Equilibrium in Stars (Edexcel GCSE Physics)

• Stars are held together by a delicate balance of inwards and outwards forces
• One of these forces is the force of gravity
  • This is an attractive force which pulls the outer layers inwards

• The other force is the force from the pressure caused by the thermal expansion
  • This is an outward force which is exerted from the expanding hot gases inside the star

• When the inward pull of gravity and the force from the outward thermal expansion acting on the star are equal, the star will be in equilibrium
• This is how the stars in the main sequence remain stable for millions of years

Forces acting within a star. The centre red circle represents the star’s core and the orange circle represents the stars outer layers

• If the temperature of a star increases, the outward pressure will also increase
  • This will cause the star to expand

• If the temperature drops the outward pressure will also decrease
  • This will cause the star to contract

• As long as these two forces balance, the star will remain stable
• Once these forces are unbalanced, then they will no longer be in equilibrium causing the star to expand or contract
  • This happens when fusion in the core of stars, and hence thermal expansion, ceases at the end of the star's life