Lesson video

Hello, my name's Dr.

George.

This lesson is called Observing the Universe and it's part of the unit, Gravity and Space.

The outcome for the lesson is I can describe what redshift is and how it's measured.

Here are the key words for the lesson, which I'll introduce as we go along.

Come back to this slide anytime if you need to remind yourself of the meanings.

The lesson has three parts.

They're called telescopes, galaxies, and redshift.

So let's start by looking at telescopes.

When you look up at the night sky, you can see thousands of stars of different brightness, and you may also be able to see another five of the eight planets apart from Earth.

And humans have known about these for thousands of years because you don't need a telescope to see them.

The planets, Uranus and Neptune are too faint to see, and they were only discovered after telescopes were invented.

In the 17th century, that's the 1600s, The optical telescope was developed and the first telescopes used lenses to focus light.

These lenses were larger than the lenses in the eyes, which meant they could collect and focus more light than the eyes can, and that allowed people to see fainter objects than you can see with the naked eye.

And having a bigger lens also lets you see things in more detail, as in this picture of the moon.

New stars were soon discovered with these new telescopes, along with the moons of other planets like Jupiter.

And the two most distant planets, Uranus and Neptune, were also found using telescopes.

Now, here's a question for you.

Why do telescopes allow us to see objects that are too faint to see with the naked eye? When I ask a question, I'll wait five seconds, but if you need longer, press Pause, and press Play when you have your answer ready.

And the correct answer is B, they have large lenses, larger than the lenses in the eye, so they collect more light than the eye does.

The light from stars needs to pass through the atmosphere before it can reach you or a telescope that's on the ground.

We'd call that a ground-based telescope.

But the air in the atmosphere is always moving and it varies in density, and this causes refraction of the light, which changes its direction slightly over time.

This is what makes the stars twinkle.

The atmosphere absorbs some of the light, so not all of the light can reach a ground-based telescope.

And the atmosphere also absorbs other types of electromagnetic radiation that stars emit.

Telescope technology has developed significantly over the past 400 years and large ground-based telescopes now often use large, curved mirrors instead of lenses.

It's easier to build bigger mirrors.

Very large lenses are so heavy that they can start to deform under their own weight.

Larger telescopes collect more light and provide sharper and brighter images.

These large ground-based telescopes are typically built on the tops of tall mountains, as this lifts them above a lot of the atmospheric distortion and the clouds.

The photo shows an example.

But not all telescopes are built in isolated places like this.

The Griffith Observatory was built on the outskirts of the city of Los Angeles in the USA, and it was specifically built for the public to be able to use the telescope.

It gets millions of visitors each year, but it is affected by light pollution from the city and by cloud and traffic vibrations.

Now, which of these are likely to be problems when making observations at the Griffith Observatory in Los Angeles? And three of these are correct answers: light pollution, vibration from traffic, and it's below cloud level.

But B is not correct.

The observatory was specifically built to give easy access to people from the city.

Now, several of the most powerful telescope systems we have are based in space.

Here, there's no atmosphere in the path of the light, so clearer images can be produced.

We call these space telescopes and they can also detect radiation that can't pass through our atmosphere, allowing them to detect things that would be invisible from the surface of the earth.

This includes some frequencies of X-rays, ultraviolet radiation, and infrared radiation.

Here are a couple of examples of space telescopes, probably the two most well known.

The Hubble Space Telescope; it orbits Earth and it can observe UV, ultraviolet, visible light, and infrared.

Its largest mirror is four-and-a-half square metres in area.

More recently, the James Webb Space Telescope has been launched and that actually orbits the sun.

It observes infrared radiation and its largest mirror has an area of 25 square metres.

Space telescopes have a range of advantages and disadvantages when compared to ground-based telescopes.

Some advantages are that there's no atmospheric distortion.

The light reaches these telescopes without passing through the atmosphere.

There's no light pollution.

There are no nearby cities, for example.

There are no vibrations due to traffic of course, and they can detect radiation, which cannot pass through our atmosphere, including infrared.

Disadvantages though is that they're very expensive to construct and to launch, and they're difficult to maintain and repair because that has to be done in space, and they have a limited lifespan.

