Beyond the solar system

Chapter overview

3 weeks

Thus far, the learners have only been exposed to solar system astronomy. In this chapter learners will now be introduced to astronomy outside the solar system, which focuses on the studies of galaxies and the Universe.

The main aims of this chapter are to ensure that learners understand the following:

  • The Sun is our closest star, but if it were farther away it would appear just like all the other stars in the sky at night.
  • Stars are arranged in galaxies, held together by the force of gravity.
  • Our own galaxy is called the Milky Way Galaxy.
  • There are billions of other galaxies in the Universe and they come in a variety of shapes and sizes.
  • The distances between stars and galaxies are enormous and so new units of measurement are needed because familiar units like kilometres are too small to be useful.
  • On the largest scale, matter in the Universe is arranged rather like a bath sponge, into thin filamentary structures with large voids between them.

If you have internet access and a projector in your class, an interesting and fun way to introduce what lies beyond our solar system, and beyond the Milky Way, is to use this interactive animation 'Scale of the Universe', where you use a sliding scale to either zoom in or zoom out, available here: Start with the human sized scale and zoom out. For interest, you can also go back to the start and zoom in to get to the microscopic level and even smaller for learners to appreciate the size of atoms.

2.1 The Milky Way Galaxy (2.5 hours)




Activity: Draw the Milky Way

observing, identifying, drawing

CAPS suggested

Activity: Make the Milky Way

observing, identifying, (modelling)

CAPS suggested

2.2 Our nearest star (1 hour)

2.3 Light years, light hours and light minutes (3 hours)




Activity: Travelling fast



Activity: Scale of the solar system

calculating, reading tables, analysing


Activity: Our closest stars

reading tables, analysing


2.4 What is beyond the Milky Way Galaxy? (2,5 hours)




Activity: Comparing galaxies

observing, identifying, describing, ranking


Note: There are two optional, extension activities included in this section. They are:

  • Activity: Wine glass gravitational lens
  • Activity: The expanding Universe

  • How far is our second closest star, Proxima Centauri?
  • What is a galaxy and how many different types of galaxy are there?
  • Where is our Sun located within our own Milky Way Galaxy?
  • How do galaxies arrange themselves on the largest scales in the Universe?
  • How large is the observable Universe and how many galaxies does it contain?

The Milky Way Galaxy

In this section learners will discover that the Sun is one of about 200 billion stars in our home galaxy, the Milky Way. Learners will be introduced to the main features of the Milky Way Galaxy which include its central bulge, flat disk and spiral arms. Students will also learn the Sun's place within the Milky Way: we are not in the centre of our galaxy, but rather are out on the edge of our galaxy, about halfway out from the centre.

Some learners have difficulty in envisioning what they are actually looking at when they see the Milky Way in the sky at night. In fact, every individual star that we see in the sky at night is part of our Milky Way. If the Milky Way were spherical in shape, then we would not see the thin band of the Milky Way across the sky, stars would be more uniformly distributed across the whole sky. However, because the Milky Way is flat, when you look at the band of the Milky Way across the sky at night you are actually looking along the plane of the disk of the galaxy in towards the centre where there is a high density of stars. The density of stars is so high that they cannot be individually distinguished by the naked eye, and so the Milky Way appears as a white band of light across the sky.

  • galaxy
  • galaxy disk
  • galaxy bulge
  • spiral arm

At the darkest places on Earth, far away from city lights, you can see thousands of stars at night using nothing but your eyes. In fact there are many more stars in the sky which are too faint for us to see.

All of the individual stars that you can see are members of our Milky Way Galaxy. A galaxy is a massive collection of stars, gas and dust all held together by gravity. The Milky Way has about 200 billion stars and our Sun is just one of those stars in the Milky Way Galaxy.

From the Earth, the Milky Way looks like a bright hazy band of light across the sky, mixed in with dark dusty patches. This was called Galaxies Kuklos by the Greeks which means the Milky Circle because they thought it looked like milk spilled across the sky. The Romans changed the name to Via Lactea which means the Milky Road or the Milky Way.

Time lapse video of the Milky Way.

The Milky Way stretching across the sky viewed from Sutherland. The dark shape of the SALT telescope can be seen in the foreground with the night sky in the background (SAAO)

If you could travel outside the Milky Way and look down on it from above, the galaxy would look like a giant spiral in space as shown in the following image.

This is what the Milky Way would look like if you could see it from far away in space. Scientists only know this from many observations made from Earth. No one has actually been that far away from our galaxy to look at it. The structure is what we have inferred from other observations.

Discover more online and read about missions beyond our solar system.

The image shows what scientists think our galaxy looks like. You can see the spiral arms of our Milky Way. These are bluish in colour and are filled with dust and gas and hot young stars. The thin dark wisps in the image are dust lanes, regions where the gas is very dusty. The central part of the galaxy is more orangey in colour than the spiral arms. This is because the stars found at the centre of the galaxy tend to be older and cooler than the young hot blue stars.

Scientists think that there are five major spiral arms in our galaxy. These are the Norma Arm, the Scutum-Crux Arm, the Sagittarius Arm, the Perseus Arm and the Cygnus Arm.

Some of the arms have alternative names, a table is included here for reference in case other names are listed in books or online.

Common Name

Alternative Name

Norma Arm

3 kiloparsec Arm

Scutum-Crux Arm

Centaurus Arm

Sagittarius Arm

Sagittarius-Carina Arm

Orion Arm

Local Arm

Perseus Arm


Cygnus Arm

Outer Arm

Our Sun is located in a small spiral arm called the Orion (or Local) Arm which lies between the Sagittarius Arm and the Perseus Arm. Our Sun is about halfway out from the centre of the galaxy.

