The life cycle of stars (AQA GCSE Physics): Revision Notes
📚Revision Notes
The life cycle of stars
Stars change throughout their lives. What happens to a star depends on how much mass it has when it forms.
How stars are born
All stars begin the same way. A cloud of dust and hydrogen gas called a nebula gets pulled together by gravity. As the dust and gas squeeze together, they heat up. When it gets hot enough, nuclear fusion starts. This creates energy and makes the star shine.
Note
The process from nebula to shining star can take millions of years. The temperature in the core must reach about 10 million degrees Celsius before nuclear fusion can begin.
Low-mass stars
Low-mass stars have up to four times the mass of our Sun. Our Sun is a low-mass star. It formed about 4.6 billion years ago and is roughly halfway through its life.
Stage 1: Main sequence star
- Nuclear fusion turns hydrogen into helium in the star's core
- This releases huge amounts of energy
- The star gives out light and heat
- Most stars spend most of their lives in this stage
- There's a balance between gravity pulling inwards and fusion energy pushing outwards
Stage 2: Red giant
- The hydrogen fuel starts to run out
- The star expands and becomes much larger
- The outer layers cool down and turn red
- Other heavier elements start to form through fusion
Example
Example: Our Sun's Future
In about 5 billion years, our Sun will enter the red giant phase. It will expand so much that it may engulf Mercury and Venus. The Earth's oceans will boil away as the Sun becomes hundreds of times brighter than it is today.
Stage 3: White dwarf
- All nuclear fusion eventually stops
- The star collapses under gravity
- It becomes very small and dense
- A white dwarf is very hot but gradually cools down
Stage 4: Black dwarf
- The white dwarf cools completely
- It stops glowing
- This takes billions of years to happen
High-mass stars
High-mass stars are much more massive than our Sun. They live shorter lives but die more dramatically.
Important
High-mass stars live much shorter lives than low-mass stars. While our Sun will live for about 10 billion years, a star 20 times more massive might only live for 10 million years - a thousand times shorter!
Early stages
- High-mass stars start the same way as low-mass stars
- They go through the main sequence stage much faster
- Everything happens more quickly because they have more mass
Red supergiant stage
- Instead of becoming a red giant, they become a red supergiant
- These are much bigger than red giants
- They burn through their fuel very quickly
Supernova explosion
- When fusion stops, the star collapses rapidly
- This causes a massive explosion called a supernova
- The explosion is incredibly bright and can be seen from far away
- Heavy elements get scattered throughout space
Final stage
After the supernova, what's left depends on the original mass:
- Neutron star: If the remaining core is less than four times the Sun's mass
- Black hole: If the remaining core is more massive
Why stars matter
Stars are element factories. Nuclear fusion in stars creates all the naturally occurring elements. When massive stars explode as supernovae, they spread these elements throughout the universe. This is how heavier elements like iron and gold formed.
Note
Studies show that every element in your body (except hydrogen) was created inside a star. The calcium in your bones, the iron in your blood, and the oxygen you breathe were all forged in the nuclear furnaces of ancient stars. As astronomer Carl Sagan famously said, "We are made of star stuff."
Remember!
Summary
Key Points to Remember:
- Star fate depends on mass: Low-mass stars become white dwarfs, high-mass stars explode as supernovae
- All stars start as nebulae: Gravity pulls dust and gas together until fusion begins
- Main sequence is the longest stage: This is when stars burn hydrogen and shine steadily
- Heavy elements come from stars: Fusion in stars creates elements, supernovae spread them around
- Our Sun is middle-aged: It's a low-mass star about halfway through its life cycle