Particle Model of Matter

Energy

Electricity

Atomic Structure

Magnetism

Forces

Waves

Space

Physics

Y11 Physics

Key Stage 4

Year 11

UNIT

In this lesson we will be introduced to the components of our solar system. We will also discuss the physics that led to its formation and how our sun maintains its balance.

Solar system

In this lesson we will explore the factors that affect an orbit, using objects in our solar system to demonstrate.

Orbits

﻿Welcome back physicists. Before we start here are a few essentials. So, as before make sure your phone is on silent and you've closed down any tabs you might have opened. Obviously, don't give up. Complete all the lesson activities even if they are hard. And remember, if you are not thinking you are not learning. As ever, in this lesson, we are going to need a pen or pencil, paper or a calculator. And if you haven't got these, what on earth were you thinking? So pause the lesson now and go and get them. Okay, so let's get started. So in this lesson, we are going to be looking at the differences between celestial bodies, revisiting what is meant by an orbit, and why they occur including the factors affecting orbits. And then we will look into a little bit more about non-circular orbits before again as ever, finishing off with some questions for you to do. Okay, so this is a photograph of the earth from a satellite in low orbit. In other words it's close to the earth. The moon, is quite a bit further away from the earth than this. And what I'd like you to do, is I'm going to show you a question in a second and I'd like you to fill in the gaps as a memory jogger. So, pause the lesson now, and just take the time to fill in the gaps. Okay, so let's look at the solutions. I'll show them and just take the time to check you've got it right, and make any corrections. Okay, so don't forget to use capital letters to show that Earth, Moon and Sun are the names of those specific bodies. Now this is because, sun can refer to any star in any solar system. The moon, or so a moon rather, is just any natural satellite. And earth, is a common name for the soil or even electrical ground. So, if you're talking about the Earth, that needs to be a capital E. The Sun needs to be capital S, and the Moon needs to be at capital M. But if you're talking about all those things in the context we just talked about, then there'll be lower case. Okay. So remember, gravity is a non contact force, which means there is no physical contacts between bodies. There is to use the phrase, that hopefully you've heard before, action at a distance. That's really really important. So please remember that. That's the end of the recap. So what we'll do now is we'll look at some more celestial bodies. Okay, so just take a second to look at this diagram. Right, so we'll have a quick fire question. Basically, what I would like you to do is to identify what those four objects are using that diagram. And so just state what they are writing down, just write the letter, followed by the name and in the case of confusion, just so you know, A represents a distant luminous body and the sizes and the distance are not to scale. Okay, so A represents a distant luminous body. Pause the lesson now and write down the answers. Okay, so here are the answers. So just take a second to check. And you may want to write these orbits down in full sentences. So in other words, a planet orbits a star, et cetera. So if you need to do that, pause the lesson now and just write each one down in a full sentence. Making a summary table is a good way of organising information, helps you easily compare and contrast similar items. So, copy and complete the table on this slide, pause the video until you have finished and then restart for the solutions. Here are the solutions. So just pause the video now and check that you've got everything right, and make any necessary corrections that you need to. Okay, I'm just going to just discuss a couple of things on this table. So there are distinctions to be made between natural and artificial satellites. So don't think that a satellite is always artificial. A satellite is any body that orbits another. So, for example, the Earth is a satellite of the Sun. And also remember that stars, planets, moons and dwarf planets originated in Nebula. Okay, so a quick review of something that you hopefully would have done in forces. Pause the lesson now and answer these questions in full sentences. Okay, so let's look at the solutions. So the first question, what's speed and velocity? Second question. Okay, so pause the video now. Make any necessary corrections if you need to. Okay, so well done if you've got that right. And just a couple of things to note. So in physics, we can define a lot of things via their equation. So in the case of the first one, speed is defined as being distance over time. We could say the rate of change of distance if we wanted to, if we wanted to put that in words. And then notice the difference between how we define velocity. So that would be the rate of change of displacement or displacement over time or displacement divided by time. And then I hope you remember this from your, forces unit. Speed is a scalar and velocity is a vector. And the difference between those is that scalars have a magnitude only. And some of you may have written the word size, which is fine, but it's not as good. If you're going to be a proper physicist, if you're going to use language the way I'd like to use it, please use the word magnitude. It's so much better term. And the point that is important to make is that speed changes when the value changes. So for example, 20 metres per second to 40 metres per second is a change of speed. And velocity changes when either the value or the direction changes or both. So if you go from 20 to 40 metres per second, you've changed your speed, but you you've also changed your velocity. But if you keep your speed constant and you change your direction, then you are also changing your velocity. Okay, so let's look at some factors affecting orbits. Take a second to look at this diagram. So let us assume that the Moon is orbiting anticlockwise around the planet in a circular orbit. Bodies remain in circular orbit because they're travelling at the right speed, perpendicular, that's at right angles to the circular path. And the force of gravity is always into the centre of the circle, along the radius. So you can see here, that the direction of the gravitational force points towards the centre of the circle and the direction of velocity is perpendicular to the direction of the gravitational force. Now this is the situation with planets in our own solar system. The same could happen with moons around planets. Even objects in orbit, round a neutron star or a black hole, would do the same. Okay, so just take a second to look at this animation. So this means that the resultant force due to gravitational attraction is always changing the direction of the orbiting body. So in other words, its velocity is changing. The magnitude of the velocity is constant. So the speed is constant, but the direction is changing. So hence, we say that velocity is changing. So I mentioned this before, when either the velocities, magnitude or direction change or both of them, the velocity is changing. So circular motion like orbits is a rare occurrence of a body accelerating, even though its speed is unchanged. This can be seen in this animation, which imagines a body being fired from a tall tower, you can see that at the top, on the equator at the, at the right speed. And it's orbiting the earth in a circle. And you can see, as it goes round, the speed is constant. The direction is constantly changing and therefore the velocity is changing. Okay, so let's do some quick fire quizzes. So velocity is, Speed with direction. Well done if you got that right. I hope so. For a circular orbit, the direction of motion and the direction of gravity are, perpendicular. Okay. Well done. If you got that right. Right, so let's look at non circular orbits. So, so far we've talked about all this as being circled. Let's look at examples of where they're not circular. Okay, so take a second to look at the images on this slide. So these show top and side views of orbiting bodies around our Sun. Now the planets' orbits are pretty much circular. So the arrow there is pointing at the orbit of Uranus and it's pretty circular. But the dwarf planets are