Our Solar System is orbiting around the centre of the Milky Way at thousands of kilometres per hour. But even at that speed, it still takes over 200 million years for us to make one complete orbit around the Milky Way Galaxy.

All the stars in this galaxy are revolving around the centre of the galaxy. Just as the Earth travels around the Sun, the Sun and our entire solar system is traveling around the centre of the Milky Way Galaxy at a speed of 250 km/s. Even though we are travelling incredibly fast, it takes the Sun about 225 million years to complete one orbit around the galaxy centre. The Milky Way is truly massive, measuring a staggering 950 000 000 000 000 000 km across!

If learners are familiar with scientific notation, then the above diameter of the Milky Way can be written as 9.5 x 1017 km.

If you could shrink the solar system so that the distance from the Sun to Pluto is 2.5 cm, the Milky Way would have a diameter of 2000 km (about the distance from Durban to Windhoek!)

To us the Earth seems big, but the Earth is only a very small part of the Solar System. And our Solar System is a very small part of the Milky Way Galaxy. And our galaxy is only a very small part of the whole Universe.

The Sun's position in the Milky Way.

If, instead of looking down on the Milky Way Galaxy, you looked at it from one side you would see that the Galaxy looks like this:

Looking at the Milky Way from the side.

The Milky Way is shaped like a giant fried egg. It is about a hundred times wider than it is thick, and it bulges in the middle. The central lump is called the bulge and the rest of the galaxy outside the bulge is called the disk.

As you know, we are inside the Milky Way Galaxy. So when you look at the thin milky-looking band stretching across the sky at night, what do you think you are actually looking at?

The thin band of light that you see is actually the stars in the Sagittarius arm as you look inwards towards the centre of the galaxy. There are so many stars densely packed together that you cannot make out individual stars with your eyes. Therefore you just see a haze of light. Above and below the plane of the disk there are very few stars.

If you look closely at the image of the Milky Way above, you can see several round fuzzy blobs dotted about above and below the disk. These are called globular clusters and are vast collections of hundreds of thousands of ancient stars tightly packed together by gravity. The Milky Way has an estimated 160 globular clusters. The oldest stars in the galaxy are found in these globular clusters, some are almost as old as the Universe itself.

Why is it dark at night?

A globular cluster called M80. The stars in this globular cluster are around 12.5 billion years old. Our Sun is a mere 4.5 billion years old.

Draw the Milky Way

The aim of this activity is to reinforce the idea that the Milky Way Galaxy is a spiral galaxy with five major spiral arms in addition to some smaller arms. Learners will also be reminded that the Sun and Earth are not at the centre of the galaxy, but rather about half way out along a minor arm called the Orion Arm.


  • black paper
  • white crayon, pencil or paint
  • glue - optional
  • glitter or sand - optional
  • newspaper for working on
  • white or silver pencil/pen for labelling
  • sticker - optional


  1. Draw or paint a picture of the Milky Way. You can use the picture in the text above as a guide. The galaxy has five major spiral arms, and some smaller ones including our Orion Arm. The galaxy also has a bulge in the middle.
  2. If you are going to use glitter or sand, glue along your spiral arms and in the central bulge.
  3. Scatter glitter or sand over the picture, each grain represents a star in our Milky Way.
  4. Tilt the picture onto the newspaper to remove any excess glitter.
  5. Label each of the major arms of the Milky Way Galaxy.
  6. On the Orion Arm place a sticker or mark a point halfway out from the galaxy centre. This marks the position of the Sun.

The sound of interstellar space.

How do you think astronomers know what the Milky Way looks like from the outside when they have never been outside the Milky Way? The task is similar to trying to figure out the shape of a forest from outside when you are in the middle of the forest. How would you go about this?

Learner-dependent answer. Ask learners to explain their answers. A typical response could be that we count the number of stars we see in each direction.

Astronomers look at the sky in all directions and count the number of stars that they see, they also measure the distance to each of the stars so that they can build up a three dimensional map of the galaxy. One of the difficulties that astronomers have in doing this is seeing through all the dust in the galaxy which dims the optical light coming from the stars.

Video showing us zooming out from the Earth to outside our galaxy.

Make the Milky Way

In this activity learners will make a model of the Milky Way. They must come up with the best materials they can think of and obtain for their models. For example, they can use cardboard, cotton wool balls and glitter. This can be done as a group model, where learners are given the task a couple days before the lesson and they must collect the materials, or else you can supply a selection of materials in class which they can then use to build the model. Encourage learners to be creative when thinking about the materials to use to represent the different components.

The aim of this activity is to give learners a three dimensional view of the Milky Way, including the structure of the central bulge and the disk containing the spiral arms. The glitter is used to represent the distribution of stars and the colours are used to demonstrate how old and young stars are distributed in the galaxy. The life cycle of stars in not covered until Grade 9. Therefore, although you may want to mention that the stellar populations in the bulge and the disk of our galaxy are different, it is not essential to do so.


  • thick piece of black cardboard at least 30 cm across
  • other materials for your model, either collected by you or supplied by your teacher

Examples of other materials to supply are:

  • a bag of cotton balls or pillow stuffing
  • glue
  • string
  • pencil
  • red, blue, gold and silver glitter
  • star sticker

We will learn more about the life cycle of stars in Gr 9. Younger stars are hotter and bright white or blue in colour, while older stars are cooler and more yellow and red in colour.


