 Okay, all right, so it's on 11, so we're gonna get started now. Thanks for joining. So this talk is going to be about the biggest explosions in the universe. So I'm gonna give you a quick introduction on how today's session will work and then we'll jump right in. So my name is Georgie and I'm an astronomer at Mount Stromler Observatory in Canberra. So I began studying astronomy in New Zealand in 2013 and now I'm working on my PhD here in Canberra, which means my job is to research things in space and try to learn more about them. So for the past few years, I've been studying a particular type of exploding object in space, which we will learn about later in today's talk. So my work involves computer programming, mathematical modeling, applying scientific methods, writing research papers and collaborating with scientists all over the world. So I'm also part of the Mount Stromler Observatory outreach team, which means I spend some of my time teaching the public about space like we're doing today. So for today's session, I'm gonna give a presentation that should take about 20 minutes and then afterwards we can have a discussion about some of your space questions. So make sure you put your questions in the Facebook live stream comments at any time and we'll answer them at the end of the talk. So there are lots of different things in space that are interesting. There are black holes and galaxies and planets and stars and maybe even aliens. But as I said, what I really like to look at are explosions in space and these explosions in space are some of the biggest explosions that humans have ever seen. So first off, we should talk about what an explosion is. Most of us will have seen explosions on TV or maybe even in real life. And normally they're very loud and dramatic and there can be flames or big clouds of smoke and dust. But scientifically, we're going to loosely define an explosion as something that suddenly expands and releases energy. And often there's a lot of heat and gas involved too. So this picture shows a controlled explosion of 16 tons of TNT, which is an explosive material. So today we're going to be talking about really big explosions. So we're going to need to measure the size of these explosions and megatons of TNT. So a megaton of TNT is a million tons or a thousand million kilograms of explosive. That's about a hundred thousand times bigger than the explosion in this picture. So what we're measuring is the explosive force or the kinetic energy released in each explosion. But before we can talk about how big explosions are in space, we need to understand how big explosions can be here on Earth. So I want you to have a think about different things on Earth that can explode. And while you're thinking, we can watch another controlled explosion, this time with a hundred tons of TNT. So you can see we sort of have the initial explosion, which looks like a ball of flames, and then a shockwave, and then the big clouds of smoke and dust bellowing out. And that's similar to how things explode in space. But that wasn't even close to the biggest explosion on Earth. So let's see what we have. The most powerful nuclear bombs on Earth have the explosive force of 50 megatons of TNT. So that's over 500,000 times bigger than the explosion we saw in the picture and 50,000 times bigger than the explosion we saw in the video. The largest volcanic eruption in history exploded with a force of about 1,000 megatons of TNT. And the asteroid that hit the Earth and killed the dinosaurs had a force of 100 million megatons of TNT, which is absolutely huge. So now that we understand how big explosions are on Earth, we can start to have a look at explosions in space. So we're going to start with a relatively small explosion in space called a nova. So let's start with a white dwarf, which is a small star about the size of Earth, but with the weight of our Sun. So if you take all of the stuff in our Sun and cram it into a ball the size of Earth, we have a white dwarf. So a nova is an explosion on the surface of the white dwarf, which is big enough for us to see, but not big enough to blow up the whole star. So one white dwarf can have multiple nova explosions in its lifetime, but we're just going to measure the energy of one. So try and have a guess at how many megatons of TNT might equal a nova. Remember that the asteroid that hit the Earth and killed the dinosaurs had a force of 100 million megatons of TNT. So that should give you some idea. Let's see what we have. Wow, okay, it's a big number. So a nova has a force of 2 million quadrillion megatons of TNT. So that is 20 trillion times more powerful than the asteroid that killed the dinosaurs. So I'm sure you can all guess what would happen if a nova explosion happened here on Earth. It would be destroyed by Earth. So we're really lucky that these nova explosions only happen to stars in space. If you have any questions about nova, make sure to leave them in the comments because now we're going to look at some even bigger explosions in space. So what's bigger than a nova? The answer is a supernova. So I'm sure some of you could have guessed that one. Now, there are two different types of supernova