Lesson video

Hello, my name's Dr.

George.

This lesson is called Stellar evolution, and you'll be learning about what happens during the lifetimes of stars as part of the unit Gravity in space.

The outcome of the lesson is: I can describe lifecycles of stars and the variations that depend on the mass of a star.

I'll be introducing these keywords as we go along, but you can come back to this slide anytime if you need to remind yourself of the meanings.

The lesson has three parts.

They're called Main sequence stars, Red giant and white dwarf stars, and Supernova, neutron stars, and black holes.

Now you'll need to remember that stars are powered by nuclear fusion reactions that happen in their cores.

A nuclear fusion is when two small nuclei merge to become a larger nucleus, and that releases energy.

The core of a star is at an extremely high temperature and pressure, and it's that high temperature and pressure that enables fusion to happen.

Energy is transferred from the core to the outer layers of the star by radiation and by convection currents.

The fusion produces gamma radiation and heating, which causes convection currents within the star.

And while a star is still fusing hydrogen, we say it's in its main sequence stage of life.

Within a star, there are two competing forces.

There are gravitational forces pulling matter inwards, because matter attracts matter, and that compresses the core.

But at the same time, there's radiation pressure pushing matter outwards because when there's a lot of radiation being emitted, that actually applies a significant pressure.

Now here's a question for you.

What would happen to the radius of a star if the gravitational forces compressing it were larger than the forces caused by radiation pressure? 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.

The correct answer is the radius of a star would decrease because the gravitational forces act inwards.

And if the inwards forces were greater, that would make the star become smaller.

When a star is in the main sequence part of its life, the overall inwards forces and outward forces are in balance.

And so the star is in a stable state.

It'll stay at a constant size and it'll emit radiation at a steady rate.

The Sun is in its main sequence and has been for about 5 billion years and will be for around 5 billion more years.

A star stays in the main sequence as long as it has enough hydrogen fuel left in its core.

Stars with greater mass have greater pressures and temperatures in the core, and they use up fuel much more rapidly.

So even though they have more hydrogen to begin with, they use it up so much more quickly that they stay in the main sequence for a shorter time.

A star with a similar mass to our Sun is in the main sequence for about 10 billion years.

A star with half the mass of the Sun is in the main sequence for about 100 billion years, 10 times longer.

And a star with 10 times the mass of the Sun would be in the main sequence for only 20 million years.

That may still seem like a long time to us, but it's only about a 5000ths as long as the star with half the mass of the Sun.

Now is this true or false? A star with 50 times the mass of the Sun will be in the main sequence for a much shorter time than the Sun.

And I'd also like you to choose a justification for your answer from A, B, and C.

And the statement is true.

And the reason that explains that is that large stars use up fuel at a far greater rate than small stars.

So even though they have more fuel, they use it up in much less time.

Well done if you picked that answer.

And now a couple of questions for you to write written answers to, so press pause while you do that and press play when you're ready to check your answers.

And here are example answers to the questions.

Question one asked about the Sun, first of all describing the forces acting in a main sequence star.

You could say, in the Sun, gravitational forces act inwards, compressing the core.

Radiation pressure, due to the fusion reactions in the core, pushes outwards.

Overall, these forces are balanced.

And then you were asked to explain why the Sun radiates visible light.

You could say this: nuclear fusion reactions in the core release radiation, which heats the star.

Energy is transferred to the surface layers, and some is emitted as visible light.

So the key thing there is that the emitted energy is coming from nuclear fusion in the core, and the surface layers then emit visible light as a result.

They emit other types of radiation as well, including infrared, gamma rays, and radio waves.

In question two, you were asked to suggest what would happen to the size of a star first of all, if the outward forces were greater than the inward forces, and it would expand.

And if the inward forces were greater than the outward forces, so gravitational forces are stronger than the forces from radiation pressure, the star would contract, it would become smaller.

So well done if you got most or all of those answers right.

Now let's take a look at red giant and white dwarf stars.

And this is about what can happen to a star after the main sequence.

Over time, nuclear fusion in a star's core will change its composition.

It will change the types of nuclei that are there.

The concentration of helium nuclei in the core will gradually increase because fusion produces helium nuclei.

The concentration of hydrogen nuclei will decrease because the hydrogen nuclei are fusing to become helium.

Eventually it will become more difficult for hydrogen nuclei to fuse together as there'll be fewer of them, so they won't collide as frequently.

And this change in the core composition leads to some rather dramatic changes in the star as a whole.

Now a question.

Which of the following statements about the composition of the Sun's core over time are correct? And did you spot that there were two correct statements here? The concentration of hydrogen nuclei is decreasing, and the concentration of helium nuclei is increasing as hydrogen fuses into helium.

The helium produced by the fusion reactions forms a new core at the centre of the star.

As there's not enough hydrogen in this core to sustain fusion, the outwards radiation pressure decreases in the star.

