Lesson video

Hello, and welcome to this lesson called Stars, Galaxies, and the Universe.

This is from the unit called Our Solar System and Beyond.

My name's Mr. Norris.

People have always wondered what is out there, in space away from Earth.

People have looked at the night sky and seen the stars, and wondered what the stars are and where they are, and why they look the way they do.

Well, we now know the answers to lots of those questions.

So hopefully, this lesson will help to share that understanding with you so you can gain an appreciation of Earth's place in not just the solar system, but the wider universe as well.

Let's get going.

The outcome of this lesson should be that by the end of the lesson, hopefully you'll be able to describe what stars and galaxies are, and explain how they look when seen from Earth.

Some keywords we'll be talking about in this lesson are star, light-year, billion, galaxy, and universe.

Each word will be explained as it comes up in the lesson.

This lesson is divided into three parts.

The first part focuses on the sun and other stars, the second part looks at how the distance of stars explains the way they look, their appearance.

And the final section, we're really zooming out to look at galaxies and the universe on the biggest scale possible.

Let's start with the sun and other stars.

So we know that the sun is a star.

On Earth, the sun appears extremely brightly.

So bright, it's dangerous to look at directly, so a little reminder not to look at the sun directly ever.

And you might not know that the sun's true colour is white, even though it's often drawn yellow.

So why is that the case? Well, the sun can appear more yellow or orange at sunrise or sunset because of Earth's atmosphere scattering some colours of the sun's light more than others.

So this is one reason why people often think of the sun as being yellow, when in fact its true colour is white.

So I'll just mention images of the sun, like this one on the screen now.

Now, this is actually a false colour image.

So it's not a photograph, like photographs that are taken with normal cameras that you'll be used to using.

These are images which are made by taking measurements of each part of the sun, and then choosing colours for different values.

So then each part of the sun kind of gets coloured in a different colour, which you've just given to that value.

So images of the sun like this look really impressive, and they can be really useful to scientists because they're made up of thousands and thousands of individual measurements of each part of the sun.

But you've got to remember that they could be false colour images, as opposed to true colour images.

And the other things we need to know about stars are stars are extremely large and extremely hot, and they give out light and other radiation in all directions.

And that's one of the key differences between stars and planets.

Stars are so hot that they're giving out their own light, whereas planets don't give out their own light.

And remember that stars are not burning gas.

They are made of gas, but the gas in a star isn't burning, 'cause burning means chemically reacting with oxygen in the surroundings.

And there's no oxygen in the surroundings around a star, because there's no oxygen in space.

So the gas of a star is not burning, but the gas within a star, there are nuclear reactions happening within stars.

So that's what causes the high temperatures, nuclear reactions within a star.

Time for a quick check now of what we've just gone through, I would like you to just tick all the statements that are true.

So, which of these are true? A, the sun is a planet.

B, the sun is a star.

C, the sun is yellow.

D, the sun is a huge ball of burning gas.

E, nuclear reactions in the sun make it hot and bright.

Which of those are true? Five seconds to decide, pause the video if you need to.

Okay, let's see how you got on.

The statements that are true are B, the sun is a star, and E, nuclear reactions in the sun make it hot and bright.

The sun is not yellow, it's white, remember.

And the gas in the sun isn't burning, there's nuclear reactions happening within it.

So it was only B and E that were correct, well done if you got both of those.

So the sun is a star, but we're now gonna talk about all the other stars that we can see.

So the stars that appear in the night sky.

So from Earth, stars in the night sky appear much smaller and dimmer than the sun.

And in towns and cities, there's often too much background light from street lamps and offices and things like that to see many stars in the night sky.

And that effect is called light pollution, when there's too much background light to be able to see the stars in the night sky.

And, of course, in this picture, there's lots and lots of stars, but the moon is not the star.

The moon orbits Earth, relatively close to Earth on the scale of space.

So different stars in the night sky, actually they do have different sizes, brightnesses, and colours in the night sky.

And telescopes, binoculars, and cameras with long exposure times, which capture more light, will help us see that more clearly, will help us see the different sizes, brightnesses, and colours of different stars in the night sky.

So, not all stars are the same.

In fact, the colour of a star actually depends on its surface temperature, the temperature of its surface.

So the cooler stars are more red.

And if a star is warmer, then it will look more orange or yellow, or white or even blue.

And actually, that's the same sequence of colours as when you heat up anything, okay? If you heat up anything, eventually it gets to a point where it's so hot, it starts glowing.

And the colour something glows first is red when you heat it up.