They don't last as long as ground-based telescopes can, for example, because of the fuel needed to keep them in orbit.

Hubble, for example, is very gradually slowed down by the thin atmosphere that's at the height of its lower Earth orbit.

That will cause it to gradually lose altitude, lose height, if it didn't periodically use energy to raise itself back up again.

Now, there's a telescope called the Hard X-ray Modulation Telescope, HXMT.

You may not have heard of it, but can you think why it would be based in orbit rather than on the surface of the Earth? And the correct answer is because X-rays are blocked by the atmosphere, so you can't make observations of X-rays emitted by objects in space using a telescope on Earth.

And now here's a task for you.

Imagine that the UK wants to develop a new telescope for observing distant stars.

Can you compare the advantages and disadvantages of developing, constructing, and using a ground-based or a space-based telescope system and suggest what types of location in the UK will be best for building a ground-based telescope? Press Pause while you write your answers, and press Play when you're ready to check them.

Here are many of the advantages and disadvantages you could have mentioned.

Ground-based telescopes have the advantage of being cheaper to construct, operate, and maintain and repair.

They can also be larger because they don't have to fit into a space shuttle, be launched into space.

And they can be more easily upgraded over time.

There are ground-based telescopes that we're still using that have been around for 100 or even 200 years.

Disadvantages include atmospheric interference with images.

We've seen that the twinkling of stars is because of movement of the air in the atmosphere, and that leads to images being blurry, less detailed.

And light pollution interferes with images if the telescope is anywhere near the centre of population.

Space-based telescopes have an advantage that their images are not affected by the atmosphere because they're above it.

And so they can also detect radiation that doesn't reach the Earth's surface because it's absorbed by the atmosphere.

But disadvantages include that they're more expensive, they're hard to maintain and repair because that has to be done in space, there's a risk that they could be destroyed on launch if something goes wrong with the launch, and they have a more limited lifespan.

Well done if you included many of these in your answers.

And then you were asked where we might build a ground-based telescope for the UK.

The telescope needs to be built somewhere high with low levels of light pollution.

This should be a mountainous region far away from cities.

The UK doesn't actually offer many great places to build telescopes.

And another option, which you might not have thought of, is to build a telescope abroad and operate it remotely from here, which is something we've done with telescopes, for example, in Hawaii.

Now let's learn about galaxies, which we didn't understand until we had good enough telescopes to observe them in detail.

When the first telescopes were built, astronomers started to search for new stars and planets, which couldn't be seen with the naked eye.

You can imagine that people were excited to see what else they could find out there.

And so they constructed new star charts, mapping out these new discoveries.

Astronomers rushed to make new discoveries with larger and larger telescopes as they became available.

While looking for comets in the 17th century, the French astronomer, Charles Messier noticed that while most stars in the background looked like single points of light, there were some objects which were more spread out.

They were fuzzy.

He decided to catalogue these and he catalogued over 100 of what became known as Messier Objects, but he didn't know what they were.

As telescopes became more powerful, astronomers found out that there were a range of different types of Messier Object.

Some were nebulae, clouds of gas and dust.

Some were clusters of stars, now called galaxies.

Now, measuring how far away objects are in space is difficult.

The galaxies turned out to be too distant to measure directly.

People didn't know yet if there were objects within our galaxy, the Milky Way, or beyond it.

In the early 1900s, the astronomer, Henrietta Leavitt investigated stars, which changed brightness over a regular period.

These are called Cepheid variable stars and Leavitt found that there was a mathematical relationship between their brightness and their period of oscillation, how long they take to do one complete cycle of brightness changes.

She was able to find this mathematical relationship and that meant that if you made observations of a Cepheid variable star over time to find its period of oscillation, you could then know from Leavitt's relationship, how bright that star actually was.

And then knowing how bright it looks from here, enabled you to work out how far away it was.

It's a bit like if you know the actual brightness of all street lamps, and then at night, you see a street lamp from a distance.

It would be possible to work out how far away that street lamp was from you.

And this is what we can do with Cepheid variables, because we know their actual brightness, because of how it relates to the period of oscillation.

Objects like this, where we know their actual brightness and can use that to work out their distance, are called standard candles.