  1. You need to build a 3 dimensional model of the Milky Way Galaxy. You will either need to collect the most appropriate materials for your model beforehand, or else your teacher will supply you with a selection of materials to use in class.
  2. Cut out a circle of radius 15 cm from the black card and use this to build your 3D model.
  3. You must show the central bulge, the spiral arms and the different coloured stars.
  4. Mark the position of our Sun on your model.
  5. Using your model, view it from different angles and compare the view you have with the images of the Milky Way in this chapter.

Learners must come up with their own model designs. An example design is included here if you would prefer to make one which you then use to demonstrate to learners, instead of them making their own:

  1. Build a dome of cotton balls in the centre of one side of the cardboard. Use glue to keep the cotton balls in place. The dome should be about 8 cm across and 4cm high.
  2. Repeat on other side of the board. The cotton ball dome represents the bulge of our galaxy.
  3. Pull the outer cotton balls into six spirals around the cotton ball dome. These represent the five major spiral arms found in the disk of our galaxy, in addition to the minor spiral arm that our Sun is found in.
  4. Dribble glue on the spiral arms and sprinkle blue and silver glitter on the glue. These represent hot newly forming stars.
  5. Dribble glue all over the cotton wool dome ball in the middle and sprinkle this glue with gold and red glitter to represent cooler, older stars.
  6. Mark a position 8 cm from the centre inside one of the spiral arms.
  7. Stick the star sticker on the spiral arm at the marked position. This marks the position of our Sun.
  8. Make a hole in the centre of the model and thread it with a string so that it can be hung up.


What are the two main parts that make up our Milky Way Galaxy?

The disk and the bulge.

Where are the spiral arms located; in the disk or the bulge of our galaxy?

In the disk.

Is our Sun found in the central bulge or in a spiral arm in the disk?

Our Sun is located in a spiral arm.

How far from the centre of the galaxy is our Sun located?

Just over half way out from the centre.

Our nearest star

In this brief section learners will be introduced to the large distances found between stars in preparation for the following section on light hours, minutes and seconds.

  • Proxima Centauri
  • Alpha Centauri
  • constellation

The Sun is our closest star, and is only 150 million kilometres from Earth. When you look up at the sky at night, if you are lucky enough to be far from the glare of city lights, you can see thousands of stars. For those of you in a city, perhaps you can see hundreds of stars, depending on the amount of light pollution from street lights and other light sources. As you know, there are actually billions of stars in our galaxy but most of them are too faint to see from Earth.

A constellation is a group of stars that, when viewed from Earth, form a pattern in the sky.

Of the brighter stars, one famous constellation that is visible, even from big cities in South Africa, is the Southern Cross or Crux. The two bright stars at the bottom left pointing towards the cross are called the pointers.

You can find south using the Southern Cross Constellation. Just extend the long axis of the cross 4 times and then go straight down to the horizon to find south.

The Pointers (circled) and the Southern Cross.

The brightest of the Pointers looks slightly orange if you look closely. This star is called Alpha Centauri and is our closest easily visible star after the Sun. Alpha Centauri is actually part of a triple star system which is where three stars are in orbit around each other. The two main stars of the system are called Alpha Centauri A and Alpha Centauri B. They orbit close together, on average about eleven times the Earth-Sun distance from each other.

A smaller, fainter star, called Proxima Centauri, orbits much farther out. If you were to look at Alpha Centauri through a small telescope, instead of one star you would be able to make out the two separate stars Alpha Centauri A and B next to each other. Proxima Centauri is much fainter and further away from the other two so you would not see this one with the other two.

A comparison of the sizes of the Alpha Centauri star system and the Sun.

Proxima Centauri, the closest star to our own Sun, is about 40 trillion km away from the Earth. Alpha Centauri A and B are slightly farther away, at 42 trillion km away from us. Our closest star is 694 times farther away than Pluto is. These numbers are astronomically large! As the numbers are so large, astronomers do not use kilometres to measure the distances to stars, but use larger units based on the speed of light, which you will discover in the next section of this chapter.

Do you know how much a trillion or a billion is? Have a look at the following table

In words

In number format

one thousand

1 000

one million

1 000 000

one billion

1 000 000 000

one trillion

1 000 000 000

Proxima Centauri was discovered in 1915 by the Scottish astronomer Robert Innes. He was the director of what was then the Union Observatory in South Africa.

Astronomers have recently discovered a planet similar in size to the Earth orbiting around Alpha Centauri B, but we think it is too close to the star to have life on it.

The following is an optional, extension activity that you can do on scientific notation with your learners. Scientific notation is only covered in Gr 9 Mathematics, however. many of the numbers used in this chapter are very long, and so can be written in scientific notation. Also, if you do some of the subsequent activities doing calculations with a calculator, the answers will be given in scientific notation. It is therefore useful for learners to know what this is. You can use the following activity to explain scientific notation to learners and write some of the examples given in the tables on the board as examples.

Activity: Scientific notation

In science one often needs to work with very large or very small numbers. For example, we spoke about the distance from Earth to our next closest star after the Sun as being 40 trillion km. How much is a trillion?

Look at the following table:

In words

In number format

In scientific notation

one thousand

1 000

1,0 x 103

one million

1 000 000

1,0 x 106

one billion

1 000 000 000

1,0 x 109

one trillion

1 000 000 000 000

1,0 x 1012

Therefore, the distance from Earth to Proxima Centauri is 40 000 000 000 000 km. This is a very large number to work with.

