In Grades 6 and 8 learners covered material regarding the solar system including the Sun. In Grade 7, they focused on the system which includes the Sun, Earth and Moon. Learners should be familiar with the fact that the Sun is a star and produces heat and light (energy) via nuclear reactions. In this chapter the focus is on the life cycle of stars, including how they are born and die. The exact evolution that a star follows depends on the initial mass of the star. The Sun"s evolution is presented as an example. The main aims of this chapter are to ensure that learners understand the following:

stars are born in vast clouds of gas and dust

stars spend most of their lives on the main sequence fusing hydrogen gas to helium gas

stars eventually swell up to form a red giant star stars like the Sun end their lives as planetary nebulae and white dwarfs

Some learners may ask why stars look "spiky" in the photographs from telescopes, but in the diagrams shown here, they are presented as spheres. Watch this video to find out and explain to your learners:

Do you think it is important to teach astronomy to learners at school? Read this interesting and informative article detailing the benefits and applications of astronomy:

5.1 The birth of a star (0.5 hours)

5.2 Life of a star (1 hour)




Activity: Observing Orion in the spring sky


CAPS suggested

5.3 Death of a star (1.5 hours)




Activity: Life cycle of a Sun-like star

observing, investigating


Activity: The life cycle of the Sun

observing, writing


Activity: Flow diagram poster showing the lifecycle of a Sun-like star

writing, drawing, sequencing

CAPS suggested

A good way to introduce the topic of stellar evolution is to start by asking learners how long they think stars last . Many will answer forever. Many people are unaware that, like humans, stars are born, live their lives and then die. You can also ask them what is meant by "living" when referring to a star, after all, stars do not perform the seven life processes, as taught in Life and Living. Astronomers generally consider stars that are undergoing nuclear reactions in their cores to be living stars.

You are watching: Which event marks the birth of a star

Stars are also compared in terms of relative concepts, such as:

young and old cool and hot how big they are how massive they are (the mass is important in terms of looking at how stars die)

Where are stars born? Can we talk about a star as "living"? How long do stars like the Sun live? How do stars spend most of their life? Why are stars different colours? How do stars die?
Stars do not live forever, just like people. Stars are born, live their lives, changing or evolving
as they age, and eventually they die. Often stars do this in a much more spectacular way than humans do!

In this chapter, you will notice that many nouns are used as adjectives, for example, Sun is the noun and solar is the adjective. Other noun and adjective pairs include: moon and lunar, star and stellar, planet and planetary.

Scientists speak of stellar evolution when talking about the birth, life and death of stars. The lifetime of individual stars is way too long for humans to observe the evolution of a single star, so how do scientists study stellar evolution? This is possible as there are so many stars in our galaxy, so we can see lots of them at different stages of their lives. In this way, astronomers can build up an overall picture of the process of stellar evolution. In this chapter you will discover how stars are born, how they evolve, and how they die.

Lots of work went into figuring out the processes in stellar evolution. This work is still on going. What you are learning here in Natural Sciences is based on years of research, and current research is constantly taking place, updating what we know.

In this section learners will discover that stars are born in giant clouds of dust and gas, called nebulae, in space. In order to understand how collapsing gas clouds heat up to eventually form stars, learners need to understand that compressing a gas heats it up and that allowing a gas to expand cools it down. If they are unfamiliar with this concept a good analogy is to think about over-inflating a bicycle tyre (without bursting it). You could demonstrate this in class by getting learners to slightly over-inflate a tyre. They will find that the pump and tyre get hot!

In the case of inflating a tyre, you are forcing more and more molecules into a given volume (assuming that the tyre is now at full capacity). So you are compressing or squeezing the gas. Each molecule has a certain amount of kinetic energy. As more molecules are forced in by the pump, the air in the tyre is compressed and the total thermal energy increases because there are more molecules colliding inside the tyre. As more particles are contained in the same volume, the air"s temperature in the tyre increases. As you deflate the tyre, you allow the gas to expand, the molecules are more spread out. There is then less thermal energy and so the temperature decreases. You could let students feel the air as it is released from the tyre - it should be colder than the ambient air as it is rapidly expanding as it escapes from the tyre.

