky. And so one of the things we have to deal with at the start of thinking about astronomy is how to reconcile those two feelings. And one way we do it, I think, is to talk about the fact that out there amongst this vast universe and all these stars, um, there might be places that would feel familiar to us. I dunno, it's the celestial equivalent of when you're abroad and you've been away for a while, you start craving a cup of tea and a piece of toast or something like that. And so when we talk about, for example, discovering planets around other stars, a growth industry in astronomy, which we'll talk more about next time, um, we start to do things like compile lists of the 10 most earth-like places we go out into the universe to look for home, um, and ignore the fact that there are also things like lava worlds, places, unlike anything in our solar system, small, formerly rocky bodies so close to their star, uh, that they must have a molten surface. I'm reminded of Richard Feynman, the strange and complicated person. But, uh, as good with words as he was with equations who took thinking about this stuff, said, what men are poets, if they can speak of Jupiter, if he were like a man, but if he's an immense spinning sphere of methane and ammonia must be silent. Uh, and I'm no poet. Um, but we'll try and think about the grandeur of this instead of just sticking to the familiar. So the way that I deal with the billions of billions of billions that add up in this subject isn't, I should say, by having some deep understanding that comes out of being even Gresham Professor of Astronomy. When when I ask people, when I tell people I'm an astronomer, I usually get one of about three reactions. The first reaction is, have you found aliens? I think we're covering that in lecture five of this year's talks. So I will leave you in suspense for now. Says no. Um, the second <laugh>. The second question is, do you know Brian Cox, at least in this country, at the answer yes. Um, and the third thing, the third reaction isn't so much a question, but it's almost a visceral reaction. People say, oh, I never understand any of that stuff. And I think it comes from the fact that we give the impression Brian Cox and, and me and, and, and many of us who talk about astronomy, that when I say there are hundreds of billions of galaxies out in the observable universe, I think you think I know what I'm talking about. Or at least I think you think that I in some way can see those things and know what it means to envisage a hundred billion galaxies. And in truth, I just get used to saying the number more than most of you. Um, and so, you know, it's not that we have a deep understanding, but you can familiarize yourself. You could be at home in this vast universe. We can learn to live in this universe, which you all now know is very, very big, um, by remembering how we've come to understand it and how we've come to, uh, find ourselves existing in this. And you can go a long way just by looking. One of the things I like about astronomy is that it's not a subject necessarily that's about doing clever things often. It's about making really simple measurements very well and very carefully and paying careful attention to things that we see around us. So if you look at a completely gratuitous photo of the night sky, we see stars in their multitudes here. Um, we see gas from which we've come to learn that you can make more stars. And you can see there's this dark patch here, maybe on the left, just here. This is a, a not an area where are fewer stars, but this is a place where dust, um, tiny particles, much smaller than san grids made of carbon and silicon where dust is blocking out distant light. And from that dust and from other similar material, we've learned that you can make planets. And so just in this image, we've got the raw materials for our own neighborhood. But to realize that, of course we have to realize that the stars are distant from us. That was mostly, although people had talked about the stars, the suns and speculated in the Western tradition, at least of stars being the same as the suns going way back, uh, to BC uh, years. Um, really the scientific understanding, the the measurement of a distance to a star came only in the 19th century. And that wasn't from want of trying. It was because the stars were much further away than people thought they were going to be. And so the measurement proved to be difficult. One of the things that people have been doing is looking for what astronomers call parallax. So looking for a shift in position, uh, as the earth moves round the sun, if you're looking at a relatively nearby star, it should move from side to side compared to more distance stars. The traditional way to do this is to hold your hand up, uh, line up a finger on something in the distance, and you can flick between your eyes and you see your finger moves. This should you wish to try it will allow you to calculate the distance to your finger, uh, which is occasionally useful in life. So that's two things you've learned tonight. The universe is big and you now have a method for calculating the length of your arm. Should you need to. Um, when you try this with stars, right up until the 19th century, they found that very few of them moved at