not. So they're more like squashed circles, but they are stable and they persist. Now, what do you think happens to the speed of these orbiting bodies as they go around the sun, if there is a non circular orbit? what do you think happens to the direction force of gravity in these orbits? So let's see if we can have a go at predicting this from what we already know. So here are two sentences and what I'd like you to do is fill in the gaps. And in the second one we'll fill in the gap sorry, and in the second one, choose the right terms. So pause the video, read carefully, and we'll look at the solutions in a second. Okay, so here are the solutions. So I'll give you a chance just to read that and check that you've got that right. Okay, so well done if you did. Pretty straightforward. You ought to know that the gravitational force weakens with distance. Now that's according to what we call an inverse square law. That hopefully, you remember this from when you've done the photosynthesis required practical when looking at light intensity. So, if something is proportional to the inverse of a square, what we mean by that, I suppose in more simple terms is if we double the distance, the force is one quarter of its original magnitude. So, if you double your distance from the Sun, then the strength of gravity will be a quarter than what it was before. So for the dwarf planets like Ceres, Pluto, Eris, Haumea, Makemake, whose orbits are elliptical. So these are these squashed circles, elliptical, their speeds change as they orbit the Sun. And along with their orbital distances and direction of travel. And don't forget closer to home, this can be applied to artificial satellites. So here's a picture of the international space station. And this orbits the Earth in around a hundred minutes. And it's a great deal closer to the earth than a communication satellite, which whose orbital period will be about 24 hours. Okay, time for some quick maths practise. So we're going to use formulae and we're going to use standard form. Now, if you've not got your calculator and let's face it, what sort of physicist comes to a physics lesson without a calculator, then you use a hang your head in shame. You need to pause the video and you need to go and get one. Okay. So, so we're going to be using formulae and we're going to be using standard form. And that includes the formulae that you would have learned about in maths for the circumference of a circle. So, before I show you this question, you will need to use a formulae for the circumference of a circle. As the best physicist in the country, I expect you already know what that is. But if you do not, then you need to pause the video, and go find out and make sure that you learn it because you really ought to know that already. So, pause the video and answer this question. Okay, so let's look at the solution. Now, if you've been following any of the other physics lessons at Oak, for GCSE physics, then hopefully you'll be familiar with this to help you frame your answer. So we should always write down or certainly identify the values in the question, jot down the equations we're going to use. Substitute, rearrange if we need to, calculate our answer, and then add on any, units. Okay, so hopefully you knew that the distance travelled in one year is effectively the circumference of the orbital path. And the circumference can be calculated using two pie R. And so we substitute all those values in and you should hopefully have got the circumference is 9.4 times 10 to 11 metres. And then speed equals distance time divided by time. And we substitute those in. And the question said, give your answers in standard form to two significant figures. So you should end up with 2.9 times 10 to four metres per second. So well done if you've got all that, right. If you didn't pause the video now and maybe rewind, and just sort of have another go and just check where you went wrong. Okay, moving on. So what we're going to do now is finish up with some exam style questions, just to sort of give you a chance to see how the moments we talked about can be applied, in an exam question. So, before we start, any question that's going to start with a table, no matter what it says in the question STEM, which I'll show you in a minute, just take a second to look at the table. Okay, so hopefully, you know how to read a table, but I'll just kind of run through what you should be looking at there. So clearly what we've got here are a list of bodies, in other words, planets and dwarf planets and then some information about their orbital radius and their orbital period. And the orbital radius is in standard form. So for example, Mercury it says 58. So that means it's orbital radius is 58 times 10 to the six kilometres. Now notice it says kilometres, not metres. That may become useful later on, it may not. Orbital period, you'll notice is expressed in days, not months, not years, not seconds. It's in days. it's always important to pay attention to the units. Okay, so I'm going to show you the question. I want you to read it, and i want you to answer it. Pause the lesson if you need to. Okay, here's the second part of the question. Pause the lesson, press play when you're ready. Here's the third part of the question. Pause the lesson now, answer it, press play when you're ready. Okay, so hopefully you found those questions challenging, at the same time as being accessible. But let's find out, let's see how you got on. Okay. So here's the first set of answers. Pause the lesson if you need to, make any corrections that you need to. Right, so well done if you've got all that right. A couple of things I just want to highlight. One is for question B, that when we talk about velocity and define it rather, we should include both parts of that of that answer. So in other words, we talk about speed with the direction, or we can talk about speed, either specified direction. But if you're not using the word direction and speed, you can't really talk about velocity. Second thing, is that, hopefully you've noted, that they asked for a unit conversion for question C, and then you get one mark, if you just write 150 times 10 to six kilometres, if you convert that into a standard form, but you don't change the unit, you'll only get one mark for that. Okay, so let's move on to the second set of questions. Okay, so here are the solutions for the second part. Pause the video if you need to make any corrections. Okay, so that was pretty straight forward. The main thing was to sort of note there is, the reason why F is Saturn is because it's the furthest, so it's got the largest orbital radius and therefore it has the weakest or lowest gravity, gravitational force, sorry, I should say. Okay, and therefore that's why, it will have the smallest speed or the least speed. Okay, here's the third part. So pause the video if you need to and just check your answers, making any corrections if necessary. Okay, so hopefully this is quite self explanatory, this one, but just to sort of, to points how, that final part is the key here is that the equation is correct if they are accurate to two significant figures in this case. And so you can see that, while Mars came out at slightly longer, like a day and a half than what it says in the table, it's close enough. It's two significant figures and therefore the formula is correct. Okay, so that's the end of the lesson. I hope you found that valuable. I hope you found that challenging. I hope you did a lot of thinking. So what we looked at was, Orbits again, and just to sort of summarise a little few things from the lesson, that planets have circular orbits. Dwarf planets have non circular orbits. So elliptical orbits as we call it, which are like a squashed circle and that therefore their speed will vary as they orbit the Sun. For example, getting faster as they get closer, getting slower as they get further away. And that pattern of speed, is the same also within for the planets. So the planets that are further out from the Sun are travelling at a slower speed than those near the Sun. And finally, we looked also at velocity and speed and that difference. And just to sort of, to remember that any orbiting body that has a constant speed is changing direction and is therefore changing velocity, is therefore accelerating. Okay, so next thing to do is to click on the link and have a go to orbits quiz. And until next lesson, see you later.