Stars are born in vast, slowly rotating, clouds of cold gas and dust called nebulae (singular nebula). These large clouds are enormous, they have masses somewhere between 100 thousand and two million times the mass of the Sun and their diameters range from 50 to 300 light years across.

A light year is the distance that light travels in one year. Light travels extremely fast at 299 792 458 m/s. One light year is equivalent to 10 trillion kilometers.

The "Pillars of creation". These giant, dense dusty clouds of hydrogen gas are vast stellar nurseries where new stars are born. (NASA)

A famous example of one of these huge clouds is the Orion nebula in the constellation of Orion. It is visible with the naked eye if the sky is dark enough. These clouds are so massive that they can collapse under their own gravity if they are disturbed.

The collapse of a star can be triggered when the cloud is squeezed. For example if a cloud passes through a spiral arm in a galaxy it will be slowed down and compressed. This explains why lots of stars are formed in the spiral arms of galaxies.

The constellation of Orion as viewed from the southern hemisphere. The Hunter Orion is "upside down" when viewed from the south and his sword lies above the three stars in his belt. The jewel in his sword which looks like a white-pink smudge is the Orion nebula.
This diagram shows how the stars make up the constellation of Orion, as seen in the southern hemisphere.

Over time the clouds contract, become denser and slowly heat up. The clouds also break up into smaller clumps. As the clumps get smaller they begin to flatten out into a disk shape. The centre of each clump will eventually contain a star and the outer disk of gas and dust may eventually form planets around the star.

Hubble Space Telescope image of the Orion Nebula showing different protostars surrounded by a dark disk of gas and dust. These disks (called protoplanetary disks) may eventually form planets around the star.

As the contracting clump continues to heat up, a protostar is formed at the centre. A protostar is a dense ball of gas that is not yet hot enough at the centre to start nuclear reactions. This stage lasts for roughly 50 million years. As the collapse continues, the mass of the protostar increases, squeezing it further and increasing the temperature. If the protostar is massive enough for the temperature to reach 10 million degrees Celsius, then it becomes hot enough for nuclear reactions to start and the protostar will will technically be referred to as a star.

Not as well known as its star formation cousin Orion, the Corona Australis region, with the Coronet cluster at its centre, is one one of the nearest and most active star formation regions to us. This image shows the young stars at the centre, with gas and dust emissions.

The Coronet cluster, shown in the image, has a host of young stars at different life stages, which allows astronomers to gather data and pinpoint details of how the youngest stars evolve.

Do you remember that we learned about nuclear reactions last term in Energy and Change when looking at nuclear power plants?

The young star starts converting hydrogen to helium via nuclear fusion reactions. Nuclear reactions in stars produce vast amounts of energy in the form of heat and light, which is radiated into space. This energy production prevents the star from contracting further. As the star shines, the disk of dust and gas surrounding the star is slowly blown away by the star"s stellar wind which leaves behind any planets if they have already formed.

A large bubble of hot gas rising from glowing matter in a galaxy 50 million light years from Earth. Astronomers suspect the bubble is being blown by stellar winds, released during a burst of star formation.

Just like the Sun loses particles into space in the form of the solar wind, other stars also have winds called stellar winds.

Star formation in the nearest galaxy outside the Milky Way, called the Large Magellanic Cloud (LMC), taken with the Hubble Space Telescope. This image shows glowing gas, dark dust clouds and young, hot stars.

In the upper left of the image of Large Magellanic Cloud, you can see a collection of blue and white young stars. They are extremely hot and are some of the most massive stars known anywhere in the Universe.