all. In fact, there was no consistent movement. And it took telescopes and careful measurements of tiny fractions of a degree to be able to detect that the nearest stars were at what we'd call a distance of a few 10. Well, the first one that was the paradox was measured was a distance of about 10 light years. Uh, and in fact, the nearest star is a few light years away. So for scale, the sun is eight light minutes away. That's the sort of scale we have been working on with this measurement. We suddenly have, uh, a galaxy, what would've been called, and which gives us the title of tonight's talk, an island universe, a system of stars. Um, you know, they were able to measure para axes. Some of them, um, of course many were fainter and assumed to be further and quickly in the 19th century. We gotta to the point where we can make spectra of stars. We can take the light from stars just as people are done with the sun, split it up into its constituent, uh, wavelengths, split it up into a rainbow if you prefer. And you can see, uh, in these spectra of many, many stars taken very recently, um, features like these dark lines that correspond to particular elements and which match what we see in the sun. And so we can not only find stars, but we can understand that some are hotter, some are smaller, uh, some are brighter, some are less luminous than our own stars. So this is the point where the sun gets reduced to just one of many. Um, and so already 150 years ago, we've got to the point where we are lost in a sea of stars, but they've also become more familiar. And so hopefully that doesn't feel too bad. Now the question comes whether that island universe, whether that system of stars is the only one in existence. And people have been talking about this for a long time as well. In fact, they'd been arguing about it scientifically since the first great telescopes, uh, were invented telescopes that were capable of resolving objects in the night sky. Um, and amongst the first to really tackle this question to understand what the scale of the universe was, were the great observers William and Caroline Herschel working, uh, in Lau and then in in Bath. And they're famous 'cause, uh, William claims the discovery of, of Uranus, uh, a planet he wanted to call George, which would've saved all of us a lot of fuss, particularly when talking to primary school audiences. Um, so if you want to call it George, then that's fine by me. Um, he was also, he was a brilliant, um, observer, but also a telescope maker. Um, he built the finest instruments around and that gave him an advantage. Um, it, he was, he found more lucrative than his previous career, uh, which is as a, a jobing musician and composer. Um, there's a wonderful story that he met Heen, um, and compared notes. This is at the peak of both men's fame. And when I was first asked to to think about this, I had a great time talking about, you know, what Herschel and Hyden must have discussed. Herschel was writing about star formation in the creation of the universe. Maybe those ideas are echoed musically. Uh, and I foolishly said this on a TV program only to receive letters from historians that pointed out that their diaries exist. And these were essentially two freelance people. And so they talked about how much money each was making. Uh, Herschel couldn't believe how much an oratoria was going for, and Hayden couldn't believe how much Herschel was making out of building telescopes. But if you've got the best telescopes in the world, you can do interesting things. And as well as scanning the sky to look for planets, Herschel decided that he could map the extent of what we would call the Milky Way. And so you have to make some assumptions to do this. You have to assume that your telescope is capable of seeing every star in existence. It seems like a grandiose claim, but if you've got the largest telescope, you might as well make that assumption and see, um, see, uh, where it takes you. You also have to assume that the stars are evenly scattered throughout the galaxy, that there aren't places where there are more stars per cubic, uh, light year or something like that. And then all you do is you look at the sky. Or in fact, if you are the herschel's, you look carefully at patches of sky on the clearest nights you can through the biggest telescope you can build. And this is a good example of astronomy, being about simple measurements. What you do is you count the stars in different directions. They literally counted them in about 600 patches of the sky. Um, and in cing through that, those observations in catching all the faint starlight that they can and seeing stars and paying attention to them briefly, that no one else had ever seen, they're able to work out the rough shape of the Milky Way and produced the first map. So I'm gonna show you this map. I think because of that effort, because of what it represents, a first glimpse at a position in the cosmos that isn't just a scatter of stars and isn't just a, I think this is one of the great images of scientific history, and it looks like rope kill <laugh>. And you could sort of see why this isn't on t-shirt. Somebody should make this as with Gresham merchandise. We'll get this out as a t-shirt if I've got anything to do with it. So