# Life Cycle of Stars

In this worksheet, students will look at how new stars are born and the different end they may have according to their size.

Key stage:  KS 4

Curriculum topic:  Physics: Space Physics

Curriculum subtopic:  Features of the Solar System

Difficulty level:

### QUESTION 1 of 10

Space is amazing. It is so big that if you were able to run at the speed of light, then it would take you 100,000 years just to reach the other side of our galaxy. you know what, if you could go to the edge of the universe, then you would see space expanding - but it is expanding faster than the speed of light, so there are places in space that we will never see - even if we are able to travel to them, we still wouldn't be able to see them! Space is weird...

In the universe, there are billions (perhaps trillions!) of stars similar to our Sun. They are of different sizes; the Sun is a medium-sized star. All stars, including the Sun, will die one day. Their life cycle is at least a million years. The smallest stars are neutron stars, they are only around 20 km in diameter.

How are stars formed?

• A star starts its life as a cloud of gas and dust; they are called nebula clouds.
• Gravity pulls nebula clouds together, as a spinning ball of gas.
• The gas ball is so compacted that it gets hot and starts to glow, forming a protostar. At the beginning, protostars glow but cannot be seen, because they are surrounded by dust.
• After millions of years, the temperature in their core is so high that nuclear fusion (joining of hydrogen nuclei to form helium) takes place. This releases a massive amount of energy.
• The star is now visible and remains so for millions of years if it is small, and even longer (billions of years) if it is a big star. A star like our Sun may live for around 10 billion years.
• What happens at the end of a star's life depends on the size of the star.

How do stars die?

Medium-sized stars:

• The core of a medium-sized star (like our Sun) will eventually contract, while the outer part will expand.
• It changes colour from yellow to red and becomes a red giant. The Sun, for example, will become so big that it will cover Mercury and Venus and even reach Earth. Shells of gas thrown out from a red giant are called planetary nebula.
• Nebula from the Sun will stretch to the edge of the Solar System.
• The core of the original star shrinks to about the size of the Earth; it is very hot and shines brightly as a white dwarf.
• It does not make any energy, so it cools down and becomes yellow and then red and then forms a black dwarf.

Large stars:

• The core of massive stars contracts and the outer part expands forming a red supergiant, before it collapses, causing the whole star to explode and be thrown outwards.
• This explosion is called supernova.
• The remaining core is a neutron star.
• The material thrown out collides with gas and dust in space and it is called supernova remnant.
• More and more gas and dust merges with the remnant and eventually a new star is formed.
• The core continues to collapse, becomes so dense and gravitational forces are so big that not even light can escape. It becomes a black hole.

What size star is the Sun?

Small

Medium

Big

What is the name of the smaller stars?

Little stars

Newton stars

Neutron stars

What is the name of the clouds that a star starts its life in?

Nebula

Nimbula

Dust

What is a protostar?

A supernova

A compacted and hot gas ball that glows

A black hole

Why can protostars not be seen at first?

They absorb light

They explode

They are covered by dust

What is nuclear fusion?

Two hydrogen nuclei join to form helium

Two helium nuclei join to form hydrogen

Two lithium nuclei separate to form helium

What size of star forms a red giant?

Large

Medium sized

Small

What happens eventually to the original core of a medium sized star?

Black dwarf

Supergiant

Red giant

What does a large star form when it first expands before it dies?

Supernova

Supergiant

Black hole

Red giant

What happens in a black hole?

A huge explosion

Gravitational forces are so big that not even light can escape.

Dust and gas swirl together as a red giant.

• Question 1

What size star is the Sun?

Medium
EDDIE SAYS
The Sun is a medium-sized star. This is good for us as it means that the earth is just the right temperature for liquid water - the thing needed for life.
• Question 2

What is the name of the smaller stars?

Neutron stars
EDDIE SAYS
Smaller stars are called neutron stars. they may be small, but they pack a punch - a single cup full of a neutron star will weight the same a Jupiter. It will also have the gravity of Jupiter and you would be immediately crushed if you tried to take a dring from it.
• Question 3

What is the name of the clouds that a star starts its life in?

Nebula
EDDIE SAYS
Stars start their life as nebula clouds. They are super pretty, and some of them are shaped like animals - we have the horse head nebula and the crab nebula - look them up and get ready to squint, because no matter how much you tell me that the crab nebula looks like a crab, I have never been able to see it.
• Question 4

What is a protostar?

A compacted and hot gas ball that glows
EDDIE SAYS
A protostar is a compacted and hot gas ball that glows. It is just on the verge of becoming a star - this is why it is proto (meaning before).
• Question 5

Why can protostars not be seen at first?

They are covered by dust
EDDIE SAYS
Protostars cannot be seen when they are first formed because they are covered by dust. When they become a star, they push all of this dust away and this is what forms the planets.
• Question 6

What is nuclear fusion?

Two hydrogen nuclei join to form helium
EDDIE SAYS
Nuclear fusion takes place when two hydrogen nuclei join to form helium. It releases a lot of energy and heat and that is what makes the sun shine.
• Question 7

What size of star forms a red giant?

Medium sized
EDDIE SAYS
Medium sized stars form red giants, whereas large stars form supergiants. The bigest stars we know of (such as Beetlejuice) are red supergiants.
• Question 8

What happens eventually to the original core of a medium sized star?

Black dwarf
EDDIE SAYS
The core of the original star shrinks to about the size of the Earth; it is very hot and shines brightly as a white dwarf. It does not make any energy, so it cools down and becomes yellow and then red and then forms a black dwarf.
• Question 9

What does a large star form when it first expands before it dies?

Supergiant
EDDIE SAYS
The core of large stars contracts and the outer part expands, forming a red supergiant, before it collapses, causing the whole star to explode and be thrown outwards.
• Question 10

What happens in a black hole?

Gravitational forces are so big that not even light can escape.
EDDIE SAYS
Gravitational forces are so big that not even light can escape from a black hole.
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