 Okay, so now that we are on time I will get started. So today, I will be talking to you about turbulence in astrophysics. The picture that you see here is a picture of the Goma cluster, sorry, the Grave Nebula, and I will get back to this one. We are going about the different kinds of turbulent structures that you see in astrophysics. So first of all, let's start, let's start with who I am. Well, I am a PhD student from India and I study at the Astronomy Department at A&U. Other than that, I also like riding my motorcycle around in the streets of Canberra. I really love hiking, which is something I miss nowadays, even though we all are staying at home. And today, I will do what I do best. I'll talk to you about turbulence and I'll just say that turbulence is everywhere. Okay, so let's get started with something that many of us do every morning. We either have hot chocolate, tea, coffee. So what we do is we add, so while making coffee or tea or hot chocolate, we add milk to our hot chocolate, right? And when you add milk, you do see that there are all these words of milk. So the white things actually make these circular patterns, which is a feature of turbulence. You also see similar disturbances in water when you're riding in a speedboat. And in the wake of the speedboat, behind your speedboat, you see all these wave features, wave-like features behind you, which is, again, due to turbulence. So how do we actually know if the fluid is turbulent and is there a way to measure it? So all that is decided by one number, which I call RE here. It stands for Reynolds number. Now I will try to formulate what Reynolds number should depend on. So if you have opened a tap of water at slow speed, then what you see is that the water actually flows quite smoothly and you see there's almost no disturbances. But as you increase the flow speed of your tap, it starts getting disturbed. So the left side, in the left side, the water is not turbulent, which means there's a laminar flow. Laminar means not turbulent. Whereas on the right side, the flow gets disturbed. So what you see is a turbulent flow. So this number, the Reynolds number, which tells us how strong turbulence is should depend on the speed you. So you is basically how fast our water is moving here. Now, let's go to the second thing that turbulence can depend on. You may have seen tiny waves in lakes and the big waves in seas and oceans. These waves are still laminar. They are not turbulent because they are really small in size. Whereas the waves in the sea, they're so big that they are actually, you can see all this bubbling and the flow and like the water breaking at different points in this wave, which means these big waves are turbulent. So these two basically differ by the size of the wave. So that's, there comes our second term in our turbulence in the Reynolds number, which is L, L stands for the size of the system. Now there is another third thing that turbulence depends on, which is called viscosity. So all of you who may have poured honey onto your food and you have seen that it's really difficult to pour honey and it flows really slowly. So viscosity is basically tells you how difficult it is for you to pour the liquid. It's like fluid friction. So viscosity of honey is much more than the viscosity of water. So if you pour water down at the same speed, water may end up being turbulent. Like here we have turbulent water coming out of a water tap, but if you pour honey down at the same speed, then honey would still be laminar. So the third thing that it depends on is nu. Nu basically stands for viscosity. So the Reynolds number, which tells us how turbulent something is, it depends on the speed Q, the size L, and the viscosity nu. And if you know these three quantities for any liquid or gas, then you can basically tell how turbulent the liquid is. So if you have some questions regarding the Reynolds number or where all we can see turbulence, please put them in the chat. And I will try to get back to them as I can. So where all do we see turbulence around us? So the first example that I'm going to show you is smoke rising from a candle. See as smoke rises, or the initial part, when it is rising slowly, it's actually not turbulent. This part is laminar. Whereas when it has risen up to quite some height as it gains speed, it becomes turbulent. So and this smoke from the candle is around one centimeter wide. So that's the L of our Reynolds number. Now, if we look at water behind a speedboat, one centimeter, okay, one centimeter is around the width of your pencil, let's say. Now, we also see turbulence as we talked about behind, in the water behind a speedboat. Now, this is turbulence in water, whereas the previous one was turbulence in air. Now, this can be as big as 10 meter, which is probably the size of your backyard in a standard house. Now, your balcony if you live in an apartment, of course. Now, yeah, so this is 10 meters in size. Now, or it can be also the size of a bus. Now, many of you may have experienced turbulence when you are traveling in an airplane. So the pilot generally announces that all of you have to put your seatbelts on and sit upright, put your three tables upright and all that, right? So that's because the airplane passes through turbulent air and this turbulent air can have these walls, which can be as big as one kilometer in size. Now, your pilot cannot really predict turbulence because turbulence is random. These walls and the way the gas or the air or liquid moves around, it's all random in all different directions. So your pilot can't really predict and pilot your plane through it. So what we have to do is we have to stay safe and put our seatbelts on. Now, can turbulence be bigger than one kilometer? Answer is very much yes, and it can be quite scary as we can see in hurricanes. Now, hurricanes are big, big weather systems. We call them cyclones around here. This is a picture of hurricane Florence taken from the international space station. So this is as big as 1000 kilometers. For reference, this can be around the distance between Melbourne to Sydney and that's how big these worlds of turbulence