And if you continue to increase the temperature, then an object will go from glowing red to glowing more orangy or yellow, and then glowing white, that's an even higher temperature.

So when something's white hot, that's hotter than if it's red hot.

If something's glowing white, it's much hotter than when it's glowing red.

And if you were to continue heating something up past white hot, it would actually glow blue, okay? So that's the same sequence of colours for anything which gets so hot that it glows.

The colour that it glows depends on the temperature it is, and stars follow this pattern too.

So that's the colour of a star.

But what about the size and brightness a star looks in the night sky? Well, the apparent size and brightness of a star in the night sky depends mainly on its distance from us in our solar system.

So the further away a star is, the smaller and dimmer it will look.

But, of course, it will look the same colour to what it actually is, as long as there's no effects from Earth's atmosphere getting in the way.

Time for a check on what we've just said.

This represents what the sun looks like at its normal distance from Earth.

So have a look at that first picture.

And then which of the four pictures, A, B, C, or D, would show what the sun would look like if it was double the distance away? Is it picture A, B, C, or D? Take a moment now to decide, five seconds to come up with your answer.

Well done if you said picture B.

if the sun was double the distance away, the sun would look smaller but the same colour.

So when stars are further away, they will look smaller and dimmer, but the colour will look the same.

If something looks dimmer, that doesn't change the colour of its light.

So the sun is a white star, so it'll still look white, but smaller if it was further away.

Okay, time now to do a task on what we've just been talking about.

So here are some stars in a small section of the night sky, in the black rectangle with the white dots.

What I'd like you to do is draw in the boxes to predict what you think these stars would look like if they were viewed from further and further away.

So you can see there's a box for to draw what you think the stars would look like if they were viewed from double the distance away, and then four times the distance away, and then eight times the distance away, 16 times the distance away, and then 32 times the distance away.

Now, this doesn't have to be exact, but try and do as accurate a drawing as you can based on what you think those stars would look like if they were viewed from further and further away.

One thing I should say is the example drawing is white dots for the stars on a black background.

Now, just to save you time, you should do your drawings probably as black dots on a white background.

So use the opposite colours, because it'll just be easier for you to do.

So draw the stars as black dots on the white background in the boxes.

And then the second part of this task, once you've done the drawings, is to write an explanation of why you think the stars would look like this from the different distances as you get further and further away.

So you should pause the video now, and have a go at completing that task with your best efforts.

Okay, off you go.

Okay, now that you've done those first two parts of the task, have a go at part three and part four.

So part three of the task is this.

So in the room you're in, you may be able to move backwards from the screen.

So what I want you to do is do that, move backwards from the screen, and describe how the stars on the screen in this black box now, how do they appear to change as you move back distances of double the distance away, four times the distance away, eight times the distance away, et cetera? As much as you can do, roughly speaking.

Perhaps you could use paces, if you're one pace away from the screen now, then try going two paces away, and then four paces away, and then eight paces away.

And if you can, 16 or 32, but you might not be able to do those larger ones.

And it can really help to take a photograph at each distance to help you compare what the stars look like, what those stars on the screen look like at different distances.

And then for part four of the task, I would like you to do a bit of writing to explain whether your prediction and explanation were correct.

Your prediction was what you did for part one, and the explanation is what you did for part two.

Does your prediction match what you actually saw when you did part three? Try to write a better explanation for what you saw, if you can see ways to improve your original explanation from step two.

So have a go at doing those two parts of the task now.

Right, time to give some feedback on what you might have predicted, and what you should hopefully have seen, and why.

So for parts one and three, you were asked to predict or describe what the stars would look like if they were viewed from further and further away.

So look really closely in all the white boxes to see how the stars would look like if viewed at double the distance, four times the distance, eight times the distance, there are four stars there in the same pattern.

16 times the distance, there are still four stars there in the same pattern, but they look smaller.

And at 32 times the distance away, look really closely in the middle of that 32 times away box to see what that collection of stars would look like from 32 times the distance away.

So what's the explanation of why the stars look like that from greater distance? Well, hopefully what you might have predicted, and what you should have found out from doing the task, is that at greater distance, stars appear smaller and dimmer.

But we need to try and explain why that is, so here's an example explanation.

Don't worry if yours isn't exactly like this, but hopefully it might be along the right lines.

And you can always add to yours to improve it, after looking at mine.

So stars appear smaller at greater distance because at greater distance, they take up less of our field of view.

They appear dimmer because less of the starlight reaches our eyes, and that's because starlight spreads out more and more the further it travels.

And just, here's some diagrams which hopefully make that really clear.

So if you were closer to a star, like in that left hand diagram, not all of the light from a star reaches you.