that are both very powerful and we are going to talk about both types, but let's start with core-collapse supernova. So we want to start with between 10 and 50 copies of our Sun. And let's take all the mass of those Suns and squeeze it down into one mass of star called a supergine. So the insides of stars are basically factories that make heavier and heavier elements through nuclear fusion. This fusion releases energy, which provides pressure, pushing outwards to counteract the force of gravity pushing in to keep the star stable. So imagine you're blowing up a balloon. The air that you blow into that balloon stops it from collapsing just like the nuclear fusion stops the star from collapsing. So over time, hydrogen fuses to helium and then to carbon, oxygen, and silicon, and finally, silicon fuses into iron. But this is where it gets interesting. So iron is an extremely stable element and instead of giving out energy to provide pressure, it actually needs to absorb energy in order to undergo nuclear fusion. So this means that the pressure that was holding the star up against gravity disappears and it begins to collapse. But things can only collapse so much. So imagine you're trying to squeeze a big ball. You can only squeeze it in so much. Because these stars are so big, the core of the star reaches a critical density where you can't squeeze it down any tighter while the outer envelope of the core is still collapsing. When this outer envelope hits the extremely dense core, it bounces off it, kind of like the biggest trampoline in the universe and causes a huge shock wave. So the explosion of this shock wave is called a core-collect supernova. So here's a simulation of Cassiopeia A. This supernova exploded in the 1600s. So what we observe today is the supernova remnant, an expanding cloud of material left over from the supernova. So this explosion was probably about 10 million times more powerful than a nova. So if anyone is keeping count in units of TNT, that's 10 octillion megatons. And it's so bright that if one exploded in our Milky Way galaxy, it would be brighter than the moon at night, and we could even see it during the day. So these supernova remnants are important because they release all of these elements that were created in the star, as well as extra ones that were created in the explosion itself, out into space. Eventually, these materials can be gathered up to form new stars. So our sun is actually a third-generation star, which means it's made from leftover materials from lots of other stars, including core-collapsed supernova, like Cassiopeia A. And I chose to show Cassiopeia A in particular, so I think it kind of looks like an alien doing this. So that's why that one's my favorite. But now let's go back and see how core-collapsed supernova can pierce explosions on Earth. So we're not looking at just Earth anymore, we're looking at the whole universe. And yes, we can see that a nova is much bigger than any explosion on Earth, and a core-collapsed supernova is bigger than a nova. But as I mentioned, there's another type of supernova that is even bigger and cooler than a core-collapsed supernova. And that is thermonuclear supernova. This is what I look at in my research, and I think they're very cool. So let's take a look now at how they work. So this time, instead of starting with a massive star, we're gonna start with a white dwarf again. So I mentioned white dwarfs before, they're where nova explosions happen, but they can also have full-sized supernova explosions too. And in these cases, the white dwarf isn't alone in the sky, it has a friend, and they orbit very close together in a binary system. But because white dwarfs are quite dense, they have a lot of gravity, and they're actually able to pull the outer parts of their companion star off and consume them in a process called accretion. So this extra fuel that they're stealing from their friend makes the white dwarf dwarf burn hotter, but they can't expand like a regular star would to cool off. So this causes a runaway burning inside the star that gets hotter and hotter and hotter. So once a white dwarf eats enough to be about 40% heavier than our sun, a critical point is reached where the burning inside the star is so strong that it explodes as a supernova. So thermonuclear supernova are about 40% more explosive than their core collapse cousins. So they are equivalent to about 14 octillion megatons of TNT. And not only are they more explosive, they are also, in my opinion at least, a lot more useful. So remember the cartoon that we just saw where the white dwarf steals star star from their companion star? It turns out that every white dwarf gets full at exactly the same point. So they always explode when they are 1.4 times heavier than the sun. So imagine blowing up lots of balloons and having them burst at the exact same size every single time. That's what this is like. And because they always explode with the same amount of fuel, thermonuclear supernova explosions are always the same brightness. And it turns out that this is actually very useful. So imagine you're on the road at night and there's a car coming towards you. When it's far away, its