can be. They can be as big as 1000 kilometers, which is really, really big. Now, we saw turbulence in our atmosphere in the weather systems on earth. Do you think there will be turbulence in the weather systems of other planets? Well, the first planet that I can think of is Jupiter and its Gendred spot. Actually, the Gendred spot is turbulent and it is as big as 40,000 kilometers. So this video that we see is a time-lapse sequence when Voyager 1 was approaching Jupiter in 1979. And you can see all these worlds of turbulence in the atmosphere of Jupiter and the big red spot over here, which is basically an anti-cyclone. It's basically similar to a hurricane, but on Jupiter. And this is around 40,000 kilometers wide now, which is almost as big as going once all around earth. So you go to Europe and back, that's around 20,000, 30,000 kilometers. And this is bigger than that. Now, the big red spot has been in Jupiter's atmosphere for almost 400 years. And it is shrinking now. And we think it will end up becoming circular by around 2040, 50. So do we think turbulence is just limited to atmosphere of planets? Or can it be in stars as well? How about we go to the nearest star from us, which is our own Sun? As a matter of fact, the surface of the Sun is also turbulent. And I mean, Sun itself is so hot and the gas is moving so fast that all of it is all, there's turbulence all around it. You can see all these words around turbulence. In fact, it's for Sun, there is another effect that also affects how gas moves around. It is magnetism, but I will not go into that in today's talk. And the turbulence in Sun can be as big as one million kilometers. I, heat waves, heat waves can be, I mean, they can cause turbulence. So I'm just trying to answer a question that appeared heat waves can be caused by turbulence, but they are not necessarily turbulence. Okay, so giving you an example of how big one million kilometers is, if we go to the moon once and come back, that is almost around a million kilometers. So it's basically how much, how far the Apollo Q travel and came back. That's how big one of these turbulent forces, who they covered around a turbulent fall in their whole Apollo mission. Okay, now can turbulence happen outside our solar system? The answer is, of course, yes. And this is coming back to the very first slide that I had in my title introduction slide. So this is a picture of Crab Nebula and it is basically a dead star. So this star went supernova in around 1000 years back and now all the gas around it has expanded and you see all these beautiful features which are again caused by turbulence. And this is as wide as six light years. Now, do you know how long a light year is? It's basically if the distance light itself can travel in one whole year. Now, light travels really, really fast. It can travel so fast that it can reach from earth to sun in eight minutes and it reaches moon in around one second. So that's how big a light year is. And this is as wide as six light years. For reference, the second nearest star from us and the nearest star from sun is Proxima Centauri which is four light years away. So if we are here, if sun was on this end of Crab Nebula and Proxima Centauri would still be inside the Crab Nebula. That's how big the structure is. In fact, if we are here, then we will be looking at a part of Crab Nebula that is in 2016 or 2014 before all this before 2020 came upon us and brought all this misfortune. Now, coming to the biggest turbulence ever that we have seen. Well, it could be bigger than this, but this is the biggest that we know of yet. Now, this is the picture of a galaxy cluster. It's basically a big group of galaxies and all these tiny white dots that you see around are basically individual galaxies and this is basically a group with a big galaxy in the center. You can see that this, the gas around it is also quite turbulent and it is as big as 250,000 light years. So light itself will take 250,000 light years to just go from this end to this end. And that's how big these walls are. They're just rotating around and they're basically doing this whirling motion in as big as 250,000 light years. And for reference, that conversion into kilometers is 3 into 10 to power 18 kilometers. Yes, I would just like to remind you that if you have any questions about these galaxy clusters or any other slides that I have talked about, Kate, please mention them in the comments on Facebook Live and I will get to them either during this talk or after the talk is over and we'll be taking questions again. Now, Perseus cluster. So this cluster, which is Perseus cluster is six times the size of our own Milky Way. So our Milky Way is around 60,000. Yeah, it's around 40,000 light years and this is 250,000 light years. So we can put six Milky Ways inside one wall of this turbulence. Now, let's try to understand turbulence a little bit more. So I have been talking, mentioning turbulent walls quite a few times. So what is a turbulent wall? So remember when we when we are pouring milk into our hot chocolate, we saw that initially we saw a lot of some spirals which slowly became smaller and smaller. So that's what turbulent walls are. What we do is we put it milk into our coffee or hot chocolate, which starts a big wall. And then this big walls have little walls that feed on that velocity. And little walls have lesser walls and so on to viscosity. Okay, so we start with this big walls, which break down and make more smaller walls. And then these break down further and make smaller and smaller walls. And if you keep going on, you will make really, really, really tiny walls. And these tiny walls will not be turbulent anymore because that's where viscosity starts dominating. So if some of you can recall the Reynolds number that I introduced to you in one of the earliest slides, the denominator in that was viscosity. So that if the Reynolds number falls much below one, if it becomes really, really small, then we won't have turbulence anymore. So another thing that we may have noticed here is that the lesser walls have little walls and the little walls have big walls to feed on. So basically