And this was a major breakthrough in enabling us to work out distances to further away objects in the universe.

Edwin Hubble then observed Cepheid variables within other galaxies.

Telescopes were good enough to enable you to observe a single bright star in another galaxy.

And he used measurements of brightness and period of variation to calculate the distance to each one using what Henrietta Leavitt had worked out.

His results showed that galaxies were much further away than previously thought, and not objects within our own Milky Way galaxy.

And his discovery of these distances showed that the galaxies must each contain billions of stars, for them to appear as bright as they do, given how very far away they are.

And he also showed, by measuring these distances, that the universe is billions of times larger than people previously thought.

Before that, people didn't know if the universe extended beyond our own Milky Way galaxy.

Now, is this statement true or false? Edwin Hubble's discovery made the universe much larger than it was previously.

And choose a justification, a reason why you decided your answer.

And the statement is false, because his discovery increased our knowledge of how large the universe is.

But of course, it didn't actually change the real size of the universe.

We just didn't know it was that big before.

So a galaxy is a collection of several million to hundreds of billions of stars.

They come in different sizes and the stars are held together by gravitational forces.

There are several common shapes of galaxy, one of which is a spiral.

There's an example shown here; that's a photograph of a real galaxy.

The stars in spiral galaxies rotate around the central point.

Our solar system is a tiny, tiny part of our galaxy, the Milky Way Galaxy.

We can't see the full shape of the Milky Way Galaxy because we're inside it, although we are able to deduce its shape by looking around from where we are.

We see a milky strip across the clear night sky as we look inwards towards the centre of the galaxy.

It's a spiral galaxy containing about 300 billion stars, 300,000 million stars, many of which we now know also have planets orbiting them.

Is this true or false? All of the stars in the Milky Way Galaxy are visible from our solar system using telescopes.

And again, choose a justification for your answer.

The answer is false, and there are two possible justifications here.

One is that some of the stars are too dim and far away for even the most powerful telescopes to detect.

Stars come in a wide range of different brightnesses, and the Milky Way also contains nebulae, clouds of gas and dust, which can block light from distance stars.

So we can't see absolutely every star in our Milky Way Galaxy.

And a couple of questions for you to write down your answers to.

How would a distant individual star look different from a galaxy when observed with a small telescope? And describe the differences between our solar system and the Milky Way Galaxy.

Press Pause while you write down your answers and press Play when you're ready.

Here are some example answers.

Through a small telescope, a star would look like a single point of light, while a galaxy would be a larger but blurry shape.

The differences between our solar system and the Milky Way Galaxy are sometimes people do confuse the two, but the solar system contains a single, average-sized star, the Sun, eight planets, several dwarf planets, and millions of asteroids and comets orbit around it.

Six of the planets have moons in orbit around them.

The Milky Way Galaxy is a spiral galaxy which has about 300 billion stars of different sizes and ages.

These rotate around the centre of the galaxy.

Many of the stars have planetary systems. The Milky Way also contains nebulae, gas and dust clouds.

Now let's take a look at redshift.

White Light, which includes light from stars and galaxies, can be split into a continuous spectrum of colours using a prism.

You may have had a chance to do this yourself at some point.

What we see is the same colours that we see in a rainbow.

At one end is violet, which is the light that has the shortest wavelength and the highest frequency, and at the other end is red, with the longest wavelength and lowest frequency.

When we split light from the sun into its spectrum, if we look closely, we see that it forms an almost continuous spectrum.

There are some very specific frequencies missing from the spectrum, leaving gaps in the form of dark lines.

And these dark lines have formed because different gases in the star's outer layers absorb light at very specific wavelengths and then scatter that light in other directions.

The gaps in a spectrum like this are known as absorption lines, and the wavelengths or frequencies of the absorption lines show which elements and compounds are actually found in the atmosphere of the star.

Different elements produce very specific sets of lines.

We can observe this in a lab.

For example, the element sodium produces a close pair of lines in the orange part of the spectrum.

All stars contain hydrogen, so all stars will have hydrogen lines in their absorption spectrum.

We can use these hydrogen lines to compare stars.

Now, how do absorption lines identify elements and compounds in stars? There are two correct answers here.