headlights don't seem very bright, but as it gets closer, the headlights seem much brighter. So the headlights haven't actually changed in brightness. The only thing that has changed is the distance between you and the car. And in fact, if you were very clever, you might be able to measure how far away the car is just by how bright its headlights are. And it's the same with thermonuclear supernova. Because we know how much fuel they explode with, we know how bright they should be. And we can compare this to how bright they seem in the sky to figure out how far away they are. So in this way, we can use thermonuclear supernova to make a 3D map of the universe. As we observe them exploding in distant galaxies all around us, we can start to figure out what's happening on the universe on its bigger scales. So in this way, thermonuclear supernova were used to make one of the most incredible discoveries in the 21st century. So imagine this balloon as our universe. The stars represent entire galaxies, each one containing billions and billions of stars and planets. The stretchy red rubber between the stars represents outer space, the emptiness between galaxies. And just like a balloon being blown up with air, our universe is expanding. It has been expanding since the Big Bang, 13.8 billion years ago. And when scientists began using supernova explosions to measure cosmic distances, they were able to measure the expansion speed of the universe for the first time ever. What we expected to see was the expansion of the universe slowing down as gravity began to pull everything closer together. Eventually we thought the universe might even start shrinking back down. But when scientists used supernova to measure distances to very far away stars, they found that supernova were much dimmer and so much further away than expected. This means that the expansion of the universe isn't slowing down, it's speeding up. This was absolutely crazy to scientists because it went against all of our theories. It would be like you throwing your keys up in the air and expecting them to come back down and instead they flew away up into the air. So the universe seems to be blowing itself apart and it's not just that things are moving away from one another, the entire fabric of space-time is being stretched with the expansion. So this phenomenon is called the exhilarating expansion of the universe and it was so amazing that the scientists that made the discovery won a Nobel Prize. But they couldn't celebrate too much because this discovery immediately led to another big question. Why is the universe behaving like this? So based on what we know here on Earth, it doesn't make any sense. Here on Earth, things don't expand for no reason. In fact, things tend to stick together, which is why you can walk around without flying off the Earth. For the expansion of the universe to be speeding up, there must be some mysterious force that is stronger than gravity over cosmic distances that is driving the expansion. This mysterious force is called dark energy. Now, scientists don't really know what dark energy is. It's just the name we give to whatever is causing the exhilarating expansion of the universe. You can imagine dark energy as the breath that is blowing up the balloon of the universe and causing it to expand faster and faster. And it was discovered using thermonuclear supernova. So to this day, dark energy remains one of the biggest mysteries in cosmology. This is actually what I work on as part of my research. I use thermonuclear supernova to measure the universe and try and figure out some of the properties of dark energy. So if anyone has any questions about how I do that, make sure to put them in the comments and now that we've taken a brief detour into cosmology, we actually have one last space explosion to look at. So let's have a think. What could possibly be bigger than a supernova? And that is a hypernova. So a hypernova is basically a super-sized, highly-energised core-collapsed supernova. So if you remember, a core-collapsed supernova was the one made from a very massive star that collapsed and went. But instead of collapsing to a very dense core, the stars that form hypernova keep collapsing into a black hole. So this rotating black hole has two jets on either side of it that shoot out into space like we see in the picture. It forms so quickly that the outer parts of the star don't have time to react. So they're hit by jets shooting off the black hole and explode in a hypernova. Hypernova can be up to 100 times more explosive than supernova and are thought to be the source of some gamma ray bursts. So gamma ray bursts are explosions of the most energetic forms of light. But instead of exploding outwards like a supernova, they are highly focused, beaming out along jets like you see in the hypernova. And this is actually very lucky for us because gamma ray bursts are probably the biggest explosions that we know of in the universe. But because their explosions are concentrated long beams, they're actually a very low chance of one ever hitting Earth. So with that reassurance, let's take a final look at all of the explosions that we've talked about today. So we have