these walls will take the speed from these walls. These walls will take speed from these walls. But where do these were the largest walls get their speed from? The answer is we have to stir them. So that's why we have to stir coffee and milk together in order to mix the biggest first. So everywhere we need to, anywhere when we need to have turbulence and something, we need to add some energy. We need to stir the biggest first so that they can feed the smaller walls and the smaller one scale feed to the smallest first. And that's how we'll have the entire turbulence. Okay, now we know that we can mix milk and milk and hot chocolate in a mug with a teaspoon. But how about these big galaxies that I was talking about? So who is there to stir the biggest first in these big galaxies? The big galaxies are actually stirred by something very interesting. And I'll come to them really shortly. So that in the picture on the left here, again, we are seeing the big galaxy clusters, all these white white dots are individual galaxies. And this blue gas all around is the gas that is turbulent. And the red gas is a different kind of gas, which is actually stirring this turbulence in this blue gas. Now, how does that work? We'll try to show you a simulation of this, which will show how we are stirring turbulence in this blue gas. Now, basically, we have a big black hole in the center of this big galaxy cluster. Now, this big galaxy black hole, what it does is it eats and eats and keeps on eating matter all around it, because black hole nothing can really escape its gravity, right? So it'll just keep on taking in as much matter and all gas and whatever is around it, it will eat and eat and keep on eating. But once it becomes full, what does it do? Well, it basically is like us when we eat a lot, a lot more than what we can, we end up puking, right? So the black hole when it eats so much, and it can't really contain itself, it starts puking. And this is what you see here, this blue gas over here is basically black hole puke. And that black hole puke drives turbulence around in the galaxy cluster. Isn't that really cool that black holes also eat like us and also they also puke and that basically causes turbulence in these big galaxy clusters. So if you have any questions regarding black holes and black hole pukes and the turbulence that can be caused by these big black holes, please mention them in the Facebook live comments. Now, how can we measure turbulence? How many of you have seen rainbows in the sky recently, since it has been raining in Canberra? So rainbows are basically breaking out light into its different parts. So our light is composed of different colors from purple all the way to red. That's basically all the part of the light that we can see our human eye can see. You can also break down light using a prison, and a prison will break down light into this continuous spectrum, which is this rainbow. Now, suppose we have light coming out of the sun and it passed through some cloud of gas, and then we pass it through a prison, then what you will see is that some parts of the rainbow are actually missing. So there are these black spots which are missing from the rainbow. So we call this an absorption line spectrum. And then if we see light that's coming directly from this cloud of cold gas, and then we pass it through a prison, what we see is there are only some colors which appear as lines in our rainbow. And that is called an emission line spectrum. So generally we look for absorption or emission lines to try to understand what this cloud of cold gas might be made up of. So it did not always be a sun, it can be some bright source that which is giving this light and it is passing through this cold gas for us to see the absorption spectrum. Now how do we measure turbulence then? So first let's consider this cloud of gas again, and we have gas particles addressed which are not moving. And if you look at the emission line spectrum, which was all the colors of the rainbow are missing, but only a few of them are appearing, then you see thin lines, really really thin lines. And whereas if these gas particles are moving all around, maybe due to turbulence, then the thickness of this line is wider. You see that in this rainbow light thing, we have the same lines, but they are much wider. That's because when gas particles are moving around, these lines become wider. So by measuring the width of these lines, we can, sorry, by measuring width of these lines, we can tell how turbulent the gas is moving. So how fast the gas is moving and from that we can calculate how turbulent the gas is. Now where all is turbulence important in astrophysics? So I will talk about a few examples. So first one is it helps to collect star dust and make cool new stars. Now in this picture, what we are seeing is this tiny black dots which are dust. Then we have blue regions, which have a lot of gas. And then we have yellow regions, which have less gas. Now we will try to mix all of this up using turbulence. And we'll see how the dust and gas are mixed up in the end. Now let's switch in turbulence and see what happens. So you may notice that all these black dots are somehow following the blue gas around. And blue gas basically means areas in which we have lots of gas. So that means wherever there is more gas, we have more dust. So what turbulence is doing is that it's pushing the dust into regions where we have lots and lots of gas. So it's trying to put gas and dust together. Now these high density regions, the regions which have lots and lots of gas, what they can do is gravity will pull this dense regions together and then it can try to form stars. And once these form stars, they will also have all this dust that they've got due to turbulence. And that will help us make really cool and new and different kinds of stars depending on how much dust and how much gas is there. Now another thing that turbulence can help us make is planets. So turbulence can also help us make planets. These are again simulations. So we are making stars and planets using computers. So the plus signs in these videos are stars. And the