In this lesson we will explore the lifecycle of stars similar to our own and those of significantly more mass. We will look at the birth and death processes for each.

Life cycle of a star

﻿Welcome back, physicists. Good to see you've all returned for another dose of astronomy. Before we start, here are the usual messages. Make sure your phone is on silent, close down any tabs you might have open, don't give up, complete all of the lesson activities, and if you ain't thinking, you ain't learning. As ever, in this lesson, we'll need a pen or a pencil, paper, and a calculator. So if you haven't got these already, pause the lesson now and go and get them. In this lesson, we'll be looking at how stars are born, determining how a star will end up in its life cycle depending on its initial mass, revisiting fusion along the way. We'll be looking at the death of low mass stars and the death of high mass stars. And then, of course, we'll finish with some questions for you to apply all that knowledge. So, just take a second to look at this picture. In the nebula, gravitational attraction pulls matter together over a long period. When enough matter comes together, fusion can begin to happen. Although this picture is false coloured, it shows stars being born in our own home galaxy. So let's have a little return back to what we've covered in the first lesson and look at fusion reactions. Okay, so pause the lesson now and write down the answer to this question. Okay, so take a second to read this, pause the video if you need to, and make any corrections. Okay, so well done. The matter needs to come together so that individual atomic nuclei can fuse. In other words, get close together to start a fusion reaction and keep going. This is hard to achieve on Earth and scientists have found this challenging. It is, however, common in the interior of stars. Okay, so here's another question. Pause the video now if you need to and write down what you think the answer is. And here's the answer, so take a second to read this. Okay, so the dwarf planet, Ceres, is covered in craters from impacts due to the gravitational attraction of small rocks. Remember, all matter attracts other matter with the force that we call gravity. This is what brings matter together to make stars. Okay, so here's a quick question. Have a read of it and then point your finger at the correct answer. Okay, well done. Hydrogen. Another question. Point your finger at the correct answer. Well done. Nebula. One more and, again, point your finger at the correct answer. Electromagnetic radiation. Well done. So now, we'll look at the life cycle of stars from their birth to their death. Okay, so take a second to look at this picture. So hydrogen and other matter in a nebula is drawn together by gravitational attraction, and the first stage, if you like, is a protostar. Now, we don't call this stage one because it's not technically a star yet. A protostar is what a star is before it becomes a star. So take a moment to remind yourself of this diagram. Now, that looks different from the previous diagram we used for fusion, but it has all the same information in. So here, what we have are isotopes of hydrogen, called deuterium and tritium, and they start to fuse to form helium and the star turns on, and it starts to emit electromagnetic radiation. So, at this point, the fusion is beginning. Take a moment to remind yourself of this diagram. So the fusion reaction becomes self-sustaining. So what we mean by this is it continues to happen by itself, and the star continues to shine because it is emitting electromagnetic radiation. Eventually, the radiation pressure, represented by the red arrows here, balances the gravitational force, the collapse represented by the blue arrows. So that outward force of radiation pressure is balanced by the gravitational collapse inwards. At this point, the star is now a main sequence star, and the star is in stable equilibrium. When the radiation pressure outwards and the gravitational force inwards balance out, the star sustains its fusion reaction for billions of years. So this is what we mean by equilibrium. The Sun, our Sun, is currently around 4.6 billion years into this part of its life. So pause the video, have a read of these questions, jot down your answers, and we'll look at the answers in a minute. Okay, so here are the solutions. Pause the video if you need to and just check your work. So, now what we'll look at is what happens at the end of a star's life, and we'll begin with the death of a low mass star, or what happens when the hydrogen runs out. And what happens depends on the initial mass of the star. So, as I said, we're going to start with a low mass, and then, later on, we're going to look at a high mass star. So how will the Sun and other similar stars die? Stars with approximately the same mass as our Sun, which is 2 times 10 to the 30 kilos, quite a lot of mass there, will follow this path. Stage one, the hydrogen nuclei have depleted, and there's not enough to sustain equilibrium. So this means that the hydrogen fuel is now depleted. Stage two, the helium nuclei start to be fused, but not as much energy is released when this occurs compared to when hydrogen is being fused. The outward radiation pressure is no longer as great, so the temperature decreases. Stage three, now this means that the star is no longer in equilibrium. The gravitational force, which are represented by these black arrows, is now greater than the radiation pressure, represented by the red arrows. Stage four, because the inward gravitational force is larger in comparison, so the star is compressed over a short space of time, heating it up rapidly. The quickly heated star expands rapidly and forms a red giant. A very much larger body that has a large surface area. Stage five, the red giant then sheds some of its outer layers, which can go on to make new planets, and shrinks down to become a white dwarf, a planet-sized, very hot body in the same location as the original star. Now, the white dwarf then cools, giving out radiation over a very long period to become a black dwarf. Now, since this takes many billions of years, some astronomers estimate 10s to the 100s, and the universe is only 13.8 billion years old, there are no known black dwarves, as the universe just hasn't been around for long enough. Now, that's actually quite amazing if you think about it. It means that every single white dwarf that has ever existed is still around today. And, as far as I know, there is nothing else above the size of an atom that we could say that about. Okay, so our first task. What I'd like you to do is to draw out this flowchart, and I'd like you to write out these statements and fill in the blanks using the keywords. If you get stuck, rewind the video and have a listen again. Pause the video now and have a go at the task. Okay, this is the second part of your flow chart, so add this onto the bottom of the one you've drawn, and, again, pause the video now and have a go at the task. Okay, so here are the solutions to the first part. Just take a second, pause the video if you need, just check your work. Here are solutions to the second part and, again, pause the video and check your work. So what happens if the star's initial mass is much greater than our Sun? So we've looked at what happens with low mass stars, and how they end up as a white dwarf and then a black dwarf. Now, we're going to look at what happens with high mass stars. So now, we're going to talk about stars that have around 10 or more solar masses. So, in other words, they're 10 times the mass of our Sun. Let's go back to see what happens when the hydrogen fuel runs out, as this is where the life cycle differs. When the hydrogen fuel decreases, the radiation pressure decreases, and as the gravitational force inwards is much