Very large and very small numbers can be written more easily (and more compactly) in scientific notation, in the general form:

N x 10n

N is a decimal number between 0 and 10 that is rounded off to a few decimal places. n is known as the exponent and is an integer.

If n is bigger than 0 it represents how many times the decimal place in N should be moved to the right. If n is smaller than 0, then it represents how many times the decimal place in N should be moved to the left.

For example, 3,24 x 103 represents 3240 (the decimal moved three places to the right) and 3,24 x 10-3 represents 0,00324 (the decimal moved three places to the left).

If we wanted to write the distance from the Earth to Proxima Centauri in scientific notation, we need to count how many times the decimal comma must move so that N is a number between 0 and 10. It must move 13 times. Therefore 40 000 000 000 000 km can be written as 4,0 x 1013 km.

Look at the following examples.

Standard number

Scientific notation


2 x 102

1 500

1,5 x 103

67 890

6,789 x 104

48 000 210

4,8 x 107


2 x 10-2

Light years, light hours and light minutes

In this section, learners will be introduced to the concept of light years, light hours and light minutes. These units of distance are used for interstellar (between stars) and interplanetary (between planets) distances because the distances involved are huge and familiar units like metres and kilometres are just too small.

Because of the references to time in each of these distance units, learners can often mistake these units as units of time rather than units of distance. It is important to address this misconception. For example, a light hour is the distance that light travels in one hour of time. Although time is involved the final measurement is actually a distance.

A useful activity to introduce the topic is to ask learners how far they estimate they could walk, run and cycle in one hour. Although they have to use time in their estimation they should understand that they are estimating a distance. This example also includes the concept of speed. Learners should understand that if they move faster they will travel further in a given hour. Starting off by using activities that they are familiar with should prove useful when then going on to deal with the rather abstract concept of the speed of light.

This section is fairly mathematical and learners will need a calculator to complete the activities. It is useful (although not essential) if learners understand scientific notation. Learners need to understand what a million, billion and trillion correspond to and so if in doubt it might prove useful to remind learners of the powers of ten involved for millions, billions and trillions. Formulae for calculations have been provided where necessary, and it is expected that most learners will be familiar with the formula speed = distance / time. If learners are unfamiliar with this concept it would be a useful exercise to explain this before starting on the exercises in this section.

  • light minute
  • light hour
  • light year

Our solar system is a pretty big place. Our nearest neighbour, the Moon, is on average 384 400 kilometres away, and the closest to us that our nearest planet Venus gets is about 42 million kilometres. The Sun is about 150 million kilometres away and the closest that Pluto can ever get to us is 4.3 billion kilometres. These large numbers are impractical to use and so we rather use much larger distance units based on the speed of light. This makes the numbers smaller and easier to deal with.

This is just like using metres instead of centimetres to make the numbers smaller when you measure a distance. For example, if you are telling a friend how far it is from your house to school, you would say it is 7.5 km, and not 7 500 000 cm. Let's begin by comparing the speed of light with the speed of some other things that move very fast.

Scale of the Universe.

Travelling fast

A cheetah, the fastest land mammal, can reach speeds of 120 km/h, as fast as cars on the highway.
A Peregrine Falcon, the fastest animal, can fly as fast as 389 km/h.
Japan's high speed train the JR-Maglev MLX01 has reached 581 km/h.
NASA's scramjet the X-43 flies at 7000 km/h.
The international space station (ISS) orbits the Earth at a speed of 27 744 km/h.

What about light? Light travels at about 1080 million km/h, or 299 792 458 m/s.

How to break the speed of light.


  1. Imagine you are going on a trip from Cape Town to Durban, which is a distance of 1753 km.
  2. Calculate how long it would take you to complete the trip travelling at the speeds of the animals and modes of transport in the examples above.
  3. Fill in your answers in the table below.

Remember the formula time = distance / speed

Mode of transport

Speed (km/h)

Distance between Cape Town and Durban (km)

Time taken for the journey




14.6 hours

peregrine falcon


________ hours

high speed train


________ hours

NASA's scramjet


________ minutes

International space station


________ seconds



________ seconds

Mode of transport

Speed (km/h)

Distance between Cape Town and Durban (km)

Time taken for the journey




14.6 hours

peregrine falcon



4.5 hours

high speed train



3.0 hours

NASA's scramjet



15 minutes

International space station

27 744


3.8 minutes


1079 252 850


0.006 seconds

Light is amazingly fast. Look at the examples below.

In one second light can travel…

Light takes…

between Cape Town and Johannesburg 214 times.

0.0000003 seconds to travel 100 m.

between Cape Town and London, England, 31 times.

1.3 seconds to travel from the Earth to the Moon.

around the Earth 7.5 times.

8 minutes to travel from the Sun to the Earth.

How far is a second?

For distances within the solar system, astronomers use units called light hours and light minutes.

A light hour is the distance that light travels in one hour. Despite its name, a light hour is not a unit of time, it is a unit of distance.

What do you think a light minute corresponds to?

It is the distance that light travels in one minute.

Which do you think is a smaller distance, a light hour or a light minute, and why?

A light minute is smaller because the light has less time to travel in a minute than an hour. So a light minute must be shorter because this represents the distance that light travels in a minute.

Astronomers use units called light years to measure the distances between stars and galaxies. One light year is almost 10 trillion kilometres. As you can see, a light year is very, very far.

