Lesson video

Hello, my name's Dr.

George, and this lesson is called, "Birth of a Star." It's part of the unit, "Gravity in space." The outcome for this lesson is, "I can describe what a star is and how it forms." I'll be using these key words and you can come back to this slide any time if you need to check the meanings.

The lesson has three parts.

It starts with an introduction to stars, and then formation of a proto star, and then a section called, "A star is born." When we look up at the night sky, we see thousands of stars, although of course we see more if we're in a dark area than if we're in the city.

The stars twinkle because light passes through moving air in the Earth's atmosphere to get to us.

The effect is a bit like looking at lights from the bottom of a swimming pool, because of the moving water, the lights would flicker and twinkle.

The stars we can see are the brightest nearby stars, and they're only a tiny fraction of billions of stars that are too faint or too far away to be seen by the naked eye.

All of the stars we can see with the naked eye are part of the Milky Way galaxy, which is a collection of around 300 billion stars, thus 300,000 million stars.

This picture shows a different galaxy as observed by a telescope.

We can't make a picture of the Milky Way from the outside of it because we've never been outside it.

Our galaxy's held together by the force of gravity.

Matter attracts other matter.

And the Milky Way Galaxy rotates around its centre, taking around 240 million years for each turn.

All the stars inside the galaxy are in constant motion.

And now here's a question for you.

Which of the following statements about stars is correct? I'll wait five seconds when I ask a question, but if you need longer, press pause and press play when you have your answer ready.

And the correct answer is B, stars are found outside the solar system and they make up the Milky Way galaxy.

There's only one star inside our solar system, and that's the sun.

The solar system is just the sun and the objects that are orbiting it.

And another question.

Which of the following statements about stars is correct? And the answer is that all stars move through the universe.

We may sometimes picture our sun as being at a fixed point, but in fact, the sun is moving within the Milky Way galaxy, as are all the other stars.

The sun is a medium sized star, and it's 5 billion years old, 5,000 million years old.

And it's just one of those 300 billion stars that make up our galaxy.

There are eight planets including Earth that orbit the sun as it travels around the Milky Way.

Every star is a ball of hot gas and plasma, just like our sun.

Stars are powered by nuclear fusion reactions in their cores, and I'll remind you about what that is later in the lesson.

These reactions release large amounts of energy, which heat up the star.

And hot stars then emit electromagnetic radiation, including visible light, which is why we can see them.

And also infrared radiation, which warms the earth.

The sun and other stars emit other kinds of electromagnetic radiation as well, including radio waves and microwaves.

Some stars are smaller and dimmer than the sun and some are much larger and brighter.

The only reason why the sun looks so different from other stars is it's so much closer to us.

Other stars are much further away.

And some stars are very much larger than the sun.

For example, Betelgeuse is a star in the constellation of Orion.

Its diameter is over 600 times the diameter of the sun.

Here's what they would look like in size next to each other.

It's a cooler star than the sun, which makes it a reddish colour.

But although it's cooler, it's so much bigger that it's about 80,000 times as bright, but it doesn't look that way from here because it's so much further away.

If you're able to spot the constellation of Orion in the sky, you'll be able to see Betelgeuse at the top left corner.

If you're wondering whether there's a connection between name Betelgeuse and the Beetlejuice films, the character Beetlejuice was named after the star, although the spelling is different.

The star's name comes from Arabic, meaning something like, "Hand of the Giant" The giant being The constellation of Orion, which Betelgeuse appears within.

A constellation, by the way, is simply a collection of stars that seem to form a picture on the night sky.

They aren't necessarily connected in any other way and maybe very different distances from Earth.

When we use Greek names for the constellations such as Orion, Cassiopeia and Gemini, we often use Arabic names for the brightest stars including Betelgeuse, Altair and Aldebaran.

Some stars are smaller than the sun.

For example, Procion B is a star in the constellation of Canis minor.

Its diameter is a hundred times less than the diameter of the sun.

Here's a comparison.

But it's a hotter star than the sun, resulting in a very white colour.

It's also about 10,000 times dimmer than our sun, because although it's hotter, it's so much smaller.

Which of the following statements about stars is correct? And the correct answer is A, all stars are approximately spherical.

They're not actually pointy.

And now some written questions for you to answer about the galaxy and about stars.

Press pause while you do this and press play when you're ready to check your answers.

Here are some example answers.

First of all, what is the Milky Way galaxy? The Milky Way Galaxy is a collection of about 300 billion stars held together by gravitational forces.

Question two asked why the Sun looks very different to all of the other stars we can see in the Milky Way.

The sun is much closer than all of the other stars in the Milky Way, and so we can see it as a very bright sphere in the sky.

All of the other stars are so far away that they just look like tiny faint points of light.

What process powers the stars? Stars are powered by nuclear fusion reactions in their cores.

And finally, why does stars emit light? Stars are very hot, and so they emit electromagnetic radiation, which include visible light.

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

And now let's look at the formation of a protostar.

Although stars can last for billions of years, our sun is 5 billion years old, they're not unchanging.

They have what we call a lifecycle that they pass through.

They're not really alive, but we use the same kind of language.

We talk about the birth life and death of a star.

Stars are formed, they have a stable period, and then they start to change before they gradually fade out, or in some cases, explode.

Our sun is about halfway through its stable period of life.

It will begin to change significantly in another 5 billion years or so.

Stars are formed from what we call a, "Nebula." And a nebula is a region of space containing traces of gas and dust.

The photo shows an example, the Orion Nebula.

It's called that because it's found in the same area of the sky as the constellation of Orion.

And it's a region of space where new stars are forming.

Most of the gas in Nebula is hydrogen that was produced at the beginning of the universe, shortly after the Big Bang.