learnt that even the smallest explosions in space can be 20 trillion times more powerful than the biggest explosions on Earth. And the biggest explosions in space are over a thousand trillion times more powerful than those on Earth. We've also learnt how thermonuclear supernova is used to discover the accelerating expansion of the universe and the mysterious force of dark energy. So now if anyone has any questions, this is the time for us to answer them. If there's anything at all that you would like to ask about, make sure you put your questions in the common sections now. And we can have a chat about it. Otherwise, thank you all for listening. Okay, it looks like we've got a question. Can you tell us a bit more about your research into dark energy? Yes, I can. So the research into dark energy started in the 1990s, which is when this accelerating expansion of the universe was discovered. And it's still going to this day. So at the moment, the research is mainly led by a group called the Dark Energy Task Force, which are in charge of sort of thinking up these experiments that can be used to look at dark energy. So my job in particular is to work with another group called the Dark Energy Survey that runs one of these experiments. And our job has been to use a telescope in Chile to have a look out into the sky for five years and see if we can find any of these thermonuclear supernova that we use to measure these distances. And we've actually found quite a few. So I think the first discovery of the accelerating expansion of the universe used about maybe 50 thermonuclear supernova, and we have possibly up to 10,000. So what that allows us to do is to take the measurements that we use to say the universe is accelerating in its expansion, and we can really tighten those up. We can say exactly how much it is accelerating, and we can say when that acceleration sort of started, when it stopped, the history of the acceleration of the universe, because even though it's accelerating now, we don't think it's always been accelerating. It was at some point when it was softened down. So what that allows us to do is it allows us to, even though we don't know what dark energy might be, it allows us to at least understand how it behaves. And that means that scientists can come up with models for what it might be, and we can compare those models' behaviour to what we actually see with these thermonuclear supernova to see if they're the same thing. So at the moment, what we think it might be is just some inherent property of space. We think space might like to stretch, and it just might be doing it itself. There might be some sort of vacuum energy pushing it apart, the opposite to gravity, and that's what I'm trying to work on figuring out now. And the way that I can do that is through my background. I can see that we've got a question about my background. So I'm not sure. I'll start from a high school, I guess, because that's sort of where you start choosing your subjects. I took physics and mathematics through high school. I also took chemistry. But none of those things were that important. You can sort of, if you want to start studying astronomy, you can sort of pick it up from a university level without having a background in any of the sciences, because normally your first year of university will teach you all of that background that you need to know. So I actually did a double major bachelor's degree in applied mathematics in astronomy. And then that was in New Zealand at AUT. And then I moved over to Canberra, where I did my honours degree, my bachelor's degree in astronomy and astrophysics. And now I'm halfway through my PhD in astronomy and astrophysics. So that's a three to four year program. And at the end of that, I will be a doctor of astrophysics. And then I'll be able to work professionally at a university or a research institute as a researcher doing this full time. So hopefully that helps. I've just seen a funny question. What happens when our balloon pops? So this is actually one of the reasons why studying this stuff is so cool and so important, is because it's going to give us some idea about the fate of the universe. So human lifetimes are actually quite short compared to the universe. You think the universe is about 13.8 billion years old and I'm 25, so that's nothing on the universe. So we won't probably be around to see the end of it, but we can make some predictions. So just like we know that the universe started with a big bang, we think eventually it's going to do a couple of different things. It could, if the acceleration of the universe continues, keep expanding outwards and sort of get so far apart that it's called the heat death of the universe where everything's sort of too far apart that nothing can live anymore. Or the accelerating expansion of the universe might stop. We know it hasn't always been accelerating. And it might just sort of stay steady. It might sort of just stop expanding and hang out. Or it might start collapsing again. So it's not doing this at the moment, but eventually it could collapse back into what's called the big crunch. So we started with the big bang. We might end with the big crunch. But