this is the map, this is the actual drawing, and it contains one big truth and one lie. And so the well lies is unfair. One falsehood. Um, the big truth is that we live in a pancake of a galaxy, that there are places on the sky. And we all know this because we see things like the Milky Way with this stripe of stars down the middle. Um, there are places on the sky where when you look, you see a higher density of stars in the summer and the winter from here in the northern Hemisphere, we look out and we see the stripe of the milky overhead in autumn and spring skies. From here, we look outta the pancake up or down on this image, and we see the distant universe and the sprinkling of Milky Way stars. And so that's right, we really do live in a pancake. Now, Herschel didn't know, or or at least didn't think hard or couldn't work out how to overcome the effect of the dust. This material that blocks out the distant light, uh, that, um, pervades the galaxy. And so he didn't realize that he wasn't seeing most of the system. And so what that means is that you, the limit to which he could see was bounded by the dust, not by the edge of the galaxy. And so he got a roughly circular shape with us in the middle. And so this splodge in the middle, the heart of the road kill is us. And so he preserved the solar system's place at the center of the galaxy. And we still do this sort of thing. We still do star counts. We can still map the galaxy. We could look at the distribution of gas, particularly with radio telescopes that can see through the dust and allow us to explore, uh, the rest of the galaxy. And when we do that, um, we get modern maps of the galaxy, which look like this. So this is an artist's impression. We haven't yet sent a probe up to take a picture. Um, but it shows a galaxy looking down on a pancake with maybe two, maybe four spiral arms depending on, on how grand you like your spiral arms. Um, we live in a spur off one of those arms about halfway between the center and the edge. There is a bulge, a small one actually compared to other galaxies at the center of the Milky Way. Um, and you might perhaps be pleased to know there's a bar at the center as well, to which the spiral arms attach. And so Herschel had most of this, not all of it, but we knew in the 18th century that there was, uh, a shape to the system that we lived in. And pretty quickly people worked out. We weren't in the center. So then the question becomes, are there other island universes? Is this the whole of creation? Is this the whole of the universe or are there other pieces to this puzzle? And using telescopes all over the world, astronomers have begun to map the presence in the sky of faint fuzzy things, um, which clearly weren't stars. So for example, um, in the constellation of Orion, uh, here, um, where you have Orion's belt, this is now rising in the early morning, but will be a familiar sight over the winter months for those of us here in the north. Um, below the belt there is a iron sword. And if you look with the naked eye, you'll see that one of those stars, even from London skies, one of those stars appears slightly fuzzy. And that fuzziness stood out to astronomers. And as they started to scan the sky with telescopes, they realized that, um, there were many such fuzzy things. Some of them, um, were close enough that their nature became clear. Here's a gratuitous modern picture of, um, that fuzzy patch in Orion. This is the lit up bit of the great Orion Nebula, which is our nearest star forming region. It's gas that's lit up and excited by tiny, well tiny. They're, they're actually quite massive stars that have just formed from this gas inside. And it's the light from the newly formed stars that are only a few million years old that lights up the galaxy, um, and casts these beautiful shadows with dust and sculpts the material. Um, this was described by Herschel, by William Herschel as, um, an unformed fiery mist, the chaotic material of future suns. And there's the essence of that is right. He's able to say this is a place, it's a nursery from which stars are forming. There's also a famous result that, um, looks at the chemistry of the Rh nebula, which tells us that it smells of raspberries, uh, which Herschel didn't know, but I think would've worked in, uh, to his poetic description if he could have done we continuing to look and study the Iran Nebula. This is a result that only came out a few weeks ago. This is a closeup of part of the nebula taken by the J W S T, our newest space telescope built by NASA and the European Space Agency, and sending about beautiful views in the infrared that allow us to peer deep into the heart of the nebula. And what you're seeing here is in detail, the effect of the young stars down here on the Nebula. So this is an eroding flank at the top. This dusty material is being Eva evaporated by hot ultraviolet radiation from the stars at the bottom. And this image is enough for somebody to spend an entire PhD trying to understand the details of what's going on here. So though they didn't have these images, our 18th and early 19th century astronomers knew that there were places that seemed to be forming stars in the universe, that some of these fuzzy patches were forming stars by the middle of the 19th century. They