greater, the star is squeezed tighter. All of the processes happen at a greater rate. When the hydrogen fuel expires and the star expands to become a red giant, the star swells to an even greater extent than we saw before and becomes what we call a red supergiant. This cools even more rapidly and the resulting rapid collapse inward causes the star to explode and ricochet matter into the cosmos. This is called a supernova. So the matter left behind has so much gravitational pull inward that atoms are crushed and only neutrons remain, and so it's become a neutron star. Now, that's all you need to know at this stage, but I thought I'd share with you some interesting facts about neutron stars because, well, they're just interesting, and I think they're just worth thinking about for a moment. So a nutrient star is so dense that one teaspoon, that's about five millimetres, of its material would have a mass of over 5.5 times 10 to the 12 kilos. So that's about 900 times the mass of the Great Pyramid of Giza. So it's an incredibly dense material. The fastest spinning neutron star rotates at a rate of 716 times a second or 43,000 revolutions per minute. So that means the linear speed on the surface is about a quarter of the speed of light. And the gravitational field of a neutron star, or at a neutron star's surface I should say, is about 2 times 10 to the 11 times that on Earth. So that's 2 followed by 11 zeros greater than that on Earth. So just to give you a sense of what that means, if you were to jump or fall from a height of one metre on a neutron star, okay, that's about 12 kilometres in radius, you would reach the ground at about 1,400 kilometres per second. So just think about the acceleration there. Not that you would survive that in any way because you would be broken up into a stream of particles by an amusingly titled process called spaghettification. Now, if the initial mass of the star is even greater, the matter left behind after the supernova has such a strong gravitational pull that even neutrons cannot withstand such a crushing force. So these are the very highest mass stars. And, at that point, the star becomes a black hole with all of the matter remaining compressed into the smallest space imaginable. Now, gravity near a black hole is so great that the path of light is bent, as shown by this animation. So gravity acts like a lens. So we can't observe these directly, but many have been discovered by observing gravitational effects of objects around them, as well as the behaviour of light that passes close to the event horizon. And there's still much that we do not know, or do not understand I should say, about black holes, as they test the limits of the two most important theories in physics, general relativity, which is the theory we use to understand space, and quantum theory, which is the theory we use, I suppose, to understand the smallest objects, atoms and subatomic particles. Now, one day, we hope, scientists will be able to unify these two theories and, in doing so, we may understand much more about black holes. Okay, so another independent task for you. Fill in the gaps with the correct names for the stages of the star's life cycle when the initial mass is much greater than that of the Sun. Pause the video to complete the steps. Okay, second slide. So, add this to the bottom of your flow chart. Again, pause the video, complete the steps. And so here are the solutions. Pause the video if you need to and assess your own work. And then, the second part. Again, pause the video and assess your own work. Okay, so well done. Let's move on to the next part. So now, what we're going to do is you're going to do some summary questions, and just put a lot of that together, and consolidate what we've learned about. So here's the first task. See if you can do this by covering up the work you've done already. See how much you can do from memory. So pause the video now and complete those questions. Okay, here's the second set of questions. Again, pause the lesson now and complete the questions. So here are the solutions. Pause the video now if you need to and check your work. Okay, so before we move on, just want to point something out that hopefully you've noticed already, but if you haven't, it's worth remembering. It's relatively easy to remember this flowchart. If you look on the flowchart there, you can see that the nebula, protostar, main sequence is exactly the same for both types of star. And while we might talk about red supergiant or a red giant, essentially that's the same stage. So really, all you need to remember is the difference in terms of what happens afterwards. So smaller mass stars, white dwarf, black dwarf. Larger mass stars go from supernova to neutron star, or if they're really big, they go from supernova to black hole. So it's actually quite a straightforward thing to remember, and what seems like a lot of information at first, you can see there are a lot of similarities, and that makes it easier for you to recall that sequence. So what we'll look at now are some exam style questions, give you a sense of what this might look like in an exam situation. So here's the first question. Again, pause the video now and answer the questions. Okay, so here's the second part. Again, pause the video now, read the question carefully, and answer the questions. Okay, so here are the solutions. Pause the video now and check your work. Okay, so before we move on, just to remind you, in case you got that first one wrong, and you were struggling a little bit. There is that mnemonic that we talked about last lesson, the previous lesson. So if I just remind you. Radio microphones are invisible under x-ray goggles. And that goes from lowest frequency first. So, in this question, you would have to obviously recall that, jot it down just like that, and then obviously flip that round. So, therefore, radio would be the forth because it has the lowest frequency out of those. Okay, moving on. So here's the solutions to the second part. Again, pause the video now and assess your work. Okay, so just one thing before we move on. There's quite a lot there in terms of the second part of that question, describing the life cycle of star. The only thing I'll add to that is you'll notice there that there's no mention of gravitational collapse and radiation pressure balancing out during main sequence, and then how that changes as we go from red star. That's because, if you'll see, the command verb here was describe the life cycle of the star. So really that's just a description of the stages in order, which is a straightforward thing that you can remember, and that flowchart we looked at earlier would be really useful for that. Now, if that question said something like, describe the life cycle of a star and explain any changes, or explain the change from main sequence to red giant, then, of course, we would be interested in that discussion around the balance forces. So just be on the lookout for that in an exam. If you see that word, explain, then you're likely to have to add in that extra information around what causes the change from main sequence to red giant. Okay, so that brings us to the end of the lesson. Again, I hope you've enjoyed it. I hope you've learned a lot. So what we looked at, if you remember, is we revisited fusion, and then we looked at the different stages within the life cycle of a star. So we went from protostar, to main sequence, to red giant, to white dwarf, and black dwarf in the case of low mass stars, and we went from main sequence to protostar. Sorry, from protostar, to main sequence, to red supergiant, and then supernova, and then either neutron star or black hole in the case of the highest mass stars, okay? So thank you very much. I hope you've enjoyed. I hope you've learned a lot and look forward to seeing you next time.

