Stability of Stars (WJEC GCSE Physics: Combined Science)

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

• The other force is due to gas and radiation pressure
  • This is an outward force which is exerted by the expanding hot gases inside the star

Equilibrium in Stars

• When the inward pull of gravity and the outward radiation pressure acting on the star are equal, the star is said to be in equilibrium

Balanced forces in a stable star

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

• If the temperature of a star increases (for example, if the rate of fusion speeds up), the outward pressure will also increase
  • This will cause the star to expand

• If the temperature drops (for example, if the rate of fusion slows down) the outward pressure will also decrease
  • This will cause the star to contract

Nuclear Fusion in Stars

• In the centre of a stable star, hydrogen nuclei undergo nuclear fusion to form helium nuclei
• The equation for the reaction is:

• These isotopes of hydrogen, and , are known as deuterium and tritium
• A huge amount of energy is released in the reaction
• This energy provides the radiation pressure that prevents the star from collapsing under its gravity

Nuclear fusion in a stable star

The fusion of deuterium and tritium to form helium with the release of energy

Formation of elements lighter than iron

• When the supply of hydrogen begins to run out, fusion reactions of heavier elements begin to take place
• For example, two helium nuclei (produced by the fusion of 2 hydrogen nuclei) could fuse together to form a beryllium nucleus

• The beryllium nucleus could then fuse with a helium nucleus to form a carbon nucleus

• Elements lighter than iron are formed in fusion reactions like the ones above

Formation of elements heavier than iron

• Elements heavier than iron are produced in supernova explosions
  • A supernova occurs at the end of a massive stars life
  • When the star explodes it releases very large amounts of energy and neutrons

• All of the elements which have been produced by the fusion reactions are ejected into space and combine with neutrons to form the heaviest elements