Light years, light hours and light minutes measure distances. They also tell us something else very interesting. If you measure the distance to a light source in light travel time, you can work out how long light emitted from the distant source takes to reach you. Light that is emitted from an object one light year away from you, takes one year to reach your eyes. Similarly, light that is emitted from an object one light hour away, takes one hour to reach your eyes. How long do you think light emitted from one light minute away takes to reach your eyes?

One minute.

How far is a light year?

This may sound very strange to you because when you switch on a lamp in your home you see the light straight away. You do not have to wait for the light from the lamp to reach you. You do not notice that it actually takes some time for the light from the lamp to reach your eyes because light travels extremely fast. Light travels so fast, that if you were standing a metre away from the lamp it would only take only three billionths of a second for the light from the lamp to reach your eyes. It is therefore no surprise that you don't notice the delay.

Scale of the Universe interactive animation.

The speed of light is special, nothing can move faster than the speed of light, it is like a cosmic speed limit.

Scale of the solar system

Question 7 in the activity is an advanced question for able learners.


  1. The table below shows the distance that each planet lies from the Sun in kilometres (km) and then in light hours or light minutes.
  2. Answer the questions.

Distances of each planet from the Sun.


Distance from the Sun (million km)

Distance from the Sun in light hours or minutes



3.2 light minutes



6.0 light minutes



8.3 light minutes



12.7 light minutes



43.3 light minutes



1.3 light hours



2.7 light hours



4.2 light hours


How far away from the Sun is Earth?

8.32 light minutes.

How long does light take to travel from the Sun to the Earth?

8.32 minutes.

What does the answer to (2) imply about our view of the Sun?

We see the Sun as it was 8.32 minutes ago.

How many times further away from the Sun than the Earth is Neptune?

30 times further. This is calculated by dividing the distance from the Sun to Neptune by the distance from the Sun to Earth: 4495/150 = 30.

How far away from the Sun is Neptune in light hours?

4.17 light hours.

How long does light from the Sun take to reach Neptune?

4.17 hours.

Imagine you have a cousin living on Neptune. You and your cousin both decide to look at the Sun, each of you using a telescope with a special solar filter so as not to damage your eyes. As you are watching the Sun you suddenly notice a big blob of gas thrown off in a massive solar flare. You cousin says she cannot see it. Why is that?

If you see the flare happen from Earth, then the flare happened 8 minutes ago. The light from the Sun showing the flare takes 4.2 hours to reach Neptune (about 4 hours 24 minutes), so your cousin will only see the flare in 4 hours 16 minutes time.

As you can see, the solar system is very large. The orbit of Neptune is over 4 light hours from the Sun and the Kuiper Belt and Oort Cloud extend out even further than this.

The distance to the next closest star, Proxima Centauri, is 40 trillion km. This corresponds to 4.24 light years. This means that light from the star takes just over four years to reach Earth. Let's investigate the distances to some of our closest stars.

Our closest stars

In this activity learners will get a feel for how "close" the nearest stars are to the Sun. The idea of this activity is to familarise learners with the idea that stellar distances are generally measured in light years (rather than light minutes or hours which apply to solar system objects).


  1. Look at the table showing our closest stars and the star map.
  2. Answer the questions below.


Distance (light years)

Proxima Centauri


Alpha Centauri


Barnard's Star


WISE 1049-5319


Wolf 359


Lalande 21185




The following map shows the Sun in the centre with the locations of our closest stars. Each solid ring represents a distance of 2, 4, 6 and 8 light years from the Sun. The dotted circle represents the Oort Cloud.

The star map is shown in two dimensions, on a flat plane. Remember that the stars are located in 3 dimensions in space.

Always remember to write the units after your answer.


Which star is our closest neighbour, excluding the Sun?

Proxima Centauri.

How far is Sirius?

Sirius is 8.58 light years away.

How long does light from Barnard's Star take to reach us?

Light takes 5.96 years to reach us from Barnard's star.

Explain in your own words what the statement "Sirius is 8.58 light years away from Earth" means.

It means that the star is at the distance that light can travel in 8.58 years. It means that light takes 8.58 years to reach us on Earth from Sirius.

The Milky Way is so large that light takes 100 000 years to cross from one side to the other side.

Our closest stars are less than ten light years away, however most stars in our galaxy are much farther away. The distances to stars are generally measured in tens, hundreds or even thousands of light years and the distances between galaxies are truly enormous as you will discover in the next section.

What is beyond the Milky Way Galaxy?

In this section learners will find out what lies beyond our own galaxy. They will learn that there are billions of other galaxies in our Universe of all shapes and sizes. They will learn about the different types of galaxies, i.e. ellipticals, spirals, barred spirals, lenticular and irregular types. Learners do not have to know the actual names of the different shapes (this is included for interest), but they must know the shape of the Milky Way Galaxy and understand that other galaxies have different shapes. The will also look at how galaxies are arranged in the Universe: into groups and clusters of galaxies, and finally they will look at the Universe on its grandest scale finding out how matter is arranged into voids and filaments.

  • galaxy
  • galaxy group
  • galaxy cluster
  • filament
  • void
  • Universe

Our galaxy, the Milky Way, is only one out of a total of about 100 to 200 billion galaxies that astronomers estimate to be in the Universe. That's more than 10 times the total number of people on Earth.

As well as stars, galaxies contain vast amounts of gas and dust. Galaxies come in a variety of shapes and sizes. The Milky Way is an average-sized spiral galaxy: it is 100 000 light years across and contains around 200 billion stars. Small galaxies may contain only a few million stars, while large galaxies can have several trillion stars.