also knew that some of them had distinctly different shapes. And the telescope that cracked this mystery is one of my favorites of all time. Um, it's called the Leviathan of Parsons Town, built by, uh, the Earl of Burr in Western Island, which continues to be a really strange place to put a telescope where you need clear knights. But there was an essential realization that Burr had that made this telescope special. He realized two things. One is that the hard bit of building a telescope when you get big enough, it's all hard, but often the hard bit isn't the mirror, it's the mount. It's how you take what's often quite a heavy mirror and swing it around the sky with great precision. But Burr also realized that if you've got the largest telescope in the world, it doesn't matter where you point it, interesting stuff is gonna happen. So you can improvise with the mount. And so his mount, um, was essentially two large stone walls with a telescope slung between it. And what you do is you wait for an object's turn over head, and you get about an hour and a half, depending on how high in the sky it is to observe something. And then you have to wait for the next night. But who cares? You, you have a larger telescope than anyone else. And what Buren Co did was demonstrate that some of these nebula, these fuzzy patches in the sky, well some of them turn out to be spiral. So here's a drawing which was made of, uh, a nearby galaxy. This is the Whirlpool Galaxy M 51, um, from this telescope. And you have to remember that somebody made that sketch while standing on a platform at the top of this structure in the cold of night, leaning over to try and look through. I've used this telescope, it's scary. Um, and I certainly wouldn't like to be standing there and sketching, but they did a brilliant job. And so here is the sketch. So this is 19th century. Here is a recent Hubble image of the same galaxy, and you can see that it's almost bang on it's really impressive piece of work. But what this meant was that there were two types of galaxy. There were two types of nebula, sorry, sorry. It's so easy to slip. Um, there were the things that seemed to be staffing, and then there were these spiral nebula. So are these spiral nebula galaxies? Are they like our Milky Way or are they some strange part of the star forming process? This debate continued into the 20th century. And in fact, there was something called the great debate, which was held at the Smithsonian Institution, um, in, uh, the early part of the 20th century. Um, the arguments on both sides were, were fierce. Um, and they're kind of interesting on, on the, everything is a nebula side. The arguments come from what might seem like common sense. So they say things like, but if these are galaxies, we don't see any individual stars in them. So they must be very, very distant. And that's implausible. I mean, that's true. Um, they say things like, we don't see any rotation in there. So how can they be nearby objects? Well, not everything rotates. The cleverest argument, I think comes from, oh, they'd also been, they'd also been in a, in a nearby spiral nebula, there'd been a bright nova, what we would call a supernova. And again, the everything is a nebula crowd said, look, a new star appeared in that object. It outshines the rest of the galaxy, the rest of the object. Um, I should have rehearsed more. Uh, the rest of the rest of the object have ruined the punchline. You now all know galaxies exist. Good job. I gave it good job. I gave it away at the beginning. Um, this new star on its own outshines the entire galaxy. Well, that's clearly ridiculous. Can't be made of stars. It must be a a gat thing. And this is a new star that that's made. And again, that's sensible except that we now know that supernova r briefly, because of the decay of the radioactive elements that are produced in the supernova, they are capable of outshining an entire galaxy. And so on the other side, there was a, I think the cleverest argument was somebody who pointed out that new Novi, these new explosions that we see sometimes from the death of massive stars, but sometimes things like binaries that interact violently, they pointed out there were more of them seen in these spiral nebula than elsewhere in the galaxy. So if they were part of our galaxy, why did they preferentially show this violent phenomenon? And the debate remained unresolved until we could actually measure the distance to galaxies. To do that, in the 20th century, people started using particular kinds of star. And the most famous of them is a famous star in its own right. It is our current Polestar Polaris, which you find from in, in the north, from the familiar astm of the plow or the big dipper depending on which side of the Atlantic you're on. You follow the pointers on the right hand side of the saucepan up to Polaris. In SSA miner. It's not particularly brightstar, but it is near the celestial pole. And so it appears that all the stars turn around it. It's useful for navigators and so on. But I think slightly ironically, given that there have been poems written and hymn sung to the constant navigator's friend Polaris, it's a variable star. It changes brightness over time. We've got a video of it here, and you can see that over the