In this lesson we will explore how scientists have deduced that all of the known elements have been made in the interiors of stars.

Element synthesis

﻿Welcome back, physicists! Glad to have you back for another stroll through the cosmos. Now before we start, here are the usual messages. Have your phone on silent or in another room. Close down any tabs you might have open. Don't get up, complete everything that I ask you to do. And of course, if you're not thinking, you're not learning. And finally, as ever, in this lesson we will need a pen or a pencil and a piece of paper. No calculator this time, so if you haven't got a pen and a paper yet, please pause the lesson now and go and get them. So let's get started. So this lesson, we'll be reminding ourselves of the meaning of nuclear fusion, linking this to the life cycle of stars, and discovering that stars are the element factories and sometimes the distribution centres of the cosmos in which we live. And then of course, we'll finish with some questions for you to consolidate what we've been talking about. So firstly, let's revisit fusion reactions by thinking about what is made in a fusion reaction. So pause the video now and write down the answer to this question. So before I show you the answer, just take a second to remind yourself of this diagram, and if you need to, pause the video and change your answer. Okay, so here's the answer, again, pause the video if you need to, just check how you got on. So well done if you said that hydrogen nuclei are fused in an extreme environment of the interior of the star and they form helium. Now matter needs to come together, so that individual atomic nuclei can fuse, in other words get close enough together to start a fusion reaction and keep going. Now this is hard to achieve on Earth, and scientists have found it's really challenging. It is, however, common in the interior of stars. So remember, matter needs to come together, so that the individual atomic nuclei can fuse. This is what we call a nuclear fusion. So we need to get closer together to start a fusion reaction and keep going. In the main stable period of a star's life, it is fusing hydrogen together to make helium, and this releases relatively large amounts of energy. Now remind yourself of this diagram. So this happens when the star is in the main sequence, when the forces of gravitation and radiation pressure from the fusion are balanced. Now this is an incredibly fine balance. For example, the sun hardly grows or shrinks at all over millions of years, so it's in equilibrium and has been so for a very long period of time. So where in the sequence are elements made or synthesised, as we like to say? And are different elements made during different times? Well let's find out! So just take a second to remind yourself of this sequence. So which route off the main sequence is for stars with a smaller mass, as the sun? So well done if you identified this part of the diagram. And of course that means greater than the sun would be this side. So where in the sequence is hydrogen being fused to make helium? So have a think, point to the part of the screen where you think that's the case. That's correct, the main sequence. So let's look in more detail about stars and how elements are manufactured. So here's a periodic table. Hopefully that looks really familiar to you. Just take a second to remind yourself of it if it doesn't. Now we know that hydrogen has been around pretty much since the birth of the universe. And helium is produced in a main sequence star as a result of nuclear fusion. So where did all the other elements come from? What I'd like you to do is just take a moment, pause if you need to, write down where in the periodic table you would find the synthetic elements, in other words elements made by scientists, by humans. So write down where you would find the synthetic elements on this table. Now the answer to that question is coming up, but let's talk about where the elements originated. So hydrogen was made soon after the Big Bang and helium is made in main sequence stars. Now the synthetic elements, which were made in the lab and earthbound reactors, are all found in these parts of the periodic table. Now did you get that question right? Don't worry if you didn't. It's not something I would expect you to know. Now what about the other approximately 90 elements? Oxygen, gold, iron, how are they created? So just take a second to look at this graph. So what this graph is showing is the abundance, so the amount of each element in the universe. Okay, so this is on a logarithmic scale, which means each of these numbers means it's going, it's 10 times bigger than before. And then this is the atomic number, in other words the size of the element. And so you can see here that hydrogen is the most abundant element in the universe. So it turns out that the vast majority of the normal matter in the universe, excluding dark matter and dark energy, which we might touch upon in later lessons, is in fact hydrogen, which was made soon after the Big Bang. The next most abundant element is helium, and for the solar system alone, you can see that in the graph, that the elements with even numbers of protons are also more common and indeed in our earth's crust, like oxygen, silicon, and iron. So how were the large elements made? how were they synthesised? So quick pause point. What is the most abundant element in the universe? Correct, hydrogen, well done if you remembered. When was the hydrogen made? Soon after the start of the universe or during the Big Bang. What is the next most abundant element in the universe? Correct, helium! Okay, so