The distances between galaxies are even larger than the sizes of galaxies and are measured in millions or even billions of light years.

Our closest galaxy neighbour is called the Andromeda Galaxy. Andromeda is 2.5 million light years away from the Milky Way. If you wanted to travel to Andromeda and could travel as fast as light, it would still take you 2.5 million years to get there.

Our closest neighbouring galaxy, Andromeda. Light from the galaxy takes 2.5 million years to reach Earth and so the light that hits your eyes now from that galaxy was emitted before there were humans on Earth.

The Milky Way and Andromeda galaxies are moving towards each other on a collision course and astronomers estimate that the two galaxies will collide in around 4 billion years time. No need to worry just yet!

This illustration shows a stage in the predicted collision between our Milky Way Galaxy and the neighboring Andromeda Galaxy, as it will unfold over the next several billion years. This image shows how we think Earth's night sky will look like in 3.75 billion years time.

What is the Universe?

How big is the Universe?

There are five main types of galaxies. You do not need to know these names. This is included for your interest.

  • spiral
  • barred spiral
  • elliptical
  • lenticular
  • irregular

Extra information on the different shapes of galaxies:

  • Spiral galaxies have a central bulge and a flat disk with spiral arms.
  • Some spiral galaxies have arms that do not start at the centre of the galaxy but start at the end of a bright straight bar that goes across the centre of the galaxy. These are called barred spiral galaxies.

  • Elliptical galaxies look smooth and are shaped like giant rugby balls with no spiral arms. Some can be round and some can be very elongated. They contain old stars and have very little gas and dust.
  • A lenticular galaxy is in between a spiral galaxy and an elliptical galaxy. They are disk galaxies (like spiral galaxies) but do not have defined arms as they have lost most of their dust and gas. As a result, there is little star formation happening and they consist of mostly old stars (like elliptical galaxies.)
  • Irregular galaxies do not look like spirals or elliptical galaxies. Some (but not all) irregular galaxies are actually two or more galaxies in the process of colliding.

The largest known galaxy in the Universe.

Spiral galaxy named NGC 4414.
Barred spiral galaxy named NGC 1300.
Elliptical galaxy NGC 1132.
A lenticular galaxy, called NGC 5866.
Irregular galaxy named NGC 1427A.

Let's do an activity to explore the different types of galaxies we see.

Galaxy Zoo - take part in some real astronomical research by classifying the shapes of different galaxies in this citizen science project.

There is a really relevant link provided in the Visit box for the citizen science project, Galaxy Zoo. This is a really great way for you and learners to become actively involved in some real science research related to what you are doing in class. If you have internet access and a projector in your class, a suggestion is to bring this site up and go through some of the galaxies with your learners and classify them according to their shapes. Find out more about incorporating real science into your classroom with Zooniverse citizen science projects at the ZooTeach website: Citizen science offers you a free, easily accessible and inspiring opportunity to bring real science into the classroom.

ZooTeach is a website where teachers and educators can share high quality lesson plans and resources that complement the Zooniverse citizen science projects.

Something fun - have a look at this picture of a cheetah created using thousands of images of galaxies from Galaxy Zoo.

Comparing galaxies

This is an optional, extension activity. In this activity learners will describe and compare the appearance of six different galaxies. They will also rank the galaxies in terms of increasing distance from Earth.


  • images of the galaxies to be compared


  1. Look at the images of the of six galaxies used in this activity.
  2. Using the information in this chapter, write down in the table what type of galaxy our Milky Way Galaxy is.
  3. Write down in the table below what type of galaxy (spiral, barred spiral, elliptical or irregular) you think each galaxy is.

Galaxy Name

Galaxy type

The Milky Way Galaxy.

Galaxy M 89. The galaxy is 60 million light years away.

Galaxy NGC 4622. The galaxy is 111 million light years away.

The Large Magellanic Cloud galaxy. This satellite galaxy of our own Milky Way is only 163 000 light years away.

The Spindle Galaxy, 44 million light years away.

Galaxy Name

Galaxy type and reason

Milky Way Galaxy.

Barred spiral galaxy (because it has spiral arms with a bright, central bar)

Galaxy M 89, 60 million light years away.

Elliptical galaxy (because it is round and smooth with no spiral arms)

Galaxy NGC 4622, 111 million light years away.

Spiral galaxy (because it has spiral arms)

The Large Magellanic Cloud galaxy. This satellite galaxy of our own Milky Way is only 163 000 light years away.

Irregular galaxy (it does not have spiral arms and is not a smooth oval shape like elliptical galaxies. It looks like an irregular shape)

The Spindle Galaxy, 44 million light years away.

Lenticular galaxy (because disk shaped, with a central bulge, but no spiral arms)


List the galaxies in the table above in increasing order of distance from our Milky Way Galaxy.

The LMC, the Spindle Galaxy, M 89, NGC 4622.

Have a look at the following diagram which shows the location of Earth in the Universe. You do not need to know this classification; this is included for your interest.

  • Most galaxies are found gathered together in gigantic galaxy neighbourhoods, called galaxy groups. Our Milky Way is found in a group of galaxies called The Local Group.

  • Galaxy clusters are even larger, spanning tens of millions of light years, and can contain hundreds or even thousands of galaxies.

  • Many clusters of galaxies come together to form superclusters of galaxies. Our own local group is part of the Virgo supercluster.

  • Gravity holds the galaxies in groups, clusters and superclusters together.

