Red Giant

 

After 10,000 million years the hydrogen is used up in the core. This means that gravity becomes stronger than the outward pressure and the core begins to collapse.

As the core collapses it becomes hotter and hotter until helium 'burning' can occur. This increases the temperature even more and prevents further collapse by increasing the outward pressure due to heat and radiation.

During this time the outer layer of the star is now heated even more by the core and expands considerably. The outer layer cools to a red colour and the star is known as a red giant.