1. The Life Cycle of Stars

2. Stars
 Stars are huge masses of gas with a massive gravitational pull that keeps the planets orbiting around them.
 They also give out massive amounts of heat, light and radiation.
 To understand how and why stars evolve over a long time, we need to understand how they generate this heat and radiation.
 The process which stars use to give out this energy is called nuclear fusion

3. Stage 1: Nebula Stars form in clouds of dust and gas called nebulae.
We think this happens in the spiral arms of galaxies
Our Sun formed about 4,600 Million years ago.
Gravitational forces cause the large masses of gas to be squashed together.

4. Stage 2: Protostar Dense regions in the clouds collapse due to gravity
As it gets smaller the protostar at its centre gets hotter
Initially the gravitational energy of the collapsing star is its source of energy
Once the star contracts enough that its central core can burn hydrogen to helium, it becomes a "main sequence" star.

5. Stage 3: Main Sequence
The protostar grows and eventually reaches equilibrium
It becomes white hot and nuclear fusion starts
These fuse H to He for a long time.
Our Sun is in this stage
Since stars have a limited supply
of hydrogen in their cores, they
have a limited lifetime as main
sequence stars.

6. Stage 4: Red Giant Once the star runs out of fuel it expands and cools
There are two directions stars can take depending on their size.
Small stars like that of our sun:
Stage 4: Red Giant
Once the star runs out of fuel it expands and cools
It becomes a red giant
When this happens to our Sun it will expand and kill any life on Earth

7. Stage 5: White Dwarf Stage 6: Black Dwarf
The red giant uses up all its fuel and collapses under gravity
It forms a hot, dense white dwarf
Stage 6: Black Dwarf
The white dwarf cools to become a black dwarf
The whole life cycle for a star like our Sun lasts for approximately 10 billion years

8. Stage 4: Red Supergiant Stage 5: Supernova
Stars with a large mass:
Stage 4: Red Supergiant
Large stars run out of fuel quicker than smaller stars
They expand and cool to form red supergiants
Stage 5: Supernova
Red supergiants run out of fuel, become unstable and collapse.
They blow off their outer layer in an explosion called a supernova
Supernova can last for weeks

9. Two things may happen:
If the star was not massive a neutron star will be the result
Not massive is about ten times the mass of the Sun!
If the star was very massive it will result in a black hole
The life cycle of a star 10 – 30 times the mass of the Sun is only 5 million years