But the further you are from a star, like in that right hand diagram, even less light reaches you if the star's further away.

So that's why stars look dimmer the further they are away.

And stars look smaller the further they are away, 'cause they take up less of our field of view.

So well done if you wrote answers along those lines.

You didn't have to draw diagrams like that, but hopefully the diagrams help explain why stars look dimmer when they're further away.

Well done for your effort on this task, and your effort in responding to the feedback I've just given and improving your work.

If you've not done that yet, pause the video and do that now.

So that completes the first section of the lesson, which was on the sun and other stars.

So now let's look at the distances that stars are away, and how that explains their appearance, which we've kind of already touched on, but we need to pull it all together.

So we know that stars in the night sky look so small and dim because they're extremely far away, far further away than the sun is.

So let's go into a bit more detail on that.

We know that the sun is the nearest star to Earth at the centre of the solar system.

Remember on this scale, Earth is so small that you can't even see it, but it's location is shown by the arrow in the diagram.

And just a reminder that the sun is being drawn too big in this diagram.

So the sun is smaller than how it looks on this diagram.

So the sun is the nearest star to Earth, it's the centre of our solar system.

Now, the next nearest star to Earth after the sun is more than 250,000 times further away from Earth than the sun is from Earth.

That's a huge distance further away, the nearest star after the sun.

Now, in that distance you could fit over 4,000 solar systems. So if that's the distance of one solar system, the next nearest star after the sun is so far away from the solar system, you could fit over 4,000 solar systems into the distance away that the next nearest star is.

That is how far away the other stars are, compared to what's in the solar system, incredibly far away.

So usually, distances are measured in metres or centimetres.

However, the distances to other stars are so large that we need a different unit, we need a larger unit, otherwise we're gonna end up with huge numbers.

It would be more convenient to have smaller numbers of a bigger unit.

So what unit do we use? Well, stars give out light, and light takes time to travel.

For example, light from the sun takes eight minutes to reach Earth.

So when the sun gives out light, after one minute, the light would be here, two minutes, three minutes, four minutes, five minutes, six minutes, seven minutes.

It takes eight minutes for the light to actually reach Earth from the sun, because, of course, of the such great distance between Earth and the sun.

So because light takes eight minutes to reach Earth from the sun, we can say that Earth is a distance of eight light-minutes away from the sun.

Because it takes light eight minutes to go that distance, we can call that distance eight light-minutes, the distance light travels in eight minutes.

Now, that distance in metres is a huge distance, that is 150 billion metres.

So you can see why we need a new unit for measuring distances in space, and we're not even outta the solar system yet.

So the speed of light through space is extremely fast.

That's how it can go that huge distance of 150 billion metres in just eight minutes.

However, the distances in space are even bigger.

So we need to give light longer amounts of time to be able to travel these greater distances to the stars outside the solar system.

So we use units called light-years, and one light-year is the distance light travels in one year.

So that is an absolutely huge distance, 'cause a year is a large amount of time, and light travels very, very, very fast.

So the distance of a light-year is a very, very, very, very long distance.

In fact, it's just under 10 million billion metres, a light-year.

So, which of these would be the longest distance? Is it A, B, C, or D? A light-second, the distance light travels in one second, a light-minute, the distance light travels in one minute.

A light-day, the distance light travels in one day, or a light-year, the distance light travels in one year.

So all of those are real distances, but which of those is the longest? Five seconds to decide now.

I'm sure you will have chosen option D.

A light-year is going to be the longest distance out of those four, because light always travels at a set very, very fast speed.

So the longer you give it, the longer you give light to travel, the further it's gonna go.

So the distance light travels in one year is gonna be far further than all the other options, 'cause it's the longest amount of time.

Therefore, it's going to be the greatest amount of distance if you give light a longer time to travel.

So a light-year is the greatest amount of distance for that reason.

Well done if you've got that.

So the next nearest stars to Earth after the sun are three stars in a small group called the Alpha Centauri system of stars.

It's actually a group of three stars kind of together in a small group.

And the closest star to Earth's solar system is called Proxima Centauri.

So that's 4.

2 light-years away.

Those three stars in the Alpha Centauri system without a telescope in the night sky, they just look like a single bright star.

'Cause all of their light kind of merges together, it just looks like one object in the night sky without using a telescope.

Here is a picture of the next nearest star, Proxima Centauri, taken with the Hubble Space Telescope.

And all the other stars in the night sky are even further away than this.

So the closest star to the solar system after the sun, Proxima Centauri, is 4.

2 light-years away.