What is dark matter?

The Hubble Extreme Deep Field is the most distant picture of the Universe ever taken. Astronomers used the Hubble Telescope to take an image of a small patch of sky. Around 5500 galaxies of all shapes, sizes and colours were discovered in the image.

Galaxies in the Hubble Extreme Deep Field. Every smudge in the image is a distant galaxy.

How do we know how many galaxies there are in the Universe?

Extension content and activity

Galaxy clusters are beautiful yet peculiar objects. They seem to be full of a mysterious unseen type of matter which has not yet been identified. From its gravitational effects on the gas and galaxies in the cluster, astronomers estimate that this strange matter could be about five times more massive than all the galaxies and hot gas in a cluster combined. Astronomers have no idea what this mysterious matter is and call it dark matter, because they cannot see it. It turns out that this strange matter is not only found in clusters of galaxies, but is spread throughout space.

The galaxy cluster called Abell 2218. Each point of light is a galaxy.

If you look closely at the image of galaxy cluster Abell 2218, in addition to the galaxies that make up the cluster you can see thin arcs. These are images of distant galaxies behind the cluster that are distorted by matter in the cluster. The cluster of galaxies acts like a giant lens, bending and distorting the light coming from the more distant galaxies. The distant galaxies are not actually this funny shape, they are usually elliptical or spiral shaped. They just appear this way because of the lensing.

Matter bends light, just like a lens does, although the effect is much weaker, otherwise our torches would have bent light beams. When matter acts to bend light astronomers refer to the matter as a gravitational lens. Clusters of galaxies make excellent gravitational lenses because they are so massive. Most of the lensing however does not come from the galaxies or the hot gas in the cluster, but from the unseen dark matter within the cluster.

Activity: Wine glass gravitational lens

Note: This activity can be done as an extension if you decide to discuss the above content on dark matter with learners. However, this is beyond the scope of CAPS and has only been included as an optional extension. This activity can be done individually, but if there are not enough wine glasses for the entire class then learners can work in small groups and take it in turns within their group to complete this activity. It can sometimes be a bit difficult to see the rings and arcs clearly. To aid this, use a bright red pen rather than a black pen, and it may help if learners close one eye and just use one eye to observe the arcs and rings produced in this activity.

In this activity you will investigate how a wine glass acts like a lens, bending light. Dark matter in the Universe also acts like a lens, bending the light from distant galaxies making their images distorted into rings or arcs. While the wine glass bends the light due to refraction, dark matter bends the light because it has mass, and is called a gravitational lens.


  • wine glass
  • paper (graph paper if possible)
  • red pen/marker
  • water


  1. Make a large dot on the graph paper using the red marker.
  2. Place the wine glass on the graph paper and look directly down at the paper through the wine glass. Observe how it distorts the grid of the graph paper.
  3. Centre the wine glass over the dot and look directly down at the paper through the wine glass. Make a note of your observations below.
  4. Move the wine glass from side to side and up and down along the paper slightly and note what you observe.
  5. Repeat steps 3 to 4, but this time raise the glass above the paper by about 3 cm. Note what you observe.
  6. Add some water to the wine glass and repeat step 5. Note what you observe.

Note: If the dot is centred below the wine glass, learners should view a ring. If it is not centred, they should see arcs.

Ask learners the following questions:

  1. When the wine glass was centred above the dot what did you observe?

    Red ring

  2. When the wine glass was not centred what did you observe?

    Red arcs

  3. If you moved the wine glass left and right, what happened?

    The arcs move. If you move the glass to the left, the arc is on the right hand side and if you move the glass to the right, the arc appears on the left hand side.

  4. Given your observations here, what can you say about the orientation of the galaxy cluster Abell 2218, shown in the last image? Is it in line with the distant galaxies or offset slightly? (Hint: do you see arcs or a ring?)

    In the picture above you can see faint arcs in the image. This means that the gravitational lens and the background galaxies cannot be lined up perfectly, otherwise you would see a ring.

The Observable Universe

How big is the Universe?

Learners do not need to know the structure of the Universe in terms of filaments and voids. This is included as enrichment content. Learners do need to know what we mean by the observable Universe though.

There is an error in the CAPS document which incorrectly states that the size of the observable Universe is 28 billion light years. In fact the size of the observable Universe is about 93 billion light years which corresponds to 28 billion parsecs - a parsec is a unit of distance used in astronomy and is equal to about 3.1x1013 km or about 3.3 light years.

Note that the observable Universe, is the region that is visible from Earth it is not the whole of the Universe. The size of the whole Universe is unknown and it may be infinite in size. "Infinite in size" is a difficult concept for most learners to grasp and so it has been deliberately omitted from this text. You should use your own judgement as to whether it is suitable to consider elaborating upon the size of the unobservable Universe within your class.

On the largest scales the Universe resembles a giant bath sponge. The galaxy clusters are arranged in thin walls called filaments. Between the filaments are huge gaps which contain very few galaxies and so are called voids.

This computer generated graphic represents a slice of the sponge-like structure of the Universe. All the galaxies lie along thin walls called filaments. The darker areas show the voids where there are no galaxies.

Astronomers estimate that the age of the Universe is 13.7 billion years old. This might make you imagine that you can see objects from as far as 13.7 billion light years away in all directions. If you were to draw a sphere around the Earth, with a radius of 13.7 billion light years, with the Earth placed at the centre, the surface of the sphere would represent the limit of how far light could travel to Earth in 13.7 billion years. The surface would represent the edge of the observable Universe as seen from Earth. You might therefore assume that the diameter of the observable Universe is 27.4 billion light years (2 times 13.7).