The image shown here of the Large Magellanic Cloud, a satellite galaxy to the Milky Way, illustrates very clearly, an example of sequential star formation, where new star birth is triggered by the previous generation of massive stars. You can point some of these observations out to learners:

Just below the cluster of hot stars in the top left, is an area of brightly emitting hydrogen gas, illuminated by the nearby hot stars. Further to the right are several smaller dark dust clouds with odd shapes. They can be seen silhouetted against the glowing gas. Several of these dark clouds have a bright rim as they are illuminated and being evaporated due to the action of radiation from neighboring hot stars. The region around the cluster of hot stars in the image is relatively clear of gas as the stellar winds and radiation from the stars have pushed the gas away. When this gas collides with and compresses surrounding dense clouds, the clouds can collapse under their own gravity and start to form new stars. The cluster of new stars in the upper left may have been formed this way, as it is located on the rim of the large central interstellar bubble of the complex. The stars in this cluster are now beginning to clear away the cloud from their birth, and are producing new opportunities for subsequent star birth.

Curious about the Universe, but don"t know where to start? Have a look at this step-by-step guide to becoming an awesome amateur astronomer.

Life of a star

This section covers the main stages of a star"s life, from infancy to old age. Learners will also discover why stars do not all look the same and why they evolve at different rates and have different lifetimes: it is a consequence of having different masses. They will learn how important the mass of a star is in determining its evolution and observable characteristics.

A star is considered to be "born" once nuclear fusion reactions begin at its centre. Initially hydrogen is converted to helium deep inside the star. A star that is converting hydrogen to helium is called a main sequence star. Stars spend most of their lives as main sequence stars, converting hydrogen to helium at their centres or cores. A star may remain as a main sequence star for millions or billions of years.

Most of the stars in the Universe, about 90 %, are main sequence stars. The Sun is a main sequence star.

Main sequence stars are not all the same. They have different masses when they are born, depending on how much matter is available in the nebula from which they formed. These stars can range from about a tenth of the mass of the Sun up to 200 times as massive. Different mass stars have different observable properties.

Main sequence stars come in different sizes and colours. Their sizes range from around 0.1 to 200 times the size of the Sun. Their surface temperatures determine their colours and can range from under 3000°C (red) to over 30 000 °C (blue).

We normally associate red with being hot and blue with being cold. But, in stars, the bluer the star, the hotter it is, and the redder it is, the older and colder it is.

Main sequence stars also have different colours, depending on the temperatures of their surfaces. Look at the following picture and correctly label the temperatures of all the stars using the list of temperatures below. Which star represents our Sun?

Temperature list: 3000 °C, 4500 °C, 6000 °C, 10 000 °C, 40 000 °C


The following image shows the correct labels for the temperatures of different stars:


The yellow star represents our Sun.

Why are hotter stars bluer in colour? Can you remember what you learnt about the spectrum of visible light in Grade 8? The colour blue corresponds to light at shorter wavelengths (higher frequencies) than the colour red. Shorter wavelengths (higher frequencies) correspond to higher energies and thus hotter temperatures. This is also seen in the flames of a fire or candle. If you look at the flames, the central regions are bluer (and hotter) than the outer regions, which are orange and yellow.

This artist"s impression shows the relative sizes of young stars, from the smallest "red dwarfs", at about 0.1 solar masses, low mass "yellow dwarfs" such as the Sun, to massive "blue dwarf" stars weighing eight times more than the Sun, as well as the 300 solar mass star named R136a1.
Observing Orion in the spring sky

Orion is an easily recognisable constellation visible in cities as well as in dark skies. In this activity learners will have to look at the night sky to spot the constellation and identify the stars Betelgeuse and Rigel and note their difference in colour. Orion is up in the east from around 00:30 at the beginning of October, however as the months progress it rises earlier. By the beginning on December Orion is visible from around 20:30 in the east. If observing the constellation is unfeasible, you could ask learners to look at the image of the constellation in this chapter instead.

This is the first direct image of a star other than the Sun, made with NASA"s Hubble Space Telescope. This is Betelgeuse, the star marking the shoulder of Orion, which we see in the bottom right of the constellation, when viewing Orion in the southern hemisphere.