That's a huge distance, and all the other stars in the night sky are even further away than that, amazing.

So Proxima Centauri, the nearest star to the sun is 4.

2 light-years away.

What does this mean? Does it mean, A, it's 4.

2 million miles away, B, it would take 4.

2 years to get there from Earth, C, it takes light from Proxima Centauri 4.

2 years to travel the distance to Earth, or D, the distance between Earth and Proxima Centauri is 4.

2 years? What does 4.

2 light-years away mean? Make your selection now.

Okay, well done if you chose the correct answer, which is C, if something is 4.

2 light-years away, that means it takes light 4.

2 years to travel the distance.

I wonder if you spotted the mistake in D.

The question is talking about a distance, but a distance can't be just 4.

2 years, because 4.

2 years is an amount of time, not a distance.

So years are an amount of time, but light-years are an amount of distance, the distance light would travel in 4.

2 years.

Let's do a last check on this section.

Why does the sun look very different to all other stars? So all other stars are seen from Earth in the night sky, and, of course, the sun looks very different to that and it's seen in the daytime sky.

So what's the best reason for that? Is it, A, the sun is a different kind of star to the stars in the night sky, B, the sun is a different colour to all the stars in the night sky, C, the sun is brighter 'cause it gives out more light than any of the stars in the night sky, or D, the sun is much closer than any star in the night sky? Which of those is the best reason why the sun looks different to all other stars that we see? Okay, you should have chosen an option.

Well done if you said option D.

That's the only reason the sun looks very different to all of the stars in the night sky, is because it's very simply so much closer than any other star that we see in the night sky.

Well done if you got that.

Okay, time for a task now on this section of the lesson.

So look at this image, it shows stars and the moon in the night sky.

And what you need to do is just take a good look at that image and then discuss the statements, which are on the next slide, and fill in the confidence grid.

So you need to decide whether each statement is correct or incorrect, and then take the box that shows what you think.

So statement A, we can see all the stars there are.

Is that correct or incorrect? How sure are you? Statement B, dimmer stars are always further away than brighter stars.

Statement C, many stars are so dim or so far away that we can't see them.

And statement D, stars in the night sky are orbiting the sun, like the planets.

Do you think that's correct or incorrect? And how sure are you? So pause the video now, have a good think about each statement, and then tick the boxes that indicate what you think.

And I'll see you in a few moments for some feedback.

Okay, well done for your effort on that task, here's some feedback.

We can see all the stars there are.

Well, that's incorrect, we can't see all the stars there are.

So many stars are so far away that they can't be seen.

Statement B, dimmer stars are always further away than brighter stars.

Now, that's not true either, because a star that is dimmer could be dimmer because it's very, very far away.

But it could be dimmer because it's just a dimmer star that happens to be a bit closer, but it is dimmer in the first place.

So dimmer stars are not always further away than brighter stars.

There's how bright or dim a star actually is, and then there's the distance away.

Both of those factors affect its brightness and dimness.

Statement C, many stars are so dim or so far away that we can't see them, and that's true.

And statement D, stars in the night sky are orbiting the sun like the planets.

That's definitely not true.

Stars in the night sky are not in the solar system, they're much, much further away.

So they're not orbiting the sun, they are their own stars a very, very large distance away from the solar system, so they're not orbiting the sun.

Well done if you've got most of those correct.

So that takes us to the final part of the lesson.

So it's time to look at galaxies and the universe.

So if you were able to zoom out from our solar system to see more of space, then you would see views a bit like this at points along the way.

So if you imagine starting looking at the sun and zooming out until you get to the next closest star to Earth, which is in the Alpha Centauri system, and then paused at that point, you might see a view that looks like this with Alpha Centauri in the foreground.

So it looks a bit bigger from this point of view.

And the sun all the way in the background, 'cause that's where we've come from to get to this point.

And then all the other spec of lights are stars, which are even further away from the sun than Alpha Centauri system is.

So what if we continued to zoom out from here? Well, then we might see a view that looks like this.

And the sun is in there, but you might not be able to work out exactly which star the sun is.

I'll put this box in.

So we can see the sun and Alpha Centauri in the original box there.

But we've now zoomed out so far that the sun and Alpha Centauri is just clearly one star amongst hundreds in this part of space.

And what if we zoom out even further? Well, from even further away, we'd start to see a view that looks like this.

So at this kind of very great distance away, there's a cloudy glow from millions of stars, which are now so small that we can't see them because we're so far away from them.

And what if we zoomed out even further away again? Well, eventually we would see that that area of space, where the sun is, is just one star amongst billions in a swirling spiral kind of cluster of stars called a galaxy.