The gravitational forces can now compress the core more than before.

And the increase in pressure caused by the helium core being compressed is enough to start up hydrogen fusion in a layer outside the core.

This larger region releases far more energy than the previous core did, so the outwards radiation pressure in the star increases, and this causes the star to expand greatly.

When the Sun reaches this stage in its lifecycle in about 5 billion years time, it will change dramatically.

Its diameter will increase to around 100 times its current diameter.

And as it expands, it will cool, so its surface temperature will decrease.

As a result, the colour will become much more red.

At this stage, the Sun will have become a red giant star.

Why will the Sun become more red in colour when it expands to become a red giant star? And the correct answer is C, because its surface temperature will be lower.

It's a little bit like if you heat a piece of metal.

If you get it extremely hot, it may glow white, but at lower temperatures it may glow red.

As fusion of hydrogen continues in the star, new helium is still being produced, and this new helium collects in the star's core.

Over time, the increased concentration of helium nuclei and the increased temperature in the core allow helium nuclei to start fusing together to form nuclei of heavier elements.

These new reactions increase the energy produced by the star's core, which increases the outward radiation pressure, causing the star to expand even further.

And because the star has expanded, the outer layers will now be much further from the centre, and that means the gravitational forces acting on the outer layers will be smaller.

And over time, the material of the outer layers will drift off into space.

Eventually, only the core of the star will remain.

This has a much smaller diameter than the original star, but also has a very high temperature.

In the photo, we can actually see a white dwarf at the centre and around it clouds of gas and dust, which are the layers that have escaped from the red giant that this used to be.

We call this a nebula because that's what we call clouds of gas and dust in space.

The full name of these objects is actually planetary nebula, which is a really unhelpful name because this isn't a planet and it's got nothing to do with planets.

It's just that when people first observed these with small telescopes, they thought that these looked like planets.

Now that we can see them in much more detail, we can see that they have a wide variety of shapes and some of them are very beautiful.

This one's called the cat's eye nebula because it looks a little bit like a cat's eye.

Eventually, a star will run out of usable materials to fuse and will stop releasing energy in the core.

The star doesn't suddenly go out, it doesn't just switch off.

It's hot and dense enough to continue to release energy for many more billions of years.

It takes a long time for all of that energy to escape into space.

The white dwarf will gradually become cooler, becoming more and more dim over time until eventually it will reach a stage where it is too cool to emit light.

This small remnant of a star has now become what's called a black dwarf.

Although interestingly, we don't think there are any in the universe yet because it will take so long for a white dwarf to cool into a black dwarf that the universe hasn't been around long enough for the first black dwarfs to form.

And another true or false question for you.

Is this true or false? Once a star runs out of fuel in its core, it will immediately stop emitting radiation.

And again, I'd like you to choose a reason, a justification for your choice of answer.

And this is false.

The star will still have a very high surface temperature, so it will continue to emit radiation for a long time until it's cooled down.

And now I'd like you to have a go at drawing a flowchart showing the stages in the lifecycle of a star that has a similar mass to the Sun.

You can use very simple diagrams for this, and name each of the stages.

And then there's a question for you about black dwarf stars.

Press pause while you answer the questions, and press play when you're ready to check your answers.

And here are the answers.

Your flowchart should be similar to the one below.

It should have the same stages in it.

So we start with a nebula, a cloud of gas and dust, then a protostar in which that gas and dust has come together under gravity, but nuclear fusion hasn't started yet.

Then after nuclear fusion starts, the star is in its main sequence stage.

And at the end of that, it becomes a red giant, then a white dwarf surrounded by a nebula.

And finally, a black dwarf.

And the reason we haven't observed black dwarfs, apart from the fact that they're black and space is also black, is that we think there hasn't been enough time since the beginning of the universe.

It'll take many times the current age of the universe, in fact, for any black draws to form.

So well done if you were able to remember all of the stages.

We're now going to look at what happens to more massive stars towards the end of their lives.

So this section is called Supernova, neutron stars, and black holes.

Any star with a mass similar to the Sun will follow the same lifecycle that we've been describing: nebula, protostar, main sequence, red giant, white dwarf, and finally, black dwarf.

But stars that have much greater mass than the Sun will follow a different path after the main sequence.

After leaving the main sequence, one of these very large stars will form into an extremely large red giant star called a red supergiant.

The diameters of these are far greater than the diameters of red giants.

To give you an idea of it, if the Sun is this size, and the Sun will form a red giant of about this size late in its life, a red supergiant will be about this size.

Red supergiants have multiple layers to their cores, each fusing different lighter nuclei into more massive ones.

These different fusion layers produce many of the lighter natural elements that exist, in a process called nucleosynthesis.

Synthesis means making, so nucleosynthesis means the making of new nuclei.

All of the elements in the periodic table, from hydrogen up to iron, are produced in this way inside red supergiants.

But the nuclei of atoms heavier than iron can't be formed by fusion in red supergiants.

So once a red supergiant has run out of nuclei that are lighter than iron in its core, no more fusion can happen.

So fusion stops, and it stops very quickly.

Once nuclear fusion has stopped, there's not enough outward force to balance the gravitational force acting inwards.

And so the material of the star will be pulled inwards in what's called a gravitational collapse.

Mass falls towards the star's core, which is now made of iron, producing temperatures and pressures far greater than ever before in the star.

So when is a red supergiant star at its highest temperature? And the answer is: after it stops fusing lighter elements into iron after the gravitational collapse.

Supernova explosions can cause fusion of the heavy elements, beyond iron, in the periodic table, producing nuclei of even heavier atoms. And these materials are spread across huge distances by the explosion.

Some of the heavy elements on Earth were produced in supernova explosions that happened before our solar system formed.

So the Sun and planets formed out of a nebula, and that nebula would have contained the remnants from ancient supernovae.

A famous example of a supernova happened about a thousand years ago, and it could be seen without a telescope and was recorded by people in different parts of the world.

The remnants of that supernova, called the Crab Nebula, can still be seen as shown in this photo.

A supernova explosion causes enormous pressure on the iron core of its star, and that pressure is great enough to cause the iron nuclei to collapse into each other.

Those nuclei consist of protons and neutrons, but the protons combine with electrons under this pressure to form neutrons.

Most of the mass of the star collapses into a sphere that's only a few kilometres across, composed of just neutrons.

So what's left is a bit like a giant nucleus, but with neutrons only.

It's called a neutron star.

The density of a neutron star is immense because whereas normal matter is made of atoms which are mostly empty space, a neutron star is made of neutrons tightly packed together.

Just one cubic centimetre of neutron star material would have a mass of around 10 to the power 12 kilogrammes.

That's a trillion kilogrammes, around the same mass as a cubic kilometre of water.

Which of the following statements about neutron stars is correct? And the answer is B.

They're far more dense than normal matter.

They certainly aren't the largest type of star.

They are only a few kilometres across.

If a red supergiant star is large enough, the neutron star it forms has so much mass that it will completely collapse in on itself.

And a black hole forms. Black holes can have masses many times that of the Sun, compressed into a single point, which is called a singularity.

Black holes are so dense that in the region of space close to them, they have an incredibly strong gravitational field.

Even light can't escape from the space near to a black hole.

Black holes don't emit light, so you can't see the black hole itself, but they do pull nearby material into them.

And as that material falls, it emits large amounts of radiation.

This picture is an artist's impression of what a black hole might look like, with material falling into it that emits radiation.

And this radiation can be detected to identify black holes.

Supermassive black holes have been discovered at the heart of galaxies, including our own galaxy, the Milky Way.

These aren't thought to have formed from the death of stars, but by a different process.

A black hole isn't some sort of cosmic vacuum cleaner.

It doesn't suck in everything around it.

Just as objects can orbit our Sun without getting sucked into it, the stars in our galaxy can orbit the black hole at the centre.

Which of the following statements about a black hole are correct? And the answers are C and D.

They can absorb matter from their surroundings, and many galaxies have black holes in their centres.

And now a final written task for you.

I'd like you to draw a flowchart showing the stages in the lifecycle of a star of much greater mass than the Sun.

And for the final stage, there'll be two different possibilities.

And then there's a question for you about the red supergiant Betelgeuse.

Press pause while you write down your answers, and press play when you're ready to check them.

And here's what your answers might look like.

In the flowchart, we start with a nebula, protostar, and then main sequence.

That's all the same as a star the size of our Sun, but then red supergiant, followed by a supernova.

And after that, there's two possibilities, a neutron star, or for the most massive stars, the end could be a black hole.

And then question two asks what you would see if the red supergiant Betelgeuse exploded as a supernova.

You could say this: the star would appear very bright for several weeks, brighter than all of the other stars, except the Sun, combined together.

It would then fade away from sight, ending up invisible as it formed a neutron star or black hole.

And you might also have mentioned that there'll be a spreading nebula, cloud of gas and dust, which we may be able to see with a telescope.

So well done if your answers included many of the same points.

And we've reached the end of the lesson.

So I'll finish with a summary.

All stars form from collapsed nebulae, but their full lifecycle depends on their initial mass.

Stars on the main sequence fuse hydrogen in their cores.

The greater their mass, the shorter the length of time they stay on the main sequence.

Stars on the main sequence are fairly stable in size as the inward gravitational forces are balanced with outwards radiation pressure.

A star similar in mass to the Sun will follow this lifecycle: main sequence, then red giant, then white dwarf, then black dwarf.

And a star of much greater mass than the Sun will follow this lifecycle: main sequence, red supergiant, supernova, and then either neutron star or black hole.

Well done for working through this lesson on stars, and I hope you found it interesting.

I hope to see you again in a future lesson.

Bye for now.