what is made in the main sequence star? Helium, let's remind ourselves that helium is made in the core of a main sequence star by fusion. Then, when all of the hydrogen fuel is exhausted and it swells to the red giant or supergiant phase, the star starts to fuse helium in its core to form larger nuclei or heavier elements. The red giant and red supergiant star fuses bigger and bigger nuclei together to form elements up to iron. Each bigger element fusion releases less energy per reaction. Now just take a second to look at this. And that's how elements up to iron are synthesised over subsequently shorter time scales. The elements are present in shells in the red giant, with the heaviest near the core, where there is enough energy for fusion. So you can see here, as we go from the centre of the star all the way to the surface, we are getting lighter and lighter elements. And if we were to compare this to the periodic table, we would see that the elements are getting lighter and lighter as they get near the surface of the star. Now pause your video and attempt these questions. Okay, so let's look at the solutions. I'll show each one in turn, and you can just correct your work as we go through. So remind yourself of this diagram. So let's reveal the star life cycles. So main sequence fuses hydrogen to make helium. Red giants and red supergiants fuse helium to make larger elements up to iron. So every element up to iron on the periodic table was made in a star in either the red giant or the red supergiant phase, unless its helium, in which case it was made in the main sequence phase. But what about elements with larger nuclei than iron? We find these on Earth, we know they're not synthetic. Where were they made? So here's our periodic table again, although just take a second to look at it, cause it's slightly different from before. So this periodic table is showing us where different elements are likely to have been made based on a colour coding. When two or more elements are fused to form nuclei bigger than iron, energy is needed from a very, very hot star. The energy need to make these heavy elements, like gold, silver, uranium, or mercury, is present in extremely energetic events, like a supernova explosion or merging neutron stars. So if you look at the periodic table here, you can see that many heavy elements are made by very violent events. Now this science is constantly under review, and some astronomers now favour neutron star merges for common heavy nuclei synthesis. But nevertheless, energetic events are essential. So this means only the most violent, energy-filled events, like supernovae, have the energy needed to synthesise the heavier elements, i.e. the ones above iron. So where in the sequence are elements made, and are different elements made during different times? And that's a supernova event is an event with a huge amount of kinetic energy. All of the elements made in the stars are distributed across the cosmos in one of the most violent events in the universe. Supernova explosions have been known to outshine whole galaxies. So our earth must be made of the leftover debris from the supernova explosions in the past, as we have heavy elements on the earth, like gold, uranium, lead, and mercury to name but four. So let's put all that together and do some questions. So pause your video and attempt these questions. Okay, so I'll show you the solutions. I'll click for each one in turn, and when I get to the end, if you need to, just pause the video so you can check your work and make any necessary corrections. So pause the video now if you need to. Okay, so let's look at some exam style questions. So pause the video and answer these questions. And again, pause the video, answer these questions. So I'll show you the solutions. I'll click through each one in turn, and then at the end just pause the video if you need to make any corrections. Okay, so pause the video now if you need to. And in case you didn't know, just at the end there where it says owtte, that stands for or words to that effect. So if you say something that means the same as radiation, more energetic without using those words, then you can still gain credit for that. Okay, so next set of questions. So same as before, pause the video if you need to to check your work more carefully. Okay, so well done if you got most of that right. That means you've done really well, so excellent, well done. Okay, so that takes us to the end of the lesson. So hope you enjoyed it, hope you learned a lot. What we looked at today was the life cycle of stars again, only this time talking about the different elements that were made at different phases. So we learned that helium was made in the main sequence phase from hydrogen and elements heavier than helium, up to and including iron, are made in the red giant and red supergiant phase. And then the elements heavier than iron are made in very energetic events, such as supernovae. We also identified some elements that were synthetic, they're made by humans, as well as finding out that hydrogen is the most abundant element in the universe. So hope you enjoyed it. Until next time, see you later, and don't forget to click on the link to do the exit quiz. Bye!

In this lesson we will explore the scientific evidence available for the origins of the Universe, with a particular focus on understanding the Doppler effect and its importance to our understanding of the formation of the Universe.

Origins of the universe

In this lesson we will focus on the understanding of the cosmic microwave background radiation and how this provides evidence for the origins of the Universe.