Lines vary in number for different elements and compounds, and absorption lines also vary in position for different elements and compounds.

And we can observe these absorption lines in a lab and then recognise them when we see them in the spectrum of a star.

The lines don't vary in shape, they're lines.

And they don't vary in colour, they're all dark.

Well done if you've got the right answer here.

When the spectrum of a star similar to the Sun but moving away from us is analysed, it turns out that the absorption lines for hydrogen are slightly different from what we might expect.

Here are some of those lines in the spectrum of the Sun, and here's what they might look like for a star that's moving away from us.

The overall pattern is the same, but the absorption lines have shifted.

They've moved towards the red end of the spectrum.

This shows us that the wavelengths of the light from the other star have all become longer and the frequencies have all decreased compared with what they would be like if the star weren't moving relative to us.

The change in the light from the moving star is called redshift.

The greater the decrease in the frequencies, the faster the star is moving away from us.

So, here's a spectrum for the Sun, a star that's relatively slowly moving away from us, and a star that's moving away from us more quickly.

This shift is caused by something called the Doppler effect.

When the source of any wave that could be light, radio waves, sound waves, is moving away from us, the wavelength gets stretched due to the movement of the source.

This increase in wavelength for a source moving away from us causes a decrease in frequency of the wave that we receive.

Here's an example for sound.

Let's say this vehicle is making a sound with a frequency of 50 Hz.

If it's stationary, and so is the receiver, the person hears a sound with frequency 50 Hz.

But if the source of the sound, the vehicle is moving away from the receiver, you could think of the waves as being stretched.

The wavelength increases, the frequency decreases, and the person actually hears a lower sound.

You may have noticed this if you've heard an ambulance or a police car drive past you quickly.

You hear the pitch of the sound change as it comes towards you, goes right by you, and then moves away from you.

That's the Doppler effect.

This figure shows the absorption spectrum of the Sun, and underneath it, the spectra for three other stars.

Which star is moving away from us at the greatest speed? And the correct answer is B.

That's the one in which those characteristic absorption lines have moved most towards the red end of the spectrum.

The spectrum of light from an entire galaxy can also be analysed to find out how fast it's moving relative to us.

Observation and analysis show that the light from other galaxies has a much greater redshift than light from stars in our own galaxy.

Here's what we might see from another galaxy.

And we also notice that nearly all galaxies are moving away from us at very high speeds.

But the absorption spectrum of light from the nearby Andromeda Galaxy shows that the frequency of the light has increased rather than decreased.

What does this suggest? So if the frequency of the light has increased, then the wavelength has decreased.

The wave has sort of squashed instead of stretched.

And that would happen for something moving towards us.

So if the source of a wave moves towards you, that decreases the wavelength that you receive and increases the frequency.

The Andromeda Galaxy must be moving quickly towards us.

Well done if you realise that.

And now here's a gap fill exercise for you.

I'd like you to fill in the gaps in these sentences which describe the evidence that galaxies are moving.

Press Pause while you think about this, and press Play when you've finished.

And here are the answers.

Light from the sun can be spread out into a spectrum of light.

It includes some absorption lines because elements and compounds in its outer layers absorb particular frequencies of light.

Other stars produce similar patterns, but the position of the lines change if the star is moving relative to us.

If the star is moving away, the light shows redshift.

The wavelengths of its light increase and its frequencies decrease.

Galaxies also produce absorption spectra.

Most of these show large redshift as they're moving away from us at high speeds.

You may now be wondering why most of the galaxies are moving away from us.

I'm not going to go into that in this lesson, so we'll have to wait for another time.

And now we've reached the end of the lesson, so I'll finish with a summary.

The development of larger telescopes allows dimmer and more distant objects to be seen.

Using space telescopes produces more detail and allows other parts of the electromagnetic spectrum to be observed.

Galaxies are large collections of stars held together by gravitational forces.

Their distances from us can be measured using the properties of variable stars that change brightness over regular periods.

Absorption spectra show redshift when the source is moving away from us.

Almost all galaxies show large redshifts, indicating that they're moving rapidly away from us.

Well done for working through the lesson, and I hope you found it interesting.

I hope to see you again in a future lesson.

Bye for now.