Betelgeuse is so huge that, if it replaced the Sun at the center of our solar system, its outer atmosphere would extend past the orbit of Jupiter (see the scale at lower left of the image).


sky map


A clear sky is necessary for this task. Look outside at night towards the east and identify the constellation of Orion. A photograph of the constellation is included in this chapter for reference. Identify the stars Betelgeuse and Rigel.

At the beginning of October Orion is visible in the east from around 00:30 until morning. From the beginning of November Orion is visible in the east from around 22:30 and from the beginning of December it is visible in the east from around 20:30.

How long a main sequence star lives depends on how massive it is. More massive stars move onto the next stages of their lives more quickly than lower mass stars. In fact they are main sequence stars for a shorter time than lower mass stars.

A higher-mass star might have more material, but it also uses up the material more quickly due to its higher temperature. For example, the Sun will spend about 10 billion years as a main sequence star, but a star 10 times as massive will last for only 20 million years. A red dwarf, which is half the mass of the Sun, can last 80 to 100 billion years.

When the hydrogen in the centre of the star is depleted, the star"s core shrinks and heats up. This causes the outer part of the star, the star"s atmosphere, which is still mostly hydrogen, to start to expand. The star becomes larger and brighter and its surface temperature cools so it glows red. The star is now a red giant star. Betelgeuse, as you observed in the last activity, is a red giant star.

A colourful view of the globular star cluster NGC 6093 in the Milky Way, containing hundreds of thousands of ancient stars. Especially obvious are the bright red giants, which are stars similar to the Sun in mass that are nearing the ends of their lives.

Globular clusters are particularly useful for studying stellar evolution, since all of the stars in the cluster have the same age (about 10-15 billion years), but cover a range of stellar masses.

It is called a giant because the outer layers have expanded outwards and the star has got much larger than it was when it was a main sequence star.


Eventually the core of the star becomes hot enough for the next nuclear reaction to start: atoms of helium collide and fuse into heavier elements such as carbon and oxygen. However, eventually the helium in the core will also be depleted. From this point onwards, the fate of the star is determined by its mass.

For medium-sized stars, such as the Sun, the temperature in their centres will never get high enough to fuse the newly-formed carbon and oxygen into heavier elements and so they do not evolve much further. Following the red giant phase, the star becomes unstable and will eventually die as you will discover in the next section.

Scroll through this interactive animation to get a sense of the scale of some of the stars and other objects in our Universe.

The animation listed in the Visit box provides a very useful tool to give learners a sense of the scale of the Universe. If possible, you can project it up in your classroom and scale through it from a human all the way out until you get to some of the massive supergiants, and then beyond. You will also be able to see the scale of some of the objects mentioned in this chapter, such as the Crab Nebula, the Large Magellanic Cloud and Pillars of Creation.

The relative sizes of the Earth, the present day Sun and a red supergiant star, Canis Majoris, in the constellation. The Sun will eventually evolve into a red giant star in about 4.5 billion years time.
In this section learners will discover how stars die. The focus is on the death of a low mass star like the Sun. However, for completeness, the way that high mass stars die is also briefly mentioned. There are two activities in this section related to the life of Sun-like stars. Both of these are intended to help learners remember and understand the sequence of phases that a star like the Sun undergoes during its life. There is a lot of unfamiliar terminology in stellar evolution and it can be confusing for learners. Hopefully by doing activities rather than simply reading about the different stages in a Sun-like star"s evolution, learners will find the subject easier to understand.

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

As a star enters the final stages of its life, after it has become a red giant, the star becomes unstable and expands and contracts over and over. This causes the star"s outer layers to become detached from the central part of the star and they gently puff off into space. When the last of the gas in the star"s outer layers is blown away, it forms an expanding shell around the core of the star called a planetary nebula. Planetary nebulae glow beautifully as they absorb the energy emitted from the hot central star. They can be found in many different shapes, as shown in the following images.

The plural of nebula is nebulae. Planetary nebulae have nothing to do with planets but were named like this in the 1700s because they resembled planets when observed with the telescopes of the time.

A planetary nebula is different to a stellar nebula. A stellar nebula is where stars are born, whereas a planetary nebula is what some stars form at the end of their lives.