Cosmic Microwave Background Radiation (CMBR)

In this lesson we will review the Space topic including; the solar system, orbits, birth and death of stars, element synthesis, the origins of the universe and CMBR.

Review: Space

Subrahmanyan Chandrasekhar - black holes and x-ray telescopes.

Case Study: S.Chandrasekhar

﻿Hello Welcome back physicists, good to see you again. Before we start, as ever, here are the usual messages. You're probably really bored of hearing them from me now but I'm going to say 'em anyway. Make sure your phone is on silent, close down any tabs you might have open, don't give up, keep going, try all the activities that are set, and as I say every single time, hopefully you can say it with me, if you're not thinking, you're not learning. So, in this lesson we're going to need the holy trinity of physics. A pen or a pencil, paper, and of course, a calculator. So if you haven't got these already, pause the lesson now and go get them. Okay, so let's get started. So, in this lesson again, we're going to recap the life cycle of a star, how they're born, and obviously how the star will end its life cycle depending upon its initial mass. And we're going to revisit fusion along the way. We're going to meet a scientist who's not on the GCSE curriculum but he's certainly one of real interest I think for physicists. His full name is Subrahmanyan Chandrasekhar, we'll probably just call him Chandra from now on. But he did a piece of science that I think was of real significance in terms of determining the fate of a star and its lifecycle and that's a figure that he came up with called the Chandrasekhar limit which we'll talk about later on. And then we're going to touch upon the Chandra X-Ray Observatory which was launched into space and bears his name. And before finally, as ever, finishing off with some questions. So we'll begin with a recap of the lifecycle of stars, again, and we'll start with stars with a similar mass to our sun. Now you've seen this before, and we've gone through the lifecycle at least three times I think now, so this is a really good opportunity for you to practise recalling that sequence and making sure you know it off by heart. So pause the lesson now, copy out this flowchart, and fill in the blanks. Okay? So here's the second part of that flowchart. Again, pause the lesson now, copy it out underneath the one you've just done, and fill in the blanks. So here's the solutions to the first one. Okay, with the key terms in red. Pause the lesson if you need to, just to check your work. Again, pause the lesson if you need to to check the second part of the flow chart. Okay, so we're going to do the same thing again only this time with stars with a much greater mass than the sun, and hopefully if you've done that first one you should find this even easier because if you remember quite a lot of it crosses over. So again, pause the lesson now, and fill in the blanks. Here is the second part, and as before, pause the lesson and fill in the blanks. Okay, so here are the solutions. Pause the lesson, check your work. And the solutions to the second part, again, pause the lesson and check your work. Okay, so, well done if you got all that right. Hopefully you did, we've gone through that quite a lot of times so now you've got hopefully a good understanding, a good descriptive understanding of the life cycle of stars both high mass and low mass. So let's look at Chandrasekhar's life. Okay, so we're going to now look at the life of Subrahmanyan Chandrasekhar. He was born in 1910 in Lahore, in India. His uncle C.V. Raman was a physics Nobel laureate. And Chandrasekhar developed a theory about black holes which was heavily doubted at the time he publicised it. He completed his PhD in 1933, and presented his work on the mass limit of white dwarfs in 1934, and something that we'll return to later on. Chandra, the name by which he was known, was an Indian-American immigrant. He not only pursued his passion for astrophysics but was also very humble, and loyal character, who worked hard and valued relationships with others. So our story includes, new discoveries about the life cycles of stars, an infamous episode of respected and credible scientists getting it wrong, and the consequences that followed. Also the field of x-ray astronomy, and how Chandra's work has impacted and been recognised by the scientific community. After leaving India in July 1930, he worked aboard the ship that was to take him to Cambridge, England. During the trip, working with textbooks and doing lengthy calculations, he arrived at the conclusion that above around 1.4 times the mass of the sun, white dwarfs would be too massive to stay stable, and would collapse further. This was known as the Chandrasekhar Limit. So let's look further at the Chandrasekhar Limit. So just take a moment to remind yourself of this flow chart, and to read what's on the left hand side of the picture. So, to calculate this value he had to take into account the strength of gravity squeezing the star together due to its mass and the interaction of the particles that make up the star. There are laws that govern the forces between protons, neutrons, and electrons, and how much energy they possess, and it was a really complicated calculation, and he was an incredibly intelligent man. So he wanted to know what the tipping point was for stars to take one path or another. So, so far in our lessons we've just said "stars of a similar mass to the sun," for that left hand side there that's in green. And "stars much more massive to the sun" for the right path that's in blue. But that's not very accurate, is it? So, what was the exact limit? What size does a white dwarf have to collapse further, i.e. into a neutron star, or a black hole? So before we answer that question let's just recap a few basic facts. As before, I'll read out a question, pause, you call out the answer, or point at the screen, or say it in your head, and then I'll reveal the answer. So, the most massive stars will end their lives as? Black holes. The force responsible for star collapse is? Gravitational. Well done if you got that right. On the journey from India to England Chandra calculated? The mass limit for white dwarves. Okay, so we're going to do some maths practise now and we've got a task to do, give you a chance to do some calculations using standard form. So all this information here is going to be useful for the next, for the questions you're going to have a look at on the next slide. This information can be found in the work sheets which you can access if you click on the link to the next task, okay? But obviously make sure you come back to the video in order to answer, look see what the questions are. Okay? If you're not able to do that or you're watching on a mobile phone its just easier just to pause the video and write all this down then please do that. So if you need to write it down, pause the video now and jot all this information down. Okay, so here are the questions, now you need the