course of a few days, in fact, this is sped up. The star seems to flicker, um, for those who prefer data to video, here is a light curve that shows that change in variation. It's very small, it's quite subtle, um, but it's changing regularly. It's pulsing. And so its brightness is changing. It is in fact a kind of star called a phe. And these CFIs have an almost magical property. They change brightness 'cause they pulse, but how fast they pulse depends on their luminosity, how much light they're putting out into the universe. And so if you can measure how fast they're pulsing, then you can work out how bright they should appear in the sky at different distances. And so then you can compare how bright they do appear to how much light you know they're giving out. You get a distance and they're bright enough to be detected with large telescopes, mostly in the US in the first half of the 20th century in distant galaxies. And this is the measurement, though it took 20 to 30 years to calibrate it. That tells us definitively that we live in a universe of galaxies. That the Milky Way is one of a whole collection of island universes, of distant galaxies. Um, and enables us to do what lots of my friends spend lots of time doing, like go and study, uh, distant systems and try and understand 'em. This is a beautiful, uh, irregular galaxy n you see, 1961, a spiral disrupted by material falling onto the black hole at the center imaged here by a team led by Julian del Canton. Um, the galaxies on the left, julianne's on the right, just to be clear. Um, but we can also though I it's tempting to linger and tell you about my 250 or so favorite galaxies in detail. Um, could do that another day. Perhaps. Maybe you're a run out of ideas in a couple of years time. Um, but the other thing you can do is take these galaxies and think on a grander scale yet, because now we've gone from the sun to stars to the Milky Way to galaxies. You can ask whether there's a bigger structure still, whether on larger scales the universe is, um, structured. And so what you do is you just as we plotted the Milky Way by plotting the positions of stars, you can take these beautiful galaxies and reduce them to a point each. And then you can make a map of the distribution of galaxies in the universe. And this was first done in, in the late 1980s. And this is the first large scale map of the universe. So another iconic image in astronomy. Um, the map has us in the bottom and we're looking at a patch of sky. So you get this wedge shape and each.here is a galaxy. Um, you can see a couple of things. One is that there is structure here. There are places where there are lots of galaxies and there are places where there are very few galaxies. And, um, please ignore the fact that it looks like a stickman waving arms. I promise that's a coincidence. Um, you can imagine the reaction to the fact that the universe looks like a stick man waving long arms. Actually it looks like those things that you have on top of car sales rooms that flail their arms around. So maybe there's a cosmic message there who can say, um, what we have done is done this on a, a much grander scale. There was a telescope called the Sloan Digital Sky Survey that for eight years did this extraordinary thing of sitting on a mountaintop in New Mexico and allowing the stars to turn over it, allowing the sky to turn over it and using its camera to record whatever floated through its field of view. On the clearest nights, it went back and measured the position, the distance of, um, about a million galaxies. And so it allowed us to create a, a, um, three-dimensional map of our neighborhood. So this is real data that you'll seeing. Now, this is, uh, does look like the opening of a a low budget sci-fi movie, but these are galaxies in their correct positions. And you can see the lumpiness of the cosmic web, which I described, the places where there are lots of galaxies, places where there are very few. On the left of your screen right now is a c shaped structure called the Sloan Great Wall, which is so large that we think it ought not to exist in our universe. And actually recent results seem to show it doesn't. There's actually three separate things, but still the 200,000 galaxies are lined in what appears to be a great wall in a minute with no expense spared. I will stop and rotate the universe for you and you'll really see this honeycomb structure. What I want you to do though, is ignore the fact that it looks like a bow tie, that there are gaps in between. Those gaps are just the places in the universe where we haven't filled in the details yet. So you can think of that as here Be Dragons on a medieval map written on the largest possible scale. They're actually mostly places where our milky way, that pancake gets in the way of viewing these distant galaxies. Now I think this data is beautiful and you already know that I love each individual galaxy, even the fuzzy ones further out. Um, but there's a slight tendency from scientists to try and reduce beautiful data to simple measurements you've already seen. You can take the galaxies and turn them into points on a map, and we've restored them a bit in this animation. I'm, I'm a big fan of some work that was done by Carl g Gladbrook and Ko around the time.