One billion is that number there, that's a one with nine zeros afterwards.

So you could think of it as 1,000 million, that's the same thing as a billion.

And the galaxy that our sun and our solar system is part of is called the Milky Way.

It's 105,000 light-years across.

So remember how big a light-year is, and our cluster of stars, the Milky Way, is 105,000 light-years across.

So you might know that you can see the Milky Way from Earth, but only in places where there's very little light pollution, and on nights where the moon's not visible in the sky, because light from the sun that's reflected from the moon can basically cause too much glare that you can't see the Milky Way.

So you can only see the Milky Way in places where there's little light pollution, and on nights where there's no moon.

So that is the Milky Way, it's a band of light in the night sky as visible from Earth.

It gets its name from an ancient myth about a god spilling milk across the sky.

So why does the Milky Way look like this from Earth? Well, it's because Earth and our solar system is in the Milky Way.

So the Milky Way is like a spinning spiral disc of billions of stars, but we are in it.

So we are in that spinning disc of stars or disc of light, and we're looking across the same disc of light, so we see it as a band of light in the sky, in the night sky.

So the part of the Milky Way where our solar system is, is on kind of a minor arm of the Milky Way called the Orion Spur.

What if we zoomed out even further away again from our galaxy, from the Milky Way? Well, if we did that, we would see that actually the Milky Way is just one galaxy amongst billions of galaxies in the universe.

Remember, each galaxy contains billions of stars, which are light-years apart.

So it's amazing to think that our galaxy is actually just one galaxy amongst billions in the universe.

So from the furthest viewpoint we can imagine going to, every point of light is not a star, but an entire galaxy of billions of stars.

So even though they're so far away, we can actually see other galaxies from Earth.

But in the night sky, they just look like stars, 'cause they're so far away.

A powerful telescope though, can show their structure, showing they're not just stars.

So let's do a quick check.

What is the Milky Way? Tick all the options that give a correct description that could apply to the Milky Way.

Pause the video now, and make your decisions for each statement.

Okay, let's see how you got on.

The Milky Way is a galaxy, not a star, not a planet.

It's a cluster of billions of stars, that's what a galaxy is.

So C and E, definitely correct.

But from Earth, the Milky Way appears as a band of light in the night sky.

So F is correct as well, just that's what the Milky Way looks like from this perspective of Earth's surface.

So C, E, and F all correctly describe the Milky Way.

Well done if you've got those.

Time for the final task of this lesson then.

There's two parts to this task, here's part one.

Use only the words galaxy and universe to fill the blanks.

You should pause the video now, and have a go at this first.

So the whole of space and everything in it is called the universe.

In the universe, stars are found in huge groups, and each huge group is called a galaxy.

There are billions of stars in one galaxy.

The galaxy that contains the sun is called the Milky Way.

And the Milky Way is just one galaxy out many billions in the universe.

Well done if you've got most of those right.

Now, here's the second part of this task now.

What I'd like you to do for the second part of this task is to write down your full address so that it describes how to find you in the whole of space.

So here are the first lines of an example address, Buckingham Palace, London, the UK.

But if you were an intergalactic postal worker, using our imaginations a bit here, what other lines would you need on that address so the postal worker would know how to find you in the whole of space? So think about the best order that each of the following could be included in the address, the planet, the galaxy, the nearest star, our solar system, and which part of the solar system we're in.

And are there any other lines you can include that might help an intergalactic postal worker deliver a letter to you from anywhere else in space? So pause the video now and have a go at that task.

Okay, I'll give you some feedback now using the example address.

So the example address could be Buckingham Palace, London, the UK, Europe, 'cause we're kind of, we need to say which part of Earth we're in.

Europe's in the Northern hemisphere of Earth.

So that's the planet.

Earth is amongst the inner planets in the solar system.

There's no need to say close to the sun, because the solar system already specifies close to the sun.

It's the solar system within the Milky Way.

You might have put that we're on the Orion Spur within the Milky Way, that would've been an excellent extra detail.

And the Milky Way is just one galaxy in the universe.

So, well done if you got something along those lines.

Well done for completing this lesson on stars, galaxies, and the universe, here's a summary.

The sun is a star, which is an extremely hot sphere of gas that emits light.

Stars in the night sky are much smaller and dimmer than the sun because they're much further away.

A light-year is the distance light travels in one year.

It is a huge distance, and is used to measure distances in space.

Beyond the sun, the next nearest star to Earth is about four light-years away.

And in the universe, stars are found in groups called galaxies, with billions of stars in each galaxy.

There are billions of galaxies in the universe.