information on the previous slide to answer these which as I said can be accessed through the link to the worksheet, or, of course, you can pause the video, rewind, and jot it down if you haven't done already. So you've got five questions here, pause the video now, and answer them. So here are the solutions. So pause the video now, and check your work. Okay, so well done if you got all those right, and if you didn't please compare your working out with the working out that's shown here, and see if you can spot where you went wrong. Here's the set, my final two questions, so again, pause the video now, check your work. Okay? And again, as before, if you made a mistake, just check the working out, and see where you went wrong. In astronomy, we often compare masses of celestial objects to other relevant objects. So for example we may compare a newly discovered planet's mass to the mass of the Earth, and say for example, the mass of Planet X is equal to 15 Earth masses, rather than using kilos. This is called a relative measure, because it compares two things, rather than an absolute measure that just gives the answer in kilos or whatever unit you're using. So you've used this many times before. So in chemistry, to compare charges of protons, neutrons, and electrons, you don't use the absolute charges in coulombs, you just give relative charges of plus one, nought, and minus one. So, let's see if we can do that for the Chandrasekhar Limit. And can we do that in terms of a number of suns as opposed to kilos. So here's the Chandrasekhar Limit. Here's the mass of the sun. So can you work out what the Chandrasekhar limit is expressed in terms of how many suns that is? Pause the video now, and have a go. Okay, so let's look at the answer. So, what you should have done, is divide the Chandrasekhar Limit by the mass of the sun, and you should come up with a figure of 1.391. Now that's approximately 1.4 times the mass of the sun, which is something that we mentioned earlier on, and just so you can see there we show this figure by a capital M, and this symbol here which is a circle with a dot through it. Now that just symbolises the sun. So, that's 1.391, capital M, for masses of the sun. And if you want to know the name of that little circle with the dot in it, it's called a circumpunct. And I'm telling you that purely because it's really good fun to say circumpunct. When Chandra presented his work he was doubted by key figures in the scientific community at the time. Chandra's calculations predicted that very massive stars would collapse down to bodies with zero radius and infinite mass. This was before clear ideas about black holes had been formulated and this idea was rejected as being a mistake. Although Chandra had informal support, he lacked support from key people, and it was only to be almost 50 years later in 1983 that he was awarded the Nobel prize for physics for his contributions. Despite having been five decades in the making he was gracious an unphased. So, just to summarise. Chandra predicted the formation of black holes for very massive stars, and he was one of the first to prove that there is no limit to density when stars die. And actually the rejection by his peers caused him to move to other fields of study, and it was only 50 years later that his work was recognised. So now let's move on to look at the Chandra X-ray Observatory. So, take a moment to look at this graph. Okay, so just to sort of, give you a bit of a run down what this means. So on our Y axis we have here atmospheric opacity, which sounds like a bit of a mouthful. Atmospheric opacity, but what it means is how much electromagnetic radiation is absorbed by the atmosphere. So if you've got an opacity of 100% then you are completely opaque, and if you have an opacity of 0%, you are completely transparent. And obviously 50% is halfway in between the two. And then across the bottom here, hopefully it's familiar, we have wavelengths of electromagnetic waves I won't ask you to do this but I expect you could probably I hope you could probably, match broadly what the wavelengths are to the different parts of the electromagnetic spectrum. So, what this is showing is the Earth's atmosphere is transparent some electromagnetic wavelengths and it's opaque to others. Now this graph shows that, for example, at a wavelength of one metre, here, the atmosphere is completely transparent. But at a wavelength of a hundred metres, here, it's completely opaque. So this means that we have to place telescopes and instruments viewing the cosmos above our atmosphere in order to detect different types of electromagnetic waves. So, we have, or we have to have sorry, x-ray observatories above the atmosphere if we want to look at x-rays, obviously. So here at the wavelength that corresponds to x-rays you can see that the atmospheric opacity is 100% and therefore the atmosphere completely absorbs x-rays and if we observe in x-rays we would have to place our observatory in space. Radio telescopes, on the other hand, can be ground based, and you can see here at the radio waves wavelength that the opacity is 0% and therefore they pass through and can be detected at ground level. Just take a moment to look at this image. So the Chandra X-Ray Observatory allows us to see cosmological phenomena in x-ray light. The most violent and energetic events emit x-rays, so for example when matter is torn apart, it spirals into a black hole, it emits x-rays. This means that although black holes are black, i.e. they emit no radiation to be seen, the events that happened due to their presence can be detected. Now, Carl Sagan, a famous 20th Century astronomer, has quite a nice analogy for this. He said that it was similar to "Watching the footprints of an invisible man walking in the snow." Here is an example of the type of events that the Chandra X-Ray Telescope can observe in conjunction with visible light from the Hubble Space Telescope. When a massive star exploded in the Large Magellanic Cloud, a satellite galaxy to the Milky Way, it left behind an expanding shell of debris called SNR0519-69.0. Here, the multi-million degree gas, so gas that is many millions of degrees hot, is seen in x-rays from Chandra, which is in blue on that picture. The outer edge of the explosion, which is in red, and stars in the field of view, are seen in visible light, from Hubble. Okay, time for a bit of work. So, here's a series of questions, pause the video now, and answer them. Okay, so here are the solutions, and again as ever, pause the video now, make any corrections or changes that you need to. Okay, so that brings us to the end of this lesson, as well as the end of the unit, as ever, I hope you've enjoyed it, I hope you've learned a lot. And I'm just going to leave you with this final word from Chandra, which is in 1987. "I never thought that the recognition that one receives in one's lifetime is a measure of what one is." So thank you very much, and I hope you do well in your GCSEs.

Unit Overview: Space

Lessons: