🌟 Outward pressure
Fusion in the core releases huge amounts of energy. This makes the hot gas push outwards.
Stellar Evolution » Life Cycle of Stars Like the Sun
What you'll learn this session
Study time: 30 minutes
Edexcel spec: 8.9
- Name the four stages in the life of a star like the Sun
- Explain how a nebula turns into a star
- Explain why a main sequence star is stable
- Describe how a star becomes a red giant and then a white dwarf
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Unlock This CourseStars are born, live and change
Stars do not last for ever. They form, they shine for a very long time, and then they change. A star's life is measured in billions of years, so no one can watch it happen. Astronomers work it out by looking at many stars at different stages.
This lesson is about stars of a similar mass to the Sun. A star's mass decides what happens to it. Stars much bigger than the Sun end their lives differently, and you will meet them in the lesson Life Cycle of Massive Stars.
A star like the Sun goes through four stages:
- nebula
- star (main sequence)
- red giant
- white dwarf
Key terms:
- Nebula: a huge cloud of dust and hydrogen gas in space.
- Main sequence star: a star in the long, stable part of its life, shining steadily.
- Red giant: a very large, cool, red star that forms when the hydrogen in the core of a main sequence star runs out.
- White dwarf: the small, hot, dense core left behind when a red giant loses its outer layers.
Stage 1: the nebula
Every star begins as a nebula. This is a cloud of dust and hydrogen gas, spread out over a huge region of space. The particles in the cloud attract each other because of gravity.
Gravity slowly pulls the cloud together. As the gas is squeezed into a smaller space, it gets hotter and hotter. Once the centre is hot enough, nuclear fusion starts. Remember, in fusion small nuclei join to make bigger ones and release energy. Here, hydrogen nuclei fuse to make helium. A star has now been born.
Learn this
Nebula: gravity pulls a cloud of dust and hydrogen gas together. It heats up until fusion starts.
Stage 2: the main sequence star
A star like the Sun spends most of its life as a main sequence star. Our Sun is a main sequence star and has been shining steadily for billions of years.
A main sequence star is stable because two effects balance each other:
🌎 Inward pull
Gravity pulls all the star's matter inwards, towards the centre.
Learn this
In a stable main sequence star, the outward pressure from fusion is balanced by the inward pull of gravity. The star stays the same size.
The star can stay like this for as long as it has hydrogen in its core to fuse.
Stage 3: the red giant
After billions of years, the hydrogen in the core runs out. Fusion in the core slows down, so the outward pressure falls. Gravity is no longer balanced by the outward pressure.
The star then swells up to become much, much larger. As it swells, the surface cools. A cooler surface glows red (remember from the lesson Star Colour and Temperature that red stars are the coolest). The star is now a red giant.
Stage 4: the white dwarf
The red giant does not last for ever. Its outer layers drift away into space. What is left in the middle is the hot, dense core of the star. This is a white dwarf.
A white dwarf is small but very dense and very hot. It slowly cools down as it gives out its remaining energy.
The whole life cycle
| Stage | What is happening |
|---|---|
| 1. Nebula | Gravity pulls a cloud of dust and hydrogen gas together. It heats up until fusion starts. |
| 2. Star (main sequence) | Outward pressure from fusion balances the inward pull of gravity. The star is stable. |
| 3. Red giant | Hydrogen in the core runs out. The star swells and cools. |
| 4. White dwarf | The outer layers drift away. The hot, dense core is left and slowly cools. |
Worked examples
Worked example 1: put the stages in order
Put these in the correct order: white dwarf, red giant, nebula, main sequence star.
Step 1: a star begins as a cloud, so the nebula is first.
Step 2: the stable, long-lived stage comes next, the main sequence star.
Step 3: when the hydrogen runs out the star becomes a red giant.
Step 4: the core left behind is the white dwarf.
Answer: nebula, main sequence star, red giant, white dwarf.
Worked example 2: explain a stable star
Explain why the Sun stays the same size for billions of years.
Step 1: fusion in the core gives an outward pressure.
Step 2: gravity gives an inward pull.
Answer: the outward pressure from fusion is balanced by the inward pull of gravity, so the Sun stays the same size.
Worked example 3: what makes a red giant
Describe what happens when a main sequence star runs out of hydrogen in its core.
Answer: the star swells and its surface cools, so it becomes a red giant.
Common mistakes
Common mistakes
- Leaving out gravity when describing how a star forms. Gravity is what pulls the nebula together.
- Saying a star is stable because "nothing is happening". Two things are happening and they balance each other.
- Saying a red giant is small. It is very large, and it is red because its surface is cool.
- Saying a white dwarf is still fusing hydrogen. It slowly cools.
- Mixing up the order of the stages. Learn it as nebula, main sequence, red giant, white dwarf.
Exam-style question
The Sun is a main sequence star.
(a) State what a nebula is. [1 mark]
(b) Explain why the Sun is stable. [2 marks]
(c) Describe what happens to a star like the Sun after the hydrogen in its core runs out. [3 marks]
Model answer
(a) A cloud of dust and hydrogen gas (1).
(b) The outward pressure from fusion (1) is balanced by the inward pull of gravity (1).
(c) The star swells and cools to become a red giant (1). The outer layers drift away (1). The hot, dense core that is left is a white dwarf, which slowly cools (1).
Exam tip
For a stable star, always name both effects that balance: outward pressure from fusion and inward pull of gravity. One alone will not get the marks.