The size of the Universe.

However, you would actually be wrong. Astronomers estimate the size of the observable Universe to be 93 billion light years in diameter, which is much, much larger. The reason that the size is much larger than expected is because the Universe is expanding and galaxies are moving further and further away from the Earth as the space between them expands. So we are able to see galaxies that are now very far away because when they emitted their light they were closer to Earth. The size of the whole Universe, which includes regions too far from Earth for us to see at this time, is unknown.

Do we expand with the Universe?

Following is a demonstration that you can perform to show learners what is meant by the expanding Universe.

Activity: The expanding Universe

Note: This is a demonstration to help learners visualize how the space between galaxies is expanding. This is a simple 2D analogy of the true 3D situation. In this demonstration the surface of the balloon is a two dimensional representation of space and circles on the surface of the balloon represent galaxies in space. As the balloon is blown up, representing the expanding Universe, the distances between neighbouring galaxies increase which is exactly what is observed in the expanding Universe.


  • one balloon
  • small circles of paper
  • glue


  1. Cut out small circles of paper and stick them onto the balloon. Each circle represents a galaxy in the Universe.
  2. Blow up the balloon halfway. Note what happens to the distance between the paper circles.
  3. Blow up the balloon fully. Note what happens to the distance between the paper circles dots.

Ask learners the following questions:

  1. What happened to the distance between the paper circles as you inflated the balloon?

    As the balloon was inflated the distance between the dots increased.

  2. What do you think would happen if you could inflate the balloon to an even larger size? The distance between the dots would increase even further.

  3. What do the paper circles represent and what does the inflating balloon represent?

The dots represent galaxies and the inflating balloon represents the expansion of the space between them. The balloon represents the expansion of the Universe.

Read interesting articles on the latest developments in astronomical research onSpace Scoop, an astronomy news service.

  • A galaxy is a collection of millions or billions of stars, together with gas and dust, held together by gravity.
  • Galaxies come in all shapes and sizes.
  • Our home galaxy, the Milky Way Galaxy, is a spiral galaxy containing around 200 billion stars. Our Sun is just one of those stars.
  • After the Sun, our nearest star is Alpha Centauri, the brighter of the two pointer stars in the Southern Cross Constellation
  • Light minutes, light hours and light years are used to measure distances in space because the distances are so immense.

    • A light minute is the distance that light can travel in one minute.
    • A light hour is the distance that light can travel in one hour.
    • A light year is the distance that light can travel in one year.
  • Beyond the Milky Way Galaxy, are many more galaxies.
  • Astronomers estimate the size of the observable Universe to be 93 billion light years in diameter.

Concept Map

Remember that you can also add your own notes to the concept maps to expand and personalise them.

Teacher's version

What is the name of our second closest star? How far away is it? [2 marks]

Proxima Centauri. 4.24 light years away.

What is the name of our second closest easily visible star? Is it really a single star? [2 marks]

Alpha Centauri. Alpha Centauri is actually a multiple star system containing the stars Alpha Centauri A and B closely orbiting each other. To the naked eye these two stars look like a single star. Proxima Centauri is also thought to be a member of this star system but it is farther away from the other two stars.

What is the definition of a light year? [2 marks]

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

What is a galaxy? [3 marks]

A galaxy is a massive collection of stars, dust and gas held together by gravity. A typical galaxy contains hundreds of billions of stars.

Where is the Sun located within the Milky Way? [2 marks]

It is located in the Orion spiral arm halfway out from the centre of the galaxy.

How many stars are in our Milky Way Galaxy? [1 mark]

200 billion.

Name the 4 main types of galaxies. [4 marks]

Elliptical galaxies, spiral galaxies, barred spiral galaxies and irregular galaxies.

What kind of galaxy is the Milky Way? [2 marks]

The Milky Way is a barred spiral galaxy.

Draw an image of the Milky Way Galaxy as viewed from the top and as viewed from the side. Note the position of the Sun in both images. Include the labels: spiral arm, bulge, disk. [8 marks]

Learners must draw the spiral shape of the galaxy from above. The exact positioning of the arms is not important, but learners must show the position of the Sun towards the edge of one of the arms, Orion. From the edge on, learners must show a flat disk with a bulge in the middle, and they must locate the position of the Sun towards the one side of the disk.

Why does it look as though the Milky Way is a splash of milk or a starry road across the sky? [2 marks]

The Milky Way Galaxy is a flat disk and when you look at the band of the Milky Way across the sky at night you are actually looking along the plane of the disk of the Galaxy in towards the centre where there is a high density of stars.

What is a group of galaxies? [2 marks]

A collection of galaxies, held together by gravity.

What is the name of the group of galaxies that the Milky Way is a member of? [1 mark]

The Local Group.

What are clusters of galaxies and superclusters of galaxies? [2 marks]

A cluster of galaxies is a collection of 50 or more galaxies held together by gravity. Clusters of galaxies often group together to form larger structures called superclusters of galaxies.

What is the size of the observable Universe? [1 mark]

The size of the observable Universe is 93 billion light years in diameter.

Bonus question: On the largest scales what does the Universe look like? Name the two types of structure which make up the Universe on the largest scales? [2 marks]

The Universe is made of thin walls called filaments which contain the galaxies and gas and dust. In between the filaments lie empty bubbles called voids.

Total [34 marks]

Total with extension [36 marks]