white areas have lots of gas. The red and black ones have really less amount of gas. In this video, the left one has almost no turbulence. And in the right one, we have lots of turbulence. Now, which area do you think planets are most likely to form? Well, I would say that they would form in the white regions around the stars because that's where we have lots of gas. And this gas can again be collected together by gravity. If you have lots of white gas, they'll be collected together by gravity. And they can also get rocks and gas all together and squeeze it together to make planets. And you see that compared to the left one, the rightmost one has more white areas. And these white areas can form planets. Now, one question that may be coming into your mind is why did we choose two stars? The answer is actually a large fraction of the stars are actually binaries. Binaries means two star systems. Around one third of all stars in our galaxy are likely to be binaries. So that's why studying binaries and how planets form around them is quite important. So coming to conclusions, turbulence is everywhere. It's from the hot chocolate mug to big, big groups of galaxies. It's in our atmosphere. It's behind your speedboat when you're really traveling fast in the speedboat. And it's in our sun. It's in Jupiter. It's in our sun. It's in dead stars. And it's in big, big groups of galaxies. Now, both gas and liquids need to be stirred to create turbulence because there are these big worlds and we need to give energy to those big worlds so that they can feed the energy to smaller and smaller worlds. And then the small worlds can basically give it away to viscosity. So turbulence always needs energy or stirring. And turbulence, as we saw, helps us make new kinds of planets and stars. Turbulence is random, which means where gas is and where it will go is not very clear. Like when you open a tap of water and open it really fast, the water can fall in all random directions. And there's no way we can really tell them. Okay, so I will go with the questions now. And if you have any further questions, please put them in the comment section. And I will get to them. I'll get to each of your questions. So we will have around 10 to 15 minutes of question answers. Okay, the first question that I have here is, is there a way to know how much a black hole can absorb before it cubes? Go back to the black hole anything side. Well, we can actually know how much a black hole can absorb by checking if we just look at the area closest to the black hole and we'll know that the black hole is going to eat all of this. And then once it eats lots and lots in basically that area, we can calculate how much mass is in that area, how much gas is that. So it will eat that and then it will try to blow off into these jet light things. So basically we look at areas really close to the black hole where the gas cannot escape. And that can give us an estimate of how much the black hole is able to eat before it starts puking. So if there are more questions, please put them in the comments. And I will get to them. Yeah, that's a very good question that turbulence can affect how well astronomers can view space through telescopes. So what can happen is turbulence is random and unpredictable. Because of that, what you see, okay, so you may have noticed that when you try to look at a distant object just over a candle, if there's a lot of flickering of gas, then the distant object also flickers. That's because the gas right above the candle is hot, so it has low density. Now, because turbulence can move all this gas all around and you can have low density and high density gas all over the place, that means the refractive index of gas, which changes the way light travels through them. So basically if you have different densities of gas due to turbulence in different parts, then the way light passes through them is different at different times. So it is really difficult to account for that. And so the images that you see are somewhat blurred or they are somewhat fluctuated. Now, there are two ways in which, there are a bunch of ways in which astronomers try to deal with them. First is using adaptive optics. So what we do is we shoot out lasers into the sky and wait for them to be reflected back. So what we do is we see how much our laser has changed after passing through the atmosphere and coming back. So this is called adaptive optics. And depending on that, we adjust our image such that we account for this change in the picture due to turbulence. And the other way is just to go to space. So once we go to space, we don't really have to care about turbulence in the atmosphere. So we have space viscoles like the Hubble Space Telescope. Now, once we are out there, we don't really have turbulence. Turbulence won't really have a lot of effects on us because there's not much gas. So we can just see in all directions without having to worry about the effects of turbulence. Now, the second question I have here is, does the ISS have turbulence? So the ISS is actually orbiting at a much larger height. So because it is at a larger height, the density of gas, so how much gas is there at that height is actually very small. So some of us call it vacuum, but I would just say there's not a lot of gas. There is still some gas, but not a lot of gas. Now, how much turbulence can shake your plane also depends on how much gas is there around your plane causing that turbulence. So because there's not that much gas around the ISS, I would imagine that the ISS doesn't really get affected that much by turbulence in the Earth's atmosphere. Okay, so if we have any more questions, please put them in the comments and I will answer them. I'll wait for a couple more minutes and if not, then we will wrap up. Okay, if there are no more questions, thank you all for listening to this talk and being online. A recording of this will be available on Mount Stromler's Facebook page if you want to get back to go back to it. And if you want, you can comment there and we can also answer your queries on the video itself. So thank you very much for listening and we will wrap up now.