The beautiful Ring Nebula. The gas is lit up by the light from the central star which is the faint white dot in the centre of the nebula.
The Boomerang Nebula is a young planetary nebula and the coldest object found in the Universe so far.
Kohoutek 4-55 Nebula contains the outer layers of a red giant star that were expelled into interstellar space when the star was in the late stages of its life.
The Butterfly Nebula. The dying central star itself cannot be seen, because it is hidden within a doughnut-shaped ring of dust.
The Dumbbell Nebula.
The Helix Nebula.

The Butterfly Nebula is a dying star that was once five times the mass of the Sun. What resembles the butterfly wings are actually hot clouds of gas tearing across space at almost 1 million km an hour - fast enough to travel from Earth to the Moon in 24 minutes!

Some time after puffing off its outer layers, the central star will run out of fuel. When this happens the central star begins to die. Gravity causes the star to collapse inwards and the star becomes incredibly dense and compact, about the size of the Earth. The star has then become a white dwarf star.

An ultraviolet image of the Helix Nebula. As the star in the centre approaches the end of its life and runs out of fuel, it shrinks into a much smaller, hotter and denser white dwarf star.

White dwarfs have this name because of their small size and because they are so hot that they shine with a white hot light. The central parts of stars are much hotter than their surfaces, and a white dwarf is made from the remaining central parts of a star which explains why they are so hot.

The following image shows the relative size of Sirius B, a nearby white dwarf star, compared to some of the planets in our solar system. Stars and stellar remains can be smaller than planets.


White dwarfs no longer produce energy via nuclear reactions and so as they radiate their energy into space in the form of light and heat. They slowly cool down over time. Eventually, once all of their energy is gone, they no longer emit any light. The star is now a dead black dwarf star and will remain like this forever.

White dwarf stars are so dense that one teaspoon of material from a white dwarf would weigh up to 100 000 kg.

This activity can be performed in pairs or small groups. This activity demonstrates the life of a Sun-like star using a yellow balloon to represent the Sun. Learners must follow the instructions to demonstrate each of the phases that a star like the Sun goes through during its life. This activity is best completed in pairs where one member "gives the orders" and the other member completes the activity. If you have time you can repeat the activity, swapping the pairs around.


yellow round balloon - one per pair or group black marker red marker scissors 2 cm small white styrofoam ball - one per pair


In this activity you will work in pairs. One of you will instruct your partner using the instructions below. Your partner will follow your instructions. Decide which of you will be the instructor and which of you will be the experimenter. Experimenter: Insert the white styrofoam ball into the deflated balloon. Instructor: Read out the step-by-step instructions from the table below (listed in order). First state the time from the star"s birth which is given in the left hand column, then tell your partner what to do with the balloon. Experimenter: Follow the instructions from your partner very carefully. You will be demonstrating how a Sun-like star evolves over time.

Step Number


1) Star is born

Blow up the balloon to about 6 cm in diameter

2) 5 million years


3) 10 million years


4) 500 million years

Wait - planets are being formed around the star.

5) 1 billion years

Blow the balloon up a little bit

6) 9 billion years

Blow up the balloon some more and colour it

red - it is now a red giant star

7) 10 billion years

Blow the balloon up a little bit. The outer layers are now being blown off. To simulate this, slowly allow the balloon to deflate. Cut the balloon into pieces and scatter them around the white ball. The star has now become a white dwarf (the ball) surrounded by a planetary nebula (the pieces of balloon).

8) 50 billion years

Move the planetary nebula farther away from the white dwarf.

9) 500 billion years

Remove the planetary nebula and colour the ball

black - the star is now a black dwarf.

The different stages of evolution of a star like the Sun are summarised in the diagram below and compared to the lifecycle of a person.

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Let"s take a closer look at the life of our star, the Sun.

The life cycle of the Sun


The diagram below shows the life of our Sun. The Sun is a common type of star of average size and mass. Complete the sentences by filling in the gaps which summarize the evolution of our Sun over time.