it might just sort of keep expanding and contracting and expanding and contracting. So this is one of the mysteries at the moment where we're not exactly sure what's going to happen. But the more we understand about dark energy, the better we'll be able to constrain these models. Okay, another question. Is there anything stronger than a hypernova? So a hypernova and the gamma ray bursts that accompany it are one of the biggest explosions that you can see in the universe. And if you want anything bigger than that, you're going to have to start redefining what you mean by an explosion. So when I think of an explosion, I sort of think of something that visually looks explosive. There might be flames or a big burst of light or something. But there are other things that can be sort of more powerful that maybe don't look like a traditional explosion. So the big bang is one of those. The big bang was a singularity that sort of not exploded, but the better term spread out into what the universe is now. So that would be more powerful than a hypernova. But also now, if anyone is familiar with gravitational waves, you get what you can see two black holes. So remember, hypernova contain a black hole. Two black holes, if they're in a binary system, they can actually merge. And because black holes have so much matter in them, they're so dense, when those two collide, it's like two really, really big trunks crashing together. So they don't have a typical explosion like we would see from a hypernova. They give off what we know as gravitational waves. So they're so powerful that they actually shake gravity itself. So that's a really cool new branch of science. It's completely different to what I do, but those would technically be more powerful than a hypernova. And it looks like we've got one more question. What is a nova like? So a nova, basically the word nova means new. So the scientists that first discovered nova thought that they were seeing a new star appear in the sky, because what previously looked like just almost nothing, like a very tiny dot or maybe even an empty patch of sky, all of a sudden was super bright, like a big bright star was there. And they thought a new star had been born. But it actually wasn't a new star. It was just a nova explosion on the surface of a white dwarf. But because they're so bright, they shine so brightly, we can see them and they look like a new star. And it wasn't until we saw that there was a white dwarf there before that we thought, okay, this can't be a brand new star because there was something in its place before. It's got to be something else. And when there was a white dwarf left there afterwards as well, we can say, all right, something on the white dwarf has exploded into a nova just on the surface. But the white dwarf is still there. So that's how they're different from supernova. But in terms of what they look like, they're actually very similar to how a supernova looks and that they're just a big burst of light. The only difference is they're not quite as bright as a supernova. So hopefully that answers some of your questions. If there are no more questions, then we will wrap up. But thank you all for coming and listening. I hope you all learnt something about space explosions and a little bit about dark energy as well. So thank you all. Okay. Looks like there's one more final question. That's all right. We can do that. What made you interested in studying about space? So lots of people have very cool answers to this question. We do a lot of these talks here. So if you wanted to check out some of the other talks by Mount Stromler astronomers, they have some very cool answers to this question. Mine isn't that exciting. I didn't really... I wasn't that interested in space growing up and it wasn't until I got to university I was looking through the different courses that I could take and I saw that I could take a course in astronomy as an elective. So basically just like a fun option. And I'd never really done anything to do with astronomy before. I didn't really know anything about it, but I thought it sounded like sort of a fun option to go along with my applied mathematics degree. So I started taking the class. The class was held at night time and we got to go out to the planetarium, the Stardome Planetarium in Auckland, New Zealand, and use the telescopes and take amazing photos of stars and space and learn all about it. And I think that class is what really got me hooked. I became really, really interested. It was my most fun class, definitely. And there are so many different things you can do with astronomy. You know, there's radio astronomy, optical astronomy, which is where I work. Lots and lots of different things you can do with it. And there's also lots of ways to get interested in it. If you're younger than what I was. So I didn't start really learning about it until I was 18 and it was just that I got lucky that I took a class that I was really interested in. So hopefully that answers that one. And I think that will be it for us today. So thanks again all for coming along. Remember to check out some of the other talks that we are hosting. And yeah, enjoy the rest of your day.