And the dust is very fine particles of material that was formed in older stars.

These stars have since broken apart or exploded and scattered dust throughout the Milky Way.

Which gas in a nebula was mostly formed at the beginning of the universe? And the correct answer is hydrogen.

A nebula can start to collapse if it's disturbed by stars or other objects passing nearby.

And then gravitational forces between the gas and dust particles begin to pull them closer together.

The closer the particles get, the stronger the gravitational forces between them, and so they accelerate towards each other.

They move faster and faster.

Over millions of years, the speed of particles in the nebula increases significantly and the nebula becomes denser.

The particles become more tightly packed.

What collapses in the first stage of star formation? And the correct answer is what's called a nebula, which is usually a cloud of both gas and dust.

As the nebula collapses, the particles of gas and dust continue to get closer and closer together.

The gravitational forces compress the material into a rotating sphere that has much a higher density than the original nebula.

The individual particles are now moving very quickly.

And so this means the temperature of this ball of gas and dust is very high.

The ball of hot dense gas is called a, "Protostar." The pressure and temperature in the core of the protostar are much higher than the temperature and pressure at the surface.

And some of the material from the nebula, instead of ending up in the protostar, forms a disc that rotates around the protostar.

This material may go on to form planets later.

Where is the temperature and pressure in a protostar greatest? And the answer is at its core.

That's where the material is most compressed because of the gravitational forces pulling the matter together.

And now another written task for you.

I'd like you to answer the question, "What is a nebula?" And then draw a series of three or four labelled diagrams that show how a protostar is formed from a nebula, and describe what's happening in each diagram.

Press pause and press play when you're ready to check your answers.

Here are some example answers.

A nebula is a region of space that contains gases and dust.

And here's a possible way to answer the second question.

The diagrams show a nebula, the cloud of glass and dust, the idea that it's collapsing because of gravitational forces and gets smaller.

And then a protostar, a rotating ball of gas.

For the first step, you could say something like, "A nebula is disturbed and starts to collapse due to gravitational forces, attracting the particles of gas and dust together.

And then the particles in the nebula get closer together and move faster and faster, causing a temperature increase and higher pressure." And finally, for the protostar, the nebula material forms a hot rotating ball of gas called a protostar.

It has high temperature and pressure at its centre.

Now let's move on to the third part of the lesson, a star is born.

A protostar isn't yet a star, but the temperature and pressure, as we've seen, are very high at the centre.

And gravitational forces pull material inwards.

The pressure from the hot core causes an outward push.

If the protest star continues to absorb more material, it'll become larger.

The gravitational forces will become larger and that will increase the pressure and temperature of the core even further.

The pressure in a protostar's core can eventually become high enough for nuclear fusion reactions to begin.

Nuclear fusion is when small nuclei begin merging together to form heavier nuclei.

For example, here are two hydrogen isotopes, hydrogen two with a proton and neutron, and hydrogen three with a proton and two neutrons.

They may collide and fuse and form helium four, two protons, two neutrons, and a separate neutron.

This process, when it happens many times, releases large amounts of radiation, heating the centre of the star further, which further increases the rate of the fusion reaction.

Which of the following elements can be produced in a star by the fusion of two hydrogen nuclei? The correct answer is Helium, as shown on the previous slide.

Each hydrogen nucleus has a proton and the new nucleus that's created will have two protons, and that makes it helium.

Gamma radiation is also released when nuclear fusion happens.

So gamma radiation's released in the core, travels through the star and heats up all the material of the star as it's absorbed.

The object's now hot enough to emit large amounts of radiation from its surface, and this includes infrared radiation and visible light.

The protostar has become a star.

The size of the new star will depend on the amount of material gathered from the original nebula.

Some stars are millions of times larger than others.

The new star is in what's called the, "Main sequence" Of its lifecycle.

Most stars are main sequence stars, including our sun, and they stay in their main sequence while they fuse hydrogen in their cores.

During the main sequence part of its life, the star is fairly stable, its size and temperature relatively constant.

This may seem surprising, but the larger a star, the faster it uses up its hydrogen fuel and the less time it spends in the main sequence.

Which of the following stars will remain in the main sequence for the longest time? The answer is the one with the least mass, A, because smaller stars stay on the main sequence longer.

And one final task for you to check what you've learned.

Have a go at answering these three questions.

Press pause while you do and press play when you're ready to see example answers.

So here are ways that you could have answered these questions.

Similarities between a protostar and a star, both the rotating balls of hot gas, their pressure and temperature increase towards the centre.

And differences.

A star has nuclear fusion processes happening in its core releasing energy that allows the star to give out large amounts of radiation from its surface.

If nuclear fusion isn't happening, we'd call that a protostar.

Describing the process of nuclear fusion.

Nuclear fusion is the merging of small nuclei to form larger nuclei with a release of some energy.

And finally, the conditions needed for nuclear fusion to happen in the core of a star.

It requires very high temperatures and pressures, which are the conditions in the core of stars.

So well done if you were able to answer those.

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

The Milky Way galaxy contains billions of stars.

A star is a hot ball of gas and plasma, which gives out radiation.

Stars have a wide range of sizes.

A star is powered by nuclear fusion reactions in its core, which release energy heating the star and allowing it to emit radiation.

Stars have life cycles, typically lasting billions of years.

A Nebula is a region of space containing gases, mostly hydrogen, and dust.

Protostars are formed when a nebula collapses due to gravitational attraction.

A protostar becomes a star if the pressure in its core is high enough to allow nuclear fusion reactions to begin.

Well done for working through this lesson.

I hope you found it interesting and perhaps it's made you want to know, if you don't already, what happens to a star later in its